Cellular and molecular aspects of drug resistance in cancers

Abstract

Objectives

Cancer drug resistance is a multifaceted phenomenon. The present review article aims to comprehensively analyze the cellular and molecular aspects of drug resistance in cancer and the strategies employed to overcome it.

Evidence acquisition

A systematic search of relevant literature was conducted using electronic databases such as PubMed, Scopus, and Web of Science using appropriate key words. Original research articles and secondary literature were taken into consideration in reviewing the development in the field.

Results and conclusions

Cancer drug resistance is a pervasive challenge that causes many treatments to fail therapeutically. Despite notable advances in cancer treatment, resistance to traditional chemotherapeutic agents and novel targeted medications remains a formidable hurdle in cancer therapy leading to cancer relapse and mortality. Indeed, a majority of patients with metastatic cancer experience are compromised on treatment efficacy because of drug resistance. The multifaceted nature of drug resistance encompasses various factors, such as tumor heterogeneity, growth kinetics, immune system, microenvironment, physical barriers, and the emergence of undruggable cancer drivers. Additionally, alterations in drug influx/efflux transporters, DNA repair mechanisms, and apoptotic pathways further contribute to resistance, which may manifest as either innate or acquired traits, occurring prior to or following therapeutic intervention. Several strategies such as combination therapy, targeted therapy, development of P-gp inhibitors, PROTACs and epigenetic modulators are developed to overcome cancer drug resistance. The management of drug resistance is compounded by the patient and tumor heterogeneity coupled with cancer’s ability to evade treatment. Gaining further insight into the mechanisms underlying medication resistance is imperative for the development of effective therapeutic interventions and improved patient outcomes.

Graphical Abstract

Introduction

Cancer represents a daunting challenge in healthcare and, globally is one of the foremost causes of human fatality [1, 2]. The prevalence of cancer is attributed to the aging population, lifestyle changes, environmental factors and advanced detection methods [3]. Recent statistics revealed that cancer caused 9.7 million deaths with 20 million new cases (in 2022) and the future predictions are not very encouraging [4]. One of the primary reasons for this is the resistance of cancer cells towards the therapeutic drugs. Drug resistance continues to be a serious barrier in the treatment of many diseases, contributing to therapy failure and disease progression [5–8]. Statistical models reveal that the drug resistance causes over 90% deaths of patients undergoing treatment of some cancers [9]. Understanding the processes causing medication resistance is critical for creating successful treatment methods and improving patient outcomes [8, 10, 11]. Several cellular and molecular factors contribute to the development of the drug resistance to cancer drugs such as changes in genetics (target modification, activation of alternative routes), drug metabolism (overexpression of efflux pump, modified medication metabolism), tumour heterogeneity, hypoxia, microenvironment, cancer stem cells, epigenetic modifications and some others (Fig. 1).

Fig. 1.

Different mechanisms of drug resistance in cancer cells

The genetic changes such as mutations, amplifications, and deletions, can result in the activation of alternative signaling pathways or variations in therapeutic targets, leading to resistance towards targeted therapy [12]. Changes in drug metabolism, such as increased drug efflux or greater drug inactivation by metabolic enzymes, might diminish medication efficacy [13]. Tumor heterogeneity, defined as the existence of various cell populations within a tumor, might also contribute to resistance through the survival of subpopulations with innate or acquired resistance [14]. Hypoxia, inflammation, and the presence of cancer-associated fibroblasts can all establish a protective niche for cancer cells, sheltering them from the effects of medications [15]. Drug resistance can also be caused by epigenetic alterations such as DNA methylation and histone modifications. Furthermore, the existence of cancer stem cells, which have self-renewal and tumorigenic capabilities, has been associated to drug resistance [16]. Understanding the interaction of these many medication resistance pathways is critical for developing successful treatment methods [17]. Combination therapy that target numerous pathways, personalized medicine techniques based on specific patient characteristics, and the creation of innovative medications that circumvent resistance mechanisms are all potential approaches to addressing cancer drug resistance [18]. The area of cancer drug resistance has seen substantial development, especially at the cellular and molecular levels. These developments necessitate an updated review highlighting emerging mechanisms not fully addressed earlier. This review provides an overview of the different mechanisms of medication resistance reported in cancer and the various strategies for overcoming it. Moreover, the review aims to integrate insights from related fields providing a comprehensive perspective. This integration is essential to understanding the full complexity of drug resistance. In summary, the review not only highlights the recent findings but also provides important future perspectives to improve research and clinical outcomes. To provide a thorough understanding of drug resistance in cancer treatment, the review begins with an overview of the key anticancer drugs and their mechanism of action as illustrated in Table 1; Fig. 2. Each contributing factor that leads to resistance against these cancer drugs is then explored in depth to provide readers with a comprehensive perspective on the topic.

Table 1.

