Molecular mechanisms of cisplatin resistance in ovarian cancer

Ovarian cancer is one of the most common malignant tumors of the female reproductive system. The majority of patients with advanced ovarian cancer are mainly treated with cisplatin-based chemotherapy. As the most widely used first-line anti-neoplastic drug, cisplatin produces therapeutic effects through multiple mechanisms. However, during clinical treatment, cisplatin resistance has gradually emerged, representing a challenge for patient outcome improvement. The mechanism of cisplatin resistance, while known to be complex and involve many processes, remains unclear. We hope to provide a new direction for pre-clinical and clinical studies through this review on the mechanism of ovarian cancer cisplatin resistance and methods to overcome drug resistance.


Introduction
Ovarian cancer is the seventh most commonly diagnosed cancer among women worldwide. 1According to the international unified classification established by the World Health Organization, ovarian cancer is divided into many categories from different histological sources, including epithelial tumors, germ cell tumors, sex cord interstitial tumors, and metastatic tumors. 2 Epithelial ovarian cancer is the most common type, representing about 70% of ovarian malignant tumors, and is more common in middleaged and old women and less common in pre-adolescent women. 3There are many pathological types of epithelial ovarian cancer, among which serous adenocarcinoma accounts for 70%; other types include endometrioid adenocarcinoma, clear-cell carcinoma, and mucinous adenocarcinoma. 3The incidence of ovarian cancer is lower than that of cervical cancer and endometrial cancer, which ranks third among female reproductive system malignancies, but its mortality ranks first. 4According to the American Cancer Society, it is estimated that 13,270 deaths were a result of ovarian cancer in the United States in 2023. 4Since the mid-1970s, the mortality of ovarian cancer has decreased by more than 30% due to diminutions in incidence and treatment improvements. 5However, earlystage ovarian cancer is not usually diagnosed because of a lack of specific early symptoms and effective screening strategies.About 70% of patients diagnosed with International Federation of Gynecology and Obstetrics stage III or IV ovarian cancer will experience a recurrence after treatment. 6It is the biggest barrier, with a 49.7% 5-year survival rate after diagnosis. 7s the most successful metallic anti-cancer drug, platinum shows efficient and broad-spectrum anti-tumor activities that play an important role in clinical cancer treatment. 8Cisplatin was the first and the most effective platinum-based anti-neoplastic drug. 9It is used for various solid cancers, including ovarian, cervical, bladder, and lung cancer. 10For many years, tumor cytoreductive surgery and combination chemotherapy based on cisplatin and paclitaxel have been the main treatment options for ovarian cancer. 11However, in the course of clinical treatment, cisplatin toxicity and resistance have gradually emerged as a challenge to improving the prognosis of patients.
Tumor-free interval is an important indicator of the sensitivity of chemotherapy.Patients are traditionally labeled as either platinum-sensitive (relapse >6 months since last platinum therapy) or platinum-resistant (relapse <6 months since last platinum therapy). 12Cisplatin resistance not only leads to decreased sensitivity of chemotherapeutic drugs to ovarian cancer but results in a detrimental effect on treatments.Studies have found that cisplatin resistance in ovarian cancer patients may be associated with multiple factors, including intracellular drug accumulation, cell metabolism, and DNA damage repair.In recent years, efforts have been made to develop various drugs and use new technologies to reverse cisplatin resistance.In this review, we briefly summarize the mechanism of cisplatin resistance in ovarian cancer and some strategies to overcome this resistance or restore its sensitivity, which we hope will be helpful in the treatment of ovarian cancer in the future.

Mechanisms of cisplatin-based chemotherapy resistance in ovarian cancer
The biggest drawback of cisplatin-based chemotherapy is ovarian cancer cell resistance.Many patients with ovarian cancer relapse due to cisplatin resistance.Cisplatin resistance may be caused by lowering intracellular drug accu-mulation, detoxification of glutathione (GSH), and repairing DNA.13e15 In this review, cisplatin resistance occurs at the following five stages: before cisplatin's entry into tumor cells, during the influx and efflux of cisplatin through the cell membrane, in the presence of cisplatin in the cytoplasm, during the interaction of cisplatin with nuclear DNA, and after post-targeting.

