Synthesis and immunosuppressive activities of conformationally restricted cyclosporin lactam analogues.

Cyclosporin A (CsA, 1), an immunosuppressive cyclic undecapeptide, is known to adopt predominantly a single conformation in chloroform solution, characterized in part by a type II' beta-turn encompassing Abu-Sar-MeLeu-Val (residues 2-5). In order to evaluate whether this beta-turn is bound by the receptor, we previously had prepared a conformationally restricted beta-turn analogue, (delta-lactam) CsA (2), which was found to retain only weak immunosuppressive activity, a result that could indicate that steric hindrance between receptor and the lactam atoms in 2 diminished activity or that the type II' beta-turn is not a feature in the bioactive conformation of CsA. In an attempt to distinguish between these two possibilities, we have synthesized two new CsA analogues, (gamma-lactam) CsA (3) and (des-N-methyl-lactam) CsA (4), which contain less sterically demanding conformational restrictions. The immunosupressive activity of each analogue (4-13% and 7-17%, respectively, relative to CsA), measured in an assay that determined the inhibition of concanavalin A stimulated thymocytes, is essentially equipotent with that of the delta-lactam. The chemical shifts and temperature dependencies of the protons in analogues 3 and 4 are very similar to the corresponding protons in CsA and in 2, which suggest that the solution conformations of the small lactam analogues are very similar to that of the delta-lactam 2. The synthesis of the lactam components and the corresponding CsA derivatives is described. Reduction in the size of the lactam ring does not lead to enhanced immunosuppressive activity.