Received for publication March 7, 2008.
Revised September 4, 2008.
Accepted for publication September 4, 2008.

Covariation of human microsomal protein per gram of liver with age: Absence of influence of operator and sample storage may justify inter laboratory data pooling

Abstract

Abstract Scaling of metabolic clearance values from liver microsomal data or recombinantly expressed cytochrome P450 enzymes to predict human hepatic clearance requires knowledge of the amount of microsomal protein per gram of liver (MPPGL). Identification of physiological covariates of MPPGL requires analysis of values from large diverse populations which necessitates pooling of data from numerous sources. To ensure compatibility between results obtained within and between studies the impact of inter operator differences and sample storage on values of MPPGL were investigated. Using triplicate samples from one liver (HL86), no statistically significant difference was detected between values of MPPGL prepared from samples stored at -80°C (23.5±1.2 mg.g^-1) and those determined using fresh tissue (21.9±0.3 mg.g^-1). Whilst there was a significant difference in the yield of microsomal protein obtained from another liver sample (HL43) by 3 different operators (17±1, 19±2 and 24±1 mg.g^-1, p=0.004 ANOVA) no difference was observed in the estimated MPPGL after applying appropriate correction factors for each operator (28±1, 30±5 and 31±4 mg.g^-1). The result provided justification for pooling reported values of MPPGL for use in covariate analysis. Investigation of the relationship between age and MPPGL provided preliminary evidence that MPPGL values increase from birth to a maximum of 40 mg.g^-1(95% CI mean_geo: 37-43 mg.g^-1) around 28 years followed by a gradual decrease in older age (mean of 29 mg.g^-1) at 65 years; 95% CI mean_geo : 27-32 mg.g^-1). Accordingly, appropriate age adjusted scaling factors should be used in extrapolating in vitro clearance values to clinical studies.

Key words: drug clearance, hepatic elimination, human CYP enzymes, in vitro-in vivo scaling, liver microsomes, recombinant proteins