Summary of cancer drugs and their mechanism of action

S.No	Class of the Medications	Name of the Drug	Mechanism of action
1.	Tyrosine kinase inhibitors	Imatinib	Inhibits the BCR-ABL protein by binding to the ATP pocket in the active site
		Erlotinib	Inhibits the intracellular phosphorylation of tyrosine kinase associated with the epidermal growth factor receptor (EGFR)
		Gefitinib	Inhibits EGFR tyrosine kinase and further downstream signaling cascades are also inhibited, resulting in inhibited malignant cell proliferation.
		Crizotinib	Inhibits ALK by inhibiting its phosphorylation and creating an inactive protein conformation
		Lapatinib	Restricts phosphorylation of HER1 and HER2 by reversibly and competitively inhibiting ATP-binding sites of the intracellular kinase region
		Dasatinib	Inhibits the active and inactive conformations of the ABL (Abelson Murine Leukemia Viral Oncogene Homolog) kinase domain
2.	Monoclonal antibodies	Trastuzumab	Binds to the extracellular ligand-binding domain and blocks the cleavage of the extracellular domain of HER-2 and inhibits HER-2-mediated intracellular signaling cascades
3.	BRAF inhibitors	Vemurafenib	Blocks downstream processes to inhibit tumour growth and eventually trigger apoptosis
		Dabrafenib	Selectively inhibits some mutated forms of the protein kinase B-raf (BRAF)
4.	Estrogen receptor (ER) antagonist	Tamoxifen	Competitively inhibits estrogen binding to its receptor
5.	Androgen biosynthesis inhibitors	Abiraterone	Inhibits CYP17 to block androgen production
6.	mTOR inhibitors	Everolimus	Binds with high affinity to the FK506 binding protein-12 (FKBP-12), which leads to a blockage in the progression of cells from G1 into S phase.
7.	BCL-2 inhibitors	Venetoclax	Helps restore the process of apoptosis by binding directly to the BCL-2 (B-Cell Lymphoma 2) protein
8.	Vinca alkaloids	Vincristine	Disrupts microtubule dynamics, particularly during the M and S phases, which is essential for cell division and several cellular processes
		Vinblastine	Binds to the microtubular proteins of the mitotic spindle, leading to crystallization of the microtubule and mitotic arrest or cell death
9.	Anthracyclines	Doxorubicin	Exerts its antineoplastic activity through 2 primary mechanisms: intercalation into DNA and disrupt topoisomerase-mediated repairs and free radicals-mediated cellular damages
		Daunorubicin	Forms complexes with DNA by intercalation between base pairs, and it inhibits topoisomerase II activity
10.	Taxanes	Paclitaxel	Interferes with the normal function of microtubule growth and also arrests their function by having the opposite effect.
		Docetaxel	Binds to the β-subunit of tubulin which results in microtubule/docetaxel complex which does not have the ability to disassemble
11.	DNA topo isomerase inhibitors	Topotecan	Exerts its cytotoxic effects during the S-phase of DNA synthesis
		Etoposide	Inhibits DNA topoisomerase II, thereby inhibiting DNA re-ligation and can lead to apoptosis of the cancer cell.
		Irinotecan	Inhibits DNA topoisomerase I, acting on the S and G2 phases of the cell cycle.
12.	Oestrogen receptor antagonist	Fulvestrant	Binds to the receptors and degrades the estrogen receptors to which it is bound
13.	EGFR inhibitors	Gefitinib	Inhibits EGFR tyrosine kinase and further downstream signaling cascades are also inhibited, resulting in inhibited malignant cell proliferation
		Afatinib	Covalently binds to and irreversibly blocks signaling from all homo and heterodimers formed by the ErbB family members EGFR (ErbB1), HER2 (ErbB2), ErbB3 and ErbB4
14.	Purine antagonists	Mercaptopurine	Works as an antagonist to endogenous purines required for DNA replication during the S-phase of the cell cycle and inhibition of RNA and protein synthesis
15.	Antimetabolites	5-Fluorouracil	Undergoes conversion into fluorodeoxyuridine monophosphate (FdUMP), further resulting in the generation of double-stranded DNA breaks
		Methotrexate	Inhibits enzymes responsible for nucleotide synthesis
16.	Androgen receptor inhibitors	Enzalutamide	Inhibits androgen binding to its receptor, androgen receptor nuclear translocation, and subsequent interaction with chromosomal DNA to upregulate oncogenes
17.	DNA methyltransferase inhibitors	Azacitidin	Induces antineoplastic activity by inhibiting DNA methyltransferase and inducing cytotoxicity by incorporating itself into RNA and DNA
		Decitabine	It hypomethylates DNA by inhibiting DNA methyltransferase
18.	Histone Deacetylase Inhibitors	Vorinostat	Inhibits the enzymatic activity of histone deacetylases HDAC1, HDAC2 and HDAC3 (Class I) and HDAC6 (Class II)
		Romidepsin	Interacts with zinc ions in the active site of class 1 and 2 HDAC enzymes, resulting in inhibition of its enzymatic activity
		Belinostat	Inhibits the activity of histone deacetylase (HDAC) thus prevents the removal of acetyl groups from the lysine residues of histones and some non-histone proteins
		Panobinostat	Responsible for regulating the acetylation of about 1750 proteins in the body; their functions are involved in many biological processes
19.	EZH2 (Enhancer of Zeste Homolog 2) Inhibitors	Tazemetostat	Inhibits the transcription of genes associated with cell cycle arrest
20.	HDAC (Histone Deacetylase) and DNA Methyltransferase dual Inhibitors	Guadecitabine	Resists the degradation by cytidine deaminase, an enzyme that rapidly degrades decitabine

Fig. 2.

Chemical structures of selected cancer drugs

Genetic changes

Genetic changes play a pivotal role in the genesis and progression of cancer. These changes can occur in a various genes and genomic locations, resulting in dysregulated cell proliferation, poor deoxyribo nucleic acid (DNA) repair systems, and therapy resistance [19, 20]. In this section, the most prevalent genetic changes observed in cancer are discussed here.

Gene mutations

Gene mutations are alterations in a gene’s DNA sequence. They can cause aberrant proteins to be produced or protein function to be lost. Oncogene mutations such as those in Kirsten rat sarcoma (KRAS), B- rapidly accelerated fibrosarcoma (BRAF), and human epidermal growth factor receptor 2 (HER2) are reported to promote uncontrolled cell development. Conversely, alterations in tumour suppressor genes such as tumor protein 53 (TP53), breast cancer gene 1 (BRCA1), and adenomatous polyposis coli (APC) can impair normal tumor-suppressing capabilities [21]. Recent reports highlighted that these mutations are complex and could occur simultaneously or evolve resulting in multidrug resistance in cancer like NSLSC [22, 23].

Chromosomal abnormalities

Chromosomal abnormalities are changes in the structure or number of chromosomes such as translocations, deletions, amplifications, and inversions that can result in gene fusions, amplifications, or the loss of tumour suppressor genes [24]. For instance, fibroblast Growth Factor (FGF) ligands and their receptors are responsible for drug resistance and chromosomal translocation helps in FGFR production that leads to anti-cancer drug resistance [25].

Copy number variations (CNVs)

CNVs are sections of the genome in which the number of copies of a certain DNA sequence is increased or decreased in comparison to the normal diploid state [26]. CNVs that can contribute to cancer formation include oncogene amplifications, such as Erb-B2 receptor tyrosine kinase 2, ERBB2 (HER2) in breast cancer, and tumour suppressor gene deletions, such as phosphatase and tensin homolog (PTEN) in different malignancies [27].

Epigenetic modifications

Epigenetic alterations refer to DNA and histone modifications that regulate gene expression without changing the DNA sequence itself such as DNA methylation, histone acetylation, and histone methylation [28]. Tumour suppressor genes can be silenced or activated by abnormal epigenetic regulation that promotes cancer initiation and progression [29]. Resistance to nucleoside based drugs, decitabine and azacitidine that are hypomethylating agents, used in the treatment is myelodysplastic syndromes and AML are reported and the underlying mechanism being investigated [30].

Microsatellite instability (MSI)

MSI refers to variations in the length of microsatellites, which are repeated DNA sequences [31]. Defects in the DNA mismatch repair mechanism can result in MSI, which increases the mutation rate in afflicted cells. MSI is frequent in some malignancies, such as colorectal cancer, and can alter tumour behaviour and therapy response [32]. The colorectal cancers with MSI are treated with immune check-point inhibitors, however primary and secondary resistance are reported for these inhibitors [33].

Gene rearrangements and fusions

Gene rearrangements and fusions are generated by chromosomal rearrangements and entail the fusion of two usually distinct genes [34]. Breakpoint cluster region (BCR)- Abelson 1 (ABL1) fusion genes in chronic myeloid leukaemia (CML) and Echinoderm microtubule-associated protein 4- Anaplastic lymphoma kinase (EML4-ALK) fusion genes in non-small cell lung cancer (NSCLC) can drive oncogenic processes and act as therapeutic targets [35]. Recently, secondary mutations in these genes or signaling pathway lead to resistance, thereby necessitating the generation of next generation of inhibitors[36, 37].

The mentioned genetic changes can be acquired through various pathways, including carcinogen exposure, faults in DNA replication and repair, or inherited predispositions [38]. Different cancer drugs are susceptible to genetic changes like imatinib to genetic mutations, trastuzumab to chromosal aberration and copy number variations, azatidine epigenetic modification, pembrolizumab to microsatellite instability, olaparib to CNV and gene rearrangement and fusions. Table 1 illustrates the important cancer drugs that cause genetic changes. Understanding the specific genetic abnormalities that cause a certain cancer can aid in diagnosis, treatment options, and the development of targeted medicines [39]. On the other hand, the drug used in the treatment of cancer are prone to genetic changes initiated by the cancer cells rendering them ineffective, thereby leading to the development of the drug resistance. Cancer drug resistance by genetic changes is a multifaceted and complex phenomenon that is being extensively investigated for various types of cancers. Recent literature points toward that the development of resistance is rarely a single genetic change; rather it is combination of different genetic alterations like mutation, chromosomal abnormalities, epigenetic modifications etc. For instance, trastuzumab is used in the treatment of HER2-positive breast cancer and 70% of the patients develop resistance in a year. The resistance is attributed to various underlying mechanism that includes ERBB2 (HER2) mutations and nuclear localization, alteration of the gene at transcriptional and post-translation level, activation of alternative signaling pathways [40]. Moreover, in recent decades, the role of epigenetic modifications in development of cancer resistance is in the spotlight [41].Table 2 Investigations revealed that modifications in histone protein and aberration in its DNA methylation pattern leads to silencing of tumor suppressor gene and activation of oncogene that is crucial for development of resistance towards conventional and targeted chemotherapies [42, 43].