Before cisplatin's entry into tumor cells
Blood circulation is a major obstacle prior to cisplatin's entry into cells.A large amount of cisplatin administered intravenously to patients is rapidly bound by sulfhydrylcontaining proteins such as blood plasma proteins, transporters, and cysteine, particularly human serum albumin. 16he binding of cisplatin to human serum albumin reduces the anti-tumor activity of cisplatin. 17s a critical cell-cell communication medium, extracellular vesicles help regulate tumor metastasis and proliferation, including apoptotic bodies, microvesicles, and exosomes. 18It has been reported that human mesenchymal stem cell derived-extracellular vesicles regulate cisplatin resistance in ovarian cancer cells by releasing miR-18a-5p. 19In addition, a study focused on the expression of circFoxp1 in the serum of ovarian cancer found that the expression of exosomal circFoxp1 in cisplatin-resistant patients was significantly higher than that in cisplatin-sensitive patients, 20 suggesting the possibility of using other circulating exosomes as biomarkers and potential therapeutic targets for ovarian cancer.
The extracellular matrix is an important part of the milieu surrounding cells.Matrix metalloproteinases (MMPs) can degrade various proteins in the extracellular matrix and are currently being studied as potential therapeutic targets in cancer cells.The relationship between progression, invasiveness, and metastatic capacity of ovarian cancer and the activity of MMPs has been described. 21Wang et al 22 found increased MMP-19 and MMP-20 expression were correlated with platinum resistance.MMP-2 and MMP-9 are frequently identified as potential biomarkers of chemoresistance in ovarian cancer. 23Matrix adhesion is also implicated in resistance mechanisms.One study has shown that cisplatin resistance can be enhanced by the specific stiffness of extracellular matrix substrates through focal adhesion kinase and b1 integrin-phosphorylated myosin light chain-yes-associated protein signaling, while inhibiting apoptosis. 24Moreover, cisplatin resistance of ovarian cancer cells can be significantly increased due to overexpression of collagen VI, an extracellular matrix protein with growth factor-like characteristics. 25

During the influx and efflux of cisplatin through the cell membrane
Before entering the cell, interactions between cisplatin and membrane proteins, lipids, and other biomolecules affect the influx and efflux of cisplatin, resulting in changes in intracellular cisplatin accumulation, which can result in poor therapeutic effects and cause cisplatin resistance (Fig. 1).
First, the lower accumulation of intracellular cisplatin may be related to reduced drug uptake.Passive diffusion and transport proteins are the primary actors of cisplatin influx.As permeable enzymes located on the cell membrane, copper transporters regulate cellular copper homeostasis. 26High copper transporter 1 expression has been reported in advanced ovarian cancer with elevated survival of patients and improved cisplatin sensitivity. 27Solute carriers, as membrane transporters, play a role in cisplatin resistance to ovarian cancer. 28Nearly 30 kinds of solute carriers are involved in chemotherapy resistance; some are down-regulated in cisplatin-resistant ovarian cancer cells while others are not. 29In addition, organic cation transporters facilitate cisplatin's entry into cells; their downregulation reduces cisplatin nephrotoxicity 30 and delays cisplatin incorporation. 31he drug efflux system is also an important reason for decreased effective cisplatin accumulation in tumor cells, meaning that cisplatin is partly pumped out before reaching its intracellular target.Copper-transporting adenosine triphosphatases (ATPases), especially ATP7A and ATP7B, mediate the excretion of cisplatin. 32High expression of ATP7A and ATP7B are correlated with an inferior response to cisplatin, thus inducing the development of cisplatin resistance in ovarian cancer. 33Multidrug resistance protein 1, also known as P-glycoprotein, acts as an efflux pump, which outputs substrates to cells using energy from ATP hydrolysis. 34Overexpression of multidrug resistance protein 1 is also one of the main factors of cisplatin resistance in ovarian cancer. 35At present, it is necessary to develop appropriate efflux pump inhibitors to weaken drug efflux, to reverse cisplatin resistance.

