Smart and Multi-Functional Magnetic Nanoparticles for Cancer Treatment Applications: Clinical Challenges and Future Prospects

Iron oxide nanoparticle (IONPs) have become a subject of interest in various biomedical fields due to their magnetism and biocompatibility. They can be utilized as heat mediators in magnetic hyperthermia (MHT) or as contrast media in magnetic resonance imaging (MRI), and ultrasound (US). In addition, their high drug-loading capacity enabled them to be therapeutic agent transporters for malignancy treatment. Hence, smartening them allows for an intelligent controlled drug release (CDR) and targeted drug delivery (TDD). Smart magnetic nanoparticles (SMNPs) can overcome the impediments faced by classical chemo-treatment strategies, since they can be navigated and release drug via external or internal stimuli. Recently, they have been synchronized with other modalities, e.g., MRI, MHT, US, and for dual/multimodal theranostic applications in a single platform. Herein, we provide an overview of the attributes of MNPs for cancer theranostic application, fabrication procedures, surface coatings, targeting approaches, and recent advancement of SMNPs. Even though MNPs feature numerous privileges over chemotherapy agents, obstacles remain in clinical usage. This review in particular covers the clinical predicaments faced by SMNPs and future research scopes in the field of SMNPs for cancer theranostics.


Introduction
Cancer is a serious burden of disorder which has become one of the greatest dilemmas to tackle globally [1]. As stated by GLOBOCAN 2020, the World Health Organization and the American Cancer Society Database for 36 cancer types in 185 countries, there has been an approximated 19.3 million new incidences in 2020. The death rate of cancer is about 10.0 million new cases each year, with the lung carcinoma in the lead, followed by colorectal, liver, stomach, and breast carcinoma, (18%, 9.4%, 8.3%, 7.7%, and 6.9%, respectively). It is predicted there will be about 28.4 million new annual cases by 2040 worldwide. The rise of about 47% in cancer cases from 2019 to 2020, with a significant expansion in developing nations (64% to 95%) compared to advanced nations (32% to 56%) could be subsequently ascribed to globalization and the economy [2][3][4][5][6][7]. The main risk elements of carcinoma formation are genetic and epigenetic modification [8,9]. Epidemiological reports have highlighted that 35% of the mortalities are due to mode of living, e.g., smoking, alcohol, unhealthy diet, repetitive application of solarium/tanning equipment, or subjection to chemical poisoning, infectious agents, or radiation [10,11].
Despite gaining exceptional knowledge about the initiation, progression, and resistance to treatment, our failure or incapability to permanently cure metastatic cancer indicates an inadequate understating of its intricacy [12,13]. Anti-neoplastic medicines often

Inorganic Coating
The application of inorganic coatings such as gold, silver, or silica can improve the functionality and stability of MNPs in an aqueous solution. For instance, coating IONPs with gold can provide many advantages due to the unique characteristics of gold, such as magnetism, low toxicity, a capability to react with biological molecules, and surface plasmonic resonance which can facilitate optical features [95,96].
Si coatings have proven to be highly biocompatible and chemically stable in an aqueous environment [97,98] and have received Food and Drugs Administration (FDA) approval, e.g., food additives [99]. Si shell prevents oxidation and erosion at the same time reduces the cytotoxicity of IONPs [100][101][102]. For example, Si-layered Fe3O4 did not pro-

Inorganic Coating
The application of inorganic coatings such as gold, silver, or silica can improve the functionality and stability of MNPs in an aqueous solution. For instance, coating IONPs with gold can provide many advantages due to the unique characteristics of gold, such as magnetism, low toxicity, a capability to react with biological molecules, and surface plasmonic resonance which can facilitate optical features [95,96].
Si coatings have proven to be highly biocompatible and chemically stable in an aqueous environment [97,98] and have received Food and Drugs Administration (FDA) approval, e.g., food additives [99]. Si shell prevents oxidation and erosion at the same time reduces the cytotoxicity of IONPs [100][101][102]. For example, Si-layered Fe 3 O 4 did not produce a major toxicity effect to osteoblast cells and also did not modify the secretion of collagen by cells. In addition, shielding superparamagnetic IONPs (SIONPs) with Si reduced the deterioration of the core, subsequently extending practice in MRI utilization [103]. Nevertheless, there is spreading apprehension regarding their toxicity to the immune cells. Some studies identified the toxicity induced by Si-NPs to monocytes [104], microglia [105], and Kupffer cells [106] which are all size dependent. The immunotoxicity to organs was also assessed by the intravenous administration of Si-NPs which raised the abundance of mast cells in the lung [107] and heart [108]. Oxidative stress [109], pro-inflammatory effects [110], and autophagy [111] are recognized as fundamental systems provoking immune toxicity. Efforts have been made to minimize the toxicity, induced by Si. Park et al. [112] who developed a simple and efficacious pathway to graft Si-NPs with a purified protein layer to alleviate intrinsic immune responses [112].

Organic Coating
The application of organic materials to coat IONPs such as polyethylene glycol (PEG) and dextran (Dex) has gained high interest amongst other polymers and organic materials. They are regarded as safe agents, and will not be quickly identified by macrophages in the liver/spleen and have longer BCT. Although, the direct cytotoxicity of Dex has not been reported, its degradation may have a direct effect on specific cellular processes [67,113]. In addition, biopolymer chitosan is a non-toxic, biocompatible, biodegradable compound, and is viewed as a sustainable and economical material. Plus, it has immense chemical structural possibilities, e.g., its hydroxyl and amino groups can form complexes with Fe 3 O 4 NPs, increasing the hydrophilicity, stability, and biocompatibility of IONPs [114].
Surfactants can form nanocomposites with IONPs, making them sensitive towards external stimuli/internal, e.g., MF, electric fields, optical sources. The utilization of surfactants during the preparation process of IONPs facilitates a suitable coating and de-aggregates the particles. For example, the attachment of citric acid on the surface of IONPs during physical gas-phase synthesis relatively decumulated the particles [115]. In addition, the encapsulation of IONPs by surfactants can control their content release [116,117]. OlA as a capping agent can form a hydrophobic coating and its polar end can bond to the surface of IONPs, forming strong monolayer nanocomposites that can increases the consistent dispersion of MNPs in a polymer matrix of surfactant solution [118,119]. Furthermore, Kockar et al. [120] investigated the effect of tartaric and ascorbic acids as biocompatible surfactants on the characteristics of SIONPs. The surfactants increased the magnetic saturation but remained superparamagnetic, thus holding potential for biological utilization [120].
In addition to classical IONPs surface-coating agents, stimuli-sensitive/smart polymers have been designed to have fast physiochemical transitions in the surrounding tumor microenvironment (TME). Their smart chemistry is highly appealing to fabricate SMNPs since it allows a controlled and targeted distribution of pharmaceutical cargo at TME [121,122]. They can form conjugations or complexes, or become attached to biologically active molecules, e.g., nucleic acids, proteins, peptides, and carbohydrates for the purpose of wound-healing, tissue regeneration, and neoplastic medicine [123][124][125]. For example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. example, a small hydrodynamic-size citric acid coating improved the heating efficacy of IONPs, equipped it as heat mediator in MHT, and reduced the particles' aggregation while increasing the magnetization saturation [115].

Stimuli-Triggered SMNPs
SMNPs are sensitive to the differences between the intra/extracellular surroundings of malignant cells and are smart enough to implement structural transitions in response to a stimulant [128]. They could be equipped with coating agents/materials with sensitive linkers that have an innate sensitivity to the internal triggers in TME such as redox, concentration, pH, and enzyme levels [129]. Moreover, exogenic stimuli, e.g., light, heat, MF, and US can restrict early cargo release and facilitate an effective site-specific agent release [130]. The most common stimuli-responsive functional groups are collected in Table 1. In the following sections, different categories of stimuli-sensitive (pH, redox, enzyme, light and ultrasound, dual/multi-modal stimuli) SMNPs are described in detail.

Thermo-Responsive MNPs
Temperature is a vital element to vent the drug into the TME which has a temperature greater (~40-42 °C) than healthy tissues (37 °C) [131]. Thermo-responsive (TR) SMNPs are engineered via incorporation with polymers that can perform a volume phase transition at a critical solution temperature (CST) [132]. Polymers with a lower CST (LCST) have a reduced solubility when heated, whilst upper CST (UCST) polymers act in the opposite way [133]. The UCST polymers demonstrated a higher rate of agent release in response to slight temperature variation and seem promising for photo-thermal utilization. However, their control is challenging, while LCST polymers have minor adverse effects, enhanced therapeutic efficacy, and low drug doses needed [134][135][136]. For example, PNIPAAm, a TR polymer, can transit between the hydrophilic state and hydrophobic state at LCST (hydrophilic below LCST and hydrophobic above LCST) [137]. In the following sections, different categories of stimuli-sensitive (pH, redox, enzyme, light and ultrasound, dual/multi-modal stimuli) SMNPs are described in detail.

