Abrogation of gemcitabine (GEM) induced [18F]FLT flare response in tumor cells by checkpoint inhibitors (CPIs) as a potential pharmacodynamic biomarker

Objectives CPIs, have been shown to potentiate antitumor effects of chemotherapy. A GEM induced [¹⁸F]FLT(3'-deoxy-3'[¹⁸F]fluorothymidine) flare response in PC-3 mouse xenografts has been shown to be abrogated by CHK1. Using this abrogation of flare response paradigm, the potential of [¹⁸F]FLT as a pharmacodynamic biomarker for CPIs (CPI1 or CPI2) was examined by evaluating human prostate (PC-3), lung (A549) and colon (WiDr) tumor cells and xenografts.

Methods [¹⁸F]FLT uptake was determined in cell lines at 24 and 48 hours post-GEM exposure in control and CPI-treated samples. CPIs were added after 17h of GEM exposure. Xenograft mice were imaged with [¹⁸F]FLT at 24 and 48h after GEM.

Results GEM induced a bell shaped concentration dependent [¹⁸F]FLT flare response (FR) (6-fold increase) at 24h in PC-3 treated cells vs controls, while at 48h the FR decreased (< 2-fold). In WiDr, the maximal [¹⁸F]FLT FR (4 to 6-fold) occurred at 48h. In A549, maximal [¹⁸F]FLT FRs were achieved at 24h (7.5 nM GEM) and 48h (20nM and 40 nM GEM). A concentration dependent abrogation was observed with CPI1 and CPI2 in all the cell lines with A549 the most sensitive to CPI2 and PC3 the least sensitive. GEM dose dependent [¹⁸F]FLT FRs were also observed in xenografts at 24 and 48h.

Conclusions The GEM [¹⁸F]FLT flare response and its abrogation by CPI1 or CPI2 was dose, time, and cell line dependent. These results support the hypothesis that the [¹⁸F]FLT flare response and its abrogation should be clinically applicable as a pharmacodynamic biomarker of CPIs actions.