SCALING FACTORS TO RELATE DRUG METABOLIC-CLEARANCE IN HEPATIC MICROSOMES, ISOLATED HEPATOCYTES, AND THE INTACT LIVER - STUDIES WITH INDUCEDLIVERS INVOLVING DIAZEPAM
Dj. Carlile et al., SCALING FACTORS TO RELATE DRUG METABOLIC-CLEARANCE IN HEPATIC MICROSOMES, ISOLATED HEPATOCYTES, AND THE INTACT LIVER - STUDIES WITH INDUCEDLIVERS INVOLVING DIAZEPAM, Drug metabolism and disposition, 25(8), 1997, pp. 903-911
Microsomal protein recovery and hepatocellularity have been determined
and investigated as scaling factors for interrelating clearance by he
patic microsomes, freshly isolated hepatocytes and whole liver from un
treated (UT) rats and rats treated with either the cytochrome P450 ind
ucer phenobarbital (PB) or dexamethasone (DEX). Hepatocellularity in U
T rats (1.1 x 10(8) hepatocytes/g liver) was not significantly differe
nt after either PB or DEX induction (1.1 and 1.3 x 10(8) hepatocytes/g
liver, respectively), However the microsomal protein recovery index,
which provides a scaling factor that is inversely related to the effic
iency of the microsomal preparation procedure, was 47 mg/g liver in bo
th PB and DEX microsomes and differs from UT rats (60 mg/g liver), The
se contrasting findings are consistent with the interlaboratory trends
in the literature, indicating that, although hepatocellularity estima
tes are in good accord, microsomal recovery can vary 2-fold; this has
implications for scaling, The oxidation of diazepam to its three prima
ry metabolites was measured in PB and DEX microsomes and hepatocytes a
nd the scaling factors were applied to these data and previously repor
ted UT data, Marked changes in kinetics occur on induction resulting i
n a shift in the major pathway. In particular, 3-hydroxylation is indu
ced over 20-fold by DEX. Diazepam CLint was determined in vivo after a
dministration of a bolus dose into the hepatic portal vein of UT, PB,
and DEX rats; values of 127, 191, and 323 ml/min/SRW (where SRW is a s
tandard rat weight of 250 g), respectively, were obtained. Using these
scaling factors, the hepatocyte predictions of CLint were excellent (
99, 144, and 297 ml/min/SRW for UT, PB, and DEX, respectively), wherea
s only the DEX prediction (248 ml/min/SRW) was accurate for the micros
omal system, with a substantial underprediction for UT and PB (46 and
68 ml/min/SRW, respectively), Evidence is presented for product inhibi
tion, resulting from accumulation of primary metabolites within the mi
crosomal preparation, as the mechanism responsible for this underpredi
ction, These results illustrate that the scaling factor approach is ap
plicable to induced livers in which both cytochrome P450 complement an
d zonal distribution are altered, These data, together with our previo
us studies, demonstrate that CLint in cells (2.4-297 ml/min/SRW), micr
osomes (2.7-248 ml/min/SRW), and in vivo (1.5-323 ml/min/SRW) are rela
ted in a linear fashion and hence inherently both in vitro systems are
of equal value in predicting in vivo CLint.