Thermo-Responsive MNPs
Temperature is a vital element to vent the drug into the TME which has a temperature greater (~40-42 °C) than healthy tissues (37 °C) [131]. Thermo-responsive (TR) SMNPs are engineered via incorporation with polymers that can perform a volume phase transition at a critical solution temperature (CST) [132]. Polymers with a lower CST (LCST) have a reduced solubility when heated, whilst upper CST (UCST) polymers act in the opposite way [133]. The UCST polymers demonstrated a higher rate of agent release in response to slight temperature variation and seem promising for photo-thermal utilization. However, their control is challenging, while LCST polymers have minor adverse effects, enhanced therapeutic efficacy, and low drug doses needed [134][135][136]. For example, PNIPAAm, a TR polymer, can transit between the hydrophilic state and hydrophobic state at LCST (hydrophilic below LCST and hydrophobic above LCST) [137]. In the following sections, different categories of stimuli-sensitive (pH, redox, enzyme, light and ultrasound, dual/multi-modal stimuli) SMNPs are described in detail.

Thermo-Responsive MNPs
Temperature is a vital element to vent the drug into the TME which has a temperature greater (~40-42 °C) than healthy tissues (37 °C) [131]. Thermo-responsive (TR) SMNPs are engineered via incorporation with polymers that can perform a volume phase transition at a critical solution temperature (CST) [132]. Polymers with a lower CST (LCST) have a reduced solubility when heated, whilst upper CST (UCST) polymers act in the opposite way [133]. The UCST polymers demonstrated a higher rate of agent release in response to slight temperature variation and seem promising for photo-thermal utilization. However, their control is challenging, while LCST polymers have minor adverse effects, enhanced therapeutic efficacy, and low drug doses needed [134][135][136]. For example, PNIPAAm, a TR polymer, can transit between the hydrophilic state and hydrophobic state at LCST (hydrophilic below LCST and hydrophobic above LCST) [137].
Mainly, the MNPs are constructed to maintain their payloads in a physiological tem- In the following sections, different categories of stimuli-sensitive (pH, redox, enzyme, light and ultrasound, dual/multi-modal stimuli) SMNPs are described in detail.

Thermo-Responsive MNPs
Temperature is a vital element to vent the drug into the TME which has a temperature greater (~40-42 °C) than healthy tissues (37 °C) [131]. Thermo-responsive (TR) SMNPs are engineered via incorporation with polymers that can perform a volume phase transition at a critical solution temperature (CST) [132]. Polymers with a lower CST (LCST) have a reduced solubility when heated, whilst upper CST (UCST) polymers act in the opposite way [133]. The UCST polymers demonstrated a higher rate of agent release in response to slight temperature variation and seem promising for photo-thermal utilization. However, their control is challenging, while LCST polymers have minor adverse effects, enhanced therapeutic efficacy, and low drug doses needed [134][135][136]. For example, PNIPAAm, a TR polymer, can transit between the hydrophilic state and hydrophobic state at LCST (hydrophilic below LCST and hydrophobic above LCST) [137].
Mainly, the MNPs are constructed to maintain their payloads in a physiological temperature and deplete upon exposure to higher temperatures. The delivery of therapeutic In the following sections, different categories of stimuli-sensitive (pH, redox, enzyme, light and ultrasound, dual/multi-modal stimuli) SMNPs are described in detail.

Thermo-Responsive MNPs
Temperature is a vital element to vent the drug into the TME which has a temperature greater (~40-42 °C) than healthy tissues (37 °C) [131]. Thermo-responsive (TR) SMNPs are engineered via incorporation with polymers that can perform a volume phase transition at a critical solution temperature (CST) [132]. Polymers with a lower CST (LCST) have a reduced solubility when heated, whilst upper CST (UCST) polymers act in the opposite way [133]. The UCST polymers demonstrated a higher rate of agent release in response to slight temperature variation and seem promising for photo-thermal utilization. However, their control is challenging, while LCST polymers have minor adverse effects, enhanced therapeutic efficacy, and low drug doses needed [134][135][136]. For example, PNIPAAm, a TR polymer, can transit between the hydrophilic state and hydrophobic state at LCST (hydrophilic below LCST and hydrophobic above LCST) [137].
Mainly, the MNPs are constructed to maintain their payloads in a physiological temperature and deplete upon exposure to higher temperatures. The delivery of therapeutic Acetal In the following sections, different categories of stimuli-sensitive (pH, redox, enzyme, light and ultrasound, dual/multi-modal stimuli) SMNPs are described in detail.

Thermo-Responsive MNPs
Temperature is a vital element to vent the drug into the TME which has a temperature greater (~40-42 • C) than healthy tissues (37 • C) [131]. Thermo-responsive (TR) SMNPs are engineered via incorporation with polymers that can perform a volume phase transition at a critical solution temperature (CST) [132]. Polymers with a lower CST (LCST) have a reduced solubility when heated, whilst upper CST (UCST) polymers act in the opposite way [133]. The UCST polymers demonstrated a higher rate of agent release in response to slight temperature variation and seem promising for photo-thermal utilization. However, their control is challenging, while LCST polymers have minor adverse effects, enhanced therapeutic efficacy, and low drug doses needed [134][135][136]. For example, PNIPAAm, a TR polymer, can transit between the hydrophilic state and hydrophobic state at LCST (hydrophilic below LCST and hydrophobic above LCST) [137].
Mainly, the MNPs are constructed to maintain their payloads in a physiological temperature and deplete upon exposure to higher temperatures. The delivery of therapeutic cargo could be performed either via (i) thermo-sensitive drug carriers, releasing the drug in response to temperatures above the physiological temperature which is an intrinsic characteristic of malignancy cells/tissues (internal stimuli) or, (ii) the malignancy cells/tissues could be heated by an external stimulus such as MF, light, etc., to enhance the release of the pharmaceutical cargo [138,139].
In this regard, Ferjaoui et al. [140] synthesized TR IONPs carrier coated with 2-(2methoxy) ethyl methacrylate and oligo (ethylene glycol) methacrylate TR co-polymer for the sustained release of doxorubicin (DOX). The results showed 100% drug release after 52 h at 42 • C (LCST at 41 • C). The cytotoxic tests unveiled that the core/shell of IONPs had high toxic effects on human ovary carcinoma SKOV-3 cells at a very low drug concentrations [140]. Moreover, Zhang and his colleagues [141] designed nano-in-micro TR micro-spheres theranostic tools for HT and chemotherapy in cultured Caco-2 and A549 cells. In vitro, the results revealed the chemo-agent, methotrexate (MTX) or 5-fluorouacil (5-FU), had a slow release and the release of the microspheres was over the range of 37 to 43 • C, and the relaxivity (r2) value was distinctive at temperatures between 35 and 46 • C, which approved the particle characteristics as TR [141].
Although TR MNPS are deemed as low-risk and capable of efficiently loading and discharging therapeutic cargo when heated, they have not been effectively tailored to meet the clinical context. They remain unable to be induced in real-time and at the location of malignancy [142].

Magnetic-Responsive NPs
Magnetic field is a non-invasive energy. The revolution in nano-medicine has endorsed magnetic fields for cancer theranostic applications, including targeted drug release (TDR), MHT, and MRI. The external static or dynamic MF can apply a force greater than the blood flow force to drag drug-carrying MNPs through the complex physiological system and deliver the cargo to the site of malignancy. MF regulates the motion of MNPs and facilitates controlled and TDR. Magnetically guided pharmaceutical cargo delivery has a high therapeutic efficacy and low toxicity [143]. However, one of the limitations of MF-guided delivery is that MNPs are unable to hold maximum magnitude inside the physiological system when they are further away from the external magnetic force. Although this prevents the tumor from being targeted in the deeper region, externally magnetic-guided cargo delivery remains more effective in comparison to passive targeting (EPR effect) [144].

Targeted Drug Release
IONPs can efficiently transport and selectively release pharmaceutical cargo with fewer side effects at TME via an external MF, and this is one of the critical fields of research in DDS. Special consideration should be given to the pharmacokinetic and in vivo characteristics of the generated IONPs and the exerted magnetic force [145]. The potential use of IONPs as a DDS to deliver DOX to a glioblastoma cancer (GMC) site guided via an external MF in a rat was investigated by Lee et al. [146]. In this drug delivery methodology, the N-hydroxy succinimide (NHS), PEG and free thiol (SH) (NHS-PEG-SH) were conjugated to modify the surface of IONPs and improve the particles' EPR effect on GMC cells. The presence of an external MF increased the local concentration of IONPs within the GMC cells which improved the retention and accumulation of the DOX [146]. Wang et al. [147] designed a biocompatible nano-carrier with a uniform size distribution for in vivo application based on IONPs guided via an extraneous MF source. The nano-carrier demonstrated successful TDR via an extraneous MF to the rat brain, and was proven to have a potential for therapeutic application in the therapy of brain disease [147].
Magnetically TDR is promising strategy to guide the therapeutic cargo to the specified sites. Therefore, in order to succeed, an appropriate magnetic system (e.g., MF and MNPs) is a prerequisite.

