Abstract P1-19-46: A phase Ib dose escalation study evaluating the mutant selective PI3K-alpha inhibitor GDC-0077 (G) in combination with letrozole (L) with and without palbociclib (P) in patients with <i>PIK3CA</i>-mutant HR+/HER2- breast cancer

Abstract Background: Dysregulation of the PI3K/AKT/mTOR signaling pathway occurs in solid tumor malignancies. GDC-0077 (G) is a potent p110α-selective, p110α-mutant degrading inhibitor with anti-tumor activity in PIK3CA-mutant breast cancer xenograft models as a single agent and in combination with endocrine therapies (ET) with or without a CDK4/6 inhibitor (i). An open-label, Phase I dose escalation study of Galone and in combination with ET and P is underway in patients (pts) with locally advanced or metastatic PIK3CA-mutant solid tumors. Data from the combinations of G and L with and without P in pts with PIK3CA-mutant HR+/HER2- breast cancer are presented herein. Methods: This study (NCT03006172) assesses the safety (NCI-CTCAE v4), pharmacokinetics (PK), and preliminary anti-tumor activity (RECIST v1.1) of G administered daily (QD) orally at 6 or 9 mg with L (G+L) or at 3, 6, or 9 mg in combination with P+L (G+P+L). L is administered QD orally at 2.5 mg. P is administered QD orally at 125 mg on Days 1-21 followed by 7 days off in 28-day cycles. For dose expansion, prior CDK4/6i was prohibited (G+P+L) and up to one prior metastatic chemotherapy allowed (G+L and G+P+L). Tumor and ctDNA samples (collected before and after 2 weeks of G) are profiled for relevant signaling and pharmacodynamic (PD) biomarkers. Results: As of 29 March 2019, 70 pts had enrolled (G+L: 37 pts [7 at 6 mg and 30 at 9 mg]; G+P+L: 33 pts [3 at 3 mg, 3 at 6 mg, and 27 at 9 mg]). Overall, 33 pts in G+L and 31 pts in G+P+L received ≥ 1 prior metastatic therapy; 70% and 21% of pts received prior CDK4/6i in G+L and G+P+L arms, respectively. No DLTs were reported in pts receiving either combination with G. In G+L, the most frequent treatment-related AEs (TRAEs) occurring in ≥10 (27%) pts included hyperglycemia (25 pts, 68%), nausea (14 pts, 38%), and diarrhea (10 pts, 27%). Grade ≥3 TRAEs in ≥2 (5%) pts included hyperglycemia (7 pts, 19%), and fatigue and hypokalemia (2 pts each, 5%). In G+P+L, the most frequent TRAEs occurring in ≥13 (41%) pts included neutropenia (25 pts, 78%), hyperglycemia (17 pts, 52%), anemia (17 pts, 52%), diarrhea (16 pts, 49%), and nausea and thrombocytopenia (13 pts each, 39%). Grade ≥3 TRAEs in ≥2 (6%) pts included neutropenia (21 pts, 64%), hyperglycemia (5 pts, 15%), lymphopenia (3 pts, 15%), and leukopenia and thrombocytopenia (2 pts each, 6%). Hyperglycemia was manageable with oral anti-hyperglycemic medication. Stomatitis (grouped term) occurred in 11 pts (30%) in G+L and 21 pts (64%) in G+P+L and responded to treatment with dexamethasone mouthwash. 46 pts discontinued treatment, mainly due to disease progression; one due to Grade 3 hyperglycemia in G+P+L (no discontinuation due to AE in G+L). Median G treatment duration: 5.7 months (range 0.2-14.5) in G+L and 9.7 months (range 1.3-23.1) in G+P+L, with a cumulative G dose intensity of 98% for both. In G+ L, PR was reported in 6 pts (16%), confirmed PR in 3 pts (8%) and CBR 35% (13 pts). In G+P+L, PR was reported in 14 pts (42%), confirmed PR in 12 pts (36%) and CBR 76% (25 pts). G PK parameters were similar to those observed as a single agent, and P and L showed exposures comparable with historical data, suggesting an absence of PK drug-drug interaction (DDI). Robust PD downregulation of PI3K pathway effectors (pAKT, pS6) was observed in available paired biopsies and PIK3CA mutant allele frequency decreased in ctDNA on-treatment in most patients. Conclusion: This Phase Ib study of GDC-0077 in combination with letrozole with and without pabocicilb demonstrated a manageable safety profile combining GDC-0077 at its single agent recommended Phase II dose of 9 mg with letrozole with and without palbociclib at standard doses, with no evidence for DDI, high GDC-0077 dose intensity, and promising preliminary anti-tumor activity. Citation Format: Komal Jhaveri, Kevin Kalinsky, Philippe Bedard, Andres Cervantes, Cristina Saura, Ian Krop, Erika Hamilton, Peter Schmid, Andrea Varga, Nick Turner, Antoine Italiano, Valentina Gambardella, Zachary Veitch, Mafalda Oliveira, Leslie Dickmann, Naoki Kotani, Amy Kapp, Katie Hutchinson, Stephanie Royer-Joo, Anjali Vaze, Jennifer Schutzman, Dejan Juric. A phase Ib dose escalation study evaluating the mutant selective PI3K-alpha inhibitor GDC-0077 (G) in combination with letrozole (L) with and without palbociclib (P) in patients with PIK3CA-mutant HR+/HER2- breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-19-46.

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