INHIBITORS OF RIBONUCLEOTIDE REDUCTASE - COMPARATIVE EFFECTS OF AMINO-SUBSTITUTED AND HYDROXY-SUBSTITUTED PYRIDINE-2-CARBOXALDEHYDE THIOSEMICARBAZONES

A new series of alpha-(N)-heterocylic carboxaldehyde thiosemicarbazones (HCTs) was studied for their effects on L1210 cell growth in culture, cell cycle transit, nucleic acid biosynthesis and ribonucleotide reductase activity. 3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) and 3-amino-4-methylpyridine -2-carboxaldehyde thiosemicarbazone (3-AMP) were the most active compounds tested with respect to inhibition of cell growth and ribonucleotide reductase activity. 5-Aminopyridine-2-carboxaldehyde thiosemicarbazone (5-AP) and 4-methyl-5-aminopyridine-2-carboxaldehyde thiosemicarbazone (5-AMP) were sightly less active. 3-AP, 3-AMP, 5-AP and 5-AMP inhibited the incorporation of [H-3]thymidine into DNA without affecting the rate of incorporation of [H-3]uridine into RNA. The uptake and incorporation of [C-14]cytidine into cellular ribonucleotides and RNA, respectively, were not decreased by 3-AP or 3-AMP; however, the incorporation of cytidine into DNA via ribonucleotide reductase was inhibited markedly. Thus, a pronounced decrease in the formation of [C-14]deoxyribonucleotides from radioactive cytidine occurred in the acid-soluble fraction of 3-AP- and 3-AMP-treated L1210 cells. Consistent with an inhibition of DNA replication that occurred at relatively low concentrations of 3-AP and 3-AMP, cells gradually accumulated in the S-phase of the cell cycle; at higher concentrations of 3-AP and 3-AMP, a more rapid accumulation of cells in the G(0)/G(1), phase of the cell cycle occurred, with the loss of the 8-phase population, implying that a second less sensitive metabolic lesion was created by the HCTs. N-Acetylation of 3-AMP resulted in a compound that was 10-fold less active as an inhibitor of ribonucleotide reductase activity and 8-fold less active as an inhibitor of L1210 cell growth. N-Acetylation of either 5-AP or 5-AMP did not alter the inhibitory properties of these compounds. The results obtained provide an experimental rationale for the further development of the HCTs, particularly 3-AP and 3-AMP, as potential drugs for clinical use in the treatment of cancer.