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Vol. 297, Issue 2, 780-789, May 2001
Department of Medicine, University of Queensland, Princess
Alexandra Hospital, Woolloongabba, Queensland, Australia (D.Y.H., P.C.,
M.S.R.); and Section of Pharmacokinetics, Department of Pharmacology,
Martin Luther University Halle-Wittenberg, Halle, Germany (M.W.)
This work studied the structure-hepatic disposition relationships for
cationic drugs of varying lipophilicity using a single-pass, in situ
rat liver preparation. The lipophilicity among the cationic drugs
studied in this work is in the following order: diltiazem > propranolol > labetalol > prazosin > antipyrine > atenolol. Parameters characterizing the hepatic distribution and
elimination kinetics of the drugs were estimated using the multiple
indicator dilution method. The kinetic model used to describe drug
transport (the "two-phase stochastic model") integrated cytoplasmic
binding kinetics and belongs to the class of barrier-limited and
space-distributed liver models. Hepatic extraction ratio
(E) (0.30-0.92) increased with lipophilicity. The
intracellular binding rate constant (kon) and the equilibrium amount ratios characterizing the slowly and rapidly
equilibrating binding sites (KS and
KR) increase with the lipophilicity of drug
(kon: 0.05-0.35 s
1;
KS: 0.61-16.67;
KR: 0.36-0.95), whereas the
intracellular unbinding rate constant (koff)
decreases with the lipophilicity of drug (0.081-0.021
s
1). The partition ratio of influx
(kin) and efflux rate constant (kout),
kin/kout,
increases with increasing pKa value of the
drug [from 1.72 for antipyrine (pKa = 1.45) to 9.76 for propranolol (pKa = 9.45)], the differences in kin/kout for the
different drugs mainly arising from ion trapping in the mitochondria
and lysosomes. The value of intrinsic elimination clearance
(CLint), permeation clearance (CLpT), and
permeability-surface area product (PS) all increase with
the lipophilicity of drug [CLint (ml · min
1 · g
1 of liver): 10.08-67.41;
CLpT (ml · min
1 · g
1
of liver): 10.80-5.35; PS (ml · min
1 · g
1 of liver): 14.59-90.54].
It is concluded that cationic drug kinetics in the liver can be modeled
using models that integrate the presence of cytoplasmic binding, a
hepatocyte barrier, and a vascular transit density function.
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