Development of Polymer-Based Nanoformulations for Glioblastoma Brain Cancer Therapy and Diagnosis: An Update

Brain cancers, mainly high-grade gliomas/glioblastoma, are characterized by uncontrolled proliferation and recurrence with an extremely poor prognosis. Despite various conventional treatment strategies, viz., resection, chemotherapy, and radiotherapy, the outcomes are still inefficient against glioblastoma. The blood–brain barrier is one of the major issues that affect the effective delivery of drugs to the brain for glioblastoma therapy. Various studies have been undergone in order to find novel therapeutic strategies for effective glioblastoma treatment. The advent of nanodiagnostics, i.e., imaging combined with therapies termed as nanotheranostics, can improve the therapeutic efficacy by determining the extent of tumour distribution prior to surgery as well as the response to a treatment regimen after surgery. Polymer nanoparticles gain tremendous attention due to their versatile nature for modification that allows precise targeting, diagnosis, and drug delivery to the brain with minimal adverse side effects. This review addresses the advancements of polymer nanoparticles in drug delivery, diagnosis, and therapy against brain cancer. The mechanisms of drug delivery to the brain of these systems and their future directions are also briefly discussed.


Introduction
Cancer is one of the serious life-threatening diseases worldwide with a higher risk of mortality, around 10 million new cases are diagnosed every year [1,2]. Among different types of cancer, brain cancer is the most lethal and invasive type of central nervous system (CNS) disorder [3]. Brain cancer is characterised as a heterogeneous group of primary and metastatic cancers in the CNS [4,5]. The average incidence of both malignant and nonmalignant brain cancer is reported approximately 28.57 per 100,000 population, mostly affecting 0 to 19 years, with a mean annual morbidity rate of 5.57 per 100,000 population [6,7]. Among these, the malignant primary brain cancers with a 5-year survival rate of less than 33.3-35% and even the rate are still alleviating. The average survival span is still not the area of drug delivery as BBB prevents the transportation of a drug. Hence, higher doses are needed to attain desired therapeutic efficacy which causes undesirable side effects [27].

The Blood-Brain Barrier (BBB)
One of the main hurdles for the effective systemic treatment of brain cancer is the presence of the BBB. The BBB is a semipermeable membrane barrier between blood capillaries and cellular components of brain tissues that control the movement of ions, nutrients, and cells. The BBB also serves for the dynamic transport of nutrients, peptides, proteins and immune cells between the brain and blood [28]. The BBB consists of endothelial cells, glial cells (pericytes, astrocytes, and neurons) and basement membrane [29] (Figure 1). The endothelial cells line the interior brain capillaries forming the tight junctions that allow small molecules, gases and curb the influx of harmful toxins or pathogens such as bacteria, lipophilic neurotoxins, xenobiotics and hydrophilic substances from the blood to the brain [30]. Due to the presence of pinocytic vesicles, other carriers, transport proteins, and large numbers of mitochondria, hydrophobic and essential molecules such as O 2 , CO 2 , glucose, hormones, etc. can infiltrate either by passive diffusion or active transport mechanisms [29]. The presence of several transmembrane proteins characterises the tight junctions between the inter-endothelial cells. These protein complexes are mainly comprised of occludin, claudin, and junctional adhesion molecules. These three specialised proteins interact to develop an intricate, tight barrier that is exclusive to the cerebroendothelial cells [31]. The apical part of the endothelial cell is exposed to the brain's blood capillaries, and the basolateral part is exposed to the cerebrospinal fluid supported by the basement membrane. The basement membrane with 30-40 nm thickness consists of Type IV collagen, fibronectin, laminin, heparin sulfate proteoglycans and other extracellular matrix proteins that completely covers the endothelial cells and limits the movement of the solutes [29,31,32]. Approximately 98% of smaller molecular weight drugs and 100% of larger molecular weight drugs are reported for their inability to cross the intact BBB [33,34]. Under various brain-related pathological conditions, including brain cancers, glioma cells loose the structural integrity and the function of the BBB [35]. BBB is compromised in human glioma cells because of the leaky inter endothelial tight junction and poorly differentiated astrocytes that are unable to release essential components for BBB function [31,36]. In this case, it is termed as blood-brain tumour barrier (BBTB) or blood-tumour barrier (BTB) [14] (Figure 1).
