Synthesis and antimitogenic activities of four analogues of cyclosporin A modified in the 1-position.

Cyclosporin A (CSA, 1), an immunosuppressive cyclic undecapeptide, contains a unique N-methylated amino acid, (2S,3R,4R,6E)-3-hydroxy-4-methyl-2-(N-methylamino)-6-octenoic++ + acid, called both C-9-ene and MeBmt [(4R)-N-methyl-4-butenyl-4-methylthreonine] that may be essential for the biological activity of CSA. In order to determine the minimal portion of MeBmt needed for antimitogenic activity, four analogues of CSA specifically modified in the 1-position have been synthesized. These are (MeThr1)CSA (4), (MeAbu1)CSA (5), (MeAbu1,Sar10)CSA (6), and [(MeLeu(3-OH)1)]CSA (7). The synthesis of analogues was carried out by forming a linear undecapeptide that was cyclized at the two non-N-methylated amino acids. The structure of cyclic analogues 4-7 and their corresponding precursors were established unequivocally by 1H NMR, FAB mass spectrometry, elemental analysis, and HPLC. The inhibition of Con A stimulated thymocytes by CSA (1), DH-CSA (2), 7, 4, 5, and 6 gave IC50's (nM) of 4, 10, 600, 8 X 10(3), 15 X 10(3), and 40 X 10(3), respectively. The increase in IC50 by modification of the side chain in MeBmt suggested the importance of this amino acid in the 1-position of CSA for full antimitogenic activity.