Sanglifehrin−Cyclophilin Interaction: Degradation Work, Synthetic Macrocyclic Analogues, X-ray Crystal Structure, and Binding Data

Sanglifehrin A (SFA) is a novel immunosuppressive natural product isolated from Streptomyces
sp. A92-308110. SFA has a very strong affinity for cyclophilin A (IC50 = 6.9 ± 0.9 nM) but is structurally
different from cyclosporin A (CsA) and exerts its immunosuppressive activity via a novel mechanism. SFA
has a complex molecular structure consisting of a 22-membered macrocycle, bearing in position 23 a nine-carbon tether terminated by a highly substituted spirobicyclic moiety. Selective oxidative cleavage of the
C26C27 exocyclic double bond affords the spirolactam containing fragment 1 and macrolide 2. The affinity
of 2 for cyclophilin (IC50 = 29 ± 2.1 nM) is essentially identical to SFA, which indicates that the interaction
between SFA and cyclophilin A is mediated exclusively by the macrocyclic portion of the molecule. This
observation was confirmed by the X-ray crystal structure resolved at 2.1 Å of cyclophilin A complexed to
macrolide 16, a close analogue of 2. The X-ray crystal structure showed that macrolide 16 binds to the
same deep hydrophobic pocket of cyclophilin A as CsA. Additional valuable details of the structure−activity
relationship were obtained by two different chemical approaches:  (1) degradation work on macrolide 2 or
(2) synthesis of a library of macrolide analogues using the ring-closing metathesis reaction as the key
step. Altogether, it appears that the complex macrocyclic fragment of SFA is a highly optimized combination
of multiple functionalities including an (E,E)-diene, a short polypropionate fragment, and an unusual tripeptide
unit, which together provide an extremely strong affinity for cyclophilin A.