Isolation, identification and immunosuppressive activity of a new IMM-125 metabolite from human liver microsomes. Identification of its cyclophilin A-IMM-125 metabolite complex by nanospray tandem mass spectrometry

Liquid chromatography-mass spectrometry in in vitro drug metabolite screening

A combination of high performance liquid chromatography (HPLC) and mass spectrometry (LC/MS) has proven its status as the most powerful analytical tool for screening and identifying drug metabolites in modern drug discovery. These techniques have become irreplaceable for drug metabolism laboratories, providing high amounts of information from a wide variety of samples. This review focuses on the most common and useful applications of these techniques when working on in vitro metabolism, more specifically with screening and identification of chemically stable or reactive metabolites formed via biotransformation reactions. Matching specific tasks and suitable instruments is a recurring consideration; for many reasons, the time-of-flight or orbitrap mass spectrometry provides clearly increased efficiency in metabolite profiling compared to other types of mass spectrometry.

Spontaneous N --> O acyl shift in the [M + H](+) ions of [MeBmt1]-cyclosporins in an ion trap

In an ion trap the protonated molecules of the cyclic undecapeptides cyclosporins having 3-hydroxy-4-methyl-2-methylamino-6-octenoic acid (MeBmt) in their backbone undergo an N --> O peptidyl shift into the corresponding [M + H](+) ions of isocyclosporins. This rearrangement does not take place in cyclosporins [Bmt1]Cs and [3'-deoxy-MeBmt1]Cs. In cyclosporin [Thr2]Cs having two threonines in the molecule, only one of them participates in the N,O-acyl transfer. It can be concluded that the presence of the basic N-methylamino group of MeBmt, which can serve as the primary site of protonation, is necessary for isocyclosporin formation. A dominating ion series originating from the primary cleavage between MeBmt (first position in the cyclosporin ring) and amino acid residue at the neighbouring eleventh position is then observed in collision-induced dissociation spectra of protonated molecules of cyclosporins. This 'isocyclosporin' ion series can effectively be used for easy and complete cyclosporin sequencing using a tandem mass spectrometric (MS3) experiment in an ion trap. The paper further introduces an improved Gross mass spectral nomenclature for cyclic peptide sequencing and several techniques for the generation of protonated molecules of cyclosporins. Their preparation represents the fundamental requirement for smooth sequencing of cyclosporins by tandem mass spectral techniques.