In the presence of cisplatin in the cytoplasm
Resistance of cisplatin in the cytoplasm is mainly related to GSH (Fig. 2).Due to its high affinity for cisplatin, GSH competitively inhibits the binding of cisplatin to DNA, leading to cisplatin resistance. 36A recent study showed that the level of GSH in cisplatin-resistant cell line A2780 (A2780CIS) is much higher than that in A2780 cells. 37ncreased reactive oxygen species (ROS) generation leads to oxidative damage of biological molecules including DNA, proteins, and lipids, likely resulting in apoptosis. 38tochondria are important for cisplatin sensitivity.Cisplatin-resistant ovarian cancer cells contain lower levels of mitochondrial ROS (mtROS) compared with cisplatinsensitive cells. 39Cisplatin-induced cytotoxicity is closely related to mtROS production.In addition, the existence of antioxidants such as GSH and GSH peroxidase in mitochondria may also be important in cisplatin resistance. 40verexpression of GSH not only inactivates cisplatin but also reduces sensitivity to cisplatin by reducing mtROS. 41an et al 42 found that mitochondrial fission and cisplatin resistance were promoted in ovarian cancer cells under hypoxic conditions.However, the level of endogenous ROS in cisplatin-resistant ovarian cancer cells is higher than that in cisplatin-sensitive cells. 43The complicated mechanisms of ROS regulation affecting cisplatin resistance are still controversial and will require further exploration.
Autophagy is a complicated metabolic pathway to achieve the renewal of organelles and the stability of the intracellular environment. 44Autophagy is the core of the cellular stress response. 45For tumor cells, autophagy can provide energy for the body, thus protecting chemotherapy-resistant cancer cells.The phosphatidylinositol-3kinase, protein kinase B, and mammalian target of rapamycin signaling pathway is a major regulator of autophagy and associated with chemoresistance in cancer. 46After phosphatidylinositol-3-kinase activation, it stimulates its downstream target protein kinase B to activate mammalian target of rapamycin, finally inhibiting autophagy and improving the sensitivity of the human multidrug-resistant phenotype ovarian cancer cell line SKVCR. 47erroptosis is an iron-dependent new programmed necrosis, which differs from apoptosis, necrosis, and autophagy.The main mechanism of ferroptosis is that under the action of excessive iron or ester oxygenase and oxidative stress, unsaturated fatty acids on cell membrane are catalyzed to induce lipid peroxidation; in addition, the expression of the antioxidant system, GSH and glutathione peroxidase 4, is decreased. 48This mechanism has been reported to reduce cisplatin resistance in ovarian cancer by increasing lipid metabolism activity and driving redoxinduced ferroptosis. 49,50Combination therapy based on cisplatin and the ferroptosis inducer erastin appears to minimize ovarian cancer cell growth and overcome resistance. 51etabolic reprogramming refers to the metabolic changes made by cells in response to a variety of pressures.Metabolic reprogramming is prevalent in many diseases, such as glucose metabolism, amino acid metabolism, and other metabolic pathways.The metabolism of cancer cells is quite different from that of normal and healthy cells.Cancer cells are more metabolically active and have a higher reproduction rate. 52The Warburg effect proposes that in addition to normal mitochondrial respiration, glycolysis is crucial in cancer cells for much more energy production. 53Existing evidence suggests enhanced glycolysis can induce either intrinsic or acquired resistance to cisplatin in several cancer cell types. 54,55As a major transmembrane transporter for maintaining efficient glucose transport, glucose transporter 1 is widely expressed in human tissues and organs.Inhibitors of glucose transporter 1 (STF 31, IOM-1190) attenuated the proliferation of cisplatin-resistant high-grade serous ovarian cancer cells. 56gure 1 Influx and efflux of cisplatin through the cell membrane.The interaction of cisplatin with membrane proteins has been shown to affect the influx and efflux of cisplatin.

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Hexokinase 2 catalyzes glucose phosphorylation in glucose-6-phosphate, the rate-limiting step in glycolysis.Hexokinase 2 can promote cisplatin resistance in ovarian cancer by enhancing extracellular regulated protein kinase-mediated autophagy. 57s a classical non-essential amino acid, glutamine plays a crucial role in tumor cell growth and proliferation. 58hrough the tricarboxylic acid cycle, reprogramming glutamine plays a nutritional role in survival and contributes to the development of chemotherapy resistance in tumor cells. 59Cisplatin-resistant ovarian cells were more sensitive to killing due to glutamine deprivation. 60Therefore, inhibition of the glutamine metabolism pathway, which is associated with a poor prognosis of ovarian cancer, may become a new strategy for treating drug-resistant ovarian cancer. 61teraction of cisplatin with nuclear DNA Metallothionein DNA is the primary binding target of cisplatin therapy.Restricting DNA binding to cisplatin can lead to drug resistance.After cisplatin enters the nucleus, it binds to cysteine thiols of metallothionein due to its transition metal core, resulting in a loss of activity.As a low molecular weight protein, metallothionein plays an important role in the homeostasis and detoxification of metal ions. 62It has been reported to be one of the causes of tumor cell resistance to cisplatin, which is unfavorable for patients' treatment. 63Besides, higher nuclear metallothionein expression has been observed in A2780CIS cells compared with cisplatin-sensitive cells. 64

