Chemical, pharmacokinetic and biological aspects of platinum-based drugs

Abstract

The use of cisplatin for the treatment of metastatic testicular cancer is a medical historical landmark given that, for the first time, patients could be cured with drug therapy. A third generation platinum-based drug, oxaliplatin, has been registered for use in the treatment of colorectal cancer. Cisplatin and oxaliplatin have limited cross-resistance and display markedly different toxicity profiles. Platinum-based drugs (PBD) are chemically transformed to produce a variety of compounds that are of importance with respect to both wanted and unwanted toxicities. DNA-dependent protein kinase (DNA-PK), best known for its role in repair of DNA double-strand breaks, is involved in signalling pathways leading to cell death. It is unclear whether or not DNA-PK is a determinant of the cytotoxicity of PBD.

The monohydrated complex of cisplatin (MHC) is thought to be the main intracellular form of the drug. Based upon a HPLC technique developed at the Karolinska Hospital Pharmacy, MHC was isolated and quantified. MHC was found to be more cytotoxic than cisplatin. Low extracellular chloride concentration by itself had relatively little affect upon drug stability. In media with low chloride concentration and pH MHC was rapidly transformed. This could have practical implications given that solid tumours often have microenvironments that are extremely acidic. We describe a method for synthesizing the dihydrated oxaliplatin complex (DOC). DOC was formed by hydrolyzing oxaliplatin at 70oC in sodium hydroxide. The cytotoxicity of DOC was compared to oxaliplatin in a lung cancer cell line. DOC was found to be significantly more cytotoxic than oxaliplatin. The pharmacokinetic profile of intact oxaliplatin was determined in patients receiving treatment for metastatic colon cancer. The in vivo clearance was compared with the rate of oxaliplatin degradation in vitro in whole blood samples taken from the patients immediately prior to the start of drug infusion. A good correlation between clearance of oxaliplatin in vivo and degradation in vitro was noted.

We observed a short elimination half-life which is in sharp contrast to previous reports based upon analysis of platinum content. Trifluoperazine (TFP) has been shown to enhance the cytotoxicity of cisplatin. We tested a panel of phenothiazines for their inhibitory properties on DNA-PK. TFP was found to be an inhibitor of purified DNA-PK and DNA-PK in cell lysate. In a lung cancer cell line treated with TFP, DNA-PK activity was decreased with associated cleavage of DNA-PKcs and Ku86 after 30 minutes of incubation. Using a cell line defective in Ku86, we observed that the cytotoxicity of PBDs is enhanced especially with respect to cisplatin. While both drugs were able to induce apoptosis, no caspase-3 activation was noted after treatment with oxaliplatin. Cells lacking Ku86 had constitutional expression of activated p38. Oxaliplatin activated p38 more rapidly than cisplatin. We noted perinuclear translocation of p38 irregardless of Ku86 status in cells exposed to oxaliplatin. In cells exposed to cisplatin, p38 activation was localized to the nucleus in Ku86 deficient cells only. Our data supports a role for Ku86 in influencing the cytotoxicity of PBD. This influence differs depending upon the platinum carrier ligand in question.