PO-495 PI3K pathway upregulation mediates acquired resistance to platinum agents and polyadenoribose polymerase inhibitors (PARPi) in BRCA1-methylated ovarian cancer (OC)

Introduction
BRCA1-methylated OC (BMOC) are specifically sensitive to platinums and PARPi, though acquired resistance to these agents eventually develops. Elucidating underlying druggable resistance mechanisms is needed to enable novel therapeutic options in BMOC.
Material and methods
We developed two PARPi resistant (olaparib and talazoparib) and one carboplatin resistant OC cell line models (named OVC8RO, OVC8RT and OVC8RC, respectively) derived from the BRCA1-methylated cell line OVCAR8, following continuous (PARPi) or pulsed (carboplatin) drug exposure. Fold resistance (FR) to the parent drug (as determined by the ratio of the resistant cell line IC50 to the parent cell line IC50) suggested clinically relevant resistance models in OVC8RC (FR=4.80±0.43) and OVC8RO (FR=5.71±0.21). OVC8RT displayed higher level resistance (FR=45.61±11.10). We obtained tissue from 5 matched primary and recurrent (post platinum) BMOC patient tumours. Reverse phase protein array (RPPA) was used to examine differential expression and phosphorylation levels of 63 proteins between parent/resistant cell lines, and primary/matched recurrent tumours. 5 day acid phosphatase cytotoxicity assays were used to determine the IC50 of drugs. Synergy in drug combination assays was determined as per the Chou-Talalay method.
Results and discussions
Significant increases in PI3K p110a (except in OVC8RT) and AKT S473, along with a significant decrease in PTEN were seen in all resistant cell lines, relative to the parent cell line’s baseline levels (p<0.05), consistent with PI3K pathway upregulation. 3/5 recurrent tumours had increased phosphorylated AKT (T308 or S473), as compared to the corresponding primary BRCA1-methylated tumour. The selective a/δ isoform dominant PI3K inhibitor copanlisib (BAY 80–6946) displayed anti-proliferative effects in both the parent cell line (IC5076.6±7.4 nM) and all resistant cell lines (IC50s 44.0–102.1 nM). In both carboplatin and PARPi-resistant models, combination treatment of copanlisib and the parental drug resulted in synergistic growth inhibition (CI@ ED500.27–0.28) and restored sensitivity to the parent drug. When the parental cells were treated with copanlisib in combination with carboplatin or either PARPi, the observed synergism was markedly less (CI@ED50=0.71–0.85) than that observed in the drug resistant models.
Conclusion
The addition of copanlisib to carboplatin or PARPi could represent a novel therapeutic strategy in BMOC that has acquired resistance to either carboplatin or PARPi.