Synthesis and evaluation of Glyψ(PO2R-N)pro-containing pseudopeptides as novel inhibitors of the human cyclophilin hCyp-18

The human cyclophilin hCyp-18, an abundant peptidyl-prolyl cis-trans isomerase (PPIase) implicated in protein folding, controls the infection of CD4(+) T-cells by HIV-1, the pathologic agent of AIDS. Therefore, hCyp-18 is an interesting target for the development of novel anti-HIV-1 therapeutics. We focused on the design of transition-state analogue inhibitors of the PPIase activity of cyclophilin. Most experimental results reported in the literature suggest that hCyp-18 catalyzes the pyramidalization of the nitrogen of pyrrolidine via an H-bond network which results in the deconjugation of the amino acyl-prolyl peptide bond. We proposed the Glypsi(PO2R1-N)Pro motif (R = alkyl or H) as a selective transition-state analogue inhibitor of cyclophilin. This motif was inserted in Suc-Ala-Ala-Pro-Phe-pNA, a peptide substrate of hCyp18. The pseudopeptide Suc-Ala-Glypsi(PO2Et-N)Pro-Phe-pNA 1b bound to hCyp-18 (K-d = 20 +/- 5 muM) and was able to selectively inhibit its PPIase activity(IC50 =15 +/- muM) but not hFKBP-12, another important PPIase. Deprotection of the phosphonamidate moiety resulted in a complete lack of inhibition. We previously demonstrated that reduction of the Phe-pNA moiety caused a quantitative reduction of the affinity; however, Suc-Ala-Glypsi(PO2Et-N)Pro-Phepsi(CH2-NH)pNA 7b still bound and inhibited hCyp-18, suggesting that the Glypsi(PO2Et-N)Pro motif plays the major role in the binding to cyclophilin. Consequently, we propose compound 1b as being a novel transition-state mimic inhibitor of hCyp-18.