Table 2.

List of drugs targeting different stages of gene mutations

Drugs	Gene Mutations	Chromosomal Aberrations	Copy Number Variations (CNVs)	Epigenetic Modifications	Microsatellite Instability (MSI)	Gene Rearrangements and Fusions
Trastuzumab		✔	✔
Pertuzumab			✔
Lapatinib		✔	✔
Abemaciclib			✔
Tucatinib			✔
Ponatinib		✔
Palbociclib			✔
Olaparib			✔			✔
Imatinib	✔	✔
Erlotinib	✔
Gefitinib	✔
Osimertinib	✔	✔
Crizotinib	✔	✔				✔
Ceritinib	✔	✔
Vemurafenib	✔	✔
Dabrafenib	✔
Trametinib	✔	✔
Palbociclib	✔
Ribociclib	✔
Rucaparib	✔					✔
Niraparib	✔
Idelalisib	✔
Larotrectinib	✔	✔				✔
Alectinib		✔				✔
Lorlatinib						✔
Entrectinib		✔				✔
Brigatinib		✔				✔
Capmatinib						✔
Pembrolizumab					✔
Nivolumab					✔
Ipilimumab					✔
Atezolizumab					✔
Trifluridine/ tipiracil					✔
Azacitidine				✔
Decitabine				✔
Vorinostat				✔
Romidepsin				✔
Panobinostat				✔
Entinostat				✔
5-azacytidine				✔
Belinostat				✔
Tazemetostat				✔
Guadecitabine				✔
Cobimetinib		✔

Target modifications

In cancer, target modifications can contribute to therapeutic resistance by altering the medication target or its expression. These changes have the potential to render the targeted medication ineffective or impair its efficacy to block the desired route [44]. Modifications to the medication target or its expression can render targeted treatment ineffective. Mutations in the target protein, for example, can prohibit the medicine from binding and limit its action [45]. Some significant cancer medication resistance target changes are discussed.

Pharmacological target mutations

Genetic mutations in the target protein might cause changes in its structure or function, rendering it less sensitive to pharmacological inhibition [46]. These mutations can occur in a variety of oncogenes and tumour suppressor genes. For instance, mutations in the epidermal growth factor receptor (EGFR) gene can confer resistance to EGFR tyrosine kinase inhibitors (TKIs) in lung cancer [47].

Target overexpression

Cancer cells can increase the expression of the therapeutic target, resulting in enhanced signalling activity and decreased drug sensitivity. This can happen as a result of gene amplification, epigenetic alterations, or changes in transcriptional control. In breast cancer, overexpression of HER2 can lead to resistance to HER2-targeted therapy [48].

Alternative splicing

Alternative splicing can occur in cancer cells, leading to the creation of distinct isoforms or variations of the target protein. These isoforms may have different activities or be less vulnerable to pharmacological inhibition [49]. This may increase resistance to targeted medicines. For example, in chronic myeloid leukaemia (CML), alternate splicing of the BCR-ABL1 fusion gene might result in resistance to tyrosine kinase inhibitors [50].

Bypass pathway activation

Cancer cells can activate alternate signalling pathways or create bypass mechanisms to avoid the blocked target pathway [51]. This permits them to sustain cell survival and proliferation even while the medication is present. Bypass routes can be activated by mutations, gene amplifications, or cross-talk between signalling pathways [52].

Drug transporter depletion

Drug efflux pumps, such as P-glycoprotein (P-gp), can aggressively pump drugs out of cancer cells, lowering intracellular concentration and effectiveness [53]. Resistance to numerous medications, including chemotherapeutic treatments, can develop from increased expression or activity of drug efflux pumps [54].

Understanding the target changes that lead to medication resistance is critical for creating resistance-busting methods. This might include developing new medications or combination treatments that target al.ternative pathways, identifying biomarkers to guide therapy selection, or employing therapeutic techniques that can overcome particular target changes [55]. Target changes in cancer can have an impact on the efficacy of some medications that rely on particular targets for therapeutic effects [56]. Several medicines routinely used in cancer therapy that may be impacted by target changes are discussed here.

In chronic myeloid leukaemia (CML) and gastrointestinal stromal tumours (GIST), imatinib Gleevec) targets the BCR-ABL fusion protein. Mutations in the BCR-ABL gene can result in imatinib resistance [57]. In non-small cell lung cancer (NSCLC), erlotinib (Tarceva) targets the epidermal growth factor receptor and erlotinib resistance can be caused by EGFR mutations such as T790M [58]. Another EGFR inhibitor utilised in NSCLC is gefitinib (Iressa) and EGFR mutations leads to reduced efficacy of gefitinib [59]. Crizotinib (Xalkori) targets ALK gene rearrangements in NSCLC with ALK positivity and any mutation in ALK kinase domain can lead to crizotinib resistance [60]. In a similarl way, trastuzumab (Herceptin) targets the human epidermal growth factor receptor 2 in HER2-positive breast cancer and trastuzumab resistance can emerge as a result of HER2 mutations or alternative signalling [61]. BRAF inhibitors (vemurafenib, dabrafenib) target melanoma’s BRAF V600E mutation and resistance can occur as a result of acquired BRAF gene mutations or the activation of alternate pathways [62]. Tamoxifen is an oestrogen receptor (ER) antagonist that is used to treat ER-positive breast cancer and its responsiveness can be influenced by changes in the ER gene or signalling pathway [63]. In metastatic castration-resistant prostate cancer (CRPC), abiraterone (Zytiga) suppresses androgen production and changes in the androgen receptor (AR) pathway can have an impact on its efficacy [64]. Everolimus (Afinitor) inhibits the mTOR (mammalian target of rapamycin) pathway and any modifications in TOR or downstream signalling components can have an impact on its response [65]. In chronic lymphocytic leukaemia (CLL), venetoclax targets the BCL-2 protein and any aberration in BCL-2 can have an effect on its effectiveness [66].

It should be noted that the existence of target changes does not necessarily result in total resistance to these medications, and other treatment techniques or combination treatments may still be successful. The effect of target changes on treatment response varies between persons and cancer types [67].

Other route activation

Cancer cells can activate other signalling pathways or create bypass mechanisms to compensate for the drug-inhibited route. This enables them to continue reproducing and surviving in the presence of the medication [68]. Activation of alternative pathways in cancer can result in resistance or diminished efficacy of certain pathway-targeting medications [69]. Some of the medications routinely used in cancer therapy that may be altered by alternate route activation are discussed here.

