In vitro studies on the mechanisms of oxaliplatin resistance

Purpose: We have previously reported that elevation of glutathione mediated by gamma -glutamyl transpeptidase is one mechanism of oxaliplatin resistan ce. This study explored other potential oxaliplatin resistance mechanisms w ith two aims: (1) to identify the differences between cisplatin and oxalipl atin in terms of drug accumulation. DNA-Pt adduct formation and repair. and (2) to determine whether defects in drug accumulation and enhanced repair of the DNA-Pt adduct contribute to oxaliplatin resistance. Methods: The hum an ovarian carcinoma cell line A2780. an oxaliplatin-resistant variant A278 0/C25 and a cisplatin-resistant variant A2780 CP along with an inherently c isplatin-resistant HT-29 colon carcinoma cell line were used in the study, The methods consisted of sulforhodamine-B assays. atomic absorption spectro photometry and realtime. quantitative RT-PCR. Results: Significantly higher drug accumulation and DNA-Pt adduct formation were observed after exposure to cisplatin compared to after oxaliplatin in the parent A2780 cells and t he oxaliplatin-resistant A2780/C25 cells. The DNA-Pt adduct formed after tr eatment with either drug was repaired with equal efficiency by all cell lin es except A2780,CP. which repaired the DNA-cisplatin adduct more efficientl y than the DNA-oxaliplatin adduct. Relative to the parent line. the oxalipl atin-resistant A2780/C25 cells showed reduced Pt accumulation and DNA-Pt ad duct levels following exposure to oxaliplatin. but only reduced accumulatio n after exposure to cisplatin. The cisplatin-resistant A2780/CP cells showe d reduced accumulation and DNA-Pt adduct levels after exposure to cisplatin . but only reduced DNA-Pt adduct after exposure to oxaliplatin. In comparis on to A2780 cells. the inherently cisplatin-resistant HT-29 cells showed lo wer accumulation and DNA-Pt adduct levels after exposure to cisplatin. but displayed no difference after exposure to oxaliplatin. An enhanced repair o f the DNA-cisplatin adduct was observed only in A2780,CP cells relative to A2780 cells in an 8-h period. The steady-state levels of ERCC-1 mRNA, but n ot of XPA, were moderately elevated in the resistant cells. Exposure to eit her one of the drugs resulted in an induction of XPA in all the cell lines arid of ERCC-1 in cisplatin-resistant cells. There was no relationship betw een the level of expression of the repair genes and the DNA-Pt adduct level s or repair. Conclusions: Relative to cisplatin a lower intracellular conce ntration and fewer DNA-Pt adducts are sufficient for oxaliplatin to exert i ts cytotoxicity. Resistance to oxaliplatin is mediated by similar mechanism s of reduced drug accumulation and DNA-Pt adduct formation as resistance to cisplatin. There is no clear evidence that enhanced repair is a mechanism of oxaliplatin resistance in the cell line (A2780/C25) studied here. The fi ndings are suggestive of yet unidentified differences between the two drugs with respect to cellular uptake and/or efflux and repair of DNA-Pt adducts .