Magnetic Hyperthermia Application
Hyperthermia (HT) involves increasing the temperature of carcinoma cells (clinical temperature 42-46 • C) above physiological condition (37 • C), to induce the apoptosis/necrosis of neoplasm cells [148]. The neoplasm cells are sensitive to heat oscillations compared to healthy cells due to the lower blood supply around the tumor [149]. HT is actively utilized in pre-clinical and clinical trials as an adjuvant to treat numerous solid malignancies [150].
HT can fabricate heat via various techniques including alternating magnetic field (AMF) [151], high intensity focused ultrasound (HIFU) [152], and water bath [153]. Nonetheless, water bath fails to maintain spatially precise treatment, likewise HIFU demonstrates an inability to perform deep thermal treatment to a large specific location or ingress bones and air, while AMF can exhibit a deeper penetration competence with a higher location accuracy [154]. Since SIONPs endow a magnetocaloric effect, the exposure of SIONPs to strong frequency AMF generates heat via hysteresis loss and the heat is applicable in MHT. The technique harnesses the heat-releasing characteristics of remotely controlled SIONPs which are designed to be smart and can heat up to 42-45 • C [155][156][157]. HT can induce therapeutic cargo release by influencing the permeability of malignant vasculature, expanding the pore size of the endothelial membrane, rising perfusion, and enhancing the accumulation and toxicity of the therapeutic agent [158,159].
In clinic, the procedure can be classified into local (application of heat to small part via micro/radio waves, or US), regional (large part of the body is heated), and entire body HT [160]. The factors which impact the heating efficiency of SIONPs include concentration, magnetic characteristics, curie temperature~50 • C, and the applied field (e.g., frequency/amplitude) [161].
MHT has shown capacities in sensitizing malignancy cells to adjuvant treatment, and its applicability holds huge promise, qualifying for further consideration not only as an adjuvant but also as tumor ablation technique.

Theranostic Application of MRI and MNPs
Magnetic resonance imaging is a non-invasive and non-destructive diagnostic imaging modality that utilizes a powerful radio frequency (RF) electric field and a magnet field to visualize detailed images of the internal anatomy of human/animal. It allows the clinicians real-time monitoring of the treatment and location of malignancy as well as providing a handle to control the maneuver of therapeutic cargo and to regulate the dosage for optimum treatment results. Its superiority is related to the great spatial resolution, the contrast of sensitive tissue, and practicability in early diagnosis of malignancy which maximizes the chance of treatment and survival [162][163][164]. MRI contrast media are distinguished by their relaxivity (r1/r2) which reflects on how the medium can enhance the magnetic resonance (i) longitudinal relaxation time (T1) and (ii) transversal relaxation time (T2) in milliseconds (ms). The correlating longitudinal and transversal relaxation rates are r1 and r2, respectively, in which r1 = 1/T1 and r2 = 1/T2, and the unit is 1/ms. T1 contrast agents generate lighter/positive images whereas T2 contrast media produce darker/negative contrast images. The performance of a contrast media substantially relies on r1 and r2, which determines if there will be going to be T1 or T2-weighted images [165,166].
Currently, the heavy paramagnetic metal, Gadolinium (Gd), is broadly applied in clinic for diagnostic intention as a T1-weighted MRI contrast media, due to its strong magnetic moment, high relaxation time, and low r2/r1 ratio [167,168]. The toxicity of free Gd can be eliminated to some extent by terminating free Gd 3+ ions using organic chelates (e.g., diethyl-enetriaminepenta acetic acid) [169]. Although a Gd-consisting-contrast-medium (GdCCM) is widely applicable in clinic, it has a varied BCT, and compelling evidence has shown that the repeated dose of GdCCM and in particular the less stable GdCCM accumulated in the globus pallidus and dentate nucleus of the brain. The patients with kidney and liver dysfunction are unable to eliminate the heavy Gd complexes and the metal can accumulate in the brain and result in brain lesions [170][171][172][173]. Hence, the findings have provoked attention about their safety as contrast media and efforts have been made to discover safer alternatives.
The concept of the unique magnetism behavior of SIONPs and their biocompatibility has made them powerful nominees to be utilized as contrast media [174][175][176]. Ultra-small SIONPs (≤5 nm) demonstrated an encouraging performance as T1 contrast media since they possess a larger surface-to-volume ratio, expanding the accessibility of surface of iron ions to the neighboring water or hydrogen [177,178]. They are used as blood-pool contrast media for magnetic resonance angiography and perfusion imaging [179]. Although SIONP T1 contrast media (≤5 nm) have highly favorable properties, the reproducibility in mass production and the complexity of the interrelated factors impact their enhancement and make their fabrication/utilization challenging; hence, they are yet to be approved for clinical applications [180].
Larger SIONPs (>8 nm) could predominantly perform as T2 contrast media and generate T2-weighted images due to the magnetic heterogeneity produced by their powerful magnetic moment and high signal/noise ratio [168]. However, the results of T2-weighted MRI can misguide the clinicians, because of the formation of black signals or the "Bloom Effect" phenomenon which might occur due to bleeding, or the deposition of metal (Fe) [177,181]. In addition, the IONPs might degrade and perform inversely. For example, Lu et al. [182] employed IONPs (ca 20 nm) for the diagnosis of hepatocellular carcinoma (HCC). The T2 IONP contrast agent injected into healthy mice generated darker images while in the HCC tumor-bearing mice, no T2-weighted/dark images were detected. Due to the acidic TME, in less than 40 s, T2 contrast agents de-agglomerated (~3 nm) and started to degrade, intelligently reversed from T2 to T1 contrast agents, and produced positive/bright images [182].
Sridharan et al. [184] constructed bio-mineral Fe-doped nano-calcium phosphate (nCP:Fe-CA) contrast media for the in vivo detection of liver cirrhotic and HCC nodules at an early stage. The intravenously administered nCP:Fe-CA (sphere, size: 137.6, r: 63 mM −1 s −1 , colloidal stability: 48 h) detected the lesions as tiny as 0.25 cm, while the current clinical diagnosis limit is~1 cm [184]. In another in vivo study, an nCP:Fe-CA stem cell label was constructed as MRI contrast media to track the embedding, migration, and bio-distribution of the therapeutic agent in the brain. The intracerebral implantation of a nano-formula in a healthy rat's brain was highly biocompatible with an efficiency of~87% and no effect on mesenchymal stem cells. In addition, T2 relaxation time considerably reduced from 195 to 89 ms and distinctive dark T2-weighted images were observed up to 30 days. The bio-compatible nCP:Fe-CA showed potential as a T2-weighted MRI contrast agent for monitoring stem cells in vivo [185].
From a diagnostic and therapeutic point of view, IONPs used in MRI applications have displayed optimistic results in imaging, selective TDD and CDR in particular, T2 negative contrast agents. IONPs have paved the way as a desirable choice in clinic competing to replace Gd-based contrast media. In addition, new horizons of innovation in designing T1 contrast media which have been intelligently converted from T2 to T1 contrast in acidic TME seems a promising modality for the construction of next-generation smart MRI contrast media. Nonetheless, further in vivo studies are necessary to assure their credibility and ultimate translation for clinical applications.
Thus, designing appropriate detection modalities which permit in vivo studies and real-time mapping is a vital aspect in order to enhance the practicality of MNPs and empower real translational methods.

Electric Field-Responsive MNP
Electric field-responsive (EFR) stimulus has revolutionized treatment, since an electric field can be exploited endogenously/exogenously for DDS [186]. Endogenous electric field are generated by injured tissue that can influence the proliferation/division/migration of cells, e.g., tissue repair after injury [187]. External electric field pulses can facilitate TDD by triggering the cell membrane permeability to allow drug entry. It can also stimulate wound healing or tissue restoration [188,189]. Electric field can be synchronized with MNPs to assist drug delivery to the desired location. In this regard, Viratchaiboota et al. [190] put to use the technology of electric field, MF, and IONPs to deliver 5-FU to ablate cancer cells. The results indicated that the therapeutic release time decreased but the diffusion coefficient rose [190]. Although the downside of electric field application for DDS is the generation of heat, even this phenomenon can be utilized in tumor-treating field therapy to treat malignancy [191].

pH-Responsive MNPs
The pH-responsive (pHR) IONPs are designed to detect the differences in pH environments of normal body tissues (pH~7.4), the tumor extracellular matrix (pH~6.5-7.0), and organelles, e.g., endosomes (pH~4.5-6.5) and function accordingly. The low pH level in malignancy tissue is due to the excess production of lactic acid, (particularly in endosomes) and reactive oxygen species (ROS) which stimulates the generation of Glutathione (GSH) to deal with ROS [192]. Molaei et al. [193] formulated an iron oxide nano-system, enveloped with pHR polyethyleneimine (PEI) and amphiphilic poly-maleic anhydride-alt-1-octadecene and functionalized with FA for curcumin (CUR) delivery. The characteristics of the final NPs are collected in Table 2. The drug release at the acidic condition of TME was improved as compared to physiological pH due to the swelling property of cationic PEI via proton absorption and repulsion effects between positive charges. Furthermore, the nano-system could be a prospective candidate for theranotics purposes as MRI contrast media and also CDR [193].  Glutaraldehyde cross-linked chitosan-coated IONPs were prepared and loaded with epirubicin (EPI) and temozolomide (TMZ) drugs for cancer treatment by Nalluri et al. [194]. The release of EPI and TMZ was much higher at a lower pH compared to the physiological pH because of the flexibility of the polymer network. As EPI contains an amine group formed an imine bond, this bond was sensitive to cleavage at a lower pH (4.4-6.4) with glutaraldehyde while TMZ with an amide group cannot form the imine bond. At pH 4.6, the release of EPI (94.06%) was higher than TMZ (87.68%) [194].
Overall, pHR IONPs are charge-dependent, with prolonged BCT and greater accumulation in the tumor. These nano-structures demonstrate fewer adverse effects and minimum non-selective cellular uptake, and these encouraging results acknowledge their competency in therapeutic cargo delivery and targeting the specific malignancy cells/tissue [195][196][197].

