Positive Inotropic Agents in Cancer Therapy: Exploring Potential Anti-Tumor Effects

Abstract

Cancer remains a significant global health challenge despite advancements in diagnosis and treatment. Traditional cancer therapies often face limitations such as toxicity and drug resistance. Drug repurposing has emerged as a promising strategy to overcome these challenges by identifying new therapeutic uses for existing drugs. This review explores the potential of repurposing positive inotropic agents, which are traditionally used in cardiovascular medicine, for cancer therapy. Positive inotropic agents, including cardiac glycosides, β-agonists, phosphodiesterase inhibitors, and calcium sensitizers have shown preclinical evidence of anti-tumor activity through various mechanisms, such as modulation of the intracellular signaling pathways, increasing cyclic adenosine monophosphate (cAMP) levels, the production of nitric oxide, and decreasing reactive oxygen species levels. Despite the absence of specific clinical trials in this area, these findings suggest a promising avenue for further research and development of combination therapies to improve cancer treatment outcomes. However, challenges such as elucidating specific anti-tumor mechanisms, identifying predictive biomarkers, and optimizing safety profiles need to be addressed to fully realize the therapeutic potential of positive inotropic agents in oncology.

1. Introduction

Cancer is an extensive and heterogeneous disease that continues to challenge the global healthcare landscape. Characterized by uncontrolled cell growth and proliferation, cancer encompasses a diverse array of malignancies, each with its unique molecular and clinical characteristics [1]. Despite decades of intensive research efforts and significant advancements in diagnostic techniques and therapeutic modalities, cancer remains a leading cause of morbidity and mortality worldwide [2,3].

The complexity of cancer arises from its multifaceted nature, encompassing a spectrum of biological, genetic, and environmental factors that contribute to disease initiation, progression, and therapeutic response [4]. Tumors exhibit remarkable heterogeneity, both within individual malignancies and across different cancer types, leading to diverse clinical presentations and treatment outcomes [5]. This intrinsic variability poses significant challenges for the development of effective and universally applicable treatment strategies [6].

Conventional cancer therapies, including surgery, chemotherapy, and radiation therapy, have long been the cornerstone of cancer management [7]. While these modalities have demonstrated efficacy in certain contexts, they are associated with considerable limitations, such as systemic toxicity, treatment resistance, and adverse side effects [8].

In recent years, there has been growing recognition of the importance of exploring new paradigms in cancer therapy to address the limitations of traditional treatments. One promising avenue is the repurposing of existing drugs for anticancer purposes. Drug repurposing, also known as drug repositioning, involves identifying new therapeutic indications for drugs that have already been approved for other medical conditions [9,10]. By taking advantage of the existing pharmacological and safety profiles of these drugs, researchers can speed up the development of new anticancer therapies and potentially bypass the time-consuming and costly process of drug discovery and development [11,12]. A key advantage of drug repurposing lies in its speed of development. Unlike de novo drug discovery, which can take years or even decades to bring a new drug to market, repurposed drugs have already undergone safety and pharmacology tests in humans. This enables a more rapid transition from preclinical studies to clinical trials, accelerating the availability of new treatment options for cancer patients [13,14]. Furthermore, the wide diversity of molecules available for repurposing offers a wealth of potential candidates, each with unique mechanisms of action and safety profiles. This diversity provides researchers with a rich pool of compounds to explore, increasing the likelihood of identifying effective anticancer agents. Over the years, several drugs initially developed for other indications have been successfully repurposed to treat other diseases, demonstrating the potential of this strategy (

Table 1. Examples of successfully repurposed drugs.

Drug Name	Original Indication	New Indication	References
Verapamil	Heart rhythm problems	Fungal biofilms	[15]
Sildenafil	Angina	Erectile dysfunction	[16]
Carboplatin	Antitumor	Fungal biofilms	[15]
Amiloride	Hypertension	Secondary progressive multiple sclerosis (SPMS)	[17]
Minoxidil	Hypertension	Hair loss	[18]
Aspirin	Analgesia	Colorectal cancer	[19]
Thalidomide	Morning sickness	Multiple myeloma	[20]
Zidovudine	Cancer	HIV	[21]
Rituximab	Cancer	Rheumatoid arthritis	[22]
Raloxifene	Osteoporosis	Breast cancer	[23]

).

