HEPATIC DISPOSITION OF ACETAMINOPHEN AND METABOLITES - PHARMACOKINETIC MODELING, PROTEIN-BINDING AND SUBCELLULAR-DISTRIBUTION

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.