Abstract P5-11-04: A phase I/IIb study of palbociclib (PALBO) plus everolimus (EVE) and exemestane (EXE) in hormone-receptor positive (HR+)/HER2- metastatic breast cancer (MBC) after progression on a CDK4/6 inhibitor (CDK4/6i): Results of the phase II study

Abstract

Abstract Background: For patients (pts) with HR+ /HER2- MBC, the optimal regimen following disease progression on the combination of a CDK4/6i plus endocrine therapy (ET) is unknown, and there is a need to evaluate whether continued CDK 4/6 blockade is beneficial in subsequent lines of therapy. Methods: We developed an investigator initiated Phase I/II, open-label trial (NCT02871791) of triplet therapy: palbociclib (PALBO; CDK4/6i) + everolimus (EVE; mTOR inhibitor) + exemestane (EXE; ET) in men or postmenopausal women with measurable HR+ /HER2− MBC who had progressed on prior CDK4/6i and a non-steroidal aromatase inhibitor (NSAI). Pts may have received any number of prior lines of ET, and 0-1 prior line of chemotherapy. Exclusion criteria included prior use of mTORi or EXE. The primary objective was to estimate efficacy of the combination, defined as the clinical benefit rate (CR+PR+SD>6mo). With a one-sided alpha of 0.1, the study had 90% power to detect an improvement in CBR from 40% to 65%. The phase I determined RP2D to be PALBO 100 mg/day (21 of 28 days) + EVE 5mg/day + EXE 25mg/day (previously presented). For pts who entered into the phase IIa part of the study, a research biopsy at baseline and serial research blood collection for circulating tumor DNA (ctDNA) analysis were mandatory. The tumor tissue underwent whole exome sequencing and RNA sequencing and the ctDNA underwent whole exome sequencing. Here we present the first results of the phase 2 study of this triplet regimen. Results: As of June 29, 2019, after a median follow up of 3.8 (2.8-6.2) months, 32 patients were evaluable in the phase II portion of the study. The triplet combination was associated with a clinical benefit rate of 12.5% (n=4), which did not meet the predefined primary endpoint threshold (65%). There were no objective responses achieved. The median PFS was 3.8 (95%CI: 2.0-8.4) months and the 1-year PFS was 16.1% (95%CI: 3.1%-38.0%). The most common AEs were mucositis (all grades, 87.5%; grade 3/4, 15.6%), neutropenia (all grades 84.4%; grade 3/4, 71.9%), and fatigue (all grades 53.1%; grade 3/4, 0%). Data regarding genomic alterations and its association with outcomes will be presented. Conclusions: Among patients with metastatic HR+/HER2- MBC who had previous progression on a NSAI and a CDK 4/6i, there did not appear to be benefit from the addition of PALBO to EXE and EVE, but there were added toxicities. These findings suggest further evaluation of this triplet combination in patients with CDK 4/6i resistance is not warranted. Genomic data from baseline biopsies and circulating tumor DNA will be presented and may help us to explain why patients included in this study were intrinsically resistant to the triplet. Citation Format: Romualdo Barroso-Sousa, Tianyu Li, Lorenzo Trippa, Rebecca Rees, Chelsea Andrews, Arlindo R Ferreira, Karla Helvie, Ann H Partridge, Beth Overmoyer, Eric P Winer, Nikhil Wagle, Sara M Tolaney. A phase I/IIb study of palbociclib (PALBO) plus everolimus (EVE) and exemestane (EXE) in hormone-receptor positive (HR+)/HER2- metastatic breast cancer (MBC) after progression on a CDK4/6 inhibitor (CDK4/6i): Results of the phase II study [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 P5-11-04.