DNA repair
DNA repair is the most important cause of cisplatin resistance, mainly including nucleotide excision repair (NER), homologous recombination repair (HRR), mismatch repair (MMR), and nonhomologous end joining (NHEJ) (Fig. 3). 65ER aims to eliminate the damage of helical twisted DNA.NER can avoid DNA damage caused by ultraviolet radiation and electrophilic compounds, such as environmental carcinogens and cancer chemotherapy drugs. 66The changes in NER-associated genes in ovarian cancer can  improve the overall survival and progression-free survival of cisplatin-resistant patients. 67Therefore, the mRNA and protein levels of NER-related components, such as excision repair cross-complementing group genes and xeroderma pigmentosum (XP) group genes, have been quantified to investigate the relationship between NER and cisplatin resistance mechanisms in ovarian cancer and evaluate their prognostic value for patients. 68Du et al 69 found that the cisplatin sensitivity of cisplatin-resistant cell line SKOV-3 (SKOV-3CIS) could be remarkably increased by excision repair cross-complementing group 1 gene-specific silencing.XPF, in combination with excision repair cross-complementing group 1, can cleave bulky DNA lesions and improve cisplatin resistance in A2780CIS cells. 70XPA is an important scaffold for NER, which has a high affinity with damaged DNA.XPD is a key member of the human general transcription factor IIH complex.Higher expression of XPA and lower expression of XPD protein levels have been observed in cisplatin-sensitive metastatic ovarian cancer cells. 71,72XPC is an essential DNA damage recognition protein. 73In the XPC gene, three single nucleotide polymorphisms were correlated with prolonged progressionfree survival in advanced cisplatin-resistant ovarian cancer patients. 74These studies underscored the importance of these genes or proteins that may become reliable indicators of cisplatin resistance.
HRR is critical for the repair of double-strand breaks in the S and G2 phases of the cell cycle and includes some vital proteins and genes such as RAD51 recombinase and breast cancer early onset gene 1/2. 75,76In HRR, 3 0 singlestranded DNA tails are formed by nucleolytic resection of DNA ends, which are used for strand invasion.Then, the double-stranded damaged segment is paired with a homologous DNA template to restore the chromosome integrity. 77Germline mutations of breast cancer early onset genes have a profound impact on the efficacy of cisplatin and survival in ovarian cancer patients.Patients carrying mutations may be more likely to respond to cyclical cisplatin-based treatment. 78The cytotoxicity of cisplatin can be enhanced and the sensitivity of cisplatin-resistant cells can be restored by silencing breast cancer early onset gene 1/2 pathway genes. 79As a recombinase, RAD51 participates in the double-strand repair of HRR, which can be a viable marker to predict the response to cisplatin-based chemotherapy due to its overexpression in cisplatin-resistant ovarian cancer cells. 80MR can correct base mismatches during DNA replication.MMR deficiency affects cisplatin sensitivity.An early study showed that MMR depended on epigenetic modifications. 65Hypermethylation of MMR gene hMSH2 enhanced the sensitivity of ovarian cancer cells to cisplatin. 81iffering from HRR, the speed of repair is faster, but some sequences may be deleted during NHEJ. 82X-ray repair cross-complementing genes in the NHEJ signaling have an effect on double-strand breaks caused by ionizing radiation. 83However, current studies have been limited to the relationship between X-ray repair cross-complementing gene expression and the prognosis of cisplatin chemotherapy in ovarian cancer.No specific mechanism of X-ray repair cross-complementing genes for cisplatin resistance has been reported. 84sistance after post-targeting
MiRNAs mediate post-transcriptional gene silencing of target genes by targeting the 3 0 untranslated region (3 0 UTR) of mRNA. 87Down-regulation of miR-29, miR-216b, miR-378a-3p, and miR-130a conferred cisplatin resistance to ovarian cancer cells.88e97 However, the down-regulation of miR-149e3p in ovarian cancer significantly inhibited cell migration and invasion but also improved the response to cisplatin. 98Confirming the miRNA targets that alter cisplatin sensitivity will be important in understanding the mechanisms of cisplatin resistance.
LncRNAs, non-coding RNAs with a length of more than 200 nucleotides, have been characterized as essential mediators of cell proliferation, invasion, apoptosis, metabolism, immune evasion, and chemoresistance in ovarian cancer cells. 99Several lncRNAs have been reported to be highly expressed in ovarian cancer and promote cisplatin resistance, such as MCF2L-AS1, ACTA2-AS1, and CASC10, 100e107 while other lncRNAs have been found to increase cisplatin sensitivity of ovarian cancer cells, including CERNA1 and SLC25A21-AS1. 108,109ifferent from traditional linear RNAs with 5 0 e3 0 polar polyadenylation tails, circRNAs have a closed-loop structure, which is not affected by RNA nucleic acid exonucleases and thus exhibits more stable expression. 110ircRNAs can form a competing endogenous RNA (ceRNA) network with miRNAs, which regulates tumor proliferation, invasion, and chemosensitivity by directly or indirectly targeting genes. 111Compared with surrounding normal tissues, the expression of circRNAs in ovarian cancer cells may be up-regulated or down-regulated.Recent studies have gradually explored the functions of circRNAs in cisplatin resistance.112e116 A comprehensive evaluation of ncRNAs in cisplatinresistant cells is necessary to determine the exact mechanism of cisplatin resistance in ovarian cancer.Current functional studies of ncRNAs have mainly focused on miR-NAs and lncRNAs.However, little is known about the whole world of ncRNAs.Therefore, it will be important to further our understanding of the regulatory network of ncRNAs, that is, the interactions of all ncRNAs and their connections with DNA.

