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Vol. 29, Issue 4, 407-414, April 2001
Institute of Pharmaceutical Chemistry (B.H., A.S., W.J.) and
Department of Pathophysiology, (T.T.), University of Vienna, Vienna,
Austria; Institute of Pharmaceutical Chemistry, University of Graz,
Graz, Austria (O.K., E.H.); Department of Surgery, University-Hospital
of Innsbruck, Innsbruck, Austria (P.K.); and National Institutes of
Health (A.M.S.) and National Cancer Institute (E.A.S.), Bethesda,
Maryland
The metabolism of flavopiridol (FLAP), a novel anticancer drug
currently undergoing clinical development, was investigated in rat and
human liver microsomes. In the presence of uridine 5'-diphosphoglucuronic acid, two biotransformation products (M1 and M2) could be detected. Formation of metabolite M1 and M2 in terms
of enzymatic efficacy
(Vmax/KM) was
about 50- and 5-fold higher in rat (1.58 ± 2.23 and 7.22 ± 1.17 µl/min/mg) as compared with human liver microsomes (0.032 ± 0.016 and 1.52 ± 0.93 µl/min/mg), indicating species-related
differences in FLAP glucuronidation. Incubation in the presence of
human recombinant UDP-glucuronosyltransferases (UGTs) demonstrated that
M1 is almost exclusively catalyzed by UGT1A1, whereas M2 is formed by
UGT1A9 and only to a minor extent by UGT1A1 and UGT1A10. Chemical
inhibition experiments further prove the involvement of UGT1A1 and
UGT1A9 in the formation of M1 and M2, as the UGT1A1 substrate bilirubin
preferably inhibited M1 over M2 (Ki: 36 and
258 µM, respectively), whereas the UGT1A9 substrate propofol showed a
more pronounced decrease in M2 but not in M1 formation
(Ki: 47 and 142 µM, respectively). Both
conjugates were purified from rat liver microsomes and analyzed by mass
spectrometry, NMR, and UV experiments. On the basis of these results,
M1 was identified as
5-O-
-glucopyranuronosyl-flavopiridol and M2 as 7-O-
-glucopyranuronosyl-flavopiridol. In conclusion,
our results elucidate the enzymatic pathways of FLAP in rat and human
liver, which must be considered during cancer therapy of patients.
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