Redox-Responsive MNPs
The redox-responsive (RR) magnetic nano-system is constructed considering the reduced TME which can perform as a unique inner signal, permitting the RR nanosystem to degrade and discharge its therapeutic payload. The oxidation/reduction state of GSH and nicotinamide adenine dinucleotide phosphate (NADPH) governs the reducing TME with each having distinctive reduction capabilities [198]. Compared to NADPH, GSH has a greater concentration in reducing TME (2-10 µM) and regulates the TME via reduction in the disulfide linkage and the reaction with excessive ROS [131,[198][199][200]. Mousavi et al. [201] created a di-block co-polymer based on PEG and poly(ε-caprolactone) (PCL) with SS-linkage for the co-delivery of IONPs and DOX ( Figure 2). The biocompatible RR nano-carriers had a high and rapid DOX release rate in the reductive environment of human foreskin fibroblast cells [201]. In another work, the researchers produced a RR protein delivery system based on IONPs and methoxy-poly(ethylene glycol)-block-poly [dopamine diethylene triamine-Lglutamate] polymer ligands to investigate redox-triggered targeted human serum albumin (HSA) as a model protein delivery and diagnostic imaging of breast cancer [202]. The average size of nano-carriers was approximately 60-70 nm and proteins were released swiftly under a high concentration of GSH (10 µM) due to the reduction-triggered disulfide bonds cleavage. The polymer-coated particles had a low cytotoxicity and biocompatibility against HeLa cells and demonstrated an effective cellular uptake. In vivo imaging analysis of breast-tumor-bearing mice showed the nano-carriers can serve as potential T2weighted MRI contrast media [202]. For delivering DOX and MRI, polydopamine (PDA)based RR IONPs were constructed by Shang et al. [203]. In the presence of GSH, a sustained and accumulative DOX release (73%) was observed, while in the absence of GSH the release rate declined (37%). In addition, the IONPs exhibited intense T2-weighted signals, a negative contrast result in MRI analysis, and an enhanced r2 value [203].
A RR magnetic star-structured micellar (MSSM) was generated using magnetite and PEG and PCL co-polymers and loaded with DOX. The MSSM was modified by phenylboronic acid (PhBA) to enhance the agent's capability to target sialic acid (SA) which is up regulated in cancerous cells, e.g., HeLa cells and HepG2 cells. The MNPs with a saturation magnetization of 15 emu/g had both active-targeting and magnetic-targeting features to accumulate around the malignant tissues and internalize HepG2 cells by the sialic acidmediated endocytosis. Moreover, the rapid DOX release under a high level of GSH improved the therapeutic efficacy. The RR MSSM systems displayed therapeutic efficacy in targeting malignancy tissue without the premature or non-specific distribution of therapeutic cargo due to the low level of reducing species in the blood. However, these studies were conducted on animal models which are dissimilar to real conditions in malignancy cells/tissues or in metastatic carcinomas. Plus, the major concern is mass production which has remained a hurdle [192,199,204].

Enzyme-Responsive MNPs
The integration of MNPs with enzyme responsive (ER) stimuli has received great interest since enzymes play essential roles in all biological and metabolic processes. Some of their advantages are substrate specificity and high selectivity, and they are capable of In another work, the researchers produced a RR protein delivery system based on IONPs and methoxy-poly(ethylene glycol)-block-poly [dopamine diethylene triamine-Lglutamate] polymer ligands to investigate redox-triggered targeted human serum albumin (HSA) as a model protein delivery and diagnostic imaging of breast cancer [202]. The average size of nano-carriers was approximately 60-70 nm and proteins were released swiftly under a high concentration of GSH (10 µM) due to the reduction-triggered disulfide bonds cleavage. The polymer-coated particles had a low cytotoxicity and biocompatibility against HeLa cells and demonstrated an effective cellular uptake. In vivo imaging analysis of breast-tumor-bearing mice showed the nano-carriers can serve as potential T2-weighted MRI contrast media [202]. For delivering DOX and MRI, polydopamine (PDA)-based RR IONPs were constructed by Shang et al. [203]. In the presence of GSH, a sustained and accumulative DOX release (73%) was observed, while in the absence of GSH the release rate declined (37%). In addition, the IONPs exhibited intense T2-weighted signals, a negative contrast result in MRI analysis, and an enhanced r2 value [203].
A RR magnetic star-structured micellar (MSSM) was generated using magnetite and PEG and PCL co-polymers and loaded with DOX. The MSSM was modified by phenylboronic acid (PhBA) to enhance the agent's capability to target sialic acid (SA) which is up regulated in cancerous cells, e.g., HeLa cells and HepG2 cells. The MNPs with a saturation magnetization of 15 emu/g had both active-targeting and magnetic-targeting features to accumulate around the malignant tissues and internalize HepG2 cells by the sialic acid-mediated endocytosis. Moreover, the rapid DOX release under a high level of GSH improved the therapeutic efficacy. The RR MSSM systems displayed therapeutic efficacy in targeting malignancy tissue without the premature or non-specific distribution of therapeutic cargo due to the low level of reducing species in the blood. However, these studies were conducted on animal models which are dissimilar to real conditions in malignancy cells/tissues or in metastatic carcinomas. Plus, the major concern is mass production which has remained a hurdle [192,199,204].

Enzyme-Responsive MNPs
The integration of MNPs with enzyme responsive (ER) stimuli has received great interest since enzymes play essential roles in all biological and metabolic processes. Some of their advantages are substrate specificity and high selectivity, and they are capable of attaining ER drug release through the bio-catalytic action at malignancy cells/tissues [131,205]. In cancerous cells, specific enzymes, including proteases, phospholipases, lipase, or glycosidase, often exhibit a higher expression than in normal cells [206]. In recent studies, two classes of enzymes have often been used as stimulants in ER drug delivery, including proteases (or peptidases) and phospholipase [207]. For instance, Li et al. [208] fabricated mesoporous silica nano particles (MPSNPs) engulfing DOX and matrix metalloproteinase-2 (MMP-2) ER peptide for chemo-drug delivery and contrast media in MRI ( Table 3). The rate of DOX release without the peptide was significantly greater; however, MMP-2-facilitated IONPs initially had a slow-release rate, and then gradually 20 min later the rate value intensely rose. The peptide on the surface of the NPs efficiently cleaved in the presence of the MMP-2 enzyme to induce DOX release. Furthermore, the results of the methyl thiazole tetrazolium assay showed that the final nano-carrier had a high specificity to HT-1080 human fibrosarcoma cells with high MMP-2 expression and limited toxicity to normal cells. The MRI results acknowledged that the exogenous MF-stimulated accumulation of nanocarriers at the tumor site improved T2 signals and r2; hence, they should be considered as candidates in a sensitive probe [208].  Similarly, Nosrati et al. [209] developed enzyme-responsive glycine-coated Fe 3 O 4 NPs functionalized with MTX for TDD to MCF-7 breast carcinoma cells (Figure 3). The MTX was released faster since the proteinase K enzyme cleaved the peptide inside lysosomes. Furthermore, the final nano-carrier with an average size of 46.82 nm demonstrated a higher cytotoxicity on the MCF-7 cell line as compared to free MTX due to the large number of enzymes in lysosomes that cleaved peptide bonds and allowed the free MTX to decrease cellular viability [209]. Rastegari et al. [210] prepared two samples, coating one with βcyclodextrin (β-CD) and the other with carboxymethyl chitosan (CMCS) to degrade and promote prodigiosin (PG) delivery in the presence of lysosomal glycoside hydrolases. The characteristics of nano-carriers such as size, saturation magnetization, release, and toxicity are collected in Table 4. Both nano-carriers displayed a relatively fast rate of PG release in the cells' lysosome and had exceptionally low drug-leakage into the bloodstream. The nano-carriers targeted glucose overexpressing cells and the PG-loaded CMCS MNPs had higher toxicity effects on MCF-7/GFP and HepG2 cells and could be more effective in the killing of cancerous cells compared to PG-loaded β-CD MNPs [210].
grade and promote prodigiosin (PG) delivery in the presence of lysosomal glycoside hydrolases. The characteristics of nano-carriers such as size, saturation magnetization, release, and toxicity are collected in Table 4. Both nano-carriers displayed a relatively fast rate of PG release in the cells' lysosome and had exceptionally low drug-leakage into the bloodstream. The nano-carriers targeted glucose overexpressing cells and the PG-loaded CMCS MNPs had higher toxicity effects on MCF-7/GFP and HepG2 cells and could be more effective in the killing of cancerous cells compared to PG-loaded β-CD MNPs [210].   Carboxymethyl chitosan (CMCS), β-cyclodextrin (β-CD), Saturation magnetization (Ms), Encapsulation efficiency of prodigiosin (EEPG), The half-maximal inhibitory concentration (IC50).
The MNPs incorporated with enzymes display tremendous diagnosis and therapeutic potency and can embellish bio-"specificity" and "selectivity" of the nano-structures. Their site-specific and selectiveness on one hand offer significantly improved accumulation at the malignancy site and decrease the uptake of nano-formulations by non-targeted tissue, and on the other hand, facilitate site-specific CDR without undermining targeting efficacy. Plus, they can overcome constrains faced by conventional therapeutic agents. Although progress has been achieved in enzyme-responsive MNPs, there are still many limitations and drawbacks that need to be addressed, such as biocompatibility, cytotoxicity, and systemic toxicity [211][212][213][214].