In low-grade gliomas, the structure and function of the BBTB resemble normal BBB, while in hi-grade glioma, BBB is significantly altered, disrupted. Although the degree of BBB disruption varies from the tumour malignancy, low-grade glioma is still a hurdle to treat due to intact BBB. Despite high-grade gliomas, the structural disruption of their vascular density and integrity is negligible to drug permeability in tumour cells [37,38]. However, BBTB is more permeable than the BBB and allows heterogeneous permeability to drugs and other components. Thus, it is a more challenging task to combat the difficulties of brain cancer [39]. Therefore, along with the existing therapeutic regimen, new approaches are required to combat the BBB. To combat these difficulties, various techniques were developed, which are mostly invasive and cause serious side effects. Nanotechnology, especially use of polymer nanoparticles, helps address the major hurdle of glioma therapy non-invasively. Polymer nanoparticles aid in the targeting and delivery of potent drug molecules to the brain. In this review paper, we will briefly summarise the up-to-date existing therapies and diagnoses in brain cancer gliomas using polymer nanoparticles.

Polymer Nanoparticles for Drug Delivery Strategy to Overcome the BBB
The BBB is the main problem in the treatment of brain cancer glioma. The chemotherapeutic drugs are mostly ineffective due to limiting permeability to BBB as it allows to pass only low molecular weight (<500 Da), electrically neutral hydrophobic drugs with lipophilicity at log P 2-3 [11,36,40]. The majority of chemotherapeutic drugs are larger in size, ionic, hydrophilic molecules and thus cannot cross the BBB that is attributed to the requirement of a higher systemic dose that results in severe side effects [11]. To overcome these drawbacks, nanoparticles can be utilised for the controlled and sustained delivery of drugs. Biodegradable polymer nanoparticles are extensively studied systems in cancer drug delivery and therapy. These nanoparticles are also highly stable and can be tuned in order to obtain the desirable characteristics for a passive or an active targeting [41]. Polymer nanoparticles can induce selective toxicity and can load ample anticancer drugs or other molecules. Various biodegradable polymeric drug delivery systems include nanogels or hydrogels, poly(ε-caprolactone) (PCL), poly (lactic-co-glycolic acid) (PLGA), chitosan [42,43], dendrimers, etc. [44]. Due to versatile tuneable properties, these nano- In low-grade gliomas, the structure and function of the BBTB resemble normal BBB, while in hi-grade glioma, BBB is significantly altered, disrupted. Although the degree of BBB disruption varies from the tumour malignancy, low-grade glioma is still a hurdle to treat due to intact BBB. Despite high-grade gliomas, the structural disruption of their vascular density and integrity is negligible to drug permeability in tumour cells [37,38]. However, BBTB is more permeable than the BBB and allows heterogeneous permeability to drugs and other components. Thus, it is a more challenging task to combat the difficulties of brain cancer [39]. Therefore, along with the existing therapeutic regimen, new approaches are required to combat the BBB. To combat these difficulties, various techniques were developed, which are mostly invasive and cause serious side effects. Nanotechnology, especially use of polymer nanoparticles, helps address the major hurdle of glioma therapy non-invasively. Polymer nanoparticles aid in the targeting and delivery of potent drug molecules to the brain. In this review paper, we will briefly summarise the up-to-date existing therapies and diagnoses in brain cancer gliomas using polymer nanoparticles.