In addition, there is also growing interest in exploring combination therapies as a strategy to increase treatment efficacy and overcome drug resistance in cancer [24]. Combination therapy involves the simultaneous or sequential administration of two or more drugs with complementary mechanisms of action [25]. By targeting multiple pathways involved in cancer growth and progression, combination therapy has the potential to achieve synergistic effects, maximizing therapeutic benefit while minimizing adverse effects [25,26]. Moreover, repurposed drugs can be combined with other conventional or experimental therapies, thereby broadening treatment options and enabling therapeutic synergies [27]. For example, metformin, originally used to treat type 2 diabetes, has been combined with chemotherapy and radiotherapy to improve outcomes in various cancers [28]. Similarly, sildenafil, known for treating erectile dysfunction, has been combined with chemotherapeutic agents like docetaxel to enhance drug delivery to tumors [29]. Thalidomide, once a treatment for morning sickness, has been repurposed for multiple myeloma and shows significant efficacy when combined with dexamethasone [30]. Digoxin, traditionally used for heart failure, enhances the effects of radiation therapy in hypoxic tumor environments by inhibiting HIF-1α [31,32]. This combined approach is particularly relevant for overcoming drug resistance and controlling the complex signaling pathways involved in cancer progression, offering new hope for improved outcomes in patients with advanced or refractory disease.

The importance of studies on new drugs for cancer treatment has grown significantly in recent decades due to its relevance [33]. Several works of our group have explored the cytotoxic effects of repurposed drugs and presented positive results regarding the cytotoxicity of some repurposed drugs, such as atorvastatin and nitrofurantoin in breast cancer and neuroblastoma [34] and CNS drugs in bladder cancer [35].

Although these studies have contributed significantly to the discovery of new possible drugs for the treatment of cancer, it is still important to study new possible treatments to try to reduce the therapeutic dose of chemotherapy and, consequently, its side effects.

In this context, our work proposes exploring the concept of repurposing positive inotropic agents in cancer therapy. Notably, some positive inotropic agents have already demonstrated anti-tumor activity in various studies, suggesting their potential as novel therapeutic agents in cancer treatment [31,36,37,38,39,40,41]. The main novelty of this study lies in the fact that this class of drugs has not been widely studied in the context of cancer treatment. By elucidating the opportunities inherent in this approach, we aim to contribute to the ongoing efforts to advance cancer treatment and improve patient care.

2. Positive Inotropic Agents

Positive inotropic agents are medications that exert a positive effect on cardiac contractility, meaning they increase the force of contraction of the heart muscle. These agents are commonly used in the management of heart failure and other cardiovascular conditions characterized by impaired cardiac function. By enhancing myocardial contractility, positive inotropic agents improve cardiac output and alleviate symptoms such as fatigue, dyspnea, and exercise intolerance. It is important to note that cardiovascular disease is currently the leading cause of global mortality [42].

The mechanism of action of positive inotropic agents primarily involves modulation of the intracellular calcium levels in cardiac myocytes (Figure 1). These agents typically act by increasing the intracellular concentration of calcium ions, which in turn enhances the interaction between actin and myosin filaments, leading to stronger and more efficient contraction of the heart muscle.

2.1. Cardiac Glycosides

One of the most well-known positive inotropic agents is digoxin, a cardiac glycoside derived from the foxglove plant, known as Digitalis lanata [43]. Digoxin works by inhibiting the sodium-potassium ATPase pump in cardiac myocytes, resulting in an increase in intracellular calcium levels and enhanced myocardial contractility [44]. Approved by the FDA in 1954, digoxin is used to treat different cardiac conditions like atrial flutter, atrial fibrillation, and congestive heart failure along with their related symptoms, as well as the termination of fetal life before an abortion [45,46]. Despite its long history of use, digoxin is now less commonly prescribed due to concerns about its narrow therapeutic window and potential for toxicity [47]. More advanced treatments with fewer side effects and improved safety profiles, such as beta-blockers, phosphodiesterase (PDE) inhibitors, and calcium-channel blockers, have superseded its use.

2.2. β-Agonists

β-agonists, also known as β-adrenergic agonists or β-adrenergic receptor agonists, exert their effects by stimulating beta-adrenergic receptors in the heart. These receptors, when activated, lead to an increase in myocardial contractility, heart rate, and conduction velocity. Dopamine and dobutamine are both positive inotropic agents that are commonly used in the management of various cardiovascular conditions, particularly in acute settings such as heart failure and shock states. β-adrenergic agonists bind to and activate beta-adrenergic receptors located on the cell membrane of target cells. Dopamine primarily acts on beta-1 adrenergic receptors in the heart. Upon receptor activation, beta-adrenergic agonists initiate intracellular signaling cascades through activation of the G proteins associated with the receptor. This leads to the activation of adenylate cyclase, an enzyme responsible for catalyzing the conversion of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP). The production of cAMP serves as a second messenger that mediates the cellular effects of beta-adrenergic receptor activation [48]. Elevated levels of cAMP activate protein kinase A (PKA), which phosphorylates various proteins within the cell. Phosphorylation of target proteins by PKA results in a wide range of cellular responses, depending on the tissue type [49]. In cardiac myocytes, beta-1 adrenergic receptor activation leads to increased calcium influx through voltage-gated calcium channels, enhancing myocardial contractility (positive inotropic effect) [50]. In vascular smooth muscle cells, beta-adrenergic receptor activation can lead to vasodilation (particularly via beta-2 receptors), relaxation of bronchial smooth muscle (bronchodilation), and other effects [51].