Light and Ultrasound-Responsive MNPs
Light-sensitive (LS) MNPs operate by an exogenic light source (i.e., ultraviolet (UV), visible (Vs), US irradiation, and near infrared light (NIR)/photothermal therapy (PTT)), and their physical and chemical structures become disrupted and destabilized, releasing the agent in the desired tissue [138]. The practice of UV and Vs lights is limited owing to their short penetration depth in vivo [138,215]. The non-invasive PTT utilizes NPs to change NIR light into heat to eradicate malignancy cells, and has demonstrated unique positive results in cancer therapy [206,216]. NIR light uses an absorbing chromophore (e.g., hemoglobin) to absorb light and increases the permeability of the tumor blood vessels, causing leakage, and annihilating malignant cells without causing damage to healthy cells and with low scattering property at the wavelengths of 700 to~1000 nm [138,206]. Hence, NIR could be more practical in biomedical utilization when it is hybridized with MNPs. The impact of NIR light on IONPs is due to the intrinsic photothermal effect of the particles and the increase in their thermal motion to discharge the therapeutic payload and cause apoptosis of malignancy cells. The hybridization of NIR light and IONPs allows the immobilization of pharmaceutical cargo at the malignancy site for precise CDR, leading to multiple therapeutic effects in a single dose [217]. Feng et al. [218] generated hollow mesoporous CuS NPs containing PEGylated Fe 3 O 4 and DOX-loaded for utilization in NIR-responsive DDS, diagnosis, and therapy of breast carcinoma. Nano-carriers displayed a high cytotoxicity on MCF-7 cells with decreased cell viability due to the effective phototherapy and synergetic effect of IONPs. Additionally, the exposure of IONPs with NIR light enhanced DOX release and destroyed the high number of malignancy cells [218]. Eyvazzadeh et al. [219] also synthesized core-shell gold-coated IONP (Au@IONP) as an LR agent for cancer PTT. Heating the nano-complex to the desired temperature with laser irradiation induced 70% cell death [219]. In another study, methylene blue (MB) photosensitiser was immobilized on Cu-Fe MNPs which resulted in an enhanced PTT effect and damaged the tumor cells efficiently since Cu-Fe MNPs acted as Fenton catalyst, changing H 2 O 2 into ROS, e.g., singlet oxygen ( 1 O 2 )/an excited form of O 2 . [220].
The US-responsive stimuli have received significant attention due to their safe profile, deep penetration into the body, non-invasiveness, and capability of unloading IONPs payload at the desired sites via thermal and mechanical effects [221]. The irradiated US waves continuously fabricate micro-bubbles (MBBs) in the form of spherical pressure waves which lead to the generation of heat, micro-jets, and oxidative radicals. The non-linear oscillations of MBBs re-radiated energy in varied frequencies. The production of low frequencies (20-100 kHz) promotes the implosion of MBB which aids the release of the therapeutic payload at the malignancy site [142,222,223]. The US-responsive magnetic mesoporous silica MBBs facilitated gene delivery guided by an external MF to malignant cells/tissues. The US assisted the cargo release and enhanced the efficiency of the plasmid DNA delivery to malignant tissue via stimulation of the blood tumor barrier to open and enhanced the membrane permeability. Furthermore, the HEK293T and SKOV3 cells treated with MMPS MBBs showed better viability than those treated with only magnetic MPSNPs (M MPSNP) due to the presence of lipid MBBs [224]. The characteristics of nano-carriers are shown in Table 5. Even though PTT which uses NIR light is capable of disrupting the scaffold of nanocarriers to induce the therapeutic agent release, the number of NIR light-absorbing chromophores are limited which restricts the progress of this procedure [225]. Additionally, US waves can be utilized to stimulate oxygen-transporting MBBs to discharge oxygen, whilst concurrently initiate a sono-sensitizer, (especially practical for treatments of hypoxic malignancies) [226]. Besides the ability of US to enhance the agent's cellular uptake, it can minimize the off-target and non-specific effects of chemo-agents [227].

Dual and Multi-Stimuli-Responsive MNPs
Single/multi stimuli-triggered MNPs have been utilized not only to improve sensitivity, but also to target and release anticancer cargo efficiently at the location of interest [228]. For example, the utilization of MHT partnered with other modalities, e.g., chemotherapy and concomitant with MRI and US has been advantages. Dual/multi modal application (i) decreases the necessity of high toxic concentration, and (ii) the therapeutic temperature is obtained in less time, preventing adverse effects (such as prolonged contact with heat causes burn/pain) [229]. Pre-clinical studies of thermo-sensitive MNPs in MHT therapy for theranostic purposes are collected in Table 6. The translation of this modality into standard clinical routine in therapy of various neoplasms has limitations including the loco-regional delivery of MNPs and real-time mapping during the procedure [230].
To tackle the aforementioned obstacles, the synergetic application of US with MHT has become one of the interesting new modalities for malignancy treatment, since it can specifically target the tumor cells without having any detrimental effect on normal cells. US-stimulated MHT is non-invasive with no ionization effect. US waves cause the vibration of tissue and as a result heat is generated [231,232]. In a pre-clinical study by Hadadian et al. [233], TR MNPs were utilized, integrating MHT with magnetomotive US imaging for localizing and temperature mapping of MNPs in a phantom study. However, further in vivo studies will be required to assess the technique in more complex and viscoelastic tissue [233]. In another study, hybridizing TR MNPs with US waves and MHT increased the rate of malignancy cell destruction and also the rate of therapeutic efficiency improved. Nonetheless, low intensity US-MHT is impractical for deep-seated malignancies and organs with air, e.g., abdomen and lungs. Since the acoustic impedance fails to distinguish between air and soft tissue, there will not be transmission in cavities with air [234].
In addition, doping Fe with other metal such as Zn and Mn which possess high saturation magnetization will improve the heating efficiency of MNPs [235]. Zn and Mn dopants in low doses have distinctive characteristics such as being non-toxic to healthy cells. Albarqi et al. [236] developed a multi dopant HR magnetic nano-carrier; using Zn, Mn, and Fe. The MNP had a high saturation magnetization and enhanced heating efficacy, suitable for MHT application [233,236].
A neoadjuvant chemo-treatment protocol using DOX synchronized with mild locoregional MHT displayed remarkable improvements in survival rate of soft tissue sarcoma patients, due to cellular modification induced by MHT, e.g., DNA repair [237]. In this regard, a number of studies employed DOX and MHT in combination therapy (Table 7) [137,[238][239][240]. The decoration of carboxylate-functionalized PNIPAAm nano-gel (NG) with Fe 3 O 4 NPs via covalent bonds generated multi-modal diagnostic imaging and a thermal therapy tool which actuated DOX release due to the affinity of Fe to the carboxylate group [137]. Under RF field, thermally triggered MNPs exhibited TDR capability, above the LCST of carboxylated PNIPAAm, LCST = 43 • C (LCST of PNIPAAm 32 • C, below body temperature). Any temperature lower than LCST will be closer to body temperature (37 • C), leading to unexpected and early agent release, likewise, above LCST can affect healthy cells and cause adverse effects [137,158,241]. In vitro studies revealed the encapsulation of DOX by magneto-liposome (thermo-responsive agent) conjugated with ferumoxytol used in MHT and drug delivery could be a powerful modality for in vivo carcinoma treatment [238]. Additionally, Pourjavadi et al. [242] used N-isopropylacrylamide (NIPAM) for the TR release of paclitaxel. The therapeutic payload release ameliorated at an elevated temperature, indicating the agent release is temperature dependent [242]. In addition, Gue et al. [239] Pramanik et al. [240] and Afzalipour et al. [243] fabricated TR MNPs, grafted with overexpressed receptor targeting functional groups, MTX, HA, and FA, respectively, for application in oncothermia (Table 7).
Furthermore, pH and heat responsive Fe 3 O 4 NPs conjugated with sodium dodecyl sulphate, aniline hydrochloride, and CUR were synthesized for CDR and MHT in vitro and in vivo studies. The rode and worm shape magnetic micelles demonstrated high colloidal stability (surface charge: −31 mV), great drug-loading affinity, satisfying heat efficiency, and high magnetization [244]. Matos et al. [245] developed Fe 3 O 4 electro-spun nano-composite, functionalized with cellulose acetate, OlA and dimercaptosuccinic acid. The pH and heat sensitive, spherical particles had a high heating capacity due to the adsorption of IONPs on the surface of fiber. They also exhibited a high efficiency in carcinoma treatment with lower adverse effects [245].     The summary of the studies based on application of MNPs in MHT therapy and CDR for theranostic purposes are collected in Table 7.
Aljani et al. [247] designed a multi-functional hybrid nano-formula, ideal for fluorescence imaging and also promising as an MRI contrast medium [247]. Additionally, Gholibegloo et al. [248] designed a smart theranostic nano-sponge for cancer treatment via the modification of Fe 3 O 4 MNPs with cyclodextrin nano-sponges (CDNSs), FA, (CDNS-FA) and loaded with CUR for TDD and T2-weighted MRI. The nano-sponge demonstrated hemo-compatibility [248]. ETB-loaded IONPs successfully performed as smart theranostic tools and contrast probe (bio-marker) for MRI, with great targeting ability against highly aggressive and metastasizing malignancy cells [249]. Moreover, the study by Abedi et al. [250] showed that increasing the concentration of iron in dual modal imidazoline-functionalized MPSNPs in MRI formed T2-weighted images (darker images), while no alteration was detected for T1-weighted images (r1 = 5.89 m/M s −1 , r2 = 144.88 m/M s −1 ) [250].
Ray et al. [164] developed a strategy for real-time mapping of MNPs by MRI, using Magnevist as a contrast agent and drug release by AMF heating. However, further in vivo and clinical assessments are needed to implement the strategy for application in clinic [164].
The solo application of PTT encounters challenges, such as uneven heat generation by laser beam energy and NPs, and also the gradual reduction in laser energy over time will cause an insufficient penetration into malignancy cells [251][252][253]. To overcome some of the issues, PTT can be synchronized with another technique, e.g., MRI. In this regard, sialic acid-functionalized mesoporous PDA SIONPs was designed for chemo-photothermal therapy and T1/T2 MRI of hepatic carcinoma. The increase in iron concentrations produced darker T2 images and lighter T1-weighted images, suggesting the nano-formula could be a potential candidate as T1/T2 dual-modal MRI contrast media [180]. In another study, arginylglycylaspartic (RGD) peptide-conjugated NBs were fabricated high relaxation value, T2-weighted MRI and ultrasound promoted the simultaneous diagnosis and therapeutic agent release to hepatocellular carcinoma cells [253].
Licciardi et al. [254] developed IONPs coated with amphiphilic inulin-based graftcopolymer as smart theranostic tools for MRI and TDD (FA conjugation permits active targeting) of DOX to colon carcinoma cells. The lipoic acid (LA) was employed as crosslinking ligand to link the polymers and to provide redox-sensitivity characteristics to stimulate CDR, due to the S-S bond which resulted in the cleavage of bonds and disturbing the stability of the molecule and releasing the agent. [254]. Similarly, Dong Li et al. [255] functionalized MNPs with FA and loaded DOX for simultaneous MRI and TDD to gastric cancer MGC-803 cells in vitro and in vivo. The MNPs displayed longer BCT ad were used for diagnosis/detection of small malignancy cell with overexpressed folate [255]. In addition, in vivo and in vitro studies revealed the conjugation of CUR with LA on the surface of Au-Fe 3 O 4 NPs and equipped with GSH ligands have potential for theranostics applications in TDD and as contrast media for MRI of brain carcinoma. Moreover, similar to previously mentioned studies, increasing the concentration of iron resulted in a decrease in signal intensity in MRI of astrocyte and U87MG cells [256]. In addition, Wang et al. [257] and Xie et al. [258] fabricated MNPs for theranostic utilization, including MRI, MHT and TDD [257,258]. Table 8 summarizes some studies for the application of SIONPs in drug delivery and imaging in single platform.       Furthermore, Gao et al. [259] reported temperature and redox-responsive poly (Nvinylcaprolactam) (PNVCL)/Fe 3 O 4 NPs, transporting 5-FU for tumor targeting and MRI. The MNPs were fabricated via inverse mini-emulsion polymerization and disulfide-bond (S-S bond) containing a cross-linker. An improvement in drug release was observed due to the reduction factor as well as increases in the temperature above the LCST of PNVCL/Fe 3 O 4 NPs [259]. The results are collected in Table 9. In other research, NIR merged with pHR, smart meso-2,3-dimercaptosuccinic acidcoated, and DOX-loaded IONPs was prepared for breast carcinoma therapy. In vitro cargo release was higher due to the high solubility of the protonated DOX at a lower pH. Likewise, NIR light irradiation induced temperature rise, aiding drug release and causing death to cancer cells [260]. Also, a triple-stimuli-responsive drug carrier was developed by conjugation of HA onto the surface of IONPs-PDA through redox-sensitive S-S bond and attaching DOX via π-π interactions ( Figure 4) [261]. Furthermore, Gao et al. [259] reported temperature and redox-responsive poly (Nvinylcaprolactam) (PNVCL)/Fe3O4 NPs, transporting 5-FU for tumor targeting and MRI. The MNPs were fabricated via inverse mini-emulsion polymerization and disulfide-bond (S-S bond) containing a cross-linker. An improvement in drug release was observed due to the reduction factor as well as increases in the temperature above the LCST of PNVCL/ Fe3O4 NPs [259]. The results are collected in Table 9. Table 9. The morphology, magnetic, loading and cytotoxicity properties of Fe3O4/PNVCL MNPs.

