Isocyclosporin A

The kinetics and mechanism of the degradation of cyclosporin A have been studied under aqueous acidic conditions. The rate of degradation was found to be specific acid-catalyzed over the pH range studied (1-4), with isocyclosporin A as the predominant degradation product. Selective reduction of the olefinic bond of the amino acid 2-N-methyl-(R)-((E)-2-butenyl)-4-methyl-L-threonine (MeBmt) did not affect the overall degradation kinetics and product distribution of cyclosporin A. These observations indicate that the alternative degradation pathway involving intramolecular alkoxy addition to the olefinic bond of amino acid MeBmt apparently does not significantly contribute to the overall degradation kinetics of cyclosporin A in the pH range 1-4. The chemical reactivity of O-acetyl-cyclosporin A was examined to probe the governing mechanism for the isomerization of cyclosporin A. Under identical conditions, O-acetyl-cyclosporin A showed a much greater chemical stability than cyclosporin A, consistent with a mechanism involving the hydroxyoxazolidine intermediate. The chemical stability of cyclosporin C, which contains two beta-hydroxyl groups, was also examined. The rate and product distribution for the degradation of cyclosporin C suggest that under aqueous acidic conditions it undergoes N,O-acyl migration solely at the amino acid residue MeBmt. Additionally, the impact of side-chain bulkiness of amino acid MeBmt was examined by studying the degradation kinetics of a series of cyclosporin A analogs.(ABSTRACT TRUNCATED AT 250 WORDS)

A direct assay method for use in studies of cyclosporin binding must be highly sensitive and selective since it must be capable of measuring the concentrations encountered in the protein-free matrix. The failure of current HPLC methods to achieve the sensitivity required for binding studies may be attributed to the use of UV detection, which relies on the relatively weak end-absorption of cyclosporin A. A method involving fluorescence derivatization was sought with the aim of increasing HPLC assay sensitivity. A method is described for producing a fluorescent derivative of cyclosporin A, a compound which has no functional groups which are easily derivatized. However, intramolecular rearrangement of cyclosporin A to form its structural isomer, isocyclosporin A, exposes a secondary amine which can be reacted with dansyl chloride to produce a fluorescent derivative. This two-step derivatization procedure was used as the basis of an HPLC fluorescence assay. Although this assay is not sufficiently sensitive to measure concentrations encountered in the protein-free matrix during plasma binding studies, the method does point to the possible development of a more sensitive assay using a derivatizing reagent other than dansyl chloride.

Cyclosporins are an invaluable class of drug used to prevent the rejection of transplanted tissue. While the most popular drug in this group is cyclosporin A, several other analogues are available, including some enantiomeric and structurally isomeric forms. Unfortunately, the presence of such isomers can make the detection and identification of these drugs by mass spectrometry (MS) alone quite challenging. Here, we demonstrate the separation and analysis of six cyclosporin analogues using liquid chromatography (LC) and differential mobility spectroscopy (DMS) coupled to MS. Using DMS, we demonstrate the separation of three isomers: CycA and CycH (cyclosporin H), which are enantiomers, and isocyclosporin A (a structural isomer of CycA and CycH). For several of the cyclosporins, we can separate different conformers for each isomeric form. After DMS separation, tandem mass spectrometry (MS/MS) analyses of the separated isomers also distinguish these isomeric forms of cyclosporin. In addition, we have probed differences between each isomer by using gas-phase hydrogen-deuterium exchange (HDX) immediately after DMS separation, which reveals differences in the levels of intramolecular hydrogen bonding between each of the cyclosporins.

We have previously reported that treatment of CsA with aqueous HCI gives rise to the formation of a number of water-soluble compounds. Two of these were identified from their FAB-MS/MS spectra as open-chain nona- and decapeptides. We describe here the identification of two other main compounds deriving from the same treatment. Identification was rendered possible from the comparison of their FAB-MS/MS spectra with those of methyl and acetyl derivatives. The two compounds are water-soluble, open-chain undecapeptides corresponding to 1.11 seco-CsA and of 4.5 seco-isoCsA, respectively.