Abstract P5-02-27: Serum thymidine kinase activity as a prognostic marker in women with metastatic breast cancer treated with two different schedules of palbociclib plus second-line endocrine therapy within the CCTG MA38 trial

Abstract Background: Thymidine kinase-1 is a cell proliferation marker downstream of the CDK4/6 pathway, whose activity can be measured in serum to reflect tumor proliferation. The CDK4/6 inhibitor palbociclib (P) is approved for the treatment of patients (pts) with hormone receptor positive metastatic breast cancer (MBC) in first or second line endocrine-based treatment settings. Approximately 10-15% of pts exhibit de novo resistance to P, with circulating levels of thymidine kinase activity (TKa) previously shown as a potential marker of early treatment resistance. Therapeutic strategies to address primary resistance to P are currently lacking. Little is known of the clinical efficacy of alternative dosing schedules of P, and its effect on TKa. Here we report serum TKa measured at different timepoints from samples collected within the MA38 (NCT02630693) study. Methods: MA38 is an open label randomised Phase 2 trial comparing two different schedules of P plus second-line ET in pts with ER-positive, HER2-negative MBC. Pts were assigned to receive physician’s choice ET plus either standard P dosing (125mg daily for 21 days on a 28-day cycle), or 100mg daily continuously. Serum samples were collected at baseline (BL; n=135), at 12 weeks (W12; n=122) and 24 weeks (W24; n=95). TKa was measured with DiviTum®, a refined ELISA-based assay (lower limit of detection [LLOD] = 100 DuA). Kaplan-Meier method estimated BL, W12 and W24 (95% CI) median PFS (mPFS; from randomization until progression by RECIST criteria or death) and overall survival (OS; from randomization until death from any cause) in groups of patients defined by dichotomizing TKa as “high” or “low” at the median. Results: MA38 enrolled 180 pts from December 2015 and February 2017 across Canada. Median follow up was 19 months. Overall, the median age was 60, and 90% of pts were post-menopausal. All pts had estrogen receptor-positive disease, and 64% had visceral metastases. On study, 56% received fulvestrant with P, 34% aromatase inhibitor and 10% tamoxifen. TKa was successfully measured in 100% of samples. Median TKa (mTKa) at BL was 234 DuA (IQR 138.5 - 438). BL TKa was not associated with clinical or pathological characteristics. TKa was prognostic at BL with mPFS of 5.5 months (mo) in pts with high TKa vs 16.3 mo with low TKa (HR=2.43; 95% CI, 1.6-3.7; p< 0.001). Similar results were obtained employing other previously reported cut off values. At multivariate analysis, BL TKa was independent from other prognostic factors including age, ECOG status and presence of visceral metastases (adjusted HR= 2.34; 95%CI 1.5- 3.6; p < 0.001). In terms of OS, BL TKa was an independent prognostic factor (adjusted HR=2.0; 95% CI, 1.1-3.7; p=0.02). At 12 mo, OS rate was 68% in pts with high BL TKa vs 92% in low TKa. Both for PFS and OS, no interaction between BL TKa and study arm was observed. At W12 mTKa was 129.5 DuA (IQR 100 - 219.8) and below LLOD (IQR 100 - 180) at W24. At these timepoints, landmark analyses showed no significant difference in PFS according to TKa. However, at W12 high TKa was significantly associated with worse OS (HR 2.0; 95%CI 1.0- 4.0; p=0.03), with a similar trend at W24 (HR 2.5; 95%CI 0.9-6.4; p=0.06). Conclusions: Baseline TKa is a reliable prognostic marker of both PFS and OS in pts treated with P and ET, further substantiating previous data. Monitoring TKa during treatment may provide important clinical information. A significant relationship between TKa and assigned treatment arm was not observed, suggesting TKa is not influenced by P treatment dose or intensity. These data confirm the role of baseline TKa as a new marker for patient stratification, and supports further investigation for the assessment of the clinical utility of TKa as a monitoring biomarker in the advanced setting. Citation Format: Amelia McCartney, Chiara Biagioni, Bingshu Chen, Lois Shepherd, Karen Gelmon, Anil A. Joy, Wendy Parulekar, Mattias Bergqvist, Ilenia Migliaccio, Angela Leo, Matteo Benelli, Emanuela Risi, Erica Moretti, Luca Livraghi, Laura Biganzoli, Luca Malorni. Serum thymidine kinase activity as a prognostic marker in women with metastatic breast cancer treated with two different schedules of palbociclib plus second-line endocrine therapy within the CCTG MA38 trial [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-02-27.