Abstract 2340: Thymidine kinase activity as a response marker for CDK 4/6 inhibition

Selective CDK4/6 inhibition is now part of standard treatment for HR+ breast cancer. There is a pressing need for a practical biomarker that can provide early indication of the biologic activity of these agents and correlate with clinical outcome. Expression of thymidine kinase (TK) occurs in actively proliferating cells, is E2F-dependent and is downregulated after CDK4/6 inhibitor-mediated G1 arrest. Here, we have investigated TK activity (TKA) as a pharmacodynamic marker for CDK4/6 inhibition in both preclinical and clinical contexts and demonstrate the potential of this assay as CDK4/6 inhibitors are further developed. TKA in response to the CDK 4/6 inhibitor palbociclib (palbo) was studied in cell culture, mouse models and samples from clinical studies. TKA was determined by the DiviTum Assay (Biovica, Sweden). In culture, the intracellular TKA and TK release in response to palbo were determined by analysis of cellular extracts and tissue culture media. In vivo, mice bearing human xenografts were treated with vehicle or palbo to determine effects on TKA in serum and in tumor extracts. Finally, serum TKA (s-TKA) was assayed in patient samples procured before and after palbo treatment in a Phase 1 study of palbo and the MEK inhibitor PD0325901 (NCT02022982). Palbo was administered on 3/4 (21 of every 28 days) or 4/4-week (continuous) schedules. In K562 cells, intracellular TKA levels exhibited a clear dose response to palbo. Reductions in TKA were seen at lower drug concentrations than those affecting cell viability. Similar results were observed in MCF-7 xenografts, where lower TKA in serum and tumor was observed after palbo treatment, whereas in E0771 xenografts, no change in TKA was observed. Clinically, serum samples were obtained before and at various time points after palbo exposure in 20 cancer patients. One patient, who discontinued study treatment at the end of cycle 1, demonstrated an increase in s-TKA. In contrast, for the other 19 patients, there was a marked decrease in s-TKA at day 21. Pre-treatment s-TKA values were higher than C1D21 (paired t-test, p Citation Format: Geoffrey Shapiro, Magnus Neumuller, Sara Lööf, Smaranda Bacanu, Takahiro Seki, John Hilton, Khan Do, Nicole Chau, Leena Gandhi, Joseph W. Gibson, Robert Distel, Pawel Niekrasz, Edward M. Suh, Mattias Bergqvist. Thymidine kinase activity as a response marker for CDK 4/6 inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2340. doi:10.1158/1538-7445.AM2017-2340