Size(nm) DLS Saturation Magnetization (emu/g) 5-FU Loading Capacity (mg/g) The Viability of SW620 Cells (%)
423.5 7.7 400 90 In other research, NIR merged with pHR, smart meso-2,3-dimercaptosuccinic acidcoated, and DOX-loaded IONPs was prepared for breast carcinoma therapy. In vitro cargo release was higher due to the high solubility of the protonated DOX at a lower pH. Likewise, NIR light irradiation induced temperature rise, aiding drug release and causing death to cancer cells [260]. Also, a triple-stimuli-responsive drug carrier was developed by conjugation of HA onto the surface of IONPs-PDA through redox-sensitive S-S bond and attaching DOX via π-π interactions ( Figure 4) [261]. In the presence of a GSH reducing agent, an NIR light, and a low pH, the therapeutic payload release was higher. The multi-modal-therapy displayed a positive response, the viability of HeLa cell was at 16.2%, while it was higher in single chemotherapy (55.3%)/PTT (52.1%). In vivo MRI results demonstrated an increased accumulation of nano-carriers in tumor tissue providing an enhanced contrast. The results are collected in Table 10 [261].
The dual and multi-responsive MNPs are composed of more than two types or complex targeting moieties in nano-platform, delivering the therapeutic cargo to the intended site. The targeting strategy offers versatile modes of response and smart control of DDS. Simultaneously, it can identify and react with more than one molecular participant of the pathological site, decrease off-target payload discharge, and improve In the presence of a GSH reducing agent, an NIR light, and a low pH, the therapeutic payload release was higher. The multi-modal-therapy displayed a positive response, the viability of HeLa cell was at 16.2%, while it was higher in single chemotherapy (55.3%)/PTT (52.1%). In vivo MRI results demonstrated an increased accumulation of nano-carriers in tumor tissue providing an enhanced contrast. The results are collected in Table 10 [261]. The dual and multi-responsive MNPs are composed of more than two types or complex targeting moieties in nano-platform, delivering the therapeutic cargo to the intended site. The targeting strategy offers versatile modes of response and smart control of DDS. Simultaneously, it can identify and react with more than one molecular participant of the pathological site, decrease off-target payload discharge, and improve therapeutic efficiency [207,[262][263][264]. Nevertheless, the system confronts an enigma because of the steric deterrent causing an insufficient/plethora level of ligand density which debilitate targeting [265].

