Bis(pivaloyloxymethyl) thymidine 5′-phosphate is a cell membrane-permeable precursor of thymidine 5′-phosphate in thymidine kinase deficient CCRF CEM cells

Bis(pivaloyloxymethyl) thymidine 5-phosphate (POM₂-dTMP) has been investigated as a membrane-permeable prodrugs of dTMP. The growth inhibitory activity of POM₂-TMP has been compared with thymidine (TdR) in wild type CCRF CEM cells (CEM) and a strain that lacks TdR kinase (CEM tk-). After 72 h incubation at 37°C, TdR showed significant antiproliferative activity (IC₅₀ = 27 μM) against CEM cells but was weakly effective (IC₅₀ = 730 μM) against the mutant cell line. By comparison, bis(pivaloyloxymethyl) thymidine 5′-monophosphate (POM₂-dTMP) was equally inhibitory (IC₅₀ = 5 μM) to both cell lines. The growth inhibitory effects were reversed by deoxycytidine. Cellular [methyl-³H]dTTP pools increased linearly over 2 h during incubation of CEM or CEM tk- with 5 μM POM₂-[methyl- ³H]dTMP. The incorporation of [methyl-³H]TdR into HClO₄-insoluble cell residue by CEM tk- was 2-dTMP into acid insoluble residue at a rate 59% that of CEM. These results demonstrate that POM₂-dTMP can penetrate into cells and serve as a source of dTMP.