INTERCONVERSION OF CYCLOSPORINE MOLECULAR-FORM INDUCING PEAK BROADENING, TAILING AND SPLITTING DURING REVERSED-PHASE LIQUID-CHROMATOGRAPHY

Reversed-phase liquid chromatography of cyclosporin A, a peptide immun osuppressant, at various temperatures produced remarkably different ch romatograms. At 60-degrees-C one sharp peak was obtained, at 23-degree s-C this became a single broad peak and between 15-degrees and 0-degre es-C this became one high sharp peak followed by a tailing or low plat eau. Remarkably different chromatograms were produced also by varying the mobile phase flow-rate. The effects of both temperature and flow-r ate on the chromatogram could be accounted for by interconversion betw een two forms of the cyclosporin molecule. Kinetic analysis showed tha t one form was converted by first-order kinetics with a half-life of 2 .0 min at 20-degrees-C and that the apparent activation energy for the conversion was about 18 kcal/mol. The other two immunosuppressants, c yclosporin C and D, were also shown to undergo interconversion. This k inetic analysis of the interconversion should be helpful in clarifying the relationship between molecular structure, and activity of the imm unosuppressants.