2.3. Phosphodiesterase (PDE) Inhibitors

Another class of positive inotropic agents comprises phosphodiesterase (PDE) inhibitors, including milrinone, amrinone, enoximone, and cilostazol. These drugs function by impeding the degradation of cAMP and cyclic guanosine monophosphate (cGMP) within cells [52]. Elevated levels of cAMP and cGMP activate PKA and protein kinase G (PKG), respectively, by binding to the regulatory subunits of these kinases. Subsequently, PKA and PKG phosphorylate various intracellular proteins, initiating a cascade of events [53]. In cardiac myocytes, heightened cAMP levels prompt the activation of PKA, leading to increased calcium influx through L-type calcium channels and augmented calcium release from the sarcoplasmic reticulum [54]. Consequently, this enhances myocardial contractility, resulting in a positive inotropic effect. Concurrently, elevated cGMP levels activate PKG, which phosphorylates proteins involved in smooth muscle relaxation and vasodilation. This leads to the relaxation of vascular smooth muscle, promoting vasodilation and a decrease in systemic vascular resistance [55].

2.4. Calcium Sensitizers

Levosimendan is another positive inotropic agent that has gained attention for its unique mechanism of action. Unlike traditional agents that primarily increase intracellular calcium levels, levosimendan exerts its effects by sensitizing cardiac myofilaments to calcium, thereby enhancing myocardial contractility without increasing the intracellular calcium concentration [56]. Additionally, levosimendan exhibits selective inhibition of PDE, which could potentially enhance its inotropic effects under specific experimental circumstances [57]. Furthermore, investigations conducted on porcine coronary endothelial cells proposed an additional mechanism for vasodilation involving the nitric oxide (NO) and cGMP-protein kinase G-myosin light chain kinase pathway [58]. This mechanism further amplifies the vasorelaxation effects of levosimendan.

3. Preclinical Evidence of Anti-Tumor Activity and Current Clinical Landscape

3.1. Dopamine

As one of the primary neurotransmitters in mammals, dopamine (DA), chemically known as 4-(2-aminoethyl)-1,2-benzenediol, plays a crucial role in neural signaling, with a widespread impact on various physiological functions within the central nervous system (CNS) [59]. These functions extend beyond voluntary movement and encompass reward processing, sleep regulation, appetite control, emotional expression, attention, cognitive abilities, sense of smell, visual processing, hormonal balance, and regulation of the sympathetic nervous system [60]. DA concentrations in tumor tissues are typically lower compared with normal tissues, and elevating dopamine levels through treatment seems to impede the proliferation of tumor cells. Furthermore, studies by Moreno-Smith et al. have demonstrated that dopamine diminishes angiogenesis induced by chronic stress, thereby mitigating tumor expansion in ovarian carcinoma [36]. Inhibition of angiogenesis was also seen in gastric cancer with dopamine treatment by reducing tumor growth [37]. In another study, DA was shown to inhibit the vascular permeabilizing strongly and selectively and the angiogenic activities of VPF and VEGF. Via DA D2 receptors, induced the endocytosis of VEGF receptor 2, which is critical for promoting angiogenesis, thus preventing VPF/VEGF binding, receptor phosphorylation, and subsequent signaling steps [61]. Sarkar et al. presented results showing that the administration of low doses of DA, similar to the doses used in human therapies, prevents the progression of breast and colon cancers in vitro. Furthermore, a synergistic result is observed when DA is combined with conventional anticancer drugs (5-fluorouracil), resulting in prolonged survival of tumor-bearing animals [62]. Recently, a group of researchers successfully fabricated cobalt-ferrite (CF) nanoparticles (NPs) as a DA delivery agent by functionalizing CF with DA and magnetically manipulated polyethylene glycol (PEG) as an anticancer drug delivery agent for the administration of DA via a thermal and pH-sensitive form. They found that CF-DA-PEG is able to successfully transport DA inside A549 lung cancer cell lines and increases cancer cell apoptosis by inducing ROS, mitochondrial dysfunction, and destruction of the actin cytoskeleton. This study shows a new way of treating cancer by releasing DA into the cancer region at a low pH and high temperature with minimal side effects [63].

