[1113] PARP-Inhibitor Olaparib in the Treatment of Ovarian Clear Cell Cancer: Predictors of Sensitivity and Resistance

Konstantin J Dedes, Paul Wilkerson, Daniel Wetterskog, Maryou B Lambros, Rachael Natrajan, David S Tan, Adriana Campion-Flora, Daniel Nava Rodrigues, Arnaud Gauthier, Frances Daley, Christopher J Lord, Stan B Kaye, Alan Ashworth, Jorge S Reis-Filho. The Institute of Cancer Research, London, United Kingdom; The Royal Marsden NHS Foundation Trust, London, United Kingdom; Institut Curie, Paris, France

Background: Ovarian clear cell carcinoma (OCCC) is an aggressive histological subtype of epithelial ovarian cancer (EOC). A potent poly(ADP) ribose polymerase (PARP)-inhibitor, olaparib, has been shown to be active in cancers with dysfunctional homologous recombination (HR) DNA repair, including tumours with loss of BRCA1, BRCA2 or PTEN function. The aims of this study were to determine whether a subset of OCCC cell lines would be sensitive to PARP inhibition and to identify potential mechanisms of loss of competent HR in OCCCs.
Design: The ability of 12 OCCC cell lines to elicit HR DNA repair in the presence of DNA double stand breaks was determined using the γH2AX (a surrogate marker for DNA double strand breaks) and RAD51 (a surrogate of marker HR repair) foci formation assay following irradiation or treatment with cisplatin or olaparib. Cell viability was assessed following treatment with cisplatin and olaparib using Celltitre Glo, BRCA1, BRCA2 and PTEN expression by western blotting. Co-treatment of cells with olaparib and verapamil (a p-glycoprotein inhibitor) was performed to evaluate the role of MDR1 expression on olaparib resistance. A tissue microarray containing 50 OCCC primary tumours was assessed for the expression of PTEN and MDR1.
Results: Of the 12 OCCC cell lines, TOV-21 and KOC-7c cells lacked PTEN expression, and none showed loss of BRCA1 or BRCA2 expression. Five of the 12 OCCC cell lines (KOC-7c, TOV-21, KK, RMG-1, and SMOV-2) were unable to elicit HR in the presence of DNA double strand breaks induced by radiation. These cells, with the exception of KOC-7c, had significantly higher sensitivity to cisplatin and olaparib than HR-competent OCCC cells. Upon treatment with cisplatin or olaparib, however, KOC-7c cells did not form γH2AX foci and RAD51 foci, but expressed high levels of MDR1. Treatment of KOC-7c with the MDR1 inhibitor verapamil sensitised these cells to olaparib. Loss of PTEN expression was noted in 8% (4/49) primary OCCCs, of which 3 expressed moderate-to-high levels of MDR1.
Conclusions: A subgroup of OCCC cells are sensitive to PARP inhibitors in vitro, and this sensitivity can be predicted by a γH2AX/RAD51 foci formation assay. PTEN loss of function was associated with a defect in HR repair in OCCCs. Expression of known drug efflux pumps (e.g. MDR1) may cause resistance to olaparib. These results provide a rationale for the testing of PARP inhibitors as a targeted therapy in a subset of OCCCs.
Category: Gynecologic & Obstetrics

Tuesday, March 20, 2012 1:00 PM

Poster Session IV # 149, Tuesday Afternoon


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