Endogenous generation of hydralazine from labile hydralazine hydrazones

The hypothesis that the pharmacologically active hydralazine hydrazones (HH) are endogenously hydrolyzed to parent hydralazine (H) was tested in a series of in vitro and in vivo systems. The stable hydrazones H alpha-ketoglutaric acid hydrazone and H pyruvic acid hydrazone did not hydrolyze to H in vitro (buffer or plasma), were inactive in vivo and did not generate urinary metabolites of parent H. By contrast, the labile HH, H acetaldehyde hydrazone and acetone hydrazone (HAH) generated H in vitro. H acetaldehyde hydrazone produced in vitro effects that were equipotent to the H concentration measured in the dose solutions. When administered to conscious rats and rabbits, the labile hydrazones reduced blood pressure. This effect was more gradual in onset than that of H. The hypotensive effects of HH were significantly greater than predicted by the amount of H contained in the dose solutions. Metabolic studies were conducted with the labile HH, HAH. After administration of HAH to rabbits, the proportional excretion of the urinary H metabolite, H pyruric acid hydrazone, was equal to that observed after the administration of H. We conclude that HH are inactive, except when hydrolyzed to H. The hydrolysis of certain HH, including HAH and H acetaldehyde hydrazone, in vivo may be nearly complete. Differences in the pharmacodynamic properties between labile HH and H may be related to the time course of generation of H, sequestration of hydrolysis in physiologically inactive sites or other unrecognized mechanisms.