Polymer Nanoparticles for Drug Delivery Strategy to Overcome the BBB
The BBB is the main problem in the treatment of brain cancer glioma. The chemotherapeutic drugs are mostly ineffective due to limiting permeability to BBB as it allows to pass only low molecular weight (<500 Da), electrically neutral hydrophobic drugs with lipophilicity at log P 2-3 [11,36,40]. The majority of chemotherapeutic drugs are larger in size, ionic, hydrophilic molecules and thus cannot cross the BBB that is attributed to the requirement of a higher systemic dose that results in severe side effects [11]. To overcome these drawbacks, nanoparticles can be utilised for the controlled and sustained delivery of drugs. Biodegradable polymer nanoparticles are extensively studied systems in cancer drug delivery and therapy. These nanoparticles are also highly stable and can be tuned in order to obtain the desirable characteristics for a passive or an active targeting [41]. Polymer nanoparticles can induce selective toxicity and can load ample anticancer drugs or other molecules. Various biodegradable polymeric drug delivery systems include nanogels or hydrogels, poly(ε-caprolactone) (PCL), poly (lactic-co-glycolic acid) (PLGA), chitosan [42,43], dendrimers, etc. [44]. Due to versatile tuneable properties, these nanoparticles can open tight junctions of BBB, shield BBB limiting properties of anticancer drugs, release the drug in a sustainable manner, prolong the systemic circulation, and protect against enzymatic degradation [1,45].
Studies showed that Resveratrol loaded PLGA: D-α-tocopheryl polyethylene glycol 1000 succinate blend nanoparticles (RSV-PLGA-BNPs) displayed significant increasing cytotoxicity and enhanced cell penetration in C6 glioma cells. Haemocompatibility evaluation is one of the critical analyses of interaction between nanoparticles and various blood components that determine any adverse effect upon nanoparticle exposure to blood. The nanoparticles should not cause haemolysis during and after infusions. The haemocompatibility analysis of RSV-PLGA-BNPs revealed safe for i.v. administration. The nanoparticles exhibited prolonged systemic circulation up to 36 h. The nanoparticles also showed higher brain accumulation, suggesting a potential system for the betterment of systemic circulation and plasma half-life with a promising anticancer effect against glioma [1]. In another study, L-carnitine-conjugated PLGA NPs were developed to target glioma cells. These NPs were found to significantly cross the BBB and showed a potential anti-glioma effect [46]. Lactoferrin decorated PEG-PLGA NPs was developed for the delivery of shikonin and the treatment of gliomas [47]. Lactoferrin coating promotes internalisation across the BBB. In vitro and in vivo experiments showed the enhanced nanoparticle uptake and distribution of NPs in the brain with effective treatment of glioblastomas.

Polymer Nanoparticles for Anticancer Drug Delivery to the Brain: Mechanism
Polymer nanoparticles can cross BBB or BBTB either passively or via active endocytosis mechanisms. The unmodified polymer NPs internalise BBB mainly through passive mechanism, the so-called enhanced permeability and retention (EPR) effect, which depends on nanoparticle size. However, the NPS internalised by a passive mechanism have comparatively lower brain uptake than ligand-functionalised polymer NPs [48]. Various strategies have been undertaken to improve the infiltration of NPs into the brain. These strategies involve modification of NPs with certain moieties or components to take benefit of BBB endocytosis pathways for drug delivery. Polymer nanoparticles are able to cross BBB/BBTB through adsorption-mediated transcytosis (AMT), carrier-mediated transport (CMT), and receptor-mediated transcytosis (RMT) [49][50][51][52] (Figure 2). The internalisation of polymer nanoparticles crossing BBB/BBTB is summarized in Table 2. Polymer nanoparticles with positively charged can electrostatically interact with a negatively charged luminal surface that is attributed to cross the BBB/BBTB. The cationic polymer nanoparticles can be achieved by various surface modification strategies, either by coating or conjugation of cationic polymer or surfactant to non-ionic or neutral polymer. These modifications of NPs have been shown to utilise the AMT mechanisms to improve brain uptake. For example, a study of cationic bovine serum albumin (CBSA) conjugated with poly (ethylene glycol)-b-poly(lactide) (PEG-PLA) nanoparticles (CBSA-NPs), loaded with 6-coumarin was reported for brain delivery. Results revealed that CBSA-NPs uptake in rat brain capillary endothelial cells (BCECs) was enhanced as compared to control group BSA conjugated with pegylated nanoparticles (BSA-NP) BSA-NPs. Fluorescent microscopy of coronal brain sections displayed increased accumulation of CBSA-NPs than of BSA-NPs [53].