3.2. Dobutamine

Dobutamine is a β-1 agonist approved by the FDA in 1978 for short-term use in patients with decreased contractility caused by heart failure or cardiac surgical procedures, leading to cardiac decompensation [64,65]. In 2011, Bao et al. found that dobutamine inhibits the yes-associated protein (YAP)-dependent gene transcription, which has been observed in a number of types of tumors [66,67,68,69,70]. With this, Yin et al. decided to study the effect of dobutamine on the proliferation, apoptosis, and invasiveness of the MG-63 human osteosarcoma cell line. Dobutamine was able to significantly inhibit the proliferation of MG-63 osteosarcoma cells in a time- and concentration-dependent manner. In addition, it was able to increase cell apoptosis and arrest the cell cycle at the G2/M transition stage, reduce migration and invasion, and induce expression of caspases 3 and 9 [38]. Another study conducted by Xie and his team was able to prove the antitumor effect of dobutamine on multiple myeloma cells, including inhibition of cell proliferation, promotion of apoptosis, and induction of cell cycle arrest at the G1 phase in a dose-dependent manner via inhibiting activation of the MAPK signaling pathway [39]. In human gastric adenocarcinoma, dobutamine was able to significantly inhibit cell growth, migration, cell colony formation, and invasion. In addition, dobutamine interrupted the cell cycle in the G1 or S phase and increased the apoptosis rate of gastric adenocarcinoma SGC-7901 cells [71].

3.3. Digoxin

Digoxin, known for its role as a cardioprotective drug due to its inhibition of Na⁺/K⁺-ATPase, hence promoting the intracellular accumulation of Ca²⁺ ions, has also garnered attention for its potential anticancer properties [31,40,72,73,74,75,76]. Interest in the relationship between cardiac glycosides (CGs) and cancer began to grow in the late 1970s due to two key discoveries. Firstly, it was observed that malignant cells display changes in the activity of the Na⁺/K⁺-ATPase [77]. Secondly, certain CGs were identified as genuine phytoestrogens, suggesting a potential impact on the development and progression of hormone-sensitive cancers such as breast carcinoma [78]. In 2012, Platz et al. conducted a study for the identification of novel indication for commonly prescribed drugs. They explored the potential of clinically used drugs for the treatment of prostate cancer through a combination of efficient, high-throughput laboratory-based screening and a large prospective cohort study. Digoxin has emerged as the leading candidate due to its significant potency in inhibiting proliferation in vitro (IC50 = 163 nM) and widespread clinical use [74]. Digoxin has been shown to significantly inhibit the growth of pancreatic cancer cells [79]. Notably, it exhibits low cytotoxicity in normal cells at low concentrations [80]. Moreover, digoxin has demonstrated efficacy in inhibiting primary tumor growth and metastasis in breast cancer cells implanted into SCID mice [81]. Another study demonstrated that digoxin can inhibit proto-oncogene tyrosine-protein kinase Src and the phosphorylation and expression of epidermal growth factor receptor (EGFR) as well as signal transducer and activator of transcription 3 (STAT3). This inhibition contributes to the suppression of cancer cell proliferation, migration, and invasion [82]. In non-small cell lung cancer (NSCLC) cells, digoxin downregulates NDRG1 and VEGF by inhibiting HIF-1α under hypoxic conditions and induces autophagy, thereby inhibiting cell growth through regulation of the mTOR and ERK1/2 signaling pathways [83,84]. In small cell lung cancer (SCLC), six cell lines were subjected to digoxin treatment, all exhibiting high sensitivity with rather low IC50 values (approximately 50 nM). It was identified that digoxin treatment alone induced cell cycle arrest in the G2 or M phase and triggered apoptosis across these cell lines [76].

3.4. Cilostazol

Cilostazol, primarily utilized to treat intermittent claudication, is a derivative of 2-oxyquinolone. Its mechanism involves inhibiting phosphodiesterase III, leading to increased levels of cyclic adenosine monophosphate (cAMP). Its multifaceted pharmacological profile includes antiplatelet, antiproliferative, vasodilatory, and ischemic-reperfusion protective effects [85]. In a study conducted in 1998 by Ikeda et al., it was found that administering cilostazol significantly prolonged the survival of rats with liver metastatic tumors [86]. The researchers proposed that the interaction between cancer cells and platelets plays a crucial role in the metastatic process. They suggested that cilostazol’s ability to inhibit platelet function could account for the observed improvement in their experimental outcomes. The data presented by Murata et al. clearly indicate that cilostazol directly inhibits colon cancer cell motility. Indeed, the study revealed that cilostazol effectively hinders trans-cellular migration of DLD-1 cells through mesothelial cell monolayers induced with 10% FBS, implying potential benefits for host cells [87]. In another study, cilostazol was shown to cause anticancer effects in human squamous cell carcinoma and colon cancer cells [88]. Using Hep3B and SK-Hep1 cells, cilostazol demonstrated the ability to inhibit hepatocellular carcinoma (HCC) cell proliferation through multiple mechanisms. It induced apoptosis, arrested the cell cycle at the G0 or G1 phase, and reduced the expression of cyclin D1. The antiproliferative effects of cilostazol have been linked to the activation of AMP-activated protein kinase (AMPK) and inhibition of extracellular signal-regulated kinase (ERK) and AKT signaling pathways (Figure 2).

