Abstract PD1-10: Randomized phase II study comparing two different schedules of palbociclib plus second line endocrine therapy in women with estrogen receptor positive, HER2 negative advanced/metastatic breast cancer: CCTG MA38 (NCT02630693) (2019)

Abstract Background: Palbociclib is approved for treatment of ER+, HER2- ABC at a standard dose of 125 mg po daily 3 weeks on/1 week off (STD). We evaluated the efficacy, safety, quality of life (QOL) and compliance of a 100 mg continuous daily dose (CDD) of palbociclib based on modeling data that suggested efficacy and tolerability. Methods: Canadian Cancer Trials Group (CCTG) led a randomized phase II trial (CCTG MA38) to estimate the efficacy of palbociclib 100 mg po on CDD schedule (Arm 1) relative to 125 mg po STD schedule (Arm 2) with physician choice endocrine therapy. Eligible patients had ER+, HER2- ABC, post progression on first line metastatic endocrine therapy or progressed while on/within 12 months of completion of adjuvant endocrine therapy. One line of palliative chemotherapy in ABC was allowed. Stratification factors were: visceral metastases, duration of prior endocrine therapy and planned endocrine therapy. Primary Outcome measure was investigator reported PFS; secondary outcome measures included: RR, OS, safety (CTCAE V4.0), and QOL (EORTC QLQ-C30). The sample size was 180 to enable estimation of the HR between arms with the upper bound of the 90% CI 1.36 times the estimated HR and the lower bound is 0.74 times the estimated HR. Results: 180 patients were enrolled across Canada from Dec 2015 to Feb 2017. The database was locked April 16 2018 after prespecified PFS events reached at a median follow-up of 19 months for all patients. For the whole population: median age was 60.5 years (21% ≥ 70 years); ECOG 0/1 95%; postmenopausal 91%; visceral metastases 67%. Planned endocrine therapy: aromatase inhibitor 33%, fulvestrant 57%, tamoxifen 10%. Efficacy analyses CDD vs STD: PFS stratified univariate HR 0.93 (90% CI 0.66-1.30); multivariate Cox model analysis (covariates ECOG PS, age, histology, grade): HR 0.92 (90% CI 0.67-1.25). Median PFS CDD: 9.33 months (90% CI 6.93-13.90) and STD: 11.30 months (90% CI 8.08- 13.83) Secondary analyses CDD vs STD: OS stratified univariate HR: 1.07 (90% CI 0.67-1.69); multivariate HR 1.14 (90%CI 0.75-1.75); median OS for CDD: 20.73 months (90% CI 19.29-23.30) and 21.39 months (90% CI 19.65 to 26.68) for STD. RR and median duration of response: 11.1% vs 12.4% (p=0.84) and 126 vs 169 days (p=0.86), respectively. Palbociclib drug exposure CDD vs STD: median daily dose (mg) 87 vs 125; median dose intensity (mg/week) 571.2 vs 613.5; ≥ 90% planned dose intensity, 41% vs 54%. Dose modification (withhold and/or reduction) rate CDD vs STD for neutropenia was 70% vs 40%. Grade 3/4 neutropenia 69% vs 53%; febrile neutropenia 3% each arm. Non-hematological toxicity profiles were comparable for both arms. No significant differences were seen in QOL domains. Conclusion: Palbociclib is active and tolerable when administered on either a 100 mg CDD or a 125 mg STD schedule. CDD schedule had higher rates of grade 3/4 neutropenia and dose modifications. Post approval evaluation of alternate dosing schedules for targeted therapies provides useful information regarding drug activity, toxicity and adherence. Citation Format: Parulekar WR, Joy AA, Gelmon K, Mates M, Desbiens C, Clemons M, Taylor S, Lemieux J, Bartlett J, Whelan T, Ayoub J-P, Cescon D, Bordeleau L, Rahim Y, Winch C, Chen BE. Randomized phase II study comparing two different schedules of palbociclib plus second line endocrine therapy in women with estrogen receptor positive, HER2 negative advanced/metastatic breast cancer: CCTG MA38 (NCT02630693) [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr PD1-10.