Prevention of thymidine and hypoxanthine rescue from MTA (LY231514) growth inhibition by dipyridamole in human lung cancer cell lines.

The novel multitargeted antifolate, MTA (N-[4[2-(2-amino-3,4-dihydro-4-oxo-7H-pyrrolo[2,3-d]pyrimidin-5-yl)-ethy l]-benzoyl]-L-glutamic acid; LY23 1514) inhibits thymidylate synthase, dihydrofolate reductase, and glycinamide ribonucleotide formyltransferase. The resultant inhibition of the de novo thymidylate and purine biosynthesis can be circumvented by salvage of extracellular thymidine and hypoxanthine. The first step in the salvage pathway is the transport of nucleosides and bases across the cell membrane. Dipyridamole inhibits nucleoside transport and in vitro studies have demonstrated that dipyridamole can prevent thymidine salvage rescue from antifolate thymidylate synthase inhibitors. More recently, dipyridamole also has been shown to prevent hypoxanthine rescue from antipurine antifolates in some cell lines but not others. The effects of dipyridamole on MTA growth inhibition and end product reversal by thymidine and hypoxanthine was investigated in two lung cancer cell lines with (A549) and without (COR L23) dipyridamole-sensitive hypoxanthine rescue. The IC50 values for MTA-induced growth inhibition were 28 and 640 nmol/L for COR L23 and A549 cells, respectively. End product reversal studies show that thymidine can completely reverse growth inhibition by IC50 concentration of MTA but only partially rescue cells from 10 times the IC50 concentration of MTA. The combination of thymidine and hypoxanthine was required for complete reversal from MTA at 10 times the IC50 concentration. Dipyridamole blocked the partial rescue from MTA-induced growth inhibition by thymidine alone as well as the complete rescue by thymidine plus hypoxanthine not only in A549 cells, which have dipyridamole-sensitive hypoxanthine transport, but also in COR L23 cells, in which hypoxanthine uptake is insensitive to dipyridamole. These studies demonstrate that nucleoside and base salvage can compromise the activity of MTA in human tumor cell lines, but that dipyridamole can readily prevent salvage and restore growth inhibition.