Magnetic Nanoparticle Targeting Methods
The key purpose in the diagnosis/therapeutics of carcinoma is the design of DDS with potency to target the lethal malignancy cells while leaving healthy cells/tissue intact. This might be attainable by the efficient delivery of MNPs loaded with anti-neoplastic agents into TME [213]. The successful targeting of nano-formulas depends on their ability to cross through a number of biological and physiological impediments such as unspecific interactions and early elimination from the bloodstream. The MNPs carcinoma-celltargeting includes passive targeting via carcinoma vasculature and active targeting via ligand-receptor binding.
Passive targeting implies the assembly of MNPs in the malignant cells/tissue via EPR effect which was discovered in 1980s by Maeda and his colleagues [266]. The performance of the EPR effect is particularly defined with cancer biology, e.g., hypoxia/inflammation, which causes angiogenesis and lymphangiogenesis. Due to the rapid growth of tumors, they generate highly permeable, leaky, and defective veins which are ideal to facilitate the transit of macromolecules greater than 40 kDa and accumulation of NPs in TME. The therapeutic cargo is required to stay in the blood circulation for ≥6 h to demonstrate an effective EPR effect [267][268][269]. The poor drainage of lymphatic fluid and the irregular and leaky lymphatic vessels of the lymphatic system [270] can assist the retention of NPs, resulting in the passive targeting of the therapeutic cargo [271]. For example, Guo et al. [253] designed spherical MNPs with diameters of 160-220 nm, indicating a good candidate for passive targeting via EPR effect [253]. In passively targeted nano-formulations, the heterogeneity of solid malignancies and the lack of ability to manage the uptake of nano-carriers can minimize the therapeutic efficiency and cause multiple-drug resistance [206,272]. Another limitation is the short BCT that can reduce therapeutic efficacy [273]. Even polymerizing MNPs has not yet been able to completely resolve the issues, so further optimization and careful assessments are required.
According to recent research, prospective nano-pharmaceutics should mainly concentrate on developing nano-carriers based on active targeting which have demonstrated improved/enhanced efficacy and capable of overcoming the challenges of passive targeting in carcinoma treatment [274,275]. For example, Ghorbani et al. [276] engineered an anti-neoplastic agent by conjugating MTX on MNPs, employing both passive and active targeting mechanism. Although the results indicated MTX penetrated the cells via passive targeting strategies, due to the target-site identification by ligand (MTX) and interaction with overexpressed receptors, the cellular uptake of MTX increased. Hence, folate receptor positive malignancies were actively targeted and a high degree of MCF-7 cells were eradicated compared to MDA-MB-231 cells (Table 11) [276]. Similarly, Avedian et al. [277] designed smart MPSNPs for the active and passive targeted delivery of Erlotinib (ETB) to Human cervical carcinoma cells (HeLa). PEI coating regulated pHR CDR in various pH and the targeting agent FA, facilitated targeted cargo delivery [277].
Active targeting can unload remarkable quantities of MNPs to TME unlike free or passively targeted anti-neoplastic agents. It will enhance the specificity and affinities of the MNPs towards malignancy cells. Since MNPs are functionalized with ligands which bind to overexpressed receptors on carcinoma cells [278]. The phenomenon was initiated in 1980 by Lee and his colleagues who grafted antibodies on liposomes surfaces and conjugated with ligands [279]. The performance of targeted anti-neoplastic agent significantly relies on several factors such as (i) the nano-carrier, (e.g., size, shape, charge, stability, degradability, etc.), (ii) the ligand (availability, characteristics, density, bindings, etc.), pharmaceutical agent (type, release, efficacy, etc.), and (iii) other factors (cancer heterogeneity, type, stage, overexpressed receptors, size/density) [280].
MNPs can be modified by targeting/homing agents that can actively react with overexpressed receptors on malignant cells/small molecules, e.g., carbohydrates and FA or macromolecules, i.e., antibodies, peptides, proteins, and aptamers (Apt) [281]. For instance, the conjugation of HA onto IONPs enabled selective binding to CD44 which is overexpressed on the surface of 4T1 breast carcinoma cell lines, permitting an efficient treatment. The distribution of DOX was improved at pH = 5.5 due to the protonation of DOX in the areas of higher acidity. The nano-carriers exhibited higher toxicity activity against 4T1 cells compared to GES-1 gastric mucosa cells [282]. Figure 5 illustrates the experimental procedure and active targeting of the DOX, and the cellular uptake by 4T1 cells, following the administration. penetration of nano-formula. Therefore, further research is needed for the successful production of cancer targeted IONPs in clinical applications. Furthermore, the construction of nano-formulas with efficiency in TDD and monitoring/imaging has a great degree of importance for both diagnosis/therapeutic intentions. Such smart targeted nano-formulas will have the preference to monitor TME and release the therapeutic cargo intracellularly to selectively eradicate the malignancy cells [248,250]. Figure 5. Schematic representation of an in vivo experimental procedure and the active targeting of DOX, and the cellular uptake by 4T1 cells following the administration. Redrawn from [282]. Figure 5. Schematic representation of an in vivo experimental procedure and the active targeting of DOX, and the cellular uptake by 4T1 cells following the administration. Redrawn from [282]. Table 11 shows the most recent examples of actively targeted smart magnetic nanopharmaceutics delivery for cancer treatment. For instance, FA can stimulate cellular internalization of MNPs in active manner [283]. In vitro studies indicated FA promoted the targeted and controlled release of DOX in a dual release mechanism (pH and redox) [284] and a redox-responsive mechanism, respectively [283]. FA is cheap and widely available, has low immunogenicity and toxicity, also it is simple to alter for application as single/dual targeted systems in neoplasm treatment [274,281]. Nevertheless, challenges continue to persist in clinical trials for FA-targeted NPs against human malignancy cells [285].
Anti mucin (MUC) aptamer (MUC1 Apt) is also an alternative targeting agent/tumor marker and is overexpressed in the majority of adenocarcinomas on the whole cell surface and sheds in the blood system. MUC1 Apt is a highly glycosylated transmembrane glycoprotein. MUC1 Apt was used for the pHR release and active targeting of DOX in vitro. The nano-medicine demonstrated capability as a potential multi-modal agent for the simultaneous detection and treatment of MUC1 overexpressing carcinoma cells in clinical application [286,287]. Another example is PhBA, which allows selective and reversible combination with polyhydroxylated compounds which contain vicinal diol or meta-diol structure and can form covalent complexes. This feature of PhBA was employed in an in vitro study to actively target DOX release via pHR release [288]. Another targeting agent called Lactoferrin (LF), an Fe-carrier glycoprotein, was utilized to bind to overexpressed receptors on C6 glioma cells and endothelial cells for active targeting [289].    Some of the targeting agents serve dual purposes. For example, a number of in vitro studies applied MTX which is an antimetabolite agent, capable of acting as targeting and chemo-therapeutic agents. Since the structure of MTX is analogous to FA, it can target the folate-receptor-positive tumors and interrupt the metabolism pathway [209,276,290,291]. The outcomes confirmed that Fe MTX NPs are efficient anticancer delivery systems and most likely play a part in future in vivo applications. Pemetrexed (PMX), similar to MTX, is a folate analog and its application was evaluated for active targeting in vitro and in vivo by Ak et al. [292]. Moreover, the fusion of the targeting ligand to smarten IONPs could lead to a reduction in non-specificity and enhance the uptake of nano-formula, resulting in increased anticancer effects and minimized toxicity to healthy cells as compared to passively targeted alternatives. Thus, identifying the specific receptors that are abundantly overexpressed on malignancy cells and ligands which bind strongly to these receptors are vital aspects of constructing smart IONPs. Although, many pre-clinical studies have been conducted on actively targeted stimuli-responsive MNPs, no nano-formula has yet been approved by the FDA. This is due to the presence of different barriers that limit the cells penetration of nano-formula. Therefore, further research is needed for the successful production of cancer targeted IONPs in clinical applications. Furthermore, the construction of nano-formulas with efficiency in TDD and monitoring/imaging has a great degree of importance for both diagnosis/therapeutic intentions. Such smart targeted nano-formulas will have the preference to monitor TME and release the therapeutic cargo intracellularly to selectively eradicate the malignancy cells [248,250].

Interaction of MNPs with Biological System
IONPs are ideal candidates for use in clinical utilization because the force of magnetism has low physical interactions with the body [294,295]. Since these NPs are foreign entities, the administration could cause some biological responses [296]. Intravenous injection is the most common route of entry in which the body's immune system as the first defense mechanism responds promptly and attempts to clear the particles from the blood stream [28,295].
The main part of the immune system is RES which includes monocytes circulating in the blood and specialized tissue-resident macrophages such as liver kupffer cells, bone osteoclasts, lung alveolar macrophages, and brain microglia. This system protects the body from pathogens or foreign particles such as IONPs via phagocytosis [28] and accumulating NPs in the liver and spleen [297]. Activation of immune cells and release of cytokines in the blood stream and tissues may cause systemic or local inflammation which is classified as a side effect [298]. Following the activation of immune cells such as macrophage, IONP can produce ROS and oxidative stress [299].
Moreover, the complement system is another part of the immune system that consists of a group of~30 proteins soluble in plasma that can attach to IONPs and influence their efficacy. It triggers a series of inflammatory processes which cause anaphylatoxin production and finally cardiac and respiratory complications [300].
Different studies have reported that a variety of factors such as size, charge, surface, and polymer conformation as well as molecular structure of MNPs can influence the protein adsorption. For instance, positively charged polystyrene NPs enhanced complement anaphylatoxin levels and negatively charged citrated IONPs attached to more serum proteins and activated the complement proteins extensively [299]. When NPs enter the blood stream, macromolecules, especially proteins, may bind to their surface and produce "protein corona (PC)" [298].The PC affects the cell recognition of NPs that is called "cell vision".
Cell responses to the NPs depend on the first contact between NP and the cell surface which differs between the protein coated NP and the intact one [295]. The bio-identity of NPs is influenced by three factors: physiochemical characteristics (shape, size, polydispersity, polymer conformation, molecular structure and etc.), biological elements (source of protein, human/animal), and experimental conditions (e.g., temperature, ionic strength) [301]. IONP tend to have a potency to attach to different plasma proteins such as immunoglobulins [295], blood coagulation, angiogenesis, complement system, and other regulatory proteins involved in protein processing, lipid metabolism, and cytoskeleton organization. For example, it has been demonstrated that IONPs attached to coagulation factor VII and fibrinogen led to the activation of the kallikrein system and induced thrombosis [300].
Moreover, there is another macrophage population called marginal zones in the spleen which are involved in blood clearance of pathogens or foreign agents such as IONPs by phagocytosis. IONP can aggregate in the liver/spleen and cause inflammation via necrosis, ROS production, secretion of pro-inflammatory cytokines [299], and lysosomal or mitochondrial damage [300]. ROS production enhancement is an initiating step which triggers an innate immune response by inflammasome activation [299]. Recently this feature of IONPs has attracted enormous interest in cancer vaccine immunotherapy as an activator of the immune system by targeting the tumor site [302].
The lymphatic system is composed of lymph nodes linked to each other by lymphatic vessels. When IONPs enter a tissue, they may move to lymph vessels as well as regional lymph nodes, where they encounter sinusoidal macrophages. Therefore, most of intravenously injected IONPs could be not only trapped by liver and spleen prior to reach any other organs but also lymph nodes, except intramuscular or subcutaneous injections in which regional lymph nodes may be the first clearance sites [28].
The renal system is another elimination pathway via the non-phagocytizing route that can clear carbohydrates, proteins, ions, and possibly NPs. Generally, the observation indicates that NPs with small sizes can be swiftly removed through the renal system [303]. However, there are no data that have reported the presence of non-degraded IONPs in urine [28].
The blood-brain barrier (BBB) is also an issue for the utilization of NPs in brain carcinomas, since only 2% of agents can cross that barrier. Size and charge of NPs should be optimized to be able to pass through BBB [28]. Additionally, the assembly of iron in the brain is linked with neurodegenerative illnesses, e.g., multiple sclerosis and Alzheimer's [304] due to its capability to stimulate the generation of oxidative stress and ROS [299,305]. Furthermore, the correlation of magnetite NPs with microvascular endothelial cells of BBB led to detrimental implications [306].
Intrapulmonary delivery of IONPs was practiced for imaging and treatment of lung carcinoma. During the procedure IONPs enter the alveolar which has macrophages that phagocytize IONPs [28]. The interaction of NPs with pulmonary surfactant proteins raised phagocytosis [307]. Ruge et al. [308] reported that MNPs highly attached to surfactant protein A with high interaction with alveolar macrophages and maximized phagocytosis [308]. Ultimately, phagocytosis of NPs by alveolar macrophages largely relays on the generation of protein corona around the NP which can determine bio-distribution and immunological fate of the NP [299]. Moreover, the intratracheal dispensing of IONPs enormously rose the quantities of neutrophils and inflammatory cells in bronchoalveolar lavage fluid [299].
Oral administration of the IONPs is another approach mainly used for MRI of gastrointestinal (GI) tract. The gastric acids and enzymes which can degrade the IONPs rapidly are the major biological barriers for their GI delivery [28]. IONPs after successive entry into TME amalgamate with tumor-associated macrophages (TAMs), (constituents of immune system). TAMs take part in malignancy formation by inhibiting the immune system, enhancing malignancy cell growth, survival, and migration. Extracellular signals may cause phenotypic modifications in macrophages, recognized as polarization [309], dividing macrophages into two subtypes: type 1 macrophages (M1) and type 2 macrophages (M2). M1 macrophages orchestrate pro-inflammatory retaliation, identifying carcinoma cells and instigating immune feedbacks, whereas, M2 macrophages establish anti-inflammatory results, inducing growth and multiplication of neoplasm. They are the dominant population within TME and targeting these cells has displayed highly improved outcomes. [310,311].
Phagocytosis of NPs can influence TAM polarization because these particles are recognized as foreign bodies. IONPs demonstrated a potent effect on TAM polarization due to iron transporter-related protein expression [311][312][313]. Kodali et al. [314] reported 1029 changes in gene expression of lung macrophages using IONPs while silica NPs with only 67 gene. It also reduced IL-10 secretion and maximized TNF-α secretion in macrophages more than silica NPs treatments. This phenomenon is caused by the metallic core of IONPs rather than the surface coating [314].
In clinical studies, results have indicated that Fe agglomeration in tumor tissues can induce M1 polarization. A study by Reichel et al. [296] on non-small cell lung carcinoma patients revealed Fe cumulated in cells due to hemolysis caused positive correlation to CD68 expression on TAMs and had negative effect on malignancy size. The TAMs displayed increase in inducible nitric oxide synthase (iNOS) and CD86 expression, decrease in CD206 expression as well as rise in level of IL-6 secretion and also reduction in IL-10 secretion, all of these are the features of M1 macrophages. It seems that IONP accumulation near tumors caused a reduction in tumor size. This hypothesis was tested in mouse models and all the results mentioned above were reported [296].
Taken together, all those biological interactions suggest that developing experimental investigations are necessary to study systemic and local effects of the NPs on biomolecules, immune cells, and other biological components of the body.