In the CMT mechanism, polymers NPs are designed to deliver drugs in order to take advantage of carrier molecules present in BBB. Polymer NPs are modified or decorated with membrane-penetrating components such as amino acids, peptides, and nutrients capable of transporting cargo across the BBB endothelial cells by utilising systemic transporters. For example, 2-deoxy-D-glucose modified poly (ethylene glycol)-co-poly (trimethylene carbonate) nanoparticles (DGlu-NPs) were studied for targeting the glioma BBB. The internalisation of DGlu-NP on RG-2 rat glioma cells was significantly higher than that of non-modified nanoparticles. This was attributed to the recognition of NPs by GLUT1 leading to enhanced cellular internalisation in glioma cells than in surrounding normal tissue and thus exhibiting promising in vivo anti-glioma activity [54]. In the CMT mechanism, polymers NPs are designed to deliver drugs in order to take advantage of carrier molecules present in BBB. Polymer NPs are modified or decorated with membrane-penetrating components such as amino acids, peptides, and nutrients capable of transporting cargo across the BBB endothelial cells by utilising systemic transporters. For example, 2-deoxy-D-glucose modified poly (ethylene glycol)-co-poly (trimethylene carbonate) nanoparticles (DGlu-NPs) were studied for targeting the glioma BBB. The internalisation of DGlu-NP on RG-2 rat glioma cells was significantly higher than that of non-modified nanoparticles. This was attributed to the recognition of NPs by GLUT1 leading to enhanced cellular internalisation in glioma cells than in surrounding normal tissue and thus exhibiting promising in vivo anti-glioma activity [54].
Similarly, L-carnitine modified PLGA nanoparticles (LC-PLGA NPs) were designed to utilise the advantage of Na-coupled carnitine transporter 2 (OCTN2) expressions on brain capillary endothelial cells as glioma cells for BBB infiltration and targeting. Results showed increased accumulation of NPs in the BBB endothelial cell line (hCMEC/D3) and the glioma cell line (T98G). This revealed the Na dependent cellular uptake that involves OCTN2 in the NPs internalisation process. Moreover, a higher accumulation of LC-PLGA NPs was also observed in the in vivo mouse model study. Furthermore, loading of drugs Taxol and paclitaxel in the LC-PLGA NPs improved anti-glioma activity in both 2D-cell and 3D-spheroid models [46].
With the RMT mechanism, polymer NPs are decorated/designed with targeting ligands that bind to specific cell surface receptors highly expressed in BBB transport pathways. For example, the Transferrin receptor (TfR) is one of the primary targets for investigating RMT across the BBB because of its high expression on BBB/BBTB endothelium [55]. To evaluate in vivo BBB penetration and targeting efficacy, transferrin modified doxorubicin (DOX) and paclitaxel (PTX) loaded magnetic silica PLGA nanoparticles (MNP-MSN-PLGA-Tf NPs) were developed. The nanoparticles were effectively accumulated in the tumour bearing mice suggesting that Tf facilitates NPs delivery across BBB [56]. Similarly, L-carnitine modified PLGA nanoparticles (LC-PLGA NPs) were designed to utilise the advantage of Na-coupled carnitine transporter 2 (OCTN2) expressions on brain capillary endothelial cells as glioma cells for BBB infiltration and targeting. Results showed increased accumulation of NPs in the BBB endothelial cell line (hCMEC/D3) and the glioma cell line (T98G). This revealed the Na dependent cellular uptake that involves OCTN2 in the NPs internalisation process. Moreover, a higher accumulation of LC-PLGA NPs was also observed in the in vivo mouse model study. Furthermore, loading of drugs Taxol and paclitaxel in the LC-PLGA NPs improved anti-glioma activity in both 2D-cell and 3D-spheroid models [46].
With the RMT mechanism, polymer NPs are decorated/designed with targeting ligands that bind to specific cell surface receptors highly expressed in BBB transport pathways. For example, the Transferrin receptor (TfR) is one of the primary targets for investigating RMT across the BBB because of its high expression on BBB/BBTB endothelium [55]. To evaluate in vivo BBB penetration and targeting efficacy, transferrin modified doxorubicin (DOX) and paclitaxel (PTX) loaded magnetic silica PLGA nanoparticles (MNP-MSN-PLGA-Tf NPs) were developed. The nanoparticles were effectively accumulated in the tumour bearing mice suggesting that Tf facilitates NPs delivery across BBB [56].  Abbreviation: Adsorption-mediated transcytosis (AMT), carrier-mediated transport (CMT), and receptor-mediated transcytosis (RMT).