Epithelial-mesenchymal transition
Epithelial-mesenchymal transition is a process in which cells gain invasive mesenchymal ability, which is closely related to cancer metastasis.Recent evidence suggests that epithelial-mesenchymal transition contributes to chemoresistance, and it is modulated by a number of transcription factors, such as SNAIL 1 and SNAIL 2. 117 The primary function of SNAIL 1 and SNAIL 2 is to induce + MODEL epithelial-mesenchymal transition by repressing E-cadherin transcription, although they can inhibit other epithelial markers (occluding) or tumor suppressors as well.Mesenchymal markers (e.g., vimentin), as well as the activity of metalloproteinases, are up-regulated simultaneously. 118hen SNAIL 1 and SNAIL 2 are knocked down in A2780CIS cells, the epithelial-mesenchymal transition phenotype is largely reversed and cisplatin sensitivity is restored. 119

Tumor heterogeneity
Tumor heterogeneity is a characteristic of malignant tumors and is manifested by changes in molecular biology or genes, resulting in changes in proliferation, invasion, and drug sensitivity. 120Tumor heterogeneity may arise from genetic mutations or epigenetic or genetic modifications.DNA methylation, an important epigenetic modification in organisms, is catalyzed by DNA methyltransferases (DNMTs).As a member of the DNA methyltransferase family proteins, DNA methyltransferase 1 has methyltransferase activity. 121It is responsible for the accurate replication of DNA methylation.Xiang et al 97 revealed that by directly targeting DNA methyltransferase 1, the sensitivity of cisplatin was changed both in vitro and in vivo.
Cancer stem cells are considered a determinant of intratumoral heterogeneity. 122As self-renewable pluripotent stem cells, ovarian cancer stem cells are more aggressive and induce disease recurrence, which has an essential role in cancer development, metastasis, and resistance to chemotherapy. 123The combination of clonal evolution and hierarchical ovarian cancer stem cell models is considered responsible for ovarian cancer development and recurrence.Identifying and functionally characterizing ovarian cancer stem cells will be essential for the development of effective therapies.Several established ovarian cancer stem cell markers, including CD44, CD117, CD133, CD24, and ALDH have been used to identify ovarian cancer stem cell populations. 124However, due to the lack of clinical samples from cisplatin-resistant ovarian cancer, it remains a limitation that most studies rely on tissues that did not receive chemotherapy.

Cellular senescence
Cellular senescence is often a response to intrinsic or extrinsic stress, such as DNA damage in the human body.The presence of senescent cells prevents the tissue from continuously proliferating and undergoing self-repair.Previous studies have found that senescent cells can generate and release cytokines, which activate the remaining cancer cells to acquire drug resistance during chemotherapy. 125

Strategies to reverse cisplatin resistance in ovarian cancer
In recent years, various drugs have been developed according to the different mechanisms of cisplatin resistance.In addition to platinum drugs, cisplatin-resistant therapeutic drugs for ovarian cancer, which are widely applied in

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the clinic, mainly include anti-angiogenic agents, immunosuppressors, and poly (ADP-ribose) polymerase (PARP) inhibitors (Table 2).In addition, changing the dosage form and drug delivery system is also important in overcoming cisplatin resistance in ovarian cancer (Table 3).

Platinum drugs and other chemotherapy drugs
Cisplatin, due to resistance and side effects such as nephrotoxicity and neurotoxicity, has little chance of working better as the first-generation platinum anti-cancer drug.+ MODEL Many cisplatin analogues have been developed, with the hope of using them as alternatives to improve cisplatin resistance in ovarian cancer.In common with cisplatin, carboplatin is effective for patients with advanced ovarian cancer with low toxicity. 126,127However, partial crossresistance between carboplatin and cisplatin limits its application in the treatment of cisplatin-resistant ovarian cancer. 128Most third-generation platinum chemotherapeutics, including oxaliplatin and lobaplatin, have no crossresistance with cisplatin, from which cisplatin-resistant patients can benefit. 129icrotubule stabilizing agents induce mitotic arrest and lead to cell death.Taxanes are already one of the most successful and widely used natural anti-cancer drugs. 130he combination of cisplatin and paclitaxel still serves as first-line therapy for advanced ovarian cancer. 131,132However, the more effective anti-tumor effects of paclitaxel are accompanied by strong toxicity, which can cause a series of adverse effects, such as body aches, limb numbness, and hair loss. 133The substitution of docetaxel for paclitaxel in this regimen is also under investigation.Docetaxel is an improved drug based on paclitaxel that inhibits the formation of invasion and migration activities by affecting microtubule remodeling and membrane protein aggregation in SKOV-3 cells. 134opotecan prevents DNA replication in cancer cells by inhibiting topoisomerase I. 135 Numerous studies have explored newly administered formulations with different doses and therapy treatments to find the optimal topotecan regimen in the treatment of ovarian cancer. 136Pegylated liposomal doxorubicin is a formulation of liposomal doxorubicin wrapped in polyethylene glycol, which reduces cardiotoxicity while maintaining good anti-tumor effects and increasing its circulating half-life. 137Gemcitabine is a synthetic cytosine nucleoside derivative used in chemotherapy, which exhibits strong radio-sensitization and weak toxicity. 138In addition, topotecan, pegylated liposomal doxorubicin, gemcitabine, and pemetrexed are commonly used chemotherapeutic drugs in the treatment of cisplatin resistance.139e143 Targeted therapy

