SPECIFIC AND IRREVERSIBLE CYCLOPEPTIDE INHIBITORS OF DIPEPTIDYL PEPTIDASE-IV ACTIVITY OF THE T-CELL ACTIVATION ANTIGEN CD26

The dipeptidyl peptidase N (DPP IV) activity of CD26 is characterized by its post-proline-cleaving capacity that plays an important but not yet understood role in biological processes. Here we describe a new fa mily of specific and irreversible inhibitors of this enzyme. Taking in to account the substrate specificity of DPP IV for P-2-P-1> <-P-1' cle avage, we have designed and synthesized cyclopeptides c[((H2N+)-H-a)-L ys-Pro-Aba-(6-CH2-S+R2)-Gly(n)] 2TFA(-) (Aba = 3-aminobenzoic acid, R = alkyl) possessing a proline at the P-1 position and a lysine in the Pt position, which allows the closing of the cycle on its side chain. These molecules show a free N-terminus, necessary for binding to the C D26 catalytic site, and a latent quinoniminium methide electrophile, r esponsible for inactivation. Treatment of c[(a)Z-Lys-Pro-Aba-(6-CH2-OC 6H5)-Gly(n)], obtained by peptide synthesis in solution, with R2S/TFA simutaneously cleaved the Z protecting group and the phenyl ether func tion and led to a series of cyclopeptide sulfonium salts. These cyclop eptides inhibited rapidly and irreversibly the DPP IV activity of CD26 , with IC50 values in the nanomolar range. Further studies were carrie d out to investigate the effect of the modification of the ring size ( n = 2 or 4) and the nature of the sulfur substituents (R = Me, Bu, Oct ). Cycle enlargement improved the inhibitory activity of the methylsul fonio cyclopeptide, whereas the increase of the alkyl chain length on the sulfur atom had no apparent effect. Other aminopeptidases were not inhibited, and a much weaker activity was observed on a novel isoform of DPP IV referred to as DPP IV-beta. Thus, this new family of irreve rsible inhibitors of DPP IV is highly specific to the peptidase activi ty of CD26.