Abstract 3746: PI3K inhibition sensitizes to PARP inhibitors in patient-derived xenograft models of triple negative breast cancer

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Hyperactivation of the PI3K pathway through mutation of the PIK3CA gene, PTEN loss, or growth factor induction is prevalent in breast cancer. The use of PI3K inhibitors have recently elucidated that molecular feedbacks become active upon loss of PI3K signaling. Furthermore, PI3K activity promotes cell-cycle entry and reduces double-strand break formation. However, little is known about whether PI3K feedbacks affect DNA integrity. To study this phenomenon in a clinically applicable scenario, we developed 14 patient-derived xenografts (PDXs) from metastatic breast cancer patients enrolling in clinical trials at our institution by subcutaneous tumor implantation in female mice. A triple negative breast cancer (TNBC) model (PDX44) was characterized with a PIK3CA H1047R mutation and PTEN loss, and subsequently evaluated for response to the pan-PI3K inhibitor NVP-BKM120 at a clinically equivalent dose of 27.5mg/kg. Inhibition of PI3K led to increased DNA damage in vivo as measured by gamma H2AX foci formation or increased expression of DNA damage response genes profiled at 4 or 21 days of exposure. DNA damage was also observed with another pan-PI3K inhibitor (GDC0941; 150mg/kg) in PDX44. Furthermore, BKM120 induction of gamma H2AX foci was observed in four additional PDX models. Along with increased DNA damage response, in vivo PI3K-inhibition resulted in concomitant BRCA1 and BRCA2 loss of gene-expression. A search for putative transcriptional regulators for both BRCA1 and BRCA2 using JASPAR predicted ETS1 as a possible negative regulator of the BRCA1/2 genes. Induction of ETS1 expression following PI3K inhibition was confirmed by western blot in several PDXs. Since PI3K inhibition has been shown to induce ERK activation and ETS1 is ERK regulated, we hypothesized that PI3K regulation of BRCA1/2 is mediated by an ERK-dependent up-regulation of ETS1 upon PI3K inhibition. As a matter of fact, MEK inhibition by AZD6244 reversed the BKM120-induced increase in ETS1 and down-regulation of BRCA1/2. These data demonstrate a functional role for ERK signaling feedback up-regulation after PI3K-inhibition. Since PI3K-inhibition led to a BRCAness-like phenotype, we investigated whether synthetic lethality with simultaneous PI3K and PARP inhibition could be achieved. Whereas PDX44 showed no response to PARP inhibition alone (AZD2281/Olaparib; 50mg/kg) and partial response to PI3K inhibition as single agent (BKM120; 10 or 27.5mg/kg), concomitant PARP and PI3K blockade resulted in enhanced and prolonged antitumor activity. Therefore, PI3K-inhibition promotes synthetic lethality with a PARP inhibitor by reducing BRCA1/2 expression. Finally, as a part of the SU2C PI3K dream team, this hypothesis is being potentially investigated in clinical trials. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3746. doi:1538-7445.AM2012-3746