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 [¹ψ², 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. [⁴ψ⁵, CSNH; ⁷ψ⁸, CS–NH]CsA exhibits a conformation very similar to crystalline CsA. However, the D‐Ala⁸NH, MeLeu⁶Co γ‐turn H‐bond is not present in this dithio analogue. Also different is the MeBmt¹side‐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.