Anti-angiogenic agents
Tumor growth is closely related to angiogenesis, in which vascular endothelial growth factor plays a dominant role. 144umanized monoclonal anti-vascular endothelial growth factor antibodies, such as bevacizumab, have been used in the treatment of ovarian cancer.A phase II trial evaluated the efficacy of bevacizumab monotherapy in platinumresistant epithelial ovarian cancer.The median progression-free survival of patients was 4.4 months (95% confidence interval [CI], 3.1e5.5),which confirmed that bevacizumab has single-agent activity in platinum-resistant patients. 145Bevacizumab combined with chemotherapy is more effective than chemotherapy alone. 146,147The progression-free survival of platinum-resistant ovarian cancer patients who received treatment of bevacizumab plus chemotherapy was 4 months (95% CI, 3.0e5.7),compared with 3.1 months (95% CI, 2.5e4.6) for the other group that received chemotherapy only. 148A phase II study of ramucirumab, a fully human anti-vascular endothelial growth factor receptor 2 antibody, showed that the median progression-free survival in the 45 cisplatin-resistant patients was 3.2 months (95% CI, 1.8e3.7)and median overall survival was 10.0 months (95% CI, 7.7e15.2). 149 Apatinib, which specifically binds to and inhibits vascular endothelial growth factor receptor 2, is a tyrosine kinase inhibitor used orally. 150Twenty-eight patients with platinum-resistant recurrent epithelial ovarian cancer were selected to conduct a real-world study on the effectiveness of apatinib monotherapy. 151The median progression-free survival was 5.0 months and overall survival was 9.5 months, indicating that the efficacy of apatinib may be better than bevacizumab.After the combination of apatinib and etoposide, the median progression-free survival was extended to 6.0 months, and the longest progression-free survival was continued to 20.0 months, which were significantly longer than those of monotherapy treatment. 152

PARP inhibitors
PARP inhibitors are clinically successful agents that induce synthetic lethality by impacting DNA repair pathways to cause apoptosis and enhanced sensitivity of chemotherapy drugs. 153A study of breast cancer early onset gene 1/2mutated ovarian cancer patients showed that the antitumor activity of olaparib was associated with platinum sensitivity. 154A GEICO phase II trial reported therapeutic effects for the combination of olaparib and pegylated liposomal doxorubicin in platinum-resistant ovarian cancer. 155The objective response rate in their trial was 29%, which is higher than that in platinum-resistant ovarian cancer patients treated with olaparib monotherapy. 156everal PARP inhibitors, such as rucaparib and niraparib, have also been approved for ovarian cancer treatment. 157,158Currently, under investigation in early-phase trials, talazoparib and veliparib are promising PARP inhibitors. 159owever, patients will eventually develop resistance to PARP inhibitors.Researchers found that through mutations and up-regulation of breast cancer early onset genes, the homologous recombination repair way was likely recovered, which could lead to resistance to PARP inhibitors. 160here was a striking finding and highly promising for patients as resistance to PARP inhibitors and platinum in ovarian cancer could be reversed by combining PARP with ataxia-telangiectasia-mutated-and-Rad3-related kinase inhibition. 161

