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Mutations in the phosphoinositide-3 kinase alpha isoform (PIK3CA) are frequent in breast cancer and activate the PI3K signaling pathway. We discovered GDC-0032, a selective, potent, orally bioavailable inhibitor of PI3Ka with a Ki = 0.2nM, and with reduced inhibitory activity against PI3Kβ. This selectivity profile, and excellent pharmacokinetic and pharmaceutical properties, allowed for greater efficacy in vivo at the maximum tolerated dose relative to a pan Class I PI3K inhibitor in PIK3CA mutant xenografts. Notably, GDC-0032 preferentially inhibited PIK3CA mutant cells relative to cells with wild-type PI3K. GDC-0032 potently inhibits signal transduction downstream of PI3K and induces apoptosis at low concentrations in breast cancer cell lines and xenograft models that harbor PIK3CA mutations. The mutant-bias of GDC-0032 is linked to unique properties of GDC-0032, including cellular potency against the mutant isoform and reduction of receptor tyrosine kinase (RTK) signaling. Endocrine therapies such as letrozole are commonly used treatment options for metastatic Hormone Receptor positive (HR+) breast cancer but many patients ultimately relapse. Due to the importance of PI3K in breast cancer, PI3K inhibitors such as GDC-0032 are attractive for combination with endocrine therapies. GDC-0032 was evaluated in breast cancer lines and models in combination with letrozole, and assayed for cellular viability, modulation of PI3K pathway, modulation of ER pathway markers, and apoptosis induction. The combination of GDC-0032 and letrozole decreased cellular viability and increased apoptosis relative to either single agent. We observed cross-talk between the PI3K and ER pathways that suggest a mechanism of action for the combination. In a secreted factor screen we found that multiple soluble factors render breast cancer cells non-responsive to letrozole. It was discovered that many of these factors signal through the PI3K pathway and GDC-0032 in combination with letrozole was able to overcome the growth inhibition caused by the soluble factor. We also established letrozole resistant cell lines that grow independently of any estrogen source. These letrozole resistant lines have elevated PI3K pathway signaling and are still sensitive to GDC-0032. Taken together, these data provide rationale for evaluating GDC-0032 in combination with endocrine therapies for ER+ breast cancer treatment in the clinic. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-17-01.

The phosphoinositide 3-kinases (PI3Ks) are lipid kinases that activate the PI3K signaling pathway and play an essential role in regulating breast tumor cell growth, migration, and survival. Activating and transforming mutations in the PIK3CA gene (PI3K alpha) are commonly found in HER2+ breast cancer and ER+ breast cancer. GDC-0032 is a selective, orally bioavailable inhibitor with a Ki of 0.29 nM for PI3K alpha with 30 fold less inhibition of PI3K beta. In addition, GDC-0032 has increased single agent activity against PI3K alpha mutant and HER2 amplified cancer models in vitro and in vivo. The aim of our study was to evaluate the efficacy of GDC-0032 in breast cancer models in combination with standard of care therapeutics including taxanes, endocrine therapies, and HER2 targeted therapies. We evaluated cell viability for a range of dose concentrations of GDC-0032, as single agent and in combinations, in eleven breast cancer cell lines that harbor mutations in PI3K alpha (K111N, E545K, H1047R) and/or over-expressed HER2. GDC-0032 was combined with taxanes (paclitaxel and docetaxel), endocrine therapies (fulvestrant and tamoxifen) or anti-HER2 agents (trastuzumab, pertuzumab or ado-trastuzumab-emtansine) and synergy quantified using the Chou and Talalay method of Combination Index (C.I.). The combination of GDC-0032 with taxanes, endocrine therapies or anti-HER2 therapies were synergistic in the breast cancer cell lines tested based on C.I. values of less than 1.0. The combination of GDC-0032 with docetaxel in vitro resulted in decreased viability as a result of increased cell death. The in vitro results translated in vivo as GDC-0032 enhanced the anti-tumor activity of docetaxel and paclitaxel in MCF7 (E545K) xenografts that resulted in increased tumor regressions when GDC-0032 was dosed daily or intermittently on the same schedule as the taxane. In addition, the combination of GDC-0032 and fulvestrant or tamoxifen resulted in greater efficacy in the MCF7 xenograft model when compared to either agent alone. Enhanced efficacy was also observed when GDC-0032 was combined with trastuzumab or ado-trastuzumab-emtansine in the HER2+ BT474M1 (K111N) xenograft model. Moreover, the triple combinations of GDC-0032/trastuzumab/pertuzumab or GDC-0032/trastuzumab/docetaxel resulted in durable tumor regressions that were sustained in the HER2+ KPL-4 (H1047R) and BT474M1 xenograft models, respectively. All drug combinations with GDC-0032 were well tolerated in vivo based on minimal changes in body weight. Collectively, our preclinical data demonstrate that GDC-0032 enhances the efficacy of standard of care therapeutics in PI3K alpha mutant breast cancer models and provides a strong rationale for further evaluation in patients. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-15-02.

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Purpose: Phosphoinositide 3-kinases (PI3K) are lipid kinases that can regulate breast tumor cell growth, migration and survival. Standard of care drugs such as estrogen receptor (ER) antagonists including fulvestrant and tamoxifen, and aromatase inhibitors such as letrozole are indicated for the treatment of hormone receptor positive breast cancer. The current study is focused on investigating preclinical activity in breast cancer models, for GDC-0941, a class I PI3K inhibitor, GDC-0032, a PI3K inhibitor, and GDC-0980, a dual mTOR kinase and class I PI3K inhibitor. Investigation into PI3K inhibitor efficacy in combination with endocrine therapies is also explored. Experimental Design: A panel of ER+ breast cancer cell lines were treated with GDC-0941, GDC-0032 and GDC-0980 either as single agents or in combination with fulvestrant or tamoxifen and assayed for cellular effects. MCF-7 cells ectopically expressing aromatase were utilized to test the efficacy of aromatase inhibitors in combination with PI3K inhibitors in vitro. In addition, human xenografts of breast cancer cell lines were employed to assess combination efficacy of PI3K inhibitors with fulvestrant and tamoxifen in vivo. Results: Combination of GDC-0941, GDC-0032 or GDC-0980 with endocrine therapies resulted in a decrease in cellular viability and an increase in cell death. Synergy of PI3K inhibitor combinations with fulvestrant or tamoxifen was assessed using Combination Index (C.I.), and C.I. values as low as 0.1 indicated strong synergy in some contexts. Combination activity of PI3K inhibitors and letrozole was also observed in MCF7 cells expressing aromatase. In MCF-7 xenografts, the combination of GDC-0980, GDC-0032 and GDC-0941 enhanced activity of fulvestrant resulting in tumor regressions and tumor growth delay (116% tumor growth inhibition (TGI) for GDC-0980 and 91% TGI for GDC-0941 and GDC-0032). In addition, the combination of GDC-0941 or GDC-0032 with tamoxifen enhanced the efficacy of tamoxifen in vivo (83%TGI for GDC-0941 and 102%TGI for GDC-0032). Mechanism of action and biomarker studies are underway. Conclusion: Collectively, the non-clinical efficacy data provide a strong rationale to evaluate the combination of PI3K inhibitors with anti-estrogen therapy in hormone receptor positive breast cancer. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-19-02.