Sd. Studenberg et Klr. Brouwer, HEPATIC DISPOSITION OF ACETAMINOPHEN AND METABOLITES - PHARMACOKINETIC MODELING, PROTEIN-BINDING AND SUBCELLULAR-DISTRIBUTION, Biochemical pharmacology, 46(4), 1993, pp. 739-746
Successful pharmacokinetic modeling often requires the ability of a si
mple model to describe a complex series of physiological processes. Ho
wever, a simple model may be inappropriate. Physiologically-relevant m
odeling may offer a more appropriate description, but requires further
support from in vitro/in vivo data. A well-stirred hepatic model with
linear processes was proposed to describe in vivo disposition of acet
aminophen and metabolites after a 100 mg/kg bolus of acetaminophen to
vehicle- or phenobarbital-pretreated, renal-ligated rats. Model simula
tions underpredicted acetaminophen glucuronide (AG) concentrations at
early time points in serum, and were inconsistent with AG biliary excr
etion-rate profiles. Intracellular binding of AG by ligandin was hypot
hesized, and a cytosolic compartment with reversible binding was incor
porated into the model. In this second model, only AG bound in the cyt
osolic compartment was available for excretion into bile. Model 2 bett
er described the AG biliary excretion rate-time profiles based on calc
ulated Akaike's information criterion values. However, no apparent cha
nge was observed in the underprediction of AG serum concentrations. Pa
rameter estimates derived from the two models also were different, The
rate constants regulating AG formation and sinusoidal egress were inc
reased significantly after phenobarbital pretreatment according to mod
el 1, while the AG biliary excretion rate constant was decreased signi
ficantly. Parameter estimates based on model 2 suggested that phenobar
bital pretreatment impaired the cytosolic binding of AG but increased
significantly the AG biliary excretion rate constant. The physiologic
relevance of model 2 was not supported by a subsequent investigation o
f the protein binding and subcellular distribution of acetaminophen an
d metabolites. Acetaminophen, AG and acetaminophen sulfate (AS) were n
ot bound extensively in hepatic cytosol (mean +/- SD unbound fractions
were 0.90 +/- 0,08, 0.97 +/- 0.08, and 0.88 +/- 0.06, respectively).
Phenobarbital pretreatment did not alter significantly the unbound fra
ctions of acetaminophen, AG or AS in hepatic cytosol. Acetaminophen wa
s distributed to a greater extent in lysosomes than in the nuclear, mi
tochondrial, microsomal and cytosolic fractions. Distribution of AS pr
edominated in cytosolic and lysosomal fractions. AG was detected only
in cytosol. Phenobarbital pretreatment decreased the content of acetam
inophen, AG and AS in all hepatic fractions. This study demonstrates t
he utility of pharmacokinetic modeling in exploring mechanistic hypoth
eses. However, these results underscore the importance of obtaining pi
votal data from in vitro/in vivo studies to validate hypothesized mech
anisms.