Abstract 2650: PI 3-Kinase inhibitors enhance the synthetic lethality of Parp inhibitors

Abstract

Abstract Introduction: Clinical trials have shown promising responses of BRCA-linked breast and ovarian cancers to PARP inhibitor therapy, but remissions are often short-lived and incomplete. Frequent loss of INPP4B and PTEN leads to activation in the PI3K pathway in TNBC, making the PI3K pathway an attractive treatment target. Here we present mechanistic data that explain how PI3K- and PARP-inhibition enhance each other. Methods: We examined the effects of PI3K-inhibitors BKM120 or BYL719 and PARP-inhibitor Olaparib on the metabolism of HCC1937 cells (BRCA1 5382C mutation and homozygous deletion of PTEN and p53) measuring lactic acid production, via seahorse analysis, carbon metabolism using targeted Mass Spectrometry (LC-MS/MS) and DNA synthesis. PI3K- but not Akt- or SGK-inhibition induced a DNA damage response in this BRCA1 mutant cell line as well as in a mouse model of BRCA1-related breast cancer (K14-Cre BRCA1f/f p53f/f) mouse model. Summary: Metabolic profiling showed that PI3K-inhibition decreased flux through glycolysis and specifically through the non-oxidative pentose-phosphate pathway, the main source of ribose-5-phosphate required for the de novo synthesis of nucleotides in BRCA1-mutant breast cancer cells. Nucleotide shortage led to replication stress with the appearance of γH2AX, increased poly-ADP-ribosylation and an acute drop in DNA synthesis in cell cultures as well as in vivo. PI3K inhibition led to a reduced and error-prone S-phase due to a decrease in nucleotide biosynthesis which could be rescued upon exogenous supply of nucleosides. In a mouse model (K14-Cre BRCA1f/fp53f/f induced breast cancers) we could confirm that both BKM120 and BYL719 enhanced the efficacy of Parp-inhibitor Olaparib (median PFS for Olaparib 50 days, for BKM120 4 days, for BYL719 6 days and for both combinations, BKM120+Olaparib and BYL719+Olaparib > 120 days). Conclusion: Preceding cell cycle arrest, PI3K-inhibition leads to a decrease in DNA synthesis due to decreased Ribose-5-phosphate production required for nucleotide synthesis. PI3K-inhibitors lower nucleotide pools, lead to impaired DNA damage repair and S-phase progression and further augment the synthetic lethality of Parp-inhibitors in BRCA1-related breast cancer. Citation Format: Ashish P. Juvekar, Sina Yadegarynia, Hai Hu, Costas A. Lyssiotis, Hui Liu, John M. Asara, Ralph Scully, Lewis C. Cantley, Gerburg M. Wulf. PI 3-Kinase inhibitors enhance the synthetic lethality of Parp inhibitors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2650. doi:10.1158/1538-7445.AM2015-2650