Polymer Nanoparticles for Brain Cancer Therapy
Polymer NPs are solid colloidal particles that can be utilised as carriers in which the therapeutic drugs or other active components are dissolved, entrapped, encapsulated, or adsorbed on the surface of the polymer matrix [69]. The structure of the polymer NPs can range from nanospheres to nanocapsules depending on the preparation procedure. Various polymers such as chitosan, gelatin, sodium alginate, albumin and polylactides (PLA), polyglycolides (PGA), poly(lactide co-glycolides) (PLGA), polyanhydrides, polyorthoesters, polycyanoacrylates, poly(ε-caprolactone), poly(glutamic acid), poly(malic acid), poly(Nvinyl pyrrolidone), poly(methyl methacrylate), poly(vinyl alcohol), poly(acrylic acid), poly(acrylamide), poly(ethylene glycol), poly(methacrylic acid) are mostly used in nanoparticle formation for both passive and ligand-functionalized actively targeted therapy [70]. Based on the nature of drugs to be loaded and their route of administration, different synthesis methods were implemented for the production of polymer NPs that include sol-vent evaporation, solvent diffusion, nanoprecipitation, emulsification, reverse salting out, nano-capsules nano-precipitation, layer-by-layer (LbL) method, etc. [71][72][73]. The molecular weight, crystallinity, and stability of polymers and the drug's physicochemical properties can be analysed to develop polymeric NPs for drug administration to the brain. Polymeric NPs have a unique ability to reach the tumour site through an active targeting route [74]. Researchers have developed docetaxel (DOC)-loaded PCL and its derivative poly (ethylene glycol)-block-poly(ε−caprolactone) methyl ether (mePEG-PCL) nanoparticles that were dispersed in a bioadhesive film and the formulation exhibited sustained release of drugs. Docetaxel-loaded nanoparticles induced more significant cytotoxicity than free docetaxel for glioma treatment [75]. The study reveals that glycopeptide-engineered poly(d,l-lactideco-glycolide (PLGA) NPs (g7-NPs) provides in vivo evidence of endocytosis of g7-NPs and transported into the endosomes, which help to cross BBB [76]. Gaudin and co-workers have demonstrated the use of convection-enhanced delivery (CED) of NPs for improved chemotherapeutic drugs to the tumour site. They successfully administered gemicitabine, a nucleoside analogue used for the wide range of solid tumours using squalene-based NPs. The study also revealed that PEGylation of the NPs with PEG dramatically improves the distribution of squalene-gemcitabine NPs in the tumours [77].
Most of the current nanomedicines approved by the FDA for clinical use for solid tumour treatment depend on the EPR effect. The enhanced permeability and retention effect or EPR effect is a feature that allows small sized nanoparticles and other active molecules or drugs to pass due to large pore size through leaky vasculature and accumulate in the tumor location. The brain endothelial cells and glioblastoma cells generally overexpressed a number of receptors, including the low-density lipoprotein receptor, IL-13 receptor, transferrin receptor (TfR), and nicotine acetylcholine receptor that used as drug delivery targets in the brain [78]. Numerous in vivo studies revealed that polymer NPs could circulate for a longer time and accumulate in the tumour site. It is possible to enhance the retention and accumulation of these useful NPs by decorating NPs with tumour-homing ligands such as peptide, aptamer, polysaccharides, saccharides, antibodies, flic acids, etc. [79]. Recently, pluronic micelles (PEG-PPG-PEG) have evolved as perfect candidates for brain therapy, as they can easily cross the BBB and prove their ability to inhibit drug efflux [17]. For instance, Sun et al. developed TfR-T12 peptide-modified PEG-PLA polymer nanoparticle micelles loaded with paclitaxel (PTX) for glioma therapy. They found that the polymeric micelles (TfR-T12-PMs) could be absorbed by tumour cells, cross across BBB monolayers, and inhibit the proliferation of U87MG cells in vitro. A better antiglioma effect with a prolonged median survival of nude mice-bearing glioma was also observed in comparison with unmodified PMs [80]. This suggests that TfR-T12 peptide-modified micelles can cross the BBB system and target glioma cells. In Tables 3 and 4 are shown various synthetic and natural polymer-based NPs for GBMs therapy or diagnosis.