Liu et al. observed that cilostazol eliminates radiation-resistant glioblastoma [41]. In glioblastoma multiforme (GBM) cells, which are radiation-resistant, there was an increase in cell mobility and an acceleration in cell proliferation. Notably, GBM cells typically exhibit high expression and activation of the large conductance calcium-activated potassium channel (BK channel). However, radiation resistance resulted in a reduction in BK channel activity, regardless of its expression level. Cilostazol has shown promising results, being able to inhibit cell motility, proliferation, and the formation of tumor spheres in GBM cells’ radiation resistance. In addition, in an in vivo model of radiation-resistant GBM cells, cilostazol slowed tumor growth in vivo and prolonged survival in this model.

3.5. Levosimendan

Levosimendan is renowned for its efficacy in managing acute decompensated heart failure (ADHF) by enhancing myocardial contractility and exerting vasodilatory effects through the stimulation of cardiac troponin C and opening of ATP-sensitive potassium channels. Despite the lack of any studies on levosimendan for cancer treatment, our group initiated a study to investigate the potential benefits of combining levosimendan with 5-FU to enhance the efficacy of 5-FU and reduce its required dose in the treatment of urological cancers. Thus, levosimendan demonstrated significant anticancer effects on its own in prostate cancer (PC-3) and bladder cancer (UM-UC-5) cells. When combined with 5-FU, levosimendan potentiated the anticancer effects in both cell lines. Levosimendan can inhibit cell migration, proliferation, and colony formation, possibly affecting cyclic adenosine monophosphate (cAMP) levels and nitric oxide (NO) production. Therefore, the combination of levosimendan with 5-FU shows promising potential as a form of therapy for urological cancers, offering an innovative approach to improving patient outcomes. This combination strategy may allow for a reduction in individual drug doses and, consequently, adverse effects while maintaining therapeutic efficacy. The inhibition of cancer cell migration and colony formation suggests a possible role for levosimendan in metastasis prevention and tumor growth control.

Despite promising findings in preclinical studies investigating the anti-tumor potential of positive inotropic agents, it is important to note that specific clinical trials in this area have not been identified to date. The absence of current clinical trials underscores the need for further research initiatives to assess the therapeutic potential of positive inotropic agents in cancer treatment. These efforts may include the development of prospective randomized clinical trials investigating the effects of positive inotropic agents in patients with different types of cancer, as well as the evaluation of their safety, tolerability, and efficacy in combination with other therapeutic modalities. While preclinical studies provide a solid foundation for investigating the anti-tumor potential of positive inotropic agents, well-designed clinical trials are essential to validate these results and determine their role in cancer treatment.

4. Repurposing Positive Inotropic Agents for Oncology

Positive inotropic agents such as dopamine, dobutamine, digoxin, cilostazol, and levosimendan have demonstrated potential anticancer properties through various mechanisms. These agents not only improve cardiac function but also exhibit anti-tumor activities by influencing cellular pathways. Based on previous research elucidating the anti-tumor effects of levosimendan, this article explores the shared mechanisms (

Table 2. Common mechanisms of action of positive inotropic agents in cancer.

Category	Drug	Indication	Mechanism of Action
Cardiac glycosides	Digoxin	Mild-to-moderate heart failure, increased myocardial contraction, and maintained control ventricular rate	Na-K ATPase enzyme inhibition
Phosphodiesterase III (PDE3) inhibitor	Enoximone	Congestive heart failure	PDE3 inhibition Stimulates NO production Increases cAMP levels
	Milrinone	Acute decompensated heart failure	PDE3 inhibition Increases cAMP levels
	Amrinone	Congestive heart failure	PDE3 inhibition Increases cAMP levels
β-agonists	Dobutamine	Cardiac decompensation	Increases cAMP levels
	Dopamine	Hypotension	Increases cAMP levels
	Isoetharine	Asthma	Increases cAMP levels
	Ritodrine	Premature labor	Increases cAMP levels
	Terbutaline	Asthma and premature labor	Increases cAMP levels
Calcium sensitizer	Levosimendan	Chronic heart failure	PDE3 inhibition Stimulates NO production Decreases ROS levels Increases cAMP levels

cAMP = cyclic adenosine monophosphate; NO = nitric oxide; ROS = reactive oxygen species.

) underlying the anticancer properties of positive inotropic agents [12].

