THIOCYCLOSPORINS - PREPARATION, SOLUTION AND CRYSTAL-STRUCTURE, AND IMMUNOSUPPRESSIVE ACTIVITY

The reaction of cyclosporin A (CsA) with Lawesson's reagent under different conditions yields various thiocyclosporins, in which carbonyl O-atoms and/or the hydroxy O-atom of the MeBmt residue are replaced by an S-atom. The position of the S-atom is determined by NMR spectroscopy, and the conformations of the products are studied by NMR spectroscopy and X-ray crystallography. Some of the thiocyclosporins show interesting conformational properties. Whereas one conformation strongly dominates for CsA in CDCl3, two conformers A and B, in a ratio 58:42 are found for [1-psi-2,CS-NH]CsA. Extensive NMR studies including new 2D and 3D heteronuclear techniques and restrained MD calculations using ROE effects demonstrate that the major conformer A is identical to CsA, while the minor conformer B contains an additional cis peptide bond between the Sar3 and MeLeu4 residues. [4-psi-5,CS-NH; 7-psi-8,CS-NH]CsA exhibits a conformation very similar to crystalline CsA. However, the D-Ala8NH, MeLeu6CO-gamma-turn H-bond is not present in this dithio analogue. Also different is the MeBmt1 side-chain conformation, the dithio conformation showing a strong MeBmt1OH, Sar3CO H-bond. Immunosuppressive activities of thiocyclosporins are measured in IL-2 and IL-8 reporter gene assays. Their activities are discussed in relation to their conformations.