Recently, much research has been carried out on combined photo-based therapy along with other conventional therapy or imaging for glioblastoma treatment. For example, a novel photoacoustic and photothermal guided semiconducting polymer nanoparticles (SPNs) using poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (PEG-b-PPG-b-PEG) and SP were reported. The SPNs displayed efficient cellular internalisation for PAI and PTT toward U87 cells and accumulated in subcutaneous as well as brain tumours upon intravenous injection and induced efficient cell death upon NIR-II light irradiation [81].
The recent updates reveal that conjugated polymer nanoparticles (CPNs) are performing well as photosensitiser (PS) in photodynamic therapy (PDT). This efficiency is achieved by CPNs due to their uniform size, biocompatibility, and outstanding ROS production due to extraordinary photo-physical properties as well as fluorescence emission. It is found that porphyrin doped CPNs can eliminate GBMs through ROS-induced apoptotic damage [82]. Table 3. Synthetic polymer-based nanoparticles for brain cancer glioma therapy. PBAEs

Method of Preparation
Step-wise synthesis DNA Cy3 dye BTICs from patient More than 60% transfection efficacy is observed. [86]

Polymer Nanoparticles in the Diagnosis of Brain Cancer
Before surgery, a high-resolution image using imaging modalities is required for glioma detection. Owing to the invasiveness of glioma cells, determining the exact tumour boundary by eye is challenging. Proper imaging of a tumour is essential for assessing the extent of tumour distribution before surgery and the response to a treatment regimen after surgery [5]. Several available techniques for visualisation and diagnosis of brain cancer glioma include optical and ultrasound (US) imaging, photoacoustic (PA) imaging, computed tomography (CT), positron emission tomography (PET), single-photon emission computed tomography (SPECT) and fluorescence (FL) imaging techniques (Figure 3) [112]. Currently, magnetic resonance imaging (MRI), a non-invasive technique that can detect the size, shape, and tumour location, is initially employed diagnostic method for patients with suspected GBM [113]. MRI can determine the boundaries of the tumour tissues and/or intraoperative to elucidate tumour outline during surgical resection by applying gadolinium (Gd). Due to a shorter half-life, Gd must be administered often to maintain blood levels for efficient scanning. The use of intraoperative ultrasonography to obtain integrated brain tissue imaging is another non-optical method. However, this approach does not provide enough information for detecting smaller or superficial brain tumours. Other invasive techniques for analysing brain tumour tissues include Raman spectroscopy, optical coherence tomography, fluorescence spectroscopy, and thermal imaging [114]. Computed tomography (CT) can also be used to determine the presence of the tumour. Still, its use is relatively lesser in clinics for diagnosing GBM due to poor resolution compared to MRI [115]. Likewise, positron emission tomography (PET) imaging with 11C-methionine could be an effective diagnostic tool for GBM patients' prognosis [116,117]. To understand cancer tumours, precise preoperative imaging and painless sensitive postimaging techniques to provide real-time data are demanded. Current imaging modalities, however, lack accuracy, sensitivity, and specificity. Nanotechnology has sparked interest in bioimaging and biosensing in recent years. 'Nanodiagnostics' combined with nanotechnology could provide a drug delivery system with traditional diagnostic and imaging procedures [118,119]. Nanotechnology has made it easier to acquire data with great precision and accuracy while avoiding invasive procedures. NPs with tunable optical, magnetic, and electrical properties are able to provide diagnostic tools for detection and imaging brain cancer/tumours [120]. Biocompatible NPs owing ideal physical characteristics, such as surface chemistry, morphology, solubility, stability, etc., facilitate drug delivery and imaging as it acts as image contrast agents [121]. Polymer NPs could be a good reservoir system for drugs and a platform for additional modification for efficient tumour targeting or imaging [122]. Polymer NPs possess various advantages in drug delivery to the brain that can entrap or carry drugs that prevent them from metabolism and excretion. Moreover, NPs can easily transport drugs across the BBB without changing the barrier properties [31,123,124]. In this section, polymer NPs utilised in the diagnosis and detection of brain cancer glioma until now are primarily focused. The imaging and diagnosis techniques currently being investigated with reference to polymer NPs are listed in Table 5.
