The cellular pharmacology of oxaliplatin resistance.

Oxaliplatin is a third generation platinum compound that differs from cisplatin and carboplatin in having a broader spectrum of antitumour activity. Molecular studies suggest that oxaliplatin adducts are recognised and processed differently than those produced by the earlier generation Pt-containing drugs. We report here studies on the kinetics of the development of oxaliplatin resistance, and the changes in the cellular pharmacology of oxaliplatin that accompany the emergence of the resistant phenotype in five parental human tumour cell lines and their sub-lines selected for acquired oxaliplatin resistance in vitro. During selection, resistance did not substantially increase until after at least six cycles of oxaliplatin treatment. Oxaliplatin demonstrated schedule-dependency with a 1-h exposure being substantially less cytotoxic than a continuous exposure. Whole cell uptake was linear with concentration, but uptake in the resistant cells averaged only 27+/-10 S.D.% of that in the sensitive cells. Pt accumulation in DNA was markedly reduced in four of the five resistant cell lines, but this did not correlate with either IC(50) or total cellular accumulation. Four of the five resistant sub-lines also demonstrated increased tolerance to adducts in DNA that ranged from 3.1 to 7.6-fold. We conclude that development of acquired resistance to oxaliplatin is accompanied by independent defects in both whole cell uptake and in adduct formation.

Analysis of MLH1 and MSH2 expression in ovarian cancer before and after platinum drug-based chemotherapy.

Preclinical studies have demonstrated a relationship between DNA mismatch repair (MMR) status and sensitivity to cisplatin and carboplatin. MMR-deficient cells are resistant to both drugs, and selection for cisplatin resistance in vitro is sometimes accompanied by loss of MMR protein expression. We used immunohistochemical staining techniques to investigate hMLH1 and hMSH2 expression in paired ovarian tumor sections from 54 ovarian cancer patients before and after platinum-based therapy. We sought associations between hMLH1 and hMSH2 protein expression and clinical parameters known to be of prognostic significance as well as response to treatment and overall survival. hMLH1 and hMSH2 staining decreased significantly after platinum-based therapy. The percent of malignant cells that stained positive correlated with the intensity of nuclear staining for both proteins; staining for hMLH1 correlated well with staining for hMSH2. Unexpectedly, expression of nuclear hMLH1 correlated negatively with response to treatment. Expression of nuclear hMLH1 and hMSH2 was positively correlated with pretreatment CA125 level, and expression of nuclear hMSH2 was positively correlated with change in CA125 level after treatment. Tumor stage was associated with expression of nuclear hMSH2, and tumor histological subtype was associated with both hMLH1 and hMSH2 staining. No association was found between expression of either protein and overall survival. These results indicate that the tumor is biologically altered after chemotherapy consistent with treatment-induced selection for cells expressing lower hMLH1 and hMSH2 levels. However, immunohistochemical staining for either hMLH1 or hMSH2 was not highly predictive of drug sensitivity as measured by response or survival.