Development of selective inhibitors of transglutaminase. Phenylthiourea derivatives

For the purpose of developing a transglutaminase inhibitor which could be effective in physiological and pharmacological studies, a series of phenylthiourea derivatives of alpha, omega-diaminoalkanes were designed, synthesized, and evaluated kinetically as inhibitors of transglutaminases. A homologous series of compounds of the structure phenylthiourea-(CH2)n-NH2, where n = 2, 3, 4, 5, and 6, were tested for the inhibition of both guinea pig liver transglutaminase-catalyzed amine incorporation into various glutamine-containing substrates and plasma transglutaminase (factor XIIIa)-catalyzed amine incorporation into fibrin and fibrin cross-linking. It was found that the inhibitory activity of the compounds increases with increasing number of methylene groups in the side chain up to a maximum of n = 5. A further increase in the length of the methylene side chain to n = 6 results in decreased activity. The Ki value (4.9 X 10(-5) M) of 1-(5-aminopentyl)-3-phenylthiourea (PPTU) (n = 5) for the inhibition of guinea pig transglutaminase-catalyzed amine incorporation into the B chain of oxidized insulin is in close agreement to its Km(app) value (7.1 X 10(-5) M) obtained using 14C-labeled PPTU. PPTU was also found to be a potent inhibitor of plasma transglutaminase-catalyzed fibrin cross-linking. The finding that the specificity of the alkylamines for inhibition is correlated with the length of their methyl side chains is compatible with those reported for aliphatic amines and monodansylcadaverine analogues (where dansyl is 5-dimethylaminonaphthalene-1-sulfonyl). The phenylthiourea derivatives, however, are far less toxic in mice than monodansylcadaverine as indicated by their LD50 values: PPTU, 400 +/- 25 mg/kg; and monodansylcadaverine, 160 +/- 20 mg/kg.