Abstract PD13-05: PD13-05 Updated results of a Phase 1b study of gedatolisib plus palbociclib and endocrine therapy in women with hormone receptor positive advanced breast cancer: Subgroup analysis by PIK3CA mutation status

Abstract Background: Addition of PI3K/mTOR inhibitor after progression on CDK4/6 inhibitor (CDK4/6i) and endocrine therapy (ET) can potentially restore sensitivity to CDK4/6i and prevent adaptive activation of the PI3K/mTOR pathway. To evaluate this hypothesis, we conducted a Phase Ib study of gedatolisib (G), a dual inhibitor of PI3K/mTOR, palbociclib (P) a CDK4/6i, and ET (with letrozole [LET] or fulvestrant [FUL]) in women with hormone receptor positive (HR+)/HER2- advanced breast cancer (ABC). Manageable toxicity and preliminary antitumor activity were observed in 35 patients(pts) enrolled in the dose escalation portion of the study (Forero-Torres, ASCO 2018) and 103 pts enrolled in the expansion portion of the study (Layman, SABCS 2021). Here, we report updated efficacy and safety data and sub-group analysis by PIK3CA mutation status in the four expansion study arms with a March 3, 2022, data cut-off. Methods: Pts with HR+/HER2- ABC were treated in four expansion arms: A) G+P+LET as first-line treatment, B) G+P+FUL as 2nd line treatment in pts without prior CDK4/6i; C & D) G+P+FUL as 2nd or 3rd line therapy in pts with prior CDK4/6i. P, LET, and FUL were administered at standard doses. G 180 mg was intravenously administered weekly in Arms A, B, and C and three weeks on/one week off in Arm D. The primary endpoint was investigator assessed objective response rate (ORR). Secondary endpoints included safety, duration of response and progression free survival (PFS). Results: Of the 103 pts treated with G+P+ ET in the expansion arms (A-D), 100% had measurable disease at baseline, 71% (73/103) lacked PIK3CA mutations (wild type; WT), 27% (28/103) had PIK3CA-mutations (MT), 70% (72/103) had evidence of bone metastases, and 59% (61/103) had liver metastases. The most frequent grade 3 and 4 treatment related AEs (TRAE) with G+P+ET included neutropenia (63%), stomatitis (27%), rash (20%), fatigue (11%) and hyperglycemia (7%). Treatment discontinuation due to TRAEs was 6.5% in Arm A, 15.4% in Arm B, 9.4% in Arm C and 3.7% in Arm D. Efficacy data for each arm is presented in Table 1. Promising ORR and PFS were seen in all arms regardless of PIK3CA mutation status. In Arm D, ORR was 63% overall, 73% in PIK3CA-MT pts, and 60% in PIK3CA-WT pts. Median PFS in Arm D was 12.9 months with a median follow up of 29 months. 60% and 48% of pts in the PIK3CA-MT and PIK3CA-WT Arm D sub-groups, respectively, were progression free at 12 months. Conclusions: These preliminary data demonstrate promising activity of G+P+ET combination in pts who were CDK4/6i-naïve and in those whose disease progressed on or after CDK4/6i therapy regardless of PIK3CA mutation status. Encouraging results in CDK4/6i treatment naïve patients warrant further evaluation of gedatolisib in combination with CDK4/6i treatment in the front-line setting. Arm D results provide a strong basis for the initiated Phase 3 study (VIKTORIA-1) in pts whose disease progressed on or after CDK4/6i therapy. Table 1. Efficacy Data by Expansion Arms Citation Format: Robert Wesolowski, Hope Rugo, Erica Stringer-Reasor, Hyo S. Han, Jennifer M. Specht, E. Claire Dees, Peter Kabos, Ulka Vaishampayan, Seth A. Wander, Janice Lu, Keerthi Gogineni, Alexander I. Spira, Anne F. Schott, Maysa Abu-Khalaf, Pratima Nayak, Brian F. Sullivan, Igor Gorbatchevsky, AND Rachel M. Layman. PD13-05 Updated results of a Phase 1b study of gedatolisib plus palbociclib and endocrine therapy in women with hormone receptor positive advanced breast cancer: Subgroup analysis by PIK3CA mutation status [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD13-05.