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Immunoscience Research, Pharmaceutical Products Division, Abbott
Laboratories
An in vitro glucuronidation assay was used to optimize
a series of N-hydroxyurea-containing 5-lipoxygenase
inhibitors for metabolic stability. The glucuronidation of these
compounds in cynomolgus monkey microsomes followed Michaelis-Menten
kinetics allowing calculation of Vmax and
KM. The Vmax values
ranged from 0.02 to 7.9 nmol/min/mg microsomal protein, a 400-fold
difference, whereas KM ranged from 204 to 2500 µM, only a 12-fold difference. In vitro intrinsic
clearance values (CLint) were calculated for 18 compounds tested in the kinetic assay and compared with the in
vivo plasma clearance (CLp) calculated
from intravenous studies done in cynomolgus monkeys. These initial
results suggested a relationship between the in vitro
CLint and in vivo duration as defined by
CLp. A more rapid in vitro assay
was developed in a 96-well format using a single concentration of
substrate (100 µM) from which a glucuronidation rate was calculated.
The results from this assay for 40 compounds correlated with in
vivo plasma clearance (r = 0.57). This more
efficient assay was used to test more than 100 compounds and develop
structure-metabolism relationships based on metabolic stability and
improved duration. The culmination of this effort contributed to the
discovery of ABT-761, a 5-lipoxygenase inhibitor with in
vivo duration in monkey improved 40-fold over the first
generation inhibitor. Further studies performed in human liver
microsomes demonstrated a similar trend that was corroborated by the
8-fold increase in duration after oral dosing in humans observed with
ABT-761.
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