4.1. Inhibition of Phosphodiesterase 3

Phosphodiesterase 3 (PDE3) is widely distributed across various tissues and exists in two distinct forms, known as isoenzymes PDE3A and PDE3B. The activity of PDE3 isoenzymes plays a crucial role in various physiological processes, spanning cardiovascular function, insulin secretion, carbohydrate metabolism, lipolysis, and platelet aggregation. PDE3A exhibits predominant expression in several key tissues including the heart, vascular smooth muscle, platelets, pancreas, gastrointestinal (GI) tract, endometrium, fallopian tubes, oocytes, and kidney [89]. In vascular smooth muscle, PDE3A plays a crucial role in modulating contractions, contributing to the regulation of vascular tone and blood pressure. Additionally, in platelets, PDE3A activity is involved in the process of aggregation, influencing hemostasis and thrombosis [90,91]. Specifically, the catalytic functions of PDE3B isoenzymes are subject to activation through phosphorylation by PKA and the PI3K/PKB pathways in response to diverse hormonal signals. This regulatory mechanism underscores the versatility of PDE3 in modulating intracellular signaling cascades in different cellular contexts. Moreover, PDE3 exerts physiological significance in numerous tissues, including the heart, smooth muscle, platelets, liver, oocytes, and adipocytes. Its presence and activity in these tissues highlight its multifaceted roles in regulating cellular responses to various stimuli and underscores its importance in maintaining homeostasis across different organ systems [92]. Inhibition of the PDE isoenzyme 3 leads to an increase in the intracellular concentrations of the second messenger cyclic adenosine monophosphate (cAMP).

Increase in cAMP Levels

Previous studies have indeed demonstrated that increasing levels of cAMP in cancer cells can inhibit their transcellular migration in vitro [93,94,95]. Overall, drugs that target cAMP signaling pathways represent a promising approach for preventing metastasis in cancer patients.

Existing extensively in cells, cAMP serves as a critical intracellular second messenger in various cellular processes, influencing cell metabolism, ion channel activity, gene expression, cell growth, differentiation, and apoptosis [96]. The major targets of cAMP include PKA, exchange proteins activated by cAMP (Epac1 and Epac2), and ion channels. PKA plays a crucial role in the regulation of cell proliferation by exerting its effects on transcription factors and other signaling molecules. One mechanism by which PKA may inhibit proliferation is interfering with the Ras-Raf-MEK-ERK signaling pathway, which is a key regulator of cell growth and proliferation. Specifically, PKA can phosphorylate and thereby inhibit the activity of c-Raf, a downstream effector of Ras. By phosphorylating c-Raf, PKA can prevent its activation and subsequent downstream signaling events, including the activation of MEK and ERK kinases. This inhibition of the Ras-Raf-MEK-ERK pathway by PKA can lead to decreased proliferation and cell growth [97,98]. Epac proteins, as a major effectors of cAMP signaling, participate in a wide array of cellular functions mediated by cAMP, such as cell adhesion [99,100], the cell-cell junction [101,102], cell differentiation [103], cell proliferation [104] and apoptosis [105]. Ion channels that are directly regulated by cAMP are permeable to cations such as sodium (Na⁺) and calcium (Ca²⁺), and their activation by cAMP leads to changes in membrane potential and cellular excitability. An increase in intracellular sodium concentration has been reported early in the cell death process, particularly in the context of apoptosis [106,107,108,109].

4.2. Production of Nitric Oxide

Nitric oxide (NO) is a ubiquitous free radical signaling molecule that plays a fundamental role in regulating numerous cellular processes. Its diverse functions include the modulation of angiogenesis, smooth muscle tone, immune response, apoptosis, and synaptic communication [110]. NO production in cancer is a multifaceted process with both pro-tumorigenic and anti-tumorigenic effects, depending on various factors such as the stage of cancer, the tumor microenvironment, and the concentration of NO (Figure 3). At low concentrations, NO functions mainly as a signaling molecule through the nitrosylation of soluble guanylate cyclase (sGC), leading to an increase in cyclic guanosine monophosphate (cGMP) levels. This activation of sGC results in vasodilation and may also promote angiogenesis through the activation of signaling pathways involving hypoxia-induced factor 1 alpha (HIF1α) and vascular endothelial growth factor (VEGF) [111]. These effects can promote tumor growth and metastasis. However, at higher concentrations, NO has been shown to inhibit cell proliferation and induce apoptosis, ultimately leading to the death of cancer cells [112,113].

NO can induce the expression of the tumor suppressor gene p53, which is a critical regulator of the cell cycle and apoptosis [114]. Meanwhile, p53 activation leads to the transcription of several pro-apoptotic genes, including Bax [115]. The pro-apoptotic protein Bax promotes the activation of caspases such as caspase 3, a critical executioner caspase that is activated downstream of mitochondrial events such as the release of cytochrome c, leading to apoptotic cell death [116]. NO-induced activation of caspase 8 can cleave Bid into tBid, which then translocates to mitochondria to promote cytochrome c release and further caspase activation [117].

