PR-104, the
phosphate ester of a dinitrobenzamide mustard [PR-104A; 2-((2-bromoethyl)-2-{[(2-hydroxyethyl) amino] carbonyl}-4,6-dinitroanilino)
ethyl methanesulfonate], is currently in clinical trial as a
hypoxia- and
aldo-keto reductase 1C3 (AKR1C3)-activated
prodrug for
cancer therapy. Here, we investigate species (human, dog, rat, mouse) differences in metabolism to the corresponding O-
glucuronide, PR-104G, and identify the human
UDP-glucuronosyltransferase (UGT)
isoforms responsible. After intravenous
PR-104, plasma area under the concentration-time curve ratios (PR-104G/PR-104A) decreased in the order of dog (2.3) > human (1.3) > mouse (0.03) > rat (0.005). The kinetics of
uridine 5'-diphosphoglucuronic
acid-dependent glucuronidation by liver microsomes in vitro fitted the single-
enzyme Michaelis-Menten equation with similar K(m) (∼150 μM) but differing V(max) (472, 88, 37, and 14 nmol/h/mg for dog, human, rat, and mouse, respectively), suggesting that facile glucuronidation is responsible for the anomalously rapid clearance of
PR-104A in dogs. In vitro-in vivo extrapolation of
PR-104A glucuronidation kinetics is consistent with this also being a major clearance pathway in humans. Recombinant UGT screening identified UGT2B7 as the only commercially available human
isoform able to conjugate
PR-104A, and UGT2B7
protein concentrations were highly correlated (r = 0.93) with
PR-104A glucuronidation by liver microsomes from 24 individuals. The active
hydroxylamine metabolite of
PR-104A, PR-104H, was also glucuronidated by UGT2B7, although with slightly lower specificity and much lower rates. UGT2B7
mRNA expression was highly variable in human
tumor databases. Glucuronidation of
PR-104A greatly suppressed nitroreduction by AKR1C3 and
NADPH-supplemented anoxic human liver S9 (9000g postmitochondrial supernatant). In conclusion,
PR-104A is glucuronidated by UGT2B7 with high specificity and seems to make a major contribution to clearance of
PR-104A in humans, but it also has the potential to confer resistance in some human
tumors.