Abstract PS10-27: A phase II proof-of-concept study of palbociclib (P) rechallenge in patients (pts) with hormone receptor (HR)[+]/HER2[-] metastatic breast cancer (MBC) and clinical benefit to prior P-based treatment (BIOPER)

Abstract

Abstract Background: The addition of a cyclin-dependent kinase 4-6 inhibitor (CDK4/6i) to letrozole or fulvestrant significantly improves progression-free survival (PFS) and overall survival (OS) in HR[+]/HER2[-] MBC pts. At present, the optimal endocrine treatment (ET) after progression on a CDK4/6i remains unknown. However, preliminary findings revealed drivers of adaptive resistance more frequently related to ET than to CDK4/6i. BIOPER explored the efficacy and safety of continuing the same CDK4/6i in combination with a different ET agent beyond progression on prior P-based regimen in HR[+]/HER2[-] MBC and assessed predictive biomarkers to identify those pts who are more likely to benefit from this strategy. Methods: BIOPER (NCT03184090) is a multicenter, non-controlled, phase II trial. Eligible pts included pre- and post-menopausal women aged ≥18 years with HR[+]/HER2[-] MBC that showed a confirmed progressive disease (PD) after having achieved clinical benefit (response or stable disease ≥24 weeks) on immediately prior P plus ET-based regimen. Up to two prior ET lines and not more than one line of prior chemotherapy for MBC were allowed. Pts received P (oral, 75/100/125 mg/day 3 weeks on/1 week off) combined with ET of physician’s choice (including tamoxifen, exemestane, fulvestrant, anastrozole, or letrozole) until PD or unacceptable toxicity. Co-primary endpoints were clinical benefit rate (CBR) -in terms of complete or partial response [PR] and stable disease lasting ≥24 weeks as per RECIST 1.1 (H0: CBR≤5% versus H1: CBR≥20%)- and tumor molecular alterations in the cyclin D-CDK 4/6-retinoblastoma pathway detected at baseline as markers of resistance and sensitivity to P rechallenge. Secondary endpoints included investigator-assessed PFS, objective response rate (ORR), OS, and safety using the Common Terminology Criteria for Adverse Events (AEs) 4.03. Results: Between June 15, 2017 and April 25, 2019, a total of 33 pts from 21 centers in 2 countries were enrolled. Among the 33 pts who were included in the safety set, 1 patient who did not achieve clinical benefit on prior P-based regimen was excluded from the efficacy analysis (n=32). The median age was 59.5 years (range 42-80 years) and all pts were post-menopausal. A total of 25 (78.1%) pts had visceral disease (56.3% of whom with liver metastases), 16 (50%) had ECOG 0, and 19 (59.4%) presented ≥3 metastatic sites. Of 32 pts, 15 (46.9%) received letrozole, 14 (43.8%) received fulvestrant, and 3 (9.4%) exemestane. The median PFS for the prior P-based regimen was 13.8 months (mo) (95% confidence interval [CI] 5.6-47.1 mo). The median number of prior ET and chemotherapy lines for MBC was 2 (range 1-4). By the data cutoff date, 26 PFS events occurred, 5 pts were still on treatment, and 1 patient discontinued treatment because of investigator’s decision. The CBR was 34.4% (95% CI 18.6-53.2%) reaching the prespecified primary endpoint. The ORR was 3.1% (95% CI 0.1-16.2%) with 1 patient with PR. The median PFS was 2.6 mo (95% CI 1.8-5.5 mo). With a median follow-up of 11.8 mo, the OS data were immature with a total of 8 deaths (25%). The incidence of all grade (G) and G 3 or 4 (G3-4) AEs were 90.9% and 48.5%, respectively. The most common G3-4 AEs were neutropenia (42.4%) and leukopenia (6.1%). No discontinuations due to AEs and treatment-related deaths occurred. A comprehensive molecular tumor profiling will be presented during the symposium. Conclusions: Prolonging CDK4/6 blockade beyond progression on prior P-based treatment achieved the prespecified clinical benefit among pts with HR[+]/HER2[-] MBC. This strategy is currently being evaluated in the randomized phase II PALMIRA trial. Further research is ongoing to identify patient subgroups who could benefit from this treatment strategy. Citation Format: Antonio Llombart-Cussac, Javier Cortés, Beatriz Bellosillo Paricio, Miguel Gil Gil, Giuseppe Curigliano, José Manuel Pérez-García, Laura Comerma Blesa, Manuel Ruíz Borrego, Enrique Espinosa, Lourdes Calvo, Begoña Bermejo, Meritxell Bellet, Federico Rojo Todo, Juan de la Haba, Vanesa Quiroga, Alessandro Minisini, Ana Santaballa, Miguel Sampayo, Andrea Malfettone, Joan Albanell. A phase II proof-of-concept study of palbociclib (P) rechallenge in patients (pts) with hormone receptor (HR)[+]/HER2[-] metastatic breast cancer (MBC) and clinical benefit to prior P-based treatment (BIOPER) [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS10-27.