4.3. Reduction in Reactive Oxygen Species Levels

Reactive oxygen species (ROS) are a group of reactive molecules that contain oxygen and are considered to be natural byproducts of cellular metabolism primarily produced in the mitochondria [118,119]. They include molecules such as superoxide anion (O₂^·−), hydroxyl radical (·OH), and hydrogen peroxide (H₂O₂) [120]. While ROS act as signaling molecules in various cellular pathways, influencing processes such as cell proliferation, differentiation, apoptosis, and inflammation and playing a role in host defense mechanisms, where excessive accumulation can lead to oxidative stress, causing damage to proteins, lipids, and DNA [121,122,123]. This imbalance between ROS production and antioxidant defense mechanisms can result from various factors, including environmental stressors (such as UV and ionizing radiation), metabolic dysregulation, and inflammation [124]. Oxidative stress contributes to cellular damage by oxidizing biomolecules, disrupting cellular function [125], and promoting the development of various diseases, including cancer [126,127,128,129,130,131,132], neurodegenerative disorders [133,134], and cardiovascular diseases [135,136]. When cancer cells undergo various cellular processes, their levels of ROS tend to increase, resulting in oxidative stress. This heightened oxidative stress triggers the activation of specific redox-sensitive transcription factors and growth regulatory proteins, including NF-kB, AP-1, HIF-1A, and NRF2. Activation of these transcription factors disrupts signaling pathways and alters the expression of oxidative and inflammatory mediators, which are crucial in cancer development and treatment complexities [137,138]. Moreover, cancer cells release various mediators, including oxidative and inflammatory components, which orchestrate changes in surrounding cells and tissues within the tumor microenvironment (TME). The TME, comprising diverse immune cells (both innate and adaptive) and fibroblasts, emerged as a critical determinant in cancer progression, metastasis, and clinical outcomes [139]. ROS play a pivotal role in driving signaling pathways associated with epithelial-mesenchymal transition (EMT), metastasis, and adverse clinical consequences [140,141]. Additionally, metabolic reprogramming is a key aspect of cancer pathogenesis, ensuring a continuous energy supply for uncontrolled growth, proliferation, and maintenance of stem cell-like properties in cancer cells. Extensive research has highlighted the significant contribution of ROS to reshaping metabolic dynamics across various cancer types, including lymphoid and myeloid cells [142,143,144,145]. In recent years, numerous clinical trials have investigated therapeutic drugs aimed at regulating ROS in cancer cells [146]. Additionally, there has been an extensive review of molecules that manage ROS homeostasis for cancer treatment [147,148]. Several FDA-approved drugs that target ROS regulation have been repurposed for cancer treatment, presenting a novel and effective approach to cancer therapy [149]. The production of intracellular ROS was markedly lowered in human microglial HMC3 cells treated with either levosimendan or dobutamine compared with cells that were not treated. This suggests that these drugs might be effective in reducing ROS buildup [150]. In another study conducted by Zhao et al., dimethyl succinate was used to trigger total ROS production, which was significantly inhibited when treated with digoxin [151].

Reducing ROS levels in cancer cells involves multiple mechanisms, each contributing to improved therapeutic outcomes. Antioxidant enzymes such as superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx), and peroxiredoxins (Prxs) play a crucial role in mitigating oxidative stress by converting harmful ROS into less reactive molecules [152]. For instance, SOD converts superoxide radicals into hydrogen peroxide, which catalase then breaks down into water and oxygen, while GPx reduces hydrogen peroxide and lipid hydroperoxides using glutathione, and Prx reduces peroxynitrite and organic hydroperoxides [153]. Reducing ROS levels sensitizes cancer cells to conventional therapies such as chemotherapy and radiotherapy. Reducing ROS alters the cellular environment, making cancer cells more susceptible to the cytotoxic effects of these treatments [154]. In addition, the tumor microenvironment undergoes significant changes when ROS levels are reduced. Lower levels of oxidative stress create a more favorable environment for the immune system to function [125]. Cancer cells often exploit ROS to evade immune surveillance and mount immunosuppressive responses. By reducing ROS, the immune system gains the upper hand, mounting more effective anti-tumor responses [155].

5. Challenges and Future Opportunities

The use of positive inotropic agents in cancer therapy presents a unique set of challenges and opportunities that demand a careful and innovative approach. One of the main challenges lies in identifying and developing effective therapeutic agents that can tackle the dual threat of heart disease, the leading cause of death worldwide, and cancer, one of the most lethal diseases, not only improving cardiac function but also displaying robust anti-tumor properties. The complexity of tumor biology and the heterogeneity of tumors require an in-depth understanding of the mechanisms by which these agents exert their anti-tumor effects, as well as their potential toxicity and bioavailability.

Understanding the anti-tumor mechanisms of these agents is crucial, as it could reveal new therapeutic targets and treatment strategies. However, elucidating these mechanisms is a significant challenge due to their multifunctionality and complex interaction with cell signaling pathways. In addition, identifying predictive biomarkers that can help select the patients most likely to respond to treatment with positive inotropic agents is essential to maximize therapeutic benefits and minimize adverse effects. The tables below (

Table 3. Positive inotropic agents used in the manuscript.

