A Novel In Vitro Assay to Assess Phosphorylation of 3′-[18F]fluoro-3′-Deoxythymidine

Abstract

Purpose 3′-[18F]fluoro-3’-deoxythymidine ([18F]FLT) is phosphorylated by thymidine kinase 1 (TK-1), a cell cycle regulated enzyme. Appropriate use of [18F]FLT tracer requires validation of the TK-1 activity. Here, we report development of a novel phosphoryl-transfer assay to assess phosphorylation of [18F]FLT both in tumor cell lysates and tumor cells. Procedures The intrinsic F-18 radioactivity was used to quantify both substrate and phosphorylated products using a rapid thin layer chromatography method. Phosphorylation kinetics of [18F]FLT in SW480 and DiFi tumor cell lysates and cellular uptake were measured. Results The apparent Michaelis–Menten kinetic parameters for [18F]FLT are Km = 4.8 ± 0.3 μM and Vmax=7.4 pmol min−1per 1×106 cells with ~2-fold higher TK-1 activity in DiFi versus SW480 lysates. Conclusions The apparent Km of [18F]FLT was comparable to the value reported with purified recombinant TK-1. The uptake of [18F]FLT by SW480 cells is inhibited by nitrobenzylthioinosine or dipyridamole indicating that uptake is mediated predominantly by the equilibrative nucleoside transporters in these tumor cells.