Erlotinib targets the EGFR in non-small cell lung cancer (NSCLC) and the erlotinib resistance is induced by the activation of alternative pathways, such as mesenchymal-epithelial transition (MET) or HER2 bypass signalling [70]. Trastuzumab (Herceptin) targets the human epidermal growth factor receptor 2 in HER2-positive breast cancer and activation of alternative pathways, such as the phosphoinositide 3 kinase/ protein kinase B (PI3K/AKT) pathway can lead to trastuzumab resistance [71]. Similarly, BRAF inhibitors (vemurafenib, dabrafenib) target the BRAF V600E mutation in melanoma and the resistance to these inhibitors is induced by alternative pathways activation, such as the mitogen-activated protein kinase/ extracellular signal-regulated kinase (MAPK/ERK) or PI3K/AKT [72]. Alternative EGFR or HER2 signalling pathways can impact fulvestrant response, which is an oestrogen receptor antagonist used to treat hormone-positive breast cancer [73].

In the treatment of chronic myeloid leukaemia (CML) and gastrointestinal stromal tumours (GIST), imatinib (Gleevec) is used that targets the BCR-ABL fusion protein. The activation of alternative mechanisms, such as SRC family kinase signalling, can contribute to imatinib resistance [74]. EGFR inhibitors (gefitinib, afatinib) are drugs that target EGFR in cancer and their responsiveness is reduced by alternative pathways, such as the MET pathway or the PI3K/AKT pathway [75]. Similarly, alternative pathways such as the EGFR or c-KIT signaling reduces the efficacy of crizotinib (Xalkori) that targets ALK gene rearrangements in NSCLC with ALK positivity [76]. Tamoxifen, an oestrogen receptor antagonist, used to treat ER-positive breast cancer can be rendered less responsive by PI3K/AKT/mTOR pathway [77]. In metastatic castration-resistant prostate cancer (CRPC), abiraterone (Zytiga) suppresses androgen production. The response to abiraterone can be influenced by the activation of alternate androgen receptor (AR) splice variants or the glucocorticoid receptor pathway [78]. Everolimus (Afinitor) inhibits the mTOR pathway and activation of alternate survival pathways, such as the PI3K/AKT pathway, can influence everolimus responsiveness [79].

Resistance or diminished sensitivity to targeted medicines might result from the activation of alternate pathways [80]. To overcome or postpone the development of resistance, combination therapy or medicines targeting various pathways may be used. Individual patient features and cancer types should be taken into account while identifying the best therapy choices [81].

Efflux pump overexpression

Activation of efflux pump overexpression in cancer cells can result in lower intracellular drug concentrations and effectiveness. Efflux pumps are proteins that actively pump medicines out of cells, limiting their accumulation and efficacy, such as P-glycoprotein (P-gp). This diminishes intracellular drug concentration and hence effectiveness (Fig. 3) [82]. Some of the medications routinely used in cancer therapy that may be altered by efflux pump activation are discussed here.

Fig. 3.

Role of p-glycoprotein in chemotherapeutic efflux in cancer cells

Vinca alkaloids (Vincristine, Vinblastine): Vinca alkaloids are extensively utilised in cancer therapy. Overexpression of efflux pumps, such as the multidrug resistance protein (MDR1)-encoded P-glycoprotein (P-gp), can result in lower intracellular concentrations of vinca alkaloids, limiting their efficiency [83]. Anthracyclines (Doxorubicin, Daunorubicin are chemotherapy medications that are used to treat a variety of malignancies. Overexpression of efflux pumps can result in lower intracellular concentrations of anthracyclines, which can contribute to drug resistance [84]. Taxanes (Paclitaxel, Docetaxel): Taxanes are routinely used to treat malignancies of the breast, lung, and ovary. Overexpression of efflux pumps such P-gp or breast cancer resistance protein (BCRP) can result in lower intracellular levels of taxanes, limiting their effectiveness [85]. Topoisomerase inhibitors (Topotecan, Etoposide): These drugs are used to treat a variety of malignancies. Overexpression of efflux pumps can result in lower intracellular drug concentrations, reducing the efficiency of topoisomerase inhibitors [86].

Imatinib (Gleevec) is a tyrosine kinase inhibitor that is used to treat chronic myeloid leukaemia and gastrointestinal stromal tumours. Overexpression of efflux pumps such P-gp or breast cancer resistance protein (BCRP) can alter imatinib intracellular concentrations, potentially leading to drug resistance [87]. Lapatinib (Tykerb) is a dual tyrosine kinase inhibitor that is used to treat HER2-positive breast cancer. Overexpression of efflux pumps can influence the intracellular levels of lapatinib, reducing its effectiveness [88]. Erlotinib (Tarceva) is an EGFR inhibitor used to treat non-small cell lung cancer (NSCLC). Overexpression of efflux pumps such P-gp or BCRP can lead to lower intracellular concentrations of erlotinib, limiting its efficacy [89]. Dasatinib (Sprycel) is a tyrosine kinase inhibitor that is used to treat chronic myeloid leukaemia (CML) and other kinds of leukaemia. Dasatinib’s intracellular concentrations can be affected by efflux pump overexpression, reducing its effectiveness [90].

Efflux pump inhibitors or alternate therapeutic techniques that are not impacted by efflux processes may be used to address efflux pump overexpression. It is crucial to highlight, however, that the impact of efflux pump overexpression varies between individuals and cancer types [91].

Modified drug metabolism

Cancer cells can increase medication metabolism and clearance by regulating drug-metabolizing enzymes. Drug resistance can be increased by changes in drug metabolism in cancer cells, which affects the pharmacokinetics and efficacy of anticancer drugs [92]. Drug metabolic alterations may result in greater drug inactivation or clearance, resulting in reduced medication concentrations at the target site [93]. The important drug metabolic mechanisms that have been altered in cancer resistance are as follows:

Cytochrome P450 expression

Cancer cells can up regulate the expression of drug-metabolizing enzymes such as cytochrome P450 (CYP) enzymes, glucuronosyltransferases (UGTs), and sulfotransferases (SULTs). These enzymes have the potential to increase drug metabolism and clearance, resulting in reduced drug exposure and efficacy [94]. Higher CYP3A4 expression in cancer cells, for example, can result in increased chemotherapeutic drug metabolism [95].

Phase II enzyme alterations

Phase II enzymes, such as UGTs and SULTs, promote drug conjugation with endogenous substances such as glucuronic acid or sulphate. Changes in the expression or activity of these enzymes can have an impact on the metabolism and effectiveness of medications. Variations in UGT1A1 expression, for example, can affect irinotecan metabolism, a commonly used anticancer medication [96].

Modified drug transporters

Drug transporters are essential for drug absorption and efflux. Cancer cells can alter the expression or function of drug transporters, such as ATP-binding cassette (ABC) transporters, hence influencing medicine concentrations within cancer cells. Overexpression of drug efflux pumps like as P-glycoprotein (P-gp) may result in decreased intracellular drug accumulation and hence reduced therapeutic efficacy [97].

Drug activation or inactivation alterations

Some anticancer drugs require metabolic activation to produce cytotoxic effects, whereas others are inactivated by metabolism. Changes in pathway activation or inactivation can impact pharmacological efficacy. Reduced activation of the prodrug capecitabine to its active form, 5-fluorouracil (5-FU), can result in resistance in specific cancer types [98].

