Mass spectrometric analysis of cyclosporine metabolites

Cyclosporine (CsA, mol. wt 1202) is a cyclic undecapeptide with potent immunosuppressive properties which have made it extremely useful for preventing the rejection of transplanted organs. The drug has severe side-effects, however, which may be related to its metabolism. Sensitive methods to determine the structure of metabolites of CsA have, therefore, been developed using mass spectrometric techniques. First, the molecular weight of the metabolite is determined using fast atom bombardment mass spectrometry or thermospray liquid chromatography/mass spectrometry. The metabolite is then hydrolysed to its component amino acids, which are esterified and acylated and identified by gas chromatography/mass spectrometry. To distinguish metabolism at the four identical N-methyl leucines, the metabolite is partially hydrolysed, the resulting peptides are derivatized to the trimethylsilyl-polyamino alcohols, and these in turn are analysed by gas chromatography/mass spectrometry. These procedures have been used to determine the structure of metabolites of CsA isolated from rabbit bile. The determination of the structure of one metabolite carboxylated on the eta-carbon of amino acid 1, and of one metabolite hydroxylated on the eta-carbon of amino acid 1 and on the gamma-carbon of N-methyl leucine 9 is presented. These procedures should be generally useful for the structural analysis of microgram amounts of CsA metabolites and analogs.