Cell-cycle kinases inhibitors
Cell-cycle kinase inhibitors, such as cyclin-dependent kinase 4/6 inhibitors, can block tumor growth by inhibiting cell-cycle progression. 162Both inhibitors, including ribociclib, palbociclib, and abemaciclib, are currently the only class of cyclin-dependent kinase inhibitors approved for clinical use.163e165 Palbociclib was the first cyclin-dependent kinase 4/6 inhibitor developed and tested in a clinical trial. 164Palbociclib showed longer progression-free survival and excellent tolerance, revealing the potential therapeutic effects of cyclin-dependent kinase 4/6 inhibitors in recurrent ovarian cancer. 166The novel compound ZC-22, a PARP and cyclin-dependent kinase 4/6 dual inhibitor, shows high therapeutic potential to significantly improve the response of ovarian cancer to cisplatin.It inhibits ovarian cancer cells by inducing cell cycle arrest and DNA damage in vitro and in vivo. 167Down-regulation of the expression of cyclin-dependent kinase 5 (involved in DNA repair) has been correlated with the sensitivity of A2780CIS cells to cisplatin. 168Dinaciclib can inhibit the expression of cyclindependent kinase 5 in cisplatin-resistant cell lines.The + MODEL combination of dinacoxib and cisplatin for ovarian cancer treatment may provide clinical benefits. 169

Immunotherapy
Immunotherapy has developed rapidly over the past decade as a new direction to overcome cisplatin resistance in ovarian cancer.It controls and kills tumor cells by mobilizing the immune system. 170The main immune checkpoint inhibitors currently under investigation include cytotoxic T lymphocyte antigen 4, programmed cell death protein 1 (PD-1), and programmed cell death 1 ligand 1 (PD-L1) inhibitors. 171The latter two inhibitors have been widely studied in drug resistance treatment of ovarian cancer, while the former is seldom.
Hamanishi et al 172 revealed the merits of nivolumab, a PD-1 inhibitor, in cisplatin-resistant ovarian cancer, showing that the median progression-free survival was 3.5 months (95% CI, 1.7e3.9months) and median overall survival was 20.0 months.In addition, the use of nivolumab in platinum-resistant patients results in partial response to other chemotherapeutic agents, such as pegylated liposomal doxorubicin and nedaplatin, as subsequent treatment. 173,174Pembrolizumab, an antibody specific for PD-1, prevents PD-1 ligation by both PD-L1 and PD-L2.175e177 Pembrolizumab combined with niraparib appeared to be well tolerated in platinum-sensitive ovarian cancer, providing new treatment options for patients. 177PD-L1 is a ligand for the PD-1 receptor.The combination of a PD-L1 inhibitor with a PD-1 inhibitor may cause apoptosis in T cells, affecting immune function. 178One study investigated 124 patients to determine whether the combination of the anti-PD-L1 inhibitor atezolizumab and bevacizumab was synergistic in cisplatin-resistant ovarian cancer. 179Proliferation, migration, and invasion of A2780CIS cells were synergistically inhibited in vitro.In addition, other immunosuppressants in combination with a variety of drugs have exerted sustained effects on cisplatin-resistant ovarian cancer patients.180e184

Drug delivery methods
Due to side effects, inactivation of anti-cancer drugs, and tumor heterogeneity, it is difficult to completely solve the problem of cisplatin resistance with drug treatment only.Improvement of drug delivery may produce surprising results in overcoming resistance.Drug delivery systems enable accurate drug delivery and directed release. 185

Nanoparticle delivery systems
Nanocarrier-based anti-cancer drug delivery systems have been intensively studied and developed.Compared with conventional anti-cancer drugs, nanocarriers have good biocompatibility and high bioavailability, which protect drugs from decomposition and promote targeting drugs to the tumor site.At present, a variety of nanocarriers, including lipids, polymers, and inorganic molecules, have been developed. 186iposomes consist of spherical lipid bilayers containing water centers that can be loaded with both hydrophilic and hydrophobic drugs. 187Carboplatin and paclitaxel are encapsulated in cross-linked multilamellar liposome vesicles and delivered to ovarian cancer cells to reduce systemic cytotoxicity and produce stronger anti-tumor effects. 188imilar to lipids, polymers can increase the intracellular accumulation of therapeutic agents and enhance therapeutic efficacy through conjugation-targeting parts. 189Delivery of poly (lactic-co-glycolic acid)-poly (ethylene glycol) (PLGA-PEG) nanoparticles loaded with cisplatin and a DNA repair inhibitor, wortmannin, significantly reduced tumor burden, enhanced chemotherapeutic effects, and reversed cisplatin resistance. 190The use of polymeric nanoparticle methoxy poly (ethylene glycol)-poly (L-lactic acid) (MPEG-PLA) packaging of doxorubicin and the multi-drugs resistant protein inhibitor verapamil, was able to avoid the systemic toxicity and effectively reverse drug resistance in ovarian cancer. 191norganic nanoparticles include gold nanoparticles, silica nanoparticles, and iron oxide nanoparticles.192e194 A new type of anti-tumor nanoparticles, virus-like nanoparticles, are protein nano-empty shell structures without viral nucleic acid. 195These nanoparticles help to increase drug cytotoxicity, reverse cisplatin resistance, and achieve good effects for ovarian cancer treatment.Many nanoparticles are under investigation in clinical trials for targeted therapy in ovarian cancer, which has practical significance for reversing cisplatin resistance in ovarian cancer.

