Abstract PS12-02: Safety and efficacy of an alternative schedule of palbociclib (PAL) in hormone receptor positive (HR+), HER2 negative (HER2-) metastatic breast cancer (MBC) and the utility of serum thymidine kinase 1 (sTK1) activity in predicting PAL response

Background: The approved 3 weeks (wks) on/1 wk off schedule of PAL results in grade (G)3+ neutropenia (ANC) up to 66%. We hypothesized that an alternative schedule (Alt Dose Pal), 5 on/2 off every 7 days, reduces the rate of G3+ ANC, allowing continued weekly dosing. In addition, based on our previous study supporting sTK1, an E2F-dependent enzyme critical for DNA synthesis, as a pharmacodynamic indicator of CDK4/6 inhibition, we hypothesized that Alt Dose Pal inhibits sTK1 and sTK1 dynamics predicts PAL response. Methods: A single arm phase II trial was conducted in HR+ HER2- MBC, ≤1 prior endocrine therapy (ET) for MBC (NCT03007979). Pts received PAL 125 mg daily, 5 on/2 off weekly, plus letrozole (LET) or fulvestrant (FUL) per physician choice, on a 28-day cycle (C). Goserelin was added if premenopausal. Complete blood count and chemistry panel were done at baseline (BL), C1&2D15, and D1 of C2+. The primary objective was to determine the rate of G3+ ANC in C1 D1-D29. Secondary objectives were to assess the rate of all cycle G3+ ANC, PAL dose reduction/discontinuation, adverse events (AE) per CTCAE v5, progression free survival (PFS), objective response rate (ORR: CR+PR (complete and partial responses)) and clinical benefit rate (CBR: no progression (PD) in 24 wks) by RECIST 1.1. The sample size of 47 provides 90% power, 1-sample binomial exact test, 5% alpha, to test the 1-sided null hypothesis of G3+ ANC rate >62% vs the alternative of 10% or G3+. The ORR was 50% (2 CR, 13 PR, 95% CI: 33.15%-66.85%) in 30 pts with measurable disease (n=29) or non-measurable but CR (n=1). The CBR was 81.63% (95% CI: 67.5%-90.76%) in 40 (2CR, 13 PR, 25 SD ≥24 wks) of 49 evaluable pts. The median PFS (mPFS) was 24.3 mo (95% CI 15~not reached (NR)) overall. The mPFS was 33.5 mo (95% CI 17.3~NR) and 12 mo (95% CI: 10.4~NR), in ET sensitive and resistant population, respectively. sTK1 was significantly reduced, 80% down to undetectable, as early as C1D15 (p=7.77E-07). BL sTK1 levels correlated with PD vs non-PD (p=0.003) as best response and negatively correlated with PFS (p=0.002). sTK1 rose significantly at PD (p=0.0003). A median lead time of 80.6 (IQ range 6.8-189.2) days was observed for rising TK before RECIST PD. Conclusion: The Alt Dose Pal trial met its primary endpoint with reduced G3+ ANC. The efficacy data is comparable to prior reports. sTK1 shows promise for PAL response prediction and monitoring. Citation Format: Jairam Krishnamurthy, Jingqin Luo, Rama Suresh, Foluso Ademuyiwa, Caron Rigden, Timothy Reardon, Katherine Clifton, Katherine Weilbaecher, Ashley Frith, Anna Roshal, Pavan Tandra, Mathew Cherian, Tracy Summa, Shana Thomas, Leonel Hernandez-Aya, Mattias Bergqvist, Lindsay Peterson, Cynthia X Ma. Safety and efficacy of an alternative schedule of palbociclib (PAL) in hormone receptor positive (HR+), HER2 negative (HER2-) metastatic breast cancer (MBC) and the utility of serum thymidine kinase 1 (sTK1) activity in predicting PAL response [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 PS12-02.