摘要：

α-(N)-Heterocyclic carboxaldehyde thiosemicarbazones have been shown to be potent inhibitors of the biosynthesis of DNA in mammalian cells. Studies with transplanted murine neoplasms have demonstrated that the enzymatic site responsible for the blockade of DNA replication is at the level of ribonucleoside diphosphate reductase, an enzyme of critical importance for the generation of the deoxynucleoside triphosphate precursors of these macromolecules. Kinetic studies were carried out with a partially purified enzyme from a rat tumor in an effort to define the molecular mechanism by which agents of this class inhibit the activity of ribonucleoside diphosphate reductase. The concentration of the nucleoside diphosphate substrate, the allosteric activator ATP, or magnesium ion did not influence the inhibition by a representative derivative of this type of agent IQ-1. Dithiothreitol, a model dithiol used in place of the natural substrate thioredoxin, was partially competitive with the inhibitor. The results appear to be most compatible with a model in which the preformed metal chelate of IQ-1 interacts with the enzyme at or near the site occupied by the dithiol substrate. To ascertain some of the structural features required for inhibition of ribonucleoside diphosphate reductase activity, the inhibitory potency of a series of methyl and benzo derivatives of PT was measured. The results suggested that position 6 of PT and position 3 of IQ-1 are equivalent with respect to orientation of the inhibitor at the enzymatic binding site and that little or no tolerance exists for structural modification at this position. In a similar manner, substitution of the terminal amino group of the thiosemicarbazone side chain decreased enzyme inhibition, supporting the presence of a low bulk tolerance zone in this position. Further studies with these substituted derivatives were interpreted to be suggestive of the existence of a hydrophobic bonding zone adjacent to the inhibitor-binding site of the enzyme. These results were employed to design MAIQ-1, a derivative of this class with strong inhibitory potency for ribonucleoside diphosphate reductase, as well as the other necessary requisite properties for clinical trial as an antineoplastic agent.

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