Epigenetic modifications of drug-metabolizing genes

Epigenetic alterations, such as DNA methylation or histone modifications, can regulate the expression of drug-metabolizing enzymes and transporters. Erroneous epigenetic alterations can have an impact on drug metabolism and resistance. Methylation-mediated silencing of the DNA repair gene O(6)-methylguanine-DNA-methyltransferase (MGMT), for example, can influence the response to alkylating medicines such as temozolomide [99]. Understanding the distinctive alterations in drug metabolic pathways in cancer cells is crucial for improving therapy choices and overcoming drug resistance. It might require developing novel pharmaceutical formulations, co-administering metabolic inhibitors, or employing customised therapeutic strategies depending on the patient’s metabolic profile [100]. Drug metabolism changes in cancer can have an influence on the efficacy of some medications that rely on normal metabolic mechanisms for activation or clearance [101]. Some of drugs routinely used in cancer therapy that may be influenced by altered drug metabolism are discussed here.

Tamoxifen is converted into its active form, endoxifen, by the liver enzyme cytochrome P450 2D6 (CYP2D6). In cancer patients, altered CYP2D6 metabolism can impact the conversion of tamoxifen into its active form, potentially lowering its effectiveness in hormone receptor-positive breast cancer [102]. Another drug irinotecan is converted to its active form, SN-38, by the liver enzyme carboxylesterase. Changes in carboxylesterase activity or expression in cancer patients might affect irinotecan conversion, resulting in varied therapeutic response and possible toxicity [103]. Mercaptopurine is metabolised in the liver by the enzyme thiopurine methyltransferase (TPMT). Changes in TPMT activity in cancer patients might influence mercaptopurine metabolism, resulting in varied therapeutic response and possible toxicity [104]. The enzyme dihydropyrimidine dehydrogenase (DPD) metabolises 5-FU. DPD activity or expression changes in cancer patients might affect 5-fluorouracil metabolism, resulting in varied medication response and possible toxicity [105]. Methotrexate is generally removed by renal excretion; however decreased kidney function in cancer patients can impair clearance. Furthermore, altered metabolism via polyglutamation can affect methotrexate intracellular availability and efficacy [106]. Docetaxel is largely metabolised in the liver by the enzyme CYP3A4. Changes in CYP3A4 activity or expression in cancer patients might affect docetaxel metabolism, resulting in varied medication response and possible toxicity [107]. Similarly, the vinca alkaloids are metabolised in the liver by many enzymes, including CYP3A4 and CYP3A5. Changes in the activity or expression of these enzymes in cancer patients might influence vinca alkaloids metabolism, resulting in varied medication response and possible toxicity [108]. Etoposide is metabolised by a number of enzymes, including CYP3A4, CYP3A5, and CYP2C8. Changes in the activity or expression of these enzymes in cancer patients can affect etoposide metabolism, altering therapeutic response and possible toxicity [109].

It is crucial to remember that altered drug metabolism can be impacted by a variety of variables such as genetic differences, medication interactions, liver or kidney failure, and specific patient characteristics. These characteristics are taken into account by healthcare experts in order to optimise the dose and treatment method for individual patients [110].

Tumour heterogeneity

Tumour heterogeneity refers to the occurrence of many cell populations inside a tumour that might differ in genetic and molecular properties. Tumour heterogeneity can lead to medication resistance in a variety of ways, complicating therapy [111]. Tumours are frequently made up of a diverse population of cancer cells with varying genetic and molecular features. This heterogeneity can result in cell subpopulations that are resistant to the treatment from the start or develop resistance over time [112]. Some examples of how tumour heterogeneity might affect cancer medication resistance are delineated as follows (Fig. 4).

Fig. 4.

Tumor heterogeneity in cancer drug resistance

Genetic heterogeneity

Within a tumour, different subpopulations of cancer cells may have diverse genetic changes. As a result, medication sensitivities varied amongst cell groups. While some cells may react to a certain therapy, others with distinct genetic changes may be resistant from the start or develop resistance over time. Incomplete or transitory responses to therapy might result from genetic variability [113].

Phenotypic heterogeneity

Tumour heterogeneity can also take the form of phenotypic variations between cancer cells such as variations in cell shape, differentiation state, proliferation rates, and signalling pathway activity. Drug sensitivity and response might differ due to phenotypic heterogeneity, with certain subpopulations being more resistant to therapy than others [114].

Spatial heterogeneity

Tumours can have spatial heterogeneity, which means that various locations within the tumour may have different genetic or molecular profiles. Because of particular microenvironmental variables or genetic abnormalities existing in such places, certain sections of the tumour may be more resistant to therapy. It can result in partial tumour elimination and eventual recurrence [115].

Temporal heterogeneity

Tumour cells can undergo dynamic changes over time as a result of clonal evolution, acquired mutations, or therapeutic selection pressures. This temporal variation might lead to medication resistance development. Subpopulations of cells that are initially responsive to therapy may acquire additional genetic changes or activate alternative survival pathways, leading to disease progression and resistance [116].

Understanding tumour heterogeneity and how it affects medication resistance is essential for establishing successful treatment methods. It emphasizes the need of combination treatments that target numerous pathways or cell populations, as well as the requirement for therapy monitoring and adaptation depending on the tumor’s growing heterogeneity. Single-cell sequencing and molecular profiling advances assist in unravelling the complexities of tumour heterogeneity and guiding personalised therapy options [117]. Tumour heterogeneity, or the existence of distinct cell populations within a tumour, can lead to treatment resistance development in cancer [118]. Major groups of medications suffering resistance owing to tumour heterogeneity include platinum-based therapies (e.g., cisplatin), taxanes (e.g., paclitaxel), and anthracyclines (e.g., doxorubicin). EGFR inhibitors, BRAF inhibitors, ALK (anaplastic lymphoma kinase) inhibitors, immune checkpoint inhibitors, chimeric antigen receptor (CAR)-T cell treatments, and endocrine therapies are among the other medications. Resistance to endocrine treatments in ER-positive breast cancer can be caused by heterogeneity in ER expression or mutations [119]. For example, NSCLC frequently exhibits a high degree of tumour heterogeneity. EGFR-mutant NSCLC responds well to EGFR inhibitors like gefitinib and erlotinib at first, but resistance frequently arises because of distinct clones with secondary mutations (like the T790M mutation) that activate different signalling pathways, like the MET or HER2 pathways, thereby avoiding the EGFR blockade [43]. On the other hand, variations in the expression of HER2 can also result from the heterogeneity within breast cancer tumours. Although heterogeneity can lead to the development of HER2-negative cells that survive treatment and contribute to disease recurrence, trastuzumab is an effective treatment for HER2-positive tumours. Furthermore, in HER2-positive tumours, activation of the PI3K/AKT pathway may enhance resistance [120].

Cancer stem cells

Cancer stem cells (CSCs) are a tiny subset of cells inside a tumour that have the potential to self-renew and develop into several cell types. They are frequently linked to therapeutic resistance and tumour recurrence. CSCs can have superior drug efflux mechanisms, survival pathway activation, and DNA repair capacities [121]. The existence of CSCs inside a heterogeneous tumour can contribute to therapeutic resistance and residual disease persistence. These cells are hypothesised to be important in the development of cancer, metastasis, and medication resistance [122]. The primary ways cancer stem cells contribute to medication resistance are as follows.

Inherent resistance

CSCs can be innately resistant to many therapy. They have improved DNA repair mechanisms, higher levels of drug efflux pumps (such as ATP-binding cassette transporters), and higher levels of anti-apoptotic proteins. CSCs are less vulnerable to the cytotoxic effects of chemotherapy and targeted treatments as a result of these variables [123].

Dormancy

CSCs can enter a latent state in which they become inactive and withstand the effects of medications that primarily target quickly proliferating cells. While traditional medicines target actively proliferating cancer cells, latent CSCs might survive removal and subsequently restart tumour development, resulting in disease recurrence [124].

