Novel mechanisms of platinum drug resistance identified in cells selected for resistance to JM118 the active metabolite of satraplatin.

PURPOSE

The goal of this study was to identify molecular determinants of sensitivity and resistance to JM118, the active metabolite of satraplatin, an orally bioavailable cisplatin analog that has activity in prostate cancer.

EXPERIMENTAL DESIGN

Human ovarian carcinoma 2008/JM118 cells were derived from parental 2008 cells by repeated exposure to JM118; the revertant 2008/JM118/REV subline was isolated from the 2008/JM118 cells by growth in the absence of drug. Drug sensitivity was determined by clonogenic assay and Pt levels were measured by ICP-MS.

RESULTS

Eight sequential rounds of selection yielded the 2008/JM118 subline that was 4.9-fold resistant to JM118 and cross-resistant at varying levels to satraplatin, cisplatin, carboplatin, and oxaliplatin. Cross-resistance to the other Pt drugs was lost as resistance to JM118 waned. The same parental 2008 cells selected for resistance to cisplatin were partially cross-resistant to JM118. The 2008/JM118 cells accumulated significantly more Pt than the 2008 cells when exposed to low concentrations of either JM118 or cisplatin indicating a detoxification process that involves intracellular sequestration. In contrast, 2008 cells selected for cisplatin resistance accumulated less cisplatin and less JM118 reflecting a mechanism involving reduced accumulation. The 2008 and 2008/JM118 cells did not differ in their uptake or efflux of 64Cu, expression of Cu efflux transporters ATP7A or ATP7B or their glutathione content. The 2008/JM118 cells exhibited 3.0-7.7-fold hypersensitivity to docetaxel, paclitaxel and doxorubicin. Expression profiling identified 4 genes that were significantly up-regulated and 19 that were down-regulated in the 2008/JM118 cells at a false discovery rate of 1 gene.

CONCLUSIONS

While the cellular defense mechanisms that protect cells against JM118 also mediate resistance to the other Pt drugs, these mechanisms are quite different from those commonly found in cells selected for resistance to cisplatin. JM118-resistant cells accumulate more rather than less Pt and rely on an intracellular detoxification mechanism different from that involved in cisplatin resistance. This is consistent with clinical evidence suggesting that satraplatin has activity in diseases in which cisplatin does not. In this model, JM118 resistance is associated with substantial collateral hypersensitivity to docetaxel, paclitaxel, and doxorubicin.