Nedaplatin: an effective derivative of cisplatin IntroductionPlatinum-based drugs have proven to be very effective in chemotherapy since their introduction into chemotherapy. It is reported that almost half of the cancer patients undergo to receive platinum-based drugs to treat advanced testicular, bladder and ovarian, neck and lung cancer types [Galanski, Jakupec and Keppler, 2005]. Cisplatin is the first-generation platinum-based drug to be used in chemotherapy and has proven to show strong anti-tumour activities against various cancer types. However, cisplatin often induces renal toxicity, vomiting, nausea, nephrotoxicity, gastrointestinal and ototoxicity toxicity and cisplatin-resistant cancers, which limited its use in clinics [Cooley et al., 1994]. This gave impetus for the development of the second analogue of cisplatin, known as nedaplatin in 1983 to show the same anti-tumour activity but with reduced toxicities induced by cisplatin.Nedaplatin structure and propertiesStructurally, nedaplatin (cis-diammine (glycolate) platinum) is very similar to cisplatin since the main purpose of nedaplatin was to show the same anti-tumour activity as cisplatin with reduced toxicities. Nedaplatin has the same ammine carrier ligands (NH3) like cisplatin but has a five-membered novel ring structure in which glycolate group is bound to the centre platinum ion as a bidentate ligand [Shimada, Kigawa and Itamochi, 2013] [see figure 1]. Presence of glycolate group as a bidentate ligand increases the lipophilicity of nedaplatin and renders bioavailability. The differences in chemical and physical properties between nedaplatin and cisplatin mean that the protein binding of nedaplatin is 50%, which is lower than that of cisplatin (90%) [Nedaplatin-Drugbank.ca, 2018] [Shimada, Kigawa and Itamochi, 2013]. Nedaplatin has a short elimination half-life of 1.1 to 4.4 hours compared to cisplatin and has a similar pharmacokinetics of carboplatin [Sasaki et al., 1989].Figure 1: Structure of nedaplatin (left) and cisplatin (right). Both cisplatin and nedaplatin have the same ammine carrier ligand but nedaplatin has a five-membered novel ring structure instead of two chloride ions. The five-membered ring structure consists of glycolate group that is bound to the doubly charged platinum ion as a bidentate ligand. Addition of glycolate group increases the lipophilicity of nedaplatin and allows for increased uptake through the plasma membrane. Once nedaplatin reaches the cell, it is hydrolysed into its active metabolites which interferes with DNA replication.Mechanism of actionBinding of nedaplatin reactive metabolites to DNANedaplatin works very similarly to cisplatin since nedaplatin was developed to show the same anti-tumour activities. Once nedaplatin is administered through bolus intravenous injections, it enters the cell via a range of mechanisms including high-affinity copper transporter 1 and passive diffusion [Shimada, Kigawa and Itamochi, 2013]. Upon entry of nedaplatin into the cell, the low chloride concentration (4-20mM) of cell results in hydrolysation of nedaplatin into its active metabolite form in which water molecules form complexes with centre platinum ion [Dilruba and Kalayda, 2016] [Wheate et al., 2010] [see figure 2]. The highly reactive metabolites of nedaplatin exist in equilibrium where metabolites interconvert between a series of other metabolites [see figure 2]. Figure 2: Hydrolysation of nedaplatin and binding of its’ active metabolites to DNA. Once nedaplatin enters the cell, low salt concentration allows hydrolysation of nedaplatin to take place. The active metabolites exist in equilibrium where metabolites interconvert between a series of other metabolites [2 and 3]. The active metabolites of nedaplatin also react with purine bases of DNA, in particular, N7 positions of adenosine and guanosine residues, which results in DNA bending and unwinding [Dilruba and Kalayda, 2016]. The active metabolites also bind to cytoplasmic nucleophiles such as glutathione, methionine and cysteine-rich proteins. This leads to an increase in oxidative stress as the cell loses antioxidant proteins [Nedaplatin-Drugbank.ca, 2018]. Since the centre platinum consists of two reactive sites, it also allows the formation of a cross-link between two adjacent guanines. Formation of intra-strand 1, 2- cross-link results in DNA unwinding. The reactive sites of platinum also allow the formation of inter-strand cross-link by coordinating to guanine bases of another DNA strand. [Obtained from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3658440/]Cellular response to DNA damageBinding of platinum-based drugs manipulates the duplex structure of DNA, including compacting the major groove and flattening and bending of the minor groove. The changes in DNA duplex are recognised by cellular proteins, leading to DNA damage repair or initiation of apoptosis. Protein families such as non-histone chromosomal high mobility group proteins 1 and 2 (HMG1 and HMG2), nuclear excision repair proteins (NER proteins) and mismatch repair proteins (MMR proteins) function as DNA damage recognition proteins