Bidentate peptides: Highly potent new inhibitors of enkephalin degrading enzymes

Three series of bidentates bearing an hydroxamic or an N-Acyl-N-hydroxy amino group on structures related to Phe-Gly or Phe-Ala exhibit strong inhibitory potency against purified enkephalinase with IC 50 values in the 4 to 15 nM range. As with thiol-containing inhibitors, such as thiorphan, the most active compounds are those in which a methylene spacer separates the benzyl P 1′ moiety from the Zn coordinating residue. Formation of a bidentate complex with the metal enzyme is clearly demonstrated by a loss of potency of three order of magnitude following the removal of one component of the bidentate group. All the compounds studied are unable to interact with angiotensin converting enzyme (IC 50 > 10,000 nM). Moreover, compounds of the general formula HONHCOCH 2CH(CH 2φ)CONHCH(R)COOH belonging to the most active series of enkephalinase blockers (IC 50 ∼ 4 nM) behave also as highly potent and competitive inhibitors (IC 50 ∼ 10 nM) of a Tyr-Gly releasing dipeptidylaminopeptidase purified from rat brain. The pure stereoisomer [(R)-3-(N-hydroxy) carboxamido-2-benzylpropanoyl]-L-alanine designated kelatorphan, exhibits also a relatively good inhibitory potency against aminopeptidases (IC 50 ∼ 10 μM)f and can be considered as the first virtually complete inhibitor of enkephalin metabolism. This very interesting property of inhibiting all three enzymes of enkephalin metabolism could enhance the required selectivity for a possible clinical use of these inhibitors as new analgesic and psychoactive drugs.