Domain-structured N-1,N-2-derivatized hydrazines as inhibitors of ribonucleoside diphosphate reductase: Redox-cycling considerations

Eight analogues of 1-[5-halogenosalicylidene]-2-[2'-pyridinoyl]hydrazine an d -[2'-pyridyl]hydrazine, four of 1-[pyridoxylidene]-2-[2'-pyridinoyl]hydra zine, seven of 1-[pyridoxylidene]2-[2'-pyridyl]hydrazine, and one each of 1 ,2-bis[pyridoxylidene]diaminoethane and bis[pyridoxylidenehydrazino]phthala zine were synthesized. Their solutions in DMF were assayed for activity aga inst the metalloenzyme ribonucleoside diphosphate reductase (RdR), prepared from a subcutaneously growing murine tumor (sarcoma 180) implanted in B6D2 F3 male mice. The C-14-labeled CDP reductase was assayed by the modified me thod of Takeda and Weber, in which [C-14]cytidine was separated from deoxyc ytidine by thin-layer chromatography (TLC) on cellulose foil. Distribution of radioactivity was assessed with an automatic TLC linear analyzer. Of the 31 compounds tested, 13 were essentially inactive, 7 were highly active ag ainst RdR, and the remaining 20 were slightly more active than hydroxyurea (used as a reference compound). The mechanism of inhibition is discussed in terms of three alternative pathways, initiated by sequestration of iron em bedded in the R1 subunit of the metalloenzyme to form a C-centered chelate radical (via redox cycling). Alternatively, the latter could either reduce the tyrosyl radical or intercept radicals generated in the reduction proces s.