Abstract P6-07-01: Anti-tumor efficacy of PI3K-mTOR pathway inhibitors in combination with PARP inhibitor plus carboplatin in BRCA1-competent TNBC, beyond PTEN: A proof of concept study

Abstract

Abstract Background: Recently we reported that PI3K-mTOR inhibition potentiated anti-tumor effects in BRCA-competent TNBC cells when combined with ABT888 (A) and carboplatin (C) (De et al., Neoplasia, 2014). Pro-proliferative and anti-apoptotic actions of this pathway constitute one of the main effector signals of RAS. Here, we tested the anti-tumor effect of either single or dual node blockade of PI3K-mTOR pathway when combined with A and C on BRCA-competent TNBC cells with WT-PTEN background and activated RAS-RAF pathway. Materials & Methods: Athymic mice bearing TNBC xenograft tumors were treated with pan PI3K inhibitor, GDC-0941 or PI3K-mTOR dual inhibitor, GDC-0980 alone or in combination with A and C. Mechanism-based in vitro studies were performed using a panel of BRCA-wt/mutants TNBC cells with varying genetic backgrounds. Results: Blocking a single nodal point of PI3K by GDC-0941 failed to significantly inhibit growth of pre-established tumors (> 20%) even in combination with A and C in MDA-MB231 xenografts, while GDC-0980 potentiated an anti-tumor effect by inhibiting tumor growth by 90%. In vitro treatment (1) decreased proliferation signals (pAKT, pP70S6K, p4EBP1, pS6RP), cell cycle progression, vascular mimicry & 2D clonogenic growth, and (2) increased apoptosis markers (cl-caspase3, 9, BIM, cl-PARP, & annexinV positivity). GDC-0980 in combination with A plus C concomitantly decreased Ki67, cl-caspse3, pVEGFR, CD31, p4EBP1, and pS6RP in vivo (IHC). In contrast, combination of GDC-0941 with A plus C failed to affect the cell cycle, apoptosis or 2D clonogenic growth in MDA-MB231 cells. Alternatively, Treatment with RAD001 increased pAKT while it perturbed the activation of S6RP/4EBP1. Conclusion: Our data indicate that following dual inhibition of PI3K-mTOR, S6RP/4EBP1 de-phosphorylation tracks more consistently with the drug’s tumor-growth inhibitory response rather than the upstream state of AKT-activation. Unlike mTORC1 inhibitor RAD001, GDC-0980 potently eliminates (in vitro & in vivo) feed-back re-activation of the pathway as, (1) it targets PI3K, reactivation of AKTT308 is blocked and (2) inhibition of the mTORC2 complex blocks the reactivation of AKTS473. TNBC tumors with PTEN independent RAS/RAF mutation-mediated activation of PI3K-mTOR pathway can be controlled by dual node blockade of the pathway when combined with a PARP inhibitor and carboplatin. Citation Format: Nandini Dey, Yuliang Sun, Jennifer Carlson, Lori Friedman, Pradip De, Brian Leyland-Jones. Anti-tumor efficacy of PI3K-mTOR pathway inhibitors in combination with PARP inhibitor plus carboplatin in BRCA1-competent TNBC, beyond PTEN: A proof of concept study [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-07-01.