Phenotypic plasticity

CSCs can flip between stem-like and non-stem-like states due to phenotypic plasticity. Non-stem cancer cells may develop stem-like features, including drug resistance mechanisms, during treatment, allowing them to survive therapy and regenerate the tumour [125].

Heterogeneity within the CSC population

CSCs can be heterogeneous, with various resistance mechanisms in distinct subsets of CSCs. This heterogeneity might contribute to therapeutic failure because distinct subgroups of CSCs may have resistance mechanisms to certain medicines, resulting in partial tumour elimination [126].

Interactions with the tumour microenvironment

The tumour microenvironment is critical in sustaining CSCs and increasing treatment resistance. The microenvironment acts as a protective habitat for CSCs, shielding them from the impacts of treatment [127]. CSC survival and maintenance can be aided by factors such as stromal cells, extracellular matrix components, and cytokines released by immune cells [128].

Active signalling pathways associated with cell survival and self-renewal

CSCs have active signalling pathways associated with cell survival and self-renewal, including as Notch, Wnt, Hedgehog, and PI3K/AKT. These mechanisms can provide treatment resistance by boosting cell survival and shielding CSCs from drug cytotoxicity [129].

It is critical to understand the function of CSCs in drug resistance in order to design successful treatment options. To overcome medication resistance associated with CSCs, techniques such as targeting CSC-specific vulnerabilities, combination treatments that target both CSCs and non-CSC populations, and disrupting CSC microenvironment interactions are being investigated. It may be able to enhance treatment results and prevent cancer by properly targeting CSCs [130]. CSCs have been linked to treatment resistance and cancer recurrence. While CSCs have a role in treatment resistance, it is crucial to highlight that resistance mechanisms differ depending on the kind of cancer and the particular patient [131]. For example, due to their increased DNA repair capabilities and capacity to produce high amounts of drug efflux pumps (such as ABC transporters), CSCs in breast cancer frequently exhibit resistance to common chemotherapies like doxorubicin and paclitaxel. These features enable CSCs to recur and spread after initial therapy [132]. In glioblastoma cells, overexpressing DNA repair enzymes like as MGMT (O6-methylguanine-DNA methyltransferase), CSCs are able to withstand the effects of temozolomide, a typical chemotherapeutic drug. This leads to therapeutic resistance because it enables the CSCs to repair the DNA damage caused by the drugs [133].

Microenvironmental variables

Drug resistance can be influenced by the tumour microenvironment, which includes variables such as low oxygen levels (hypoxia) and interactions with stromal cells [134]. It can produce a safe haven for cancer cells, promote medication inactivation, or activate signalling pathways that lead to resistance [135]. Drug resistance in cancer can be greatly affected by microenvironmental variables within the tumour. Stromal cells, extracellular matrix, blood arteries, immune cells, and signalling chemicals are all components of the tumour microenvironment [136]. These factors can provide a safe haven for cancer cells, promote drug inactivation, or activate signalling pathways that lead to resistance [137]. The main microenvironmental elements that contribute to cancer medication resistance are discussed here.

Hypoxia

In the tumour microenvironment, low oxygen levels (hypoxia) can induce treatment resistance. Hypoxic conditions can hinder medication delivery and increase the survival of hypoxia-adapted cancer cells, reducing the efficacy of some treatments. HIF-1, a major regulator of the physiological response to hypoxia, can activate many pathways that cause drug resistance [138]. Several cancer medicines that are used in different types of cancer therapy (radiation, chemotherapy, targeted therapy) may be affected by hypoxia and they are discussed below.

In radiation therapy, because oxygen-dependent radiation damage is decreased in hypoxic tumour cells, they are more resistant to radiation therapy. Hypoxic radiosensitizers such as Tirapazamine (TPZ) and Nimorazole have been used to specifically target and sensitise hypoxic tumour cells to radiation to overcome this [139]. In chemotherapeutic medication, hypoxia can diminish the efficacy of cisplatin, a platinum-based chemotherapy medication used to treat a variety of malignancies. Hypoxic tumour cells may have lower treatment sensitivity due to poor drug absorption and enhanced DNA repair processes [140]. Similarly, hypoxia can limit medication accumulation and penetration into hypoxic tumour areas, reducing the efficacy of the anthracycline chemotherapy agent doxorubicin [141]. A popular chemotherapy medication, 5-FU, can be resistant to hypoxia. Hypoxia can upregulate enzymes implicated in medication metabolism, resulting in decreased drug effectiveness [142]. Hypoxia-inducible factor (HIF) promotes angiogenesis as well as tumour adaptability to hypoxia. Anti-angiogenic medicines that target vascular endothelial growth factor (VEGF) or its receptor, such as bevacizumab, may help normalise tumour vasculature and increase medication delivery to hypoxic tumour areas [143]. In certain tumours, hypoxia can increase resistance to EGFR inhibitors such as gefitinib or erlotinib. In hypoxic situations, HIF activation can lead to EGFR-independent pathways that support tumour development [144]. Hypoxia can depress the immune response and reduce the effectiveness of immunotherapies such as immune checkpoint inhibitors. In the tumour microenvironment, hypoxia-inducible factor 1-alpha (HIF-1) can contribute to immune evasion mechanisms and immunosuppressive pathways [145].

Stromal cells

Within the tumour microenvironment, cancer-associated fibroblasts (CAFs), immune cells, and other stromal components can impact treatment response. CAFs have the ability to release substances that increase cancer cell survival and resistance to treatment. Tumor-associated macrophages and regulatory T cells, for example, can produce an immunosuppressive environment that restricts the efficiency of immunotherapies [146].

Extracellular matrix (ECM)

The ECM, a complex network of proteins and carbohydrates that surrounds cancer cells, can obstruct therapeutic entry into the tumour. It has the potential to erect physical barriers that prevent medications from reaching their intended recipients. Furthermore, the ECM can trigger signaling pathways, such as integrin-mediated signaling, that enhance treatment resistance and cancer cell survival [147].

Drug inactivation

Enzymes or factors that metabolise or deactivate medications can be found in the tumour microenvironment. For example, the presence of drug-metabolizing enzymes in the microenvironment, such as cytochrome P450 enzymes, might impair the bioavailability and efficacy of some medications [148].

Signaling pathways

The tumour microenvironment can activate signaling pathways that promote cancer cell survival and treatment resistance. The activation of pro-survival pathways, such as the PI3K/AKT/mTOR pathway, for example, might confer resistance to targeted therapy. Inflammatory signalling pathways, such as NF-B or STAT3, can also promote cell survival and immunological evasion, which can lead to medication resistance [149].

Blood vessel interactions

Abnormal blood vessel development and function in tumours can result in poor medication delivery and unequal drug distribution inside the tumour. Inadequate drug penetration can restrict cancer cell exposure to therapeutic medicines and lead to resistance [150].

Understanding the microenvironmental elements that contribute to medication resistance is critical for designing successful treatment methods. Overcoming microenvironment-mediated resistance may require combination therapies that target both cancer cells and the tumour microenvironment, drug delivery and penetration strategies, or interventions to modulate specific signalling pathways or immune responses within the microenvironment [151].