Magnetic Nanoparticles in Clinical Applications
A substantial number of SMNPs have been developed over the last decades [243,255,277,287,315]. However, no IONPs, passively or actively targeted malignancy cells, have yet been clinically approved for therapeutic agent delivery in treatment of carcinoma [67]. In fact, the vast number of MNPs have been approved for use in the clinic as diagnostic and imaging agents such as SIONP ferumoxytol which is in phase IV clinical trials as an MRI contrast media for the detection of lymph node metastasis. The list of MNPs under clinical trial/withdrawn from the market for cancer theranostic are collected in Table 12. Although the US FDA approved IONPs as contrast agents in MRI, most have been discontinued. This is because radiologists are not fully experienced to interpret the T2 contrast signals provided by MNPs. The only FDA-approved IONP that has not been discontinued and is the most clinically investigated, as an MRI contrast media and applicable for treatment of iron deficiency in adults with chronic kidney disease in June 2009 is ferumoxytol with tradename Feraheme ® in the US and Rienso ® in Europe [316].  Clinical applications of MNPs for cancer HT have been limited by the need for the precise placement of a large AMF within the human body. Generally, FDA-approved formulations have been evaluated and optimized over the years. Simple formulation of these NPs is the most critical requirement for utilization in clinic. However, most smart IONPs have complex structures and formulations which is a major drawback for industrial production. The notable failure of using IONPs in clinical applications is the existence of various challenges that have led to insufficient efficacy and reduced interest for medical and commercial use [273]. In the following, we will briefly discuss some of the notable challenges facing for the translation of SMNPs from production on the bench toward application in patient treatment.
As previously stated, the injected smart nano-carriers face a series of complex biological constraints from the site of injection until they reach the final target destination. These include rapid elimination, escaping from endosomal and lysosomal compartments, cellular internalization, and drug efflux pumps which hinder the assembly of nano-formulas at target site and diminish their therapeutic effects [322,323]. Although on one hand the modification of NPs might be a good solution, on the other hand adding extra synthesizing steps can create further complexities and rise the production cost. It should be noted that a positive cost-benefit balance is necessary for sustainability of the launched products in the market [324,325]. IONPs used in preclinical research are almost prepared in small scales and their large-scale synthesis may not generate same quality particles. Furthermore, the complex structural design of SMNPs make the scale up for industrial production difficult. It is essential to develop low-energy-input methods for industrial-scale production of SMNPs with simple and high reproducible formulations.
Another major challenge preventing the translation of IONPs into clinic is their safety for humans. Although IONPs were approved and practiced in clinics, e.g., as iron replacements and contrast media [295,326], various studies found that parameters such as composition, size, surface, properties, dose, and route of administration can influence their safety. Several groups reported that most IONPs were not discharged from body, accumulated in vital organs, e.g., spleen and liver and led to toxicity [324]. Furthermore, the excessive release of free Fe from Fe 3 O 4 NPs can facilitate the generation of ROS in cells and thus induce oxidative stress and disrupt liver mitochondrial function [295,324,327]. Moreover, MNPs with different coatings are toxic to brain cells and may cause neurodegenerative diseases such as Alzheimer's and Parkinson's disease [328]. Therefore, further studies on the long-term toxicity of SMNPs in the human body need to be conducted prior to their full clinical utilization. Another problem when using SMNPs as DDS is that deep organs within the body cannot be easily targeted by external magnets due to the absence of an effective MF gradient. To overcome this problem, the preparation of MNPs with high magnetic moments or the use of superconducting magnets such as SmBaCuO with the ability to produce strong magnetic gradients is necessary [328]. The dissimilarities between animal and human models create another pitfall in utilizing SMNPs for carcinoma treatment. Since an animal model, e.g., a mouse, cannot fully reflect the pathological conditions that exist in humans, and the nature of malignancy differs from person to person. Moreover, the lack of specific regulatory guidelines is final challenge toward commercialization of smart nano-carriers [329,330].
Last but not least, industrial scale-up, validation, reproducibility, and controllability of physicochemical properties of SMNPs remain a huge barricade hindering their clinical translation and public availability.

Future Perspectives
It is clear that the SMNPs have demonstrated potentials in both diagnosis and treatment of neoplasm. Although extensive research accompanied by development of advanced characterization techniques and instruments has been carried out on SMNPs during the past decades, there are still major problems related to the preparation of safe and effective smart IONPs for applications in the pharmaceutical market. Thus, the technology behind SMNPs needs further studies in order to achieve the substantial milestone in personalizing nano-medicines.
The greatest dilemma is testing on animal models which remains the fundamental key in examining the hypothesis and analyzing the safety profile of SMNPs. Hence, the way forward is to engineer research approaches and modalities which are physiologically suit-able to mimic the complicated human physiology and avoid the requirement for animals, as well as overcoming the issue of disparity between human and animal species.
IONPs demonstrated a reliable outcome in nano-platforms. Study of safety profile of approved IONPs such as ferumoxytol showed they hold substantial potential for future clinical use. The incorporation of diagnosis tools, e.g., MRI, and treatment moieties, e.g., chemo-agents, into a solo platform presents a holistic opportunity for an efficient management of malignancies. Therefore, in near future it can be expected that with development of material, pharmaceutical and biomedical sciences and the combination of nano-engineering and smart chemistry, researchers will be able to design suitable stimuli-responsive SMNPs with abilities to perform as single/multi modal theranostics tool in a single platform.

Conclusions
In the present review, an attempt was made to provide a comprehensive review of SMNPs usages for malignancy detection and effective treatment, including TDD, HT, and MRI agents. The aim was also to lessen the complications of systemic chemo-agents and controlling the therapeutic efficacy of agents in tumorous tissue.
Although SMNPs are showing to be efficient in the diagnosis and treatment of carcinoma in laboratories, there are still many challenges ahead for these NPs in relation to the translation from bench to bed. Importantly, the clinical success of the SMNPs depends upon their ability to bypass chemical and biological barriers including toxicity, biodistribution, pharmacokinetics, pharmacodynamics, as well as their industrial scale-up and reproducibility for a reliable large scale production. Hence, in the future, more extensive studies are required to address the aforementioned challenges for the development of effective and practical SMNPs in cancer theranostics.