Polymer-based superparamagnetic NPs have mainly been employed as drug delivery systems and contrast agents in MRI imaging. These NPs are highly stable and biocompatible, can prolong systemic circulation time, have drug loading ability and control 'Nanodiagnostics' combined with nanotechnology could provide a drug delivery system with traditional diagnostic and imaging procedures [118,119]. Nanotechnology has made it easier to acquire data with great precision and accuracy while avoiding invasive procedures. NPs with tunable optical, magnetic, and electrical properties are able to provide diagnostic tools for detection and imaging brain cancer/tumours [120]. Biocompatible NPs owing ideal physical characteristics, such as surface chemistry, morphology, solubility, stability, etc., facilitate drug delivery and imaging as it acts as image contrast agents [121]. Polymer NPs could be a good reservoir system for drugs and a platform for additional modification for efficient tumour targeting or imaging [122]. Polymer NPs possess various advantages in drug delivery to the brain that can entrap or carry drugs that prevent them from metabolism and excretion. Moreover, NPs can easily transport drugs across the BBB without changing the barrier properties [31,123,124]. In this section, polymer NPs utilised in the diagnosis and detection of brain cancer glioma until now are primarily focused. The imaging and diagnosis techniques currently being investigated with reference to polymer NPs are listed in Table 5.
Polymer-based superparamagnetic NPs have mainly been employed as drug delivery systems and contrast agents in MRI imaging. These NPs are highly stable and biocompatible, can prolong systemic circulation time, have drug loading ability and control of drug release, and combine with their magnetic performance for MRI [125]. Ganipineni et al. synthesised paclitaxel (PTX) and superparamagnetic iron oxide (SPIO)-loaded PEGylated PLGA-based NPs (PTX/SPIONPs) and analysed for therapeutic efficacy in an orthotopic U87MG model. The cellular internalisation of these NPs was found to be concentration dependent. The MRI scanning displayed the blood-brain barrier disruption in the glioma affected location. Moreover, enhanced accumulation was also observed in ex vivo biodistribution analysis of GBM-bearing mice with magnetic targeting [126]. Researchers have evaluated SPIO-loaded brain penetrating PLGA NPs by CED administration on rat models and visualised using positron emission tomography (PET) and MRI [127]. SPIO-loaded NPs showed excellent transverse (T2) relaxivity. After CED of NPs, the biodistribution in the brain was analysed using MRI, which revealed a period of one month longer signal attenuation of SPIO-loaded brain-penetrating PLGA NPs. The co-administration of SPIO-loaded PLGA NPs allows intraoperative monitoring of biodistribution in the brain in order to ensure the delivery to tumour location and therapeutic effect over time [127]. Researchers have developed Polysorbate 80 coated temozolomide-loaded PLGA-based superparamagnetic nanoparticles (P80-TMZ/SPIO-NPs), evaluated for anti-glioma activity and analysed as a diagnostic agent for MRI [128]. The superparamagnetic P80-TMZ/SPIO-NPs showed a significant antiproliferative effect and remarkable cellular internalisation on C6 glioma cells. Moreover, the in vitro MRI scanning revealed that P80-TMZ/SPIO-NPs could also serve as a good contrast agent [128].

Limitations and Challenges
From the past times, tremendous developments have been evidenced in brain cancer therapy. Yet, there have not been emerged significant changes in mortality rate and improving patients' quality of life. Although nanoparticle-based drug delivery systems have brought a new horizon, many challenges remain and need to be solved in the future. The development of effective polymeric NPs for drug delivery and targeting is a challenging task for clinical translations. The advantage and limitations are summarised in Figure 4. The toxicity of these systems is one of the main challenges. The slow degradation rate of polymer NPs induce a longer circulation time in the body and could cause unknown complications.