Exosomes
Exosome carriers combine the advantages of cellular drug delivery and nanotechnology.A study assessed the feasibility of cisplatin in the treatment of ovarian cancer by exosomes and confirmed exosomes were a potentially new tool for drug delivery. 196Incorporation of cisplatin into umbilical cord blood-derived M1 macrophage exosomes caused a 3.3-fold increased cytotoxicity in A2780CIS cells compared with chemotherapy alone.

Light-induced carbon monoxide delivery systems
Recently, there has been innovative research on cisplatin resistance in ovarian cancer using a light-induced carbon monoxide delivery method. 197Strong evidence suggests that exogenous carbon monoxide mediates inactivation of cystathionine b-synthase.Cystathionine b-synthase lowers the levels of GSH and metallothionein and prevents hindering the interaction between cisplatin and DNA, thus conferring sensitivity to cisplatin-resistant cells.

Folate-drug conjugates
Folic acid is a water-soluble vitamin that plays a crucial role in protein synthesis, cell division, and growth in tumor cells.Due to the overexpression of folate receptor a in ovarian cancer patients, the macro-molecular coupling drugs formed by coupling the targeting ligand folic acid and anti-tumor drugs have shown promise.Serini et al 198 constructed a cisplatin delivery hydrogel based on hyaluronic acid and folic acid, which effectively inhibited the migration of A2780CIS cells and significantly reduced epithelial-mesenchymal transition.Vintafolide (EC145), a folic acid-des-acetyl-vinblastine conjugate, shows excellent survival advantage, thus showing promise for folatereceptor-targeted therapy. 199

Conclusions and future perspectives
As the most widely used drug in various cancer chemotherapy, cisplatin brings substantial benefits to patients.It is the first-line drug used in the treatment of ovarian cancer.During long-term chemotherapy, most patients have a lower effective response and eventually suffer from severe cisplatin resistance.Cisplatin resistance in ovarian cancer is a difficult treatment point that affects the prognosis of patients.
There are many mechanisms of cisplatin resistance, and one or several changes may lead to drug resistance.Many molecular mechanisms have been linked to drug resistance in ovarian cancer, such as drug metabolism, autophagy, and DNA damage repair.However, the mechanism underlying cisplatin resistance in ovarian cancer has not been fully elucidated.For example, the ROS regulation mentioned in this review has opposite effects on cisplatin resistance.In addition, the dynamic evolution of tumor heterogeneity produces resistant subsets, which increase the difficulties of cancer treatment.The biological relationships between different subpopulations in tumors and between subpopulations and the microenvironment are also unclear.A clear definition of the molecular mechanism underlying cisplatin resistance will greatly aid in the development of treatments to address drug resistance in ovarian cancer.
Many drugs, including non-platinum chemotherapeutic drugs, anti-angiogenic agents, immunosuppressants, and PARP inhibitors, have been shown to sensitize cisplatinresistant ovarian cells.Although some drugs have been investigated in clinical trials, many have not achieved satisfactory clinical results.A considerable number of the studies have been terminated due to serious adverse reactions, which suggests that researchers should pay attention to the adverse effects of drugs in the process of exploring targeted therapy.Multi-drug combination therapy for cisplatin-resistant ovarian cancer seems to be a good therapeutic strategy to overcome cisplatin resistance.It is hoped that multiple drugs will work synergistically.However, the determination of combinations of drugs and appropriate loading methods, which can be determined based on drug metabolism and the pharmacokinetic diversity of different drugs, has also been a great challenge in the treatment of cisplatin-resistant ovarian cancer.The cost and difficulties of making the preparations also need to be considered.Overall, this review summarized the current mechanisms and therapeutic strategies of cisplatin resistance in ovarian cancer, which could pave the way for the development of resistance therapy.

Figure 3
Figure 3 Interaction of cisplatin with nuclear DNA.MT competitively binds to cisplatin after cisplatin enters the nucleus, resulting in reduced cisplatin-DNA interaction.DNA adducts restore the normal structure and confer resistance under the DNA damage repair systems, including NER, MMR, HRR, and NHEJ.

Figure 2
Figure 2 Presence of cisplatin in the cytoplasm.Oxidation-reduction, autophagy, ferroptosis-inhibition, and metabolic reprogramming impact the development of resistance in the presence of cisplatin in the cytoplasm.

Table 1
Non-coding RNAs regulate cisplatin resistance in ovarian cancer.

Table 2
Clinical studies for the treatment of cisplatin resistance in ovarian cancer.

Table 3
Role of drug delivery systems in reversing cisplatin resistance.