and leads to DNA repair [Dilruba and Kalayda, 2016] [Takahara et al., 1995]. HMG proteins specifically bind to the HMG1-DNA binding pocket of major groove of DNA adduct and the 1,2-intrastrand cross-link adduct between the adjacent guanine residues which changes the cell cycle events [Brown, Kellett and Lippard, 1993] [Huang et al., 1994]. Initially, it was thought that HMG proteins only recognise damages and promote DNA repair, but a new study has shown that these proteins also contribute to nedaplatin cytotoxicity, suggesting that HMG proteins can also inhibit DNA damage repair [Huang et al., 1994]. The DNA adducts are also recognised by proteins including hMSH2 or hMutS? components of MMR complex and activate MMR pathways to repair DNA adduct. However, failure in DNA adduct repair through NER and MMR results in the initiation of apoptosis. Resistance to Platinum-based drugsResistance to platinum drugs develops although they are very potent inducers of apoptosis of tumour cells. It is well understood that the cytotoxic effect of platinum drugs is a complex process which extends from the entry of the drug into cells to a stage of apoptosis. Resistance occurs as a consequence of changes in intracellular that prevents platinum drugs from interacting with DNA, interfering with DNA damage signals from activating apoptosis pathway [Siddik, 2003]. The level of DNA adducts induced by platinum drugs indicates the level of toxicity. Therefore, reducing the extent of DNA damages caused by platinum drugs increase the resistance against the drug. The resistance against platinum-based drugs develops through an increase in DNA adduct repair and changes in drug concentration. The increase in DNA adduct repair by MMR and NER mechanisms is described as pharmacological alternations which lead to resistance [Siddik, 2003]. [see figure 5]. Therefore, resistance to cancer drugs are multifactorial, in that many mechanisms are involved simultaneously within the same tumour cell but the single mechanism of drug resistance also possible, but it is extremely rated [Teicher et al., 1987] [Eastman et al., 1988].Figure 3: Cellular response and resistance to platinum-based drugs. Platinum-based drugs induce DNA damage, which results in the activation of many cellular pathways that lead to DNA repair. Failure in DNA damage repair through MMR and NER pathways lead to activation of the apoptotic pathway, initiated by p53 tumour suppressor protein. Failure in cell cycle arrest through p21 results in the initiation of apoptotic. However, successful DNA repair through NER and MMR adduct results in resistance to the drug. Therefore, the decrease in DNA adduct directly correlates to increase in resistance to the drug. Resistance to platinum drugs can also develop through decreased influx through copper transporter (CTR1) inactivation or increased influx through MRP2 protein. Mutations and deregulations in proteins such as p53, MAPK and ATR involved in apoptotic pathway also lead to resistance because deregulated proteins cannot phosphorylate other downstream proteins in the apoptotic pathway which results in failed initiation of apoptosisActivation of p53 and apoptosis pathwaySeveral cellular events must occur first to active p53 protein although the mediation of p53 in the cellular toxic effects of platinum-based drugs is a result of DNA damage. Platinum-based drugs activate an upstream event of kinases that regulate p53 proteins in the apoptosis pathway. Kinases such as ataxia telangiectasia mutated protein (ATM) and ATM and Rad3-related proteins (ATR) regulate the transcriptional activity and stability of tumour suppressor p53 [Siddik, 2003] [see figure 4]. Among these two kinases involved in checkpoint activation, platinum-based drugs activate the ATR kinase. The ATR kinase phosphorylates p53 protein, which can phosphorylate to active the downstream apoptotic signalling pathway. Interestingly, a recent study has shown that platinum-based drugs also activate CHK2, which is also a downstream target of ATM but CHK2 does not phosphorylate any kinases, so it appears independent of ATM [Damia et al., 2001] [see figure 4]. ATR also activates mitogen-activated protein kinase (MAPK) cascades such as ERKs, c-JUN N-terminal kinases (JNKs) and p38 kinase in response to platinum-based drugs. [Persons et al., 2000] [Appella and Anderson, 2001]. Activation of ERK is crucial for the initiation of apoptosis because ERK contributes to the regulation of p53 by phosphorylation [Siddik, 2003].Figure 4: cellular pathways activated by platinum-based drugs. The formation of platinum-DNA adducts results in activation of damage recognition proteins. The DNA damage recognition proteins activate proteins such as ATR, Fas/FasL, Chk2, c-AbI and p73. Activated ATR kinase, phosphorylate the p53 protein at serine 15 residue, which triggers the activation of p53 protein [Nghiem et al., 2001]. ATR also trigger other downstream targets by phosphorylating additional sites of p53. Activation of p53 also results in downstream initiation of DNA repair through GADD45a, lead to an increase in NER, cell cycle arrest through p21. The other ATR activated downstream targets include CHK1 kinase, which phosphorylates serine-20 residue of p53 protein [Sheih at al., 2000]. ATR also activates mitogen-activated protein kinase (MAPK) cascade which phosphorylates p53 at serine—15 and threonine- 81. The MAPK subfamily members such as Extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs) and p38 kinases are activated in response to platinum-based drugs [Persons et al., 2000] [Appella and Anderson, 2001]. Caspases act as the main regulators of apoptosis.  