A common feature of the tumour microenvironment in pancreatic cancer is hypoxia, or low oxygen levels. Hypoxia can trigger the activation of proteins known as hypoxia-inducible factors (HIFs), which in turn can enhance angiogenesis and upregulate survival pathways like VEGF (vascular endothelial growth factor), hence promoting resistance to treatments like gemcitabine [152]. Integrin signalling pathways are activated in breast cancer through interactions between cancer cells and extracellular matrix (ECM). This promotes survival and resistance to targeted medicines such as tamoxifen. A barrier that lessens drug penetration and decreases therapeutic efficacy can also be created by changes in the extracellular matrix [153].

Epigenetic modifications

Epigenetic modifications, such as DNA methylation and histone modifications, can change gene expression patterns and contribute to drug resistance by modifying the expression of drugs-response and cell-survival genes [154]. Epigenetic alterations modify gene expression patterns and cellular behaviours, which contribute to cancer medication resistance. These alterations to DNA and histones can impact gene regulation without changing the underlying DNA sequence (figure 5) [155]. Following are some instances of epigenetic alterations linked to cancer treatment resistance:

Fig. 5.

Epigenetic modifications in cancer drug resistance

DNA methylation

DNA methylation is a frequent epigenetic change in which a methyl group is added to cytosine residues in DNA. Hypermethylation of gene promoter regions can cause tumour suppressor genes to be silenced, compromising their normal activities in controlling cell growth and survival. This can impart resistance to medicines that target specific pathways or rely on tumour suppressor gene activity [156]. Mutations in DNA methylation patterns in AML may give rise to resistance to hypomethylating drugs such azacitidine. Drug resistance may result from the reactivation of suppressed genes linked to other survival pathways, which circumvents the medication's intended effects [157].

Histone modifications

Histones are proteins that coat DNA to produce chromatin. Various changes to histone proteins, including as acetylation, methylation, phosphorylation, and ubiquitination, can occur and alter gene expression [158]. Changes in gene expression patterns caused by altered histone modifications in cancer cells can develop treatment resistance. Histone deacetylase (HDAC) enzymes, for example, can remove acetyl groups from histones, resulting in chromatin compaction and gene silence [159]. Alterations in histone acetylation and methylation, can cause colorectal tumours to become resistant to chemotherapeutic drugs like 5-fluorouracil (5-FU). These modifications may lead to the expression of genes that, through increasing drug efflux, changing apoptotic pathways, or altering drug metabolism, contribute to drug resistance [160].

Chromatin remodeling

Chromatin remodeling refers to dynamic changes in chromatin structure that provide or limit DNA access for transcriptional control. In cancer cells, abnormal chromatin remodelling can result in the activation of oncogenes or the suppression of tumour suppressor genes, leading to therapeutic resistance [161]. Resistance to different anticancer medicines has been linked to dysregulation of chromatin remodeling complexes such as switch/sucrose nonfermenting (SWI/SNF) and polycomb repressive complexes [162].

Non-coding RNAs (ncRNAs)

Non-coding RNAs (ncRNAs) such as microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) have a role in post-transcriptional gene regulation [163]. Modifications in the expression of certain miRNAs or lncRNAs in cancer cells can influence drug sensitivity by changing the expression of target genes implicated in drug response or resistance. Resistance to chemotherapy, targeted treatments, and immunotherapies has been linked to miRNA dysregulation [164].

Epigenetic enzymes

DNA methyltransferases (DNMTs) and histone-modifying enzymes, which add or remove epigenetic modifications, can be dysregulated in cancer cells [165]. Increased DNMT or histone methyltransferase expression or activity can contribute to abnormal DNA methylation or histone methylation patterns, resulting in altered gene expression profiles and medication resistance [166]. Understanding the epigenetic modifications linked with drug resistance can help guide the development of anti-resistance treatment methods. To undo or regulate these epigenetic modifications and sensitise cancer cells to therapy, epigenetic treatments such as new DNMT inhibitors or HDAC inhibitors have been investigated. Furthermore, combination treatments that target both genetic and epigenetic changes in cancer may be useful in avoiding or overcoming drug resistance [167].

These medications try to restore normal gene expression patterns and perhaps correct aberrant cellular behaviour in cancer cells by targeting particular enzymes or proteins involved in epigenetic alterations. They are used to treat a variety of cancers, including haematological malignancies (such as leukaemia and lymphoma) and solid tumours (such as lung, breast, and colorectal cancers) [168]. It is crucial to note that the precise medications employed and their efficacy may vary based on the kind and stage of cancer, as well as the features of the individual patient [169].

Strategies to overcome chemotherapy resistance in cancers

Chemotherapy resistance in cancers is a major hurdle that limits the efficacy of the treatment regimen leading to recurrence of cancer. In this scenario, the cancer cells can survive and undergo unregulated division even after the administration of drugs. To enhance the chemotherapeutic outcome and limit the recurrence of cancer, different multifaceted strategies are being investigated that targets the underlying mechanistic actions.

Combination therapy approach

Combination therapy is a most utilized strategy to overcome chemotherapy resistance in human malignancies. It involves use of two or more drugs and simultaneously targeting on several pathways or mechanisms. Some of the examples are discussed here like the combination of the PARP inhibitor (olaparib) with the DNA-damaging drug (cisplatin) can improve treatment effectiveness by blocking DNA repair pathways in drug resistant cancer cells [170]. Combining PI3K inhibitor (alpelisib) with taxane (paclitaxel) has demonstrated potential in treating breast cancer resistance by focussing on the mechanisms that lead to cell survival and proliferation [171]. Similarly, combining the angiogenesis inhibitor (bevacizumab) with the topoisomerase inhibitor (irinotecan) has improved colorectal cancer outcomes by targeting the proliferation of tumour cells as well as their angiogenesis [172]. Patients with platinum-based chemotherapy and the anti-angiogenic medication, aflibercept, have shown good results in treating ovarian cancer patients with resistant tumours [173]. Furthermore, when immune checkpoint inhibitors such as pembrolizumab are combined with chemotherapeutic agents like gemcitabine can enhance immune response and reduce resistance in various cancers [174].

Targeted therapy

Unlike the conventional chemotherapy, targeted therapy is developed to specially bind to molecular targets, such as signalling pathways, which are critical to proliferation and survival of cancer cell. Transferrin, low-density lipoprotein (LDL), human epidermal growth factor receptor 2 (HER2), EGFRs, and lectins are a few of the well-known receptors linked to this endocytosis pathway. Targeted therapies frequently take use of these important endocytosis-related receptors in drug-resistant malignancies to improve drug delivery and overcome resistance [175]. For instance, EGFR inhibitors like osimertinib and HER2-targeted drugs like trastuzumab are designed to inhibit these receptors' signalling pathways to promote their destruction, and halt the growth of tumours [176]. Moreover, drugs coupled with transferrin or lipid-based nanoparticles that target the LDL and transferrin receptors enhance drug absorption in malignant resistant cancer cells [177]. By effectively avoiding drug resistance pathways, targeted therapy can enhance therapeutic outcomes for cancers that do not respond well to current treatments. For example, HER2-targeted medications like trastuzumab and EGFR inhibitors like osimertinib are designed to block the signalling pathways of these receptors, encourage their destruction, and stop tumour growth [178]. Furthermore, drugs that target the transferrin and LDL receptors, such as lipid-based nanoparticles or pharmaceuticals conjugated with transferrin, increase drug absorption in multidrug-resistant tumours. Targeted therapies can improve therapeutic outcomes for malignancies that are resistant to treatment by efficiently circumventing drug resistance mechanisms through the use of these mechanisms [177].