Drug	Type of Cancer	Anticancer Activity	Doses	Maximum Dose Used (Cardiovascular Diseases)	References
Dopamine	Breast and colon	Inhibits angiogenesis and tumor growth	50 mg/kg/day	50 µg/kg/min	[62]
Dobutamine	Gastric	Inhibits cell growth, migration, cell colony formation, and cell invasion, arrests the cell cycle at the G1 or S phase, and increases the rate of apoptosis	30 μmol/L	40 µg/kg/min	[71]
	Bone	Inhibits cell growth, migration, and cell invasion, augments cell apoptosis, and arrests the cell cycle in the G2 or M phase	10 µM		[38]
Digoxin	NSCLC	Reduces the cell viability, increases DNA damage by promoting ROS generation, and inhibitis both DNA double-strand break (DSB) and single-strand break (SSB) repair	0.2 µM– 1.0 mg/kg/day	0.75–1.5 mg/day	[31]
Cilostazol	Colon	Suppress migration	50 µM	200 mg/day	[87]
	Liver	Inhibits proliferation, induces apoptosis, induces G0 and G1 cell cycle arrest, and decreases the expression of cyclin D1 and nuclear antigen in proliferating cells	100 µM		[88]
Levosimendan	Bladder and prostate	Inhibits cell migration, cell colony formation, and proliferation	100 µM	6–12 µg/kg/10 min	Our Lab

and

Table 4. Standard chemotherapy drugs for cancer treatment. Adapted from emedicine.medscape.com.

Drug	Type of Cancer	Doses
Doxorubicin	Breast, bone, and liver	60–75 mg/m2/d
5-FU	Colon and gastric	500–2600 mg/m2/d
Cisplatin and pemetrexed	NSCLC	75 mg/m2/d + 500 mg/m2/d
Gemcitabine and cisplatin	Bladder	1000 mg/m2/d + 70 mg/m2/d
Docetaxel and prednisone	Prostate	75 mg/m2/d + 10 mg/d

) provide a comprehensive comparison of the featured inotropic agents (dopamine, dobutamine, digoxin, cilostazol, and levosimendan) and standard chemotherapy drugs used in cancer treatment.

Drug repurposing strategies can yield significant advantages in terms of overall development cost, time to market, and mitigating failure rates attributed to safety concerns. In certain instances, repurposed indications are uncovered either by the original drug developer during pre-clinical investigations or incidentally observed as a side effect in clinical trials, subsequently leading to patenting. A notable illustration of this phenomenon occurred during development of the hypertensive medication sildenafil, which was redirected toward the creation of Viagra for treating erectile dysfunction following the discovery of potent off-target effects in clinical trials [156].

The safety and tolerability of positive inotropic agents in cancer patients are paramount considerations. Given that these agents are FDA-approved and have been in use for many years, they are generally regarded as safe. However, it is crucial to acknowledge that cancer patients often experience a significant burden of toxicity from chemotherapy and radiotherapy. Therefore, careful assessment of potential side effects and effective symptom management remain essential. While positive inotropic agents are well-established medications, ongoing research efforts are still needed to optimize strategies for mitigating adverse effects and improving patients’ quality of life.

Moreover, the combination therapy of positive inotropic agents with other treatment modalities, such as chemotherapy, immunotherapy, and target therapy, offers a promising opportunity to improve clinical outcomes in cancer patients. Identifying synergistic therapeutic combinations and appropriate sequencing strategies can increase treatment efficacy and consequently reduce the required dose of chemotherapy, for example, and reduce the development of resistance.

6. Conclusions

The exploration of positive inotropic agents as potential candidates for cancer therapy represents a compelling avenue for innovation in the field of oncology. Despite the well-established use of these agents in cardiovascular medicine, their therapeutic potential in cancer treatment is an emerging area of research with promising preclinical evidence.

The multifaceted mechanisms of action exhibited by positive inotropic agents, including their ability to modulate intracellular signaling pathways, increase cAMP levels, produce nitric oxide, and decrease reactive oxygen species, highlight their potential utility as anticancer agents. Furthermore, their established safety profiles and FDA approval for other indications offer a valuable foundation for further investigation in oncology.

However, several challenges must be addressed to fully realize the therapeutic potential of positive inotropic agents in cancer treatment. These include elucidating the specific mechanisms underlying their anti-tumor effects, identifying predictive biomarkers for patient selection, and optimizing strategies to mitigate potential adverse effects in cancer patients, particularly those already burdened by chemotherapy and radiotherapy toxicity. Despite these challenges, the prospect of combining positive inotropic agents with existing cancer therapies, such as chemotherapy, immunotherapy, and targeted therapy, holds great promise for improving clinical outcomes and enhancing patient care. By leveraging the synergistic effects of combination therapy, it may be possible to maximize treatment efficacy while minimizing adverse effects and drug resistance.