Because of their low amounts, for several human metabolites of
vinorelbine chemical structures were proposed only on the basis
of LC–MS/MS measurements [50]. The complementary use of triple
quadrupole and TOF mass spectrometers enabled the identification
and structural investigation of the metabolites. Beyond the specific
fragments m/z 457 and 323, the key fragment of vinorelbine was m/z
122, which can be regarded as an analogue of fragment D observed
in the EI-induced fragmentation of VLB (Fig. 6). A desacetyl derivative
was identified by a 42 Da mass shift as compared to the parent
drug. One of the metabolites showed a molecular ion peak that was
14 Da less as compared to vinorelbine, and this alteration could be
located onto the vindoline part of the molecule. However, on the
basis of the MS/MS data it could not be determined whether it was
the N-demethylated or the aromatic O-demethylated metabolite of
vinorelbine. In the case of the hydroxylated metabolites the oxygen
atoms could be located either to the velbanamine or the vindoline
part of the parent drug. By the specific fragment m/z 122 or its
analogue of m/z 138 containing an oxygen atom, the hydroxylation
could be located to the indole aromatic part or the aliphatic part of
the velbanamine moiety. It should be noted that in this study some
of the low-resolution MS/MS spectra were of low quality, especially
for the minor metabolites, which imposes some uncertainties onto
the structural proposals. Nevertheless, this work appears to be the
single extensive structural elucidation study of unknown bisindole
derivatives based only on LC–MS/MS measurements.