Influence of Proteome Profiles and Intracellular Drug Exposure on Differences in CYP Activity in Donor-Matched Human Liver Microsomes and Hepatocytes

Human liver microsomes
(HLM) and human hepatocytes (HH) are important in vitro systems for studies of intrinsic drug clearance
(CLint) in the liver. However, the CLint values
are often in disagreement for these two systems. Here, we investigated
these differences in a side-by-side comparison of drug metabolism
in HLM and HH prepared from 15 matched donors. Protein expression
and intracellular unbound drug concentration (Kpuu) effects
on the CLint were investigated for five prototypical probe
substrates (bupropion–CYP2B6, diclofenac–CYP2C9, omeprazole–CYP2C19,
bufuralol–CYP2D6, and midazolam–CYP3A4). The samples
were donor-matched to compensate for inter-individual variability
but still showed systematic differences in CLint. Global
proteomics analysis outlined differences in HLM from HH and homogenates
of human liver (HL), indicating variable enrichment of ER-localized
cytochrome P450 (CYP) enzymes in the HLM preparation. This suggests
that the HLM may not equally and accurately capture metabolic capacity
for all CYPs. Scaling CLint with CYP amounts and Kpuu could only partly explain the discordance in absolute values
of CLint for the five substrates. Nevertheless, scaling
with CYP amounts improved the agreement in rank order for the majority
of the substrates. Other factors, such as contribution of additional
enzymes and variability in the proportions of active and inactive
CYP enzymes in HLM and HH, may have to be considered to avoid the
use of empirical scaling factors for prediction of drug metabolism.