Further, extensive investigations are required for optimization of the NPs. One of the major obstacles in clinical translation is the interaction of NPs and biological systems. Upon entering the complicated biological system, the designed polymeric NPs will instantly interact with neighboring biomolecules, leading to the formation of protein corona that alters their properties. This affects NPs size, stability, surface properties and determines the pharmacokinetics, biodistribution, cellular internalisation, intracellular trafficking, immune system, and toxicity [133][134][135][136]. In addition, more in vitro and in vivo studies are required to better understand the mechanisms in targeted nanoparticle-based therapy. Several essential factors related to the in vivo behaviour of NPs and their effect on other healthy brain cells are hence required to be extensively examined. Currently, there is still insufficient pre-clinical data of polymer-based NPs on brain delivery, data to correlate in vitro-in vivo observation, which makes it difficult to conclude about their therapeutic efficacy.  Further, extensive investigations are required for optimization of the NPs. One of the major obstacles in clinical translation is the interaction of NPs and biological systems. Upon entering the complicated biological system, the designed polymeric NPs will instantly interact with neighboring biomolecules, leading to the formation of protein corona that alters their properties. This affects NPs size, stability, surface properties and determines the pharmacokinetics, biodistribution, cellular internalisation, intracellular trafficking, immune system, and toxicity [133][134][135][136]. In addition, more in vitro and in vivo studies are required to better understand the mechanisms in targeted nanoparticle-based therapy. Several essential factors related to the in vivo behaviour of NPs and their effect on other healthy brain cells are hence required to be extensively examined. Currently, there is still insufficient pre-clinical data of polymer-based NPs on brain delivery, data to correlate in vitro-in vivo observation, which makes it difficult to conclude about their

Future Perspective and Conclusions
Glial originated brain cancers are the most aggressive gliomas that depict a threat to humans. The conventional therapies are still inefficient to overcome due to tumor heterogeneity and, specifically, the blood-brain barrier (BBB) of malignant gliomas. The polymeric nanoparticles-based brain cancer therapy approaches are currently gaining interest due to the drug safety, controllable drug release, and efficient targeting in tumors. Most importantly, reports revealed that polymer NPs could even transport across BBB. In this review article, we summarize the newest breakthroughs in the use of polymer nanocarriers for drug delivery, therapy and diagnosis of brain cancer are explored, emphasizing how they are a critical aspect of modern anticancer drug delivery strategies. Various polymer NPs have been generated to reduce anticancer drug losses, premature degradation, enhance drug availability, and reduce drug toxicity by improving drug accumulation in specific organs and tissues. Although the potential impact of polymer NPs in cancer therapy is exceedingly promising, numerous obstacles that currently limit their widespread clinical usage must be solved. For polymer NPs to be used in clinical trials, long-term safety investigations must be conducted in various animal models to eliminate the possibility of non-endogenous components accumulating in the body causing any harm. As a result, huge costs must be provided when conducting in vivo pharmacokinetic studies to evaluate the applicability in the human body. Another factor to consider is the challenges that may arise when transitioning from laboratory to large-scale production. The scaling up of the preparatory process is a major obstacle that must be surmounted. A significant number of polymer NPs are currently in the pre-clinical stage of development, but only one system has entered a clinical study. This is primarily because several challenges impede further development, such as a lack of potency in animal models and toxicity concerns. To overcome the aforementioned concerns, researchers need to focus more on new therapeutic innovations such as revising fabrication processes to modify and improve polymeric NPs in order to accommodate the demand for various anticancer drugs for effective clinical feasibility. New therapeutic innovations also include novel therapeutic strategies for combination therapy and stimuli-activated drug delivery. For example, delivering two or more anticancer drugs simultaneously might enhance the treatment of various cancer developments by targeting different tumour related signalling pathways, resulting in a synergistic therapeutic impact. In addition, the researcher needs to improve the targeting of cancer stem cells (CSCs) for effective cancer therapeutic effect as CSCs is a critical factor for tumour recurrence. In conclusion, pre-clinical experimentation and clinical trials are mandatory for an efficient polymer nanoparticle-based anticancer therapy. Hopefully, all of these developments will lead to more patient-specific and targeted anticancer therapies.