Initiation caspases 8, 9 and 10 are coupled to pro-apoptotic signals from mitochondria. Once initiator caspases are activated, they cleave and activate downstream effectors caspases 3,6 and 7, which execute apoptosis pathway by breaking down the cellular proteins.Clinical trials Phase IThe pharmacokinetics of nedaplatin was examined in phase I and the results were compared with cisplatin and carboplatin. All the drugs were given via drip infusions for 30 minutes: the doses of nedaplatin, cisplatin and carboplatin administered were 100 mg/m2, 80 mg/m2 and 450 mg/m2 [Sasaki et al., 1989]. Atomic absorption spectrometry was used to determine the platinum concentration in whole plasma, plasma ultrafiltrate and urine. The mean ratio of plasma ultrafilterable platinum to total platinum were calculated and revealed that the protein—binding abilities of nedaplatin and carboplatin were similar. More than 50% of the nedaplatin was excreted in the urine within 4 hours after administration [Sasaki et al., 1989]. The dose-limiting toxicity was mainly thrombocytopenia for both nedaplatin and carboplatin. The optimal dose of nedaplatin was found out to be 80- 100mg/m2 since there were no dose-limiting toxicities observed at this drug concentration [Ito et al., 1999].Table 1: The determined total peak plasma concentrations and AUC of nedaplatin, cisplatin and carboplatin in phase I study [Sasaki et al.,1989]. [Data obtained from: https://www.ncbi.nlm.nih.gov/pubmed/2647312]DrugAUC (µg/ml)Peak plasma concentration (µg/ml)Nedaplatin959 µg/ml5.31 µg/mlCisplatin208 µg/ml3.09 µg/mlCarboplatin3446 µg/ml19.9 µg/mlPhase II and IIINon-small cell lung cancerPhase I trials favoured a daily dosage of 80mg/m2 and this regimen was utilised in phase II trials. 44 patients with advanced non-small cell lung cancer were treated with a combination of 80mg/m2 of nedaplatin and 60mg/m2 of docetaxel for every 3-4 weeks until intolerance toxicity [Shimada et al., 2013]. Patients showed a 50% response rate (95% CI, 35.2-64.8%) with disease control rate of 75% (95% CI, 62.2-87.8%) [Teramoto et al., 2012]. There were 0 patients with complete response, 22 with partial response,11 with stable disease and 11 with progressive disease. This study also showed grade 3 hematologic toxicities including neutropenia (61.4%) and leukopenia (28.6%) [Teramoto et al., 2012]. The non-haematological toxicities were non-severe. This showed that the use of nedaplatin resulted in reduced toxicities.Esophageal cancerCisplatin with 5-FU is the most commonly used chemotherapeutic regimen for the advanced esophageal cancer. This regimen became inconvenient because it was required to administer for 5 days and causes renal dysfunction. Therefore, this regimen was replaced with nedaplatin (80 mg/m2) with 175mg/m2 of paclitaxel and showed a response rate of 41.7%, up from 33-35% [Ilson, 2003] and median time to progression of 6.1 months and 11.5 months of survival time in 46 patients [Cao et al., 2009]. This showed that the combination of nedaplatin and paclitaxel showed promising results for esophageal cancer.Uterine cervical cancerPhase III study of randomised Gynecological Oncology Group has revealed that a combination of chemotherapy of cisplatin and paclitaxel was the optimal cisplatin doublet regimen to treat patients with cervical and ovarian cancers. However, chemotherapy containing cisplatin was limited due to nephrotoxicity caused by cervical cancers. The response rate for nedaplatin chemotherapy was 46.3% for cervical cancer. The 46.3% response rate was the highest reported for a single platinum drug with reduced toxicity [Kato et al., 1992]. Nedaplatin has also shown to have a higher efficacy than carboplatin and cisplatin [Noda et al., 1987] [Noda et al., 1988] [Kato et al., 1992].Ovarian cancerIn phase II study on patients with ovarian cancer, nedaplatin achieved the same response rate of 38%, same as that of carboplatin [Kato et al., 1988]. 134 patients administered 100 mg/m2 of nedaplatin at least twice by intravenous infusion at 4-week intervals. 42 of 102 evaluable patients responded to the drug (34 partial response and 8 complete response). The overall response rate was 38% and the major side effect was hematotoxicity [Kato et al., 1988]. This study has shown that nedaplatin is an active analogue with reduced nephrotoxicity and can replace cisplatin for the treatment of ovarian cancer.ConclusionNedaplatin is a second analogue of cisplatin, which has clinically proven to have the same efficacy with less nephrotoxicity and neurotoxicity. 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