Abstract PD18-05: MEN1611, a PI3K inhibitor, combined with trastuzumab ± fulvestrant for HER2+/PIK3CA mutant advanced or metastatic breast cancer: updated safety and efficacy results from the ongoing phase 1b study (B-PRECISE-01)

Abstract Background: MEN1611 (MEN) is an oral PI3K inhibitor active on the p110α mutant and wild type, β and γ isoforms, while sparing the δ. B-PRECISE-01 is an open-label, 2-arm, phase 1b study investigating MEN1611 in combination with trastuzumab ± fulvestrant in patients with HER2 positive/PIK3CA mutated metastatic breast cancer (MBC). No dose-limiting toxicities were observed during the dose-escalation step and MEN1611 48 mg BID was selected as the recommended phase 2 dose (RP2D) for cohort expansion (CE). Methods: Eligible patients had HER2+/PIK3CA-mutated MBC and were treated with at least 2 prior lines of anti-HER2-based therapy in the advanced/metastatic setting including trastuzumab. Patients received MEN1611 + trastuzumab (MEN+T); hormone receptor positive (HR+) postmenopausal women received M+ T + fulvestrant (MEN+T+F). Recruitment was closed in December 2021. Pooled safety and efficacy data from the two subpopulations of CE are presented herein. Results: As of June 2022, 62 female patients were treated: 56 of them with MEN1611 48 mg BID (25 MEN+T and 31 MEN+T+F). Median age 55.5 years (range 34-78), 21% premenopausal, ECOG PS 0-1: 95.2%. Median metastatic regimens 4; 71.0% had prior pertuzumab and 91.9% had prior T-DM1. Common treatment-emergent adverse events (TEAEs, ≥20%) were diarrhea 66.1%, nausea 45.2%, hyperglycemia 43.6%, anemia 35.5%, asthenia 29.0%, decreased appetite 27.4%, rash 25.8%, aspartate aminotransferase increased 22.6%, vomiting 22.6%, and pyrexia 22.6%. Common TEAEs with CTCAE grade ≥3 (≥10%) were hyperglycemia (22.6%) and diarrhea (11.3%). Most treatment-related AEs (TRAEs) were reversible and manageable by supportive care. TEAEs leading to permanent treatment discontinuation occurred in 9 patients (14.5%), the only TEAE occurring in more than one patient was lipase increased (3.2%). TEAEs caused temporary treatment interruptions in 32 patients (51.6%), the most common being hyperglycemia (21.0%) and diarrhea (9.7%). TEAEs leading to dose reduction occurred in 14 patients (22.6%), the most common being diarrhea (6.5%), hyperglycemia (3.2%) and stomatitis (3.2%). Serious TRAEs were experienced by 12 patients (19.4%): hyperglycemia 6 patients, diarrhea 3 patients, anemia, general physical health deterioration, generalized edema, lipase increased, ketoacidosis and pneumonitis (1 patient each). In the efficacy-evaluable population at the RP2D (n=41) 14 patients (34.1%) showed partial response (MEN+T 5/15, MEN+T+F 9/26), 1 patient (2.4%) had a complete response (MEN+T 1/15) and 23 patients (56.1%) had stable disease (MEN+T 6/15, MEN+T+F 17/26) as best response. At the RP2D, the median (95% CI) overall survival (OS) was 21.9 (11.9, NE) months and the median (95% CI) progression free survival (PFS) 5.6 (3.7, 7.2) months. In the MEN+T group, the median OS was 11.9 (5.7, NE) months and median PFS 3.9 (2.3, 6.7) months. In the MEN+T+F group the median OS was 21.9 (16.9, NE) months and median PFS 5.7 (3.7, 11.5) months. Five patients continue on treatment. Conclusions: Updated results from B-PRECISE-01 demonstrated that MEN1611 combined with trastuzumab ± fulvestrant continued to show a manageable safety profile with encouraging anti-tumor activity and duration of response in heavily pre-treated patients with HER2+/PIK3CA-mutated advanced or metastatic breast cancer. Citation Format: Martine Piccart, Audrey Hennequin, Manuel Ruiz Borrego, Santiago Escrivá-de-Romani, Anja Williams, Begoña Jiménez Rodríguez, Gianluca Del Conte, Sacha J. Howell, Michela Palleschi, Matteo Simonelli, Francois P. Duhoux, Diego Tosi, Bernard Doger de Speville Uribe, Yolanda Jerez Gilarranz, Pierfrancesco Tassone, Giuseppe Curigliano, Simon Waters, Philippe Aftimos, Hans Wildiers, Simona Scartoni, Bartomeu Piza Vallespir, Ram Charan Shankaraiah, Krzysztof Grzegorzewski, Nassir Habboubi. MEN1611, a PI3K inhibitor, combined with trastuzumab ± fulvestrant for HER2+/PIK3CA mutant advanced or metastatic breast cancer: updated safety and efficacy results from the ongoing phase 1b study (B-PRECISE-01) [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 PD18-05.