Abstract P1-14-02: Phase II study of adjuvant endocrine therapy with CDK 4/6 inhibitor, ribociclib, for localized ER+/HER2- breast cancer (LEADER, part 1)
Authors: Spring Laura M , Scarpetti Lauren , Niemierko Andrzej , Isakoff Steven J , Moy Beverly , Wander Seth A , Smith Elisabeth , Abraham Elizabeth , Shin Jennifer , Patel Jaymin M , Comander Amy , Mulvey Therese , Bardia Aditya 
Published: March 2022

Background: CDK 4/6 inhibitors have demonstrated substantial efficacy in treating ER+/HER2- metastatic breast cancer. Therefore, there is great interest in exploring their ability to reduce recurrence risk in early breast cancer. However, conflicting results were observed in the large adjuvant phase 3 clinical trials investigating combination of endocrine therapy and CDK 4/6 inhibitor (PALLAS, MONARCH-E). While these adjuvant clinical trials evaluated upfront use of CDK 4/6 inhibitor, the optimal timing of adding CDK 4/6 inhibitor for HR+/HER2- breast cancer remains unknown. We conducted a prospective phase II clinical trial to evaluate the addition of a CDK 4/6 inhibitor, ribociclib, in patients who were already on adjuvant endocrine therapy. Methods: In part 1 of the clinical trial, eligibility included patients with localized stage I-III ER+ (≥ 10%), HER2- breast cancer; completed surgery; and were on adjuvant endocrine therapy (any number of years) with at least one year or more of treatment remaining. Patients were randomized to two different ribociclib schedules: continuous (400 mg daily of 28-day cycle; arm 1) or intermittent (600 mg daily on days 1-21 of 28-day cycle; arm 2) for one year. Patients were concurrently treated with an aromatase inhibitor (plus GnRH agonist if premenopausal). Tolerance was evaluated via CTCAE version 4.03 and proportion of subjects who discontinued CDK 4/6 treatment early. Stratification factors for statistical analysis included: disease stage (III vs lower), duration of prior endocrine therapy (within 2 years; 2-5 years vs &gt; 5 years), and whether the patient received prior chemotherapy or not. Baseline characteristics and risk factors for recurrence and for early discontinuation were compared between the arms of the study using Pearson's chi-squared test. Actuarial analysis of time to recurrence was done using the Kaplan-Meier estimator. The primary objective of part 1 was to estimate adherence to ribociclib treatment in the adjuvant setting. Results: In total, 81 patients were enrolled between February 2018 and September 2019, and 25 (31%) discontinued ribociclib treatment early, with no significant difference between study arms. The most common grade 3 or greater adverse events (AEs) leading to study discontinuation were neutropenia (44%), alanine aminotransferase increase (28%), and aspartate aminotransferase increase (16%). Among patients who discontinued early, neutropenia was more frequent in the 600 mg arm, 9 of 12 patients (75%), versus 2 of 13 patients (15%) in the 400 mg arm. No patients discontinued early due to prolonged QTc. Ribociclib was dose reduced for 22 patients (27%), with no significant difference between study arms (p = 0.12). After a median follow-up of 20 months, two patients have experienced disease recurrence with recurrence-free survival of 100% at 1 year and 97% (95% CI 88-99%) at 2 years. Biomarker (ctDNA) results will be reported at the meeting. Conclusions: Results demonstrate that while serious AEs with one year of adjuvant ribociclib are low, a substantial number of patients discontinued adjuvant CDK 4/6 inhibitor within 1 year. Overall, with limited follow-up, only two patients had recurrent disease since completion of ribociclib treatment. Tolerability and identifying patient subsets who will most benefit need to be carefully considered with CDK 4/6 inhibitors in the adjuvant setting.

Citation Format: Laura M Spring, Lauren Scarpetti, Andrzej Niemierko, Steven J Isakoff, Beverly Moy, Seth A Wander, Elisabeth Smith, Elizabeth Abraham, Jennifer Shin, Jaymin M Patel, Amy Comander, Therese Mulvey, Aditya Bardia. Phase II study of adjuvant endocrine therapy with CDK 4/6 inhibitor, ribociclib, for localized ER+/HER2- breast cancer (LEADER, part 1) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-14-02.