THE ANTIPROLIFERATIVE AND CELL-CYCLE EFFECTS OF 5,6,7,8-TETRAHYDRO-N-5,N-10-CARBONYLFOLIC ACID, AN INHIBITOR OF METHYLENETETRAHYDROFOLATE DEHYDROGENASE, ARE POTENTIATED BY HYPOXANTHINE

5,6,7,8-Tetrahydro-N-5,N-10-carbonylfolic acid (LY354899) has been dem onstrated to inhibit the dehydrogenase activity of C1-tetrahydrofolate synthase. This compound was only moderately antiproliferative toward CCRF-CEM lymphocytic leukemia cells in culture, but induced apoptosis after long incubation times. Slightly greater potency was observed in CEM cells adapted to grow in low folate media, Cell cycle alterations induced by LY354899 were unique relative to antifolates that inhibit e ither the purine or thymidine de novo biosynthetic pathways. Based on the observed changes in DNA content, we hypothesized that inhibition o f the dehydrogenase resulted in two temporally distinct events: the fi rst was a purineless-like effect and the second was a thymineless-like effect that resulted in apoptosis. To test this hypothesis, we combin ed LY354899 with the purine salvage metabolite, hypoxanthine. This com bination resulted in an earlier and more dramatic apoptotic response, indicating that the thymineless effect had been potentiated. Biochemic al analysis of ribo- and deoxyribonucleoside triphosphates confirmed t hat inhibition of the dehydrogenase activity initially resulted in dec reased pools of deoxypurines and deoxypyrimidines, followed 16 hr late r by an increase in deoxyadenosine triphosphate (dATP) and a further d ecrease in deoxythymidine triphosphate (dTTP). These studies demonstra te that the inhibition of the dehydrogenase activity of C1-tetrahydrof olate synthase may represent a viable target for the development of no vel antifolates. The results are discussed in terms of deoxypurine and deoxypyrimidine biosynthesis.