Dipyridamole potentiates the in vitro activity of MTA (LY231514) by inhibition of thymidine transport

The novel pyrrolopyrimidine-based antifolate LY231514 (MTA), inhibits multi ple folate-requiring enzymes including thymidylate synthase, glycinamide ri bonucleotide formyltransferase and dihydrofolate reductase. Both thymidine and hypoxanthine are required to reverse MTA growth inhibition in leukaemia and colon cancer cells. Prevention of MTA growth inhibition by thymidine a nd/or hypoxanthine was investigated in two human lung (A549, COR L23) and t wo breast (MCF7, T47D) tumour cell lines, and the effect of the nucleoside/ base transport inhibitor dipyridamole (DP) on thymidine and hypoxanthine re scue defined. MTA IC50 values (continuous exposure three population doublin gs) were: A549-640 nM, COR L23-28 nM, MCF7-52 nM and T47D-46 nM. Thymidine (1 mu M) completely prevented growth inhibition at the MTA IC50 in all cell lines. At 10 x IC50, growth inhibition was only partially reversed by thym idine (< 10 mu M); both thymidine and hypoxanthine (30 mu M) being required for complete reversal. reflecting the multi-targeted nature of MTA. Growth inhibition by MTA was not affected by hypoxanthine alone. A non-toxic conc entration (1 mu M) of DP prevented thymidine/hypoxanthine rescue of MTA ind icating that. DP may potentiate MTA activity by preventing nucleoside and/o r base salvage. Thymidine transport was inhibited by greater than or equal to 89% by 1 mu M DP in all cell lines, whereas hypoxanthine transport was i nhibited only in A549 and MCF7 cells. Therefore, prevention of end-product reversal of MTA-induced growth inhibition by DP can be explained by inhibit ion of thymidine transport alone. (C) 2000 Cancer Research Campaign.