Abstract PO5-18-11: Use of DiviTum®TKa as a biomarker assay for CDK4/6 inhibitor medication compliance and drug-drug interaction assessment in ER/PR positive metastatic breast cancer

Abstract Background: CDK4/6 inhibitor (CDK4/6i) with endocrine therapy (ET) is standard first line (1L) treatment for metastatic hormone positive (ER/PR+) HER2- negative breast cancer (mBC). Some patients have progression free survival (PFS) of 2 years or more whereas some patients have very short PFS on these medications. Thymidine kinase is a biomarker that reflects cell proliferation. DiviTum®TKa is an assay that was cleared by the FDA in July 2022 based on data that showed that thymidine kinase activity (TKa) value of 250 DiviTum units (DuA) is associated with the decreased likelihood of disease progression within 30 days or 60 days post testing. The purpose of this study is to assess the use of TKa as a biomarker for identifying potential medical compliance issues and drug-drug interactions (DDI) in patients on CDK4/6i and to evaluate whether counseling and optimization of concurrent medications will lead to lower TKa levels in subsequent treatment cycles and possibly longer PFS. Trial design: This is a pilot study to assess whether TKa can serve as a biomarker to identify patients with a suboptimal reduction in tumor cell proliferation caused by a potential CDK4/6i compliance or DDI issue. We will also assess whether correction of the issue can improve suppression of TKa levels in ER/PR+ mBC patients on 1L CDK4/6i and ET. For screening, we are utilizing a novel automated real-time data driven cohort identification system designed by Dr. Guannan Gong at Yale. Patients will be treated and monitored as per standard of care. An aliquot of serum obtained during routine blood draws will be sent to a CLIA-certified Biovica lab for TKa testing. TKa results will be returned in real time. Patients with a detectable TKa baseline level (above 145 DuA), followed by a sustained TKa level (145 DuA) at cycle 1 D15 and D28 will be assessed for medication compliance and DDI. TKa levels will be repeated at subsequent standard of care blood draws to assess if the intervention affected TKa levels. Patients will be followed for 2 years. Eligibility criteria: Participants must have ER+ mBC and starting 1L CDK4/6i + ET and be previously CDK4/6i -naïve. Specific aims: Primary objective: to estimate the rate of improvement in CDK4/6i response (change in TKa levels) after counseling for medication compliance and adjustment of potential deleterious DDIs. Secondary objectives: 1) estimate the rate of sub-optimal CDK 4/6i response in cycle 1 of treatment. 2) Compare clinical benefit rate (CBR) in patients with sub-optimal and optimal CDK4/6i response after both cycles 1 and cycle 3. 3) Compare PFS in patients with sub-optimal and optimal CDK 4/6i response after both cycles 1 and cycle 3. 4) Assess CDK4/6i response via TKa levels upon CDK4/6i dose reductions or changes in CDK4/6i regimens. 5) Compare CDK4/6i response profiles across the three CDK 4/6i among different patients and within the same patients if CDK4/6i is changed throughout treatment course. 6) Correlate TKa levels with tumor marker levels. 7) Assess plasma concentrations of CDK4/6i in patients with suboptimal TKa levels. Statistical methods: For binary endpoints measured at a given time point, we will estimate the two-sided 95% exact CI using Clopper-Pearson method. With a sample size of 120 patients, assuming 20% drop out rate, we predict that about 15% of patients will have suboptimal CDK4/6i response, estimating a 95% CI of 0.086-0.235 in the first cycle. Comparisons between paired outcomes will be completed using the paired t-test or Wilcoxon signed-rank test for continuous parameters of interest, or McNemar’s Chi-square test for categorical parameters of interest. Time to event endpoints will be estimated using Kaplan-Meier method. Present accrual and target accrual: This trial has just been activated. Target accrual is 120 patients. Contact information for people with a specific interest in the trial: Mariya.Rozenblit@yale.edu Citation Format: Mariya Rozenblit, Adriana Kahn, Guannan Gong, Amy Williams, Lajos Pusztai. Use of DiviTum®TKa as a biomarker assay for CDK4/6 inhibitor medication compliance and drug-drug interaction assessment in ER/PR positive metastatic breast cancer abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-18-11.