Drug repurposing

Repurposing existing, licensed medications for cancer is a more promising approach than developing drugs from scratch. The unfulfilled demand for more potent anti-cancer medications has led to an increasing awareness of therapy repurposing. Many heart diseases are treated using cardiac glycosides and they raise intracellular sodium levels and block the Na+/K+-ATPase ion pump. HeLa cells and other cancer cells are not allowed to proliferate when these drugs are used. Additionally, they cause growth arrest by eliciting the production of the cyclin-dependent kinase p21Cip1 [180]. This class of medications suppresses HIF-1α and HIF-2α, which eliminates known xenograft tumours. Beta-adrenoreceptors, which are activated by catecholamines and aid in the development of tumours, are the target of beta-blockers [181]. In experimental preclinical models of pancreatic and colorectal cancer, blocking beta-blockers can result in anti-proliferative and anti-migratory effects as well as an increase in overall survival. Metformin is used to treat type II diabetes; it does this by indirectly activating adenosine monophosphate-activated protein kinase (AMPK) via liver kinase B1 (LKB1), a tumour suppressor [182]. Tumour cell death occurs when AMPK suppresses leukemic cells' unfolded protein response. (UPR). In glioma cells, the phenothiazine derivative, chlorpromazine (CPZ) suppressed DNA polymerase activity and mitochondrial ATP synthesis in leukemic cells and promoted autophagy by blocking the AKT/mTOR pathway [183]. On the other hand, amitriptyline suppresses mitochondrial complex III, which causes caspase activation and apoptosis, which has anticancer effects. HIV treatment known as HAART has made use of protease inhibitors (PIs). One PI that can stop the advancement of the cell cycle and cause apoptosis is ritonavir, which works in a number of different ways [184, 185].

P-gp inhibitors

In multidrug resistant (MDR) cancer, P-gps are overexpressed as they actively efflux chemotherapeutic drugs from the cells, thereby lowering the efficacy of drugs. The use of P-gp inhibitors is one of the emerging strategies to treat MDR in cancer and few of the examples are discussed here. NSC23925 is a P-gp inhibitor that is noncompetitive, selective, and efficient. When NSC23925 was tested for its ability to overcome drug resistance in drug-resistant ovarian cancer cell lines, it was shown to have 20 and 50 times the potency of verapamil and cyclosporin A (CsA), respectively [186]. By precisely decreasing the expression of P-gp in vitro, NSC23925 could prevent the induction of paclitaxel resistance in osteosarcoma and ovarian cancer both in vitro and in vivo. Tetrandrine, or CBT-1, is another name for NSC77037, which inhibits P-gp function. NSC77037 has been demonstrated to stop Dox-induced MDR in human leukaemia K562 cells when combined with Dox [187]. In a dose-dependent way, NSC77037 may be able to stop Dox-induced ABCB1 expression and P-gp function. Numerous investigations have demonstrated that ABCB1 gene expression can be increased by the inducible transcription factor NF-κB. NSC77037's pharmacokinetics of Dox were not considerably changed in phase I clinical trials when compared to other P-gp inhibitors, and its side effects, which included mild nausea and sporadic vomiting, were manageable [188].

In the human leukaemia cell line K562, it has been demonstrated that dexrazoxane greatly delays the establishment of drug resistance [189]. Bexarotene, commonly referred to as LGD1069 or Targretin, is a selective retinoid X receptor ligand that has demonstrated efficacy in preclinical mouse models of breast cancer as a chemopreventive and chemotherapeutic drug [190]. In vitro and in vivo combinations with bexarotene can stop and reverse acquired drug resistance in a number of cancer cell lines. In another study, it was demonstrated that ethacrynic acid (EA), a particular inhibitor of glutathione S-transferase (GST), increases the cytotoxicity of anticancer drugs and reverses treatment resistance [191]. It has been demonstrated that selenium compounds can stop ovarian cancer patients from developing drug resistance to melphalan, cisplatin, and carboplatin in both vitro and in vivo settings [192]. Remarkably, research has demonstrated that selenium compounds can inhibit the expansion of the GST gene, which is triggered when melphalan resistance develops.

Targeted protein degraders (e.g., PROTACs)

PROTACs (PROteolysis-TArgeting Chimeras) are emerging therapeutic agents that particularly target proteins associated with drug resistance. Research shows that mutations in the target protein, which PROTACs are intended to break down, can cause cancer cells to become resistant [193]. These mutations may change the target protein's binding location, which will lessen PROTACs' ability to tag the protein for breakdown. PROTACs provide a new method of drugging "undruggable" targets because they cause their degradation [194]. This may be able to get around some of the drawbacks of small-molecule inhibitors, namely their temporary target engagement and requirement for high doses in order to produce therapeutic effects.

Epigenetic modulators

Epigenetic modulators are increasingly used in cancer drug resistance. Proteins involved in controlling chromatin structure and gene expression are the target of epigenetic modulators such as BET (bromolodomain and extra-termininal motif) inhibitors and HDAC (histone deacetylase) inhibitors [195]. When cancer cells modify their chromatin structure to rebuff the effects of these medications, resistance may arise. For instance, gene expression patterns repressed by HDAC or BET inhibitors can be restored by compensatory histone modifications or mutations in chromatin modifiers.

Conclusion

Cancer drug resistance is a daunting challenge that limits the efficacy of chemotherapeutic drugs. The resistance is caused by several factors including genetic alterations, target modifications, activation of alternative pathways, overexpression of efflux pumps, altered drug metabolism, tumor heterogeneity, cancer stem cells, and microenvironment. These factors results in intrinsic (present before treatment) and extrinsic resistance (develops after treatment) leading to cancer recurrences. The complex nature of cancer drug resistance highlights the need for multifaceted approaches to combat this formidable challenge. Research endeavors to address drug resistance must proceed with a thorough understanding of the underlying mechanism governing the resistance. In recent decades, several strategies were developed to overcome cancer drug resistance such as combination therapy, targeted therapy, development of P-gp inhibitors, PROTACs and epigenetic modulators that are promising to address this challenge. Amongst them, targeted therapies that specifically targets oncogenes and growth factors receptors such as HER2 and EGFR have shown potential in overcoming some forms of drug resistance. Genetic changes in oncogenes and tumor suppressor genes lead to overexpression of drug efflux transporters like P-gp that pump the drug out of cells, thereby reducing its therapeutic potential. This overexpression in P-gp is pronounced in MDR cancer and the use of P-gp inhibitors is an emerging strategy. The P-gp inhibitors demonstrated re-sensitize resistant cells to chemotherapeutic treatments in vitro, however limited survival advantages were observed in clinical studies. In an alternative strategy HDAC inhibitor combined with DNA-damaging drugs or targeted therapies have shown encouraging results in preclinical models and early-phase clinical trials; nevertheless, these combinations frequently result in increased toxicity and a narrow therapeutic window, making dosing regimen optimisation difficult. The clinical data on PROTACs is still in its early stages, despite encouraging preclinical study outcomes. There is currently insufficient evidence to support the transfer of efficacy from preclinical and cell line models to human patients.

In parallel, advancement in other crucial areas such as personalized medicine, high-throughput genomic and transcriptomic analyses (biomarkers identification) and nanotechnology-based drug delivery are increasingly becoming vital in combating drug resistance. It is critical to customize treatment strategies according to the unique characteristics of each patient, the type of tumor, and the particular resistance mechanisms at play. The potential of cancer therapy could be fully realized by integrating insights from cellular and molecular biology with clinical expertise to navigate through the complexities of drug resistance.