Though most drugs are metabolized by CYPs, other enzymes are crucial to a drug's clearance. For example, the cytosolic enzyme
aldehyde oxidase (AO) has been in the spotlight lately for its role in the metabolism of NCEs, particularly those of notable
clinical failure. Unlike cytochrome P450 (CYP), there is limited information on AO metabolism data. Most Phase I metabolism
is evaluated using hepatic microsomes, which are devoid of AO. Also, recent reports have noted differences in AO activity
within animal species, and that these differ considerably from humans (1). Knowing this, some pharmaceutical companies have
begun to screen NCEs using S9 fractions in parallel with purified microsomes and cytosol, while others are using human hepatocytes.
It has been recently demonstrated that cryopreserved human hepatocytes sustain AO activity near in vivo levels, enabling them to be used to screen out compounds that undergo rapid clearance due to AO metabolism (2). This makes
it both convenient and advantageous for researchers to work with a human whole-cell system that mimics in vivo effects.
Xenobiotics are moved across cell membranes by transmembrane proteins known as transporters, which may aid in the clearance
of drugs. Transporters from many protein families are responsible for the cellular uptake and excretion of xenobiotics and
can be found in the liver, kidney, blood-brain barrier, and intestine. The interactions of drugs can cause differences in
drug disposition by way of inhibition or induction, leading to the importance of transport screening in early drug development.
In hepatocytes, it has been shown that prototypical transporters are maintained after cryopreservation, validating their importance
in screening for a drug's potential as a substrate, inhibitor, or inducer of transporters.
Pooled human cryopreserved hepatocytes have long been a favorite among researchers who seek a cellular product that will provide
an average population response in studies related to transport uptake, toxicity, apoptosis, and metabolism. However, early
screening may indicate that the action of a drug will have a more profound effect in a certain subset of the population. In
this case, it may be advantageous to perform further analysis using a customized, pooled hepatocyte product that is comprised
of only those donors of interest. It is important to find a supplier with an extensive inventory of fully characterized human
cyropreserved hepatocytes to allow custom formulation of product based on donor age, demographic, genotypes, medical history,
serologies, or CYP enzyme activities.
The study of how genetic variation affects an individual's response to a drug, also known as pharmacogenomics, is an area
of specialized research that can lead to more personalized therapeutic intervention. In human hepatocytes and microsomes,
one way to identify genetic variants is by characterizing for specific single nucleotide polymorphisms (SNPs). Knowing the
genotype of the donor used to screen for pharmacokinetics can help explain an NCE's metabolic potential. Enzyme activity data
plus genotype data paint a more complete picture and help to interpret the response to the novel compound.
Investigators need to be able to evaluate the products they use to interpret and understand the results of their experiments.
Cell culture combined with molecular and histochemical techniques allow a multidisciplinary approach to unlocking differences
between donors, isolations, and systems. It may be advantageous, for example, to isolate and analyze RNA from hepatocytes
or NPCs to know the genotype of polymorphic proteins or expression levels. A cell sample can be retained from the isolation
and preserved for further investigations. If time is of the essence and fresh cells suffer loss of a function crucial to the
study, the cells alternatively can be stained and fixed before shipment.
Investigating a drug's safety and effectiveness can be accomplished with high-quality, physiologically relevant in vitro products. Success in early stage drug development requires the use of established methodologies along with openness to new,
creative approaches when faced with difficult questions. One type of answer suggests the use of customized products that fit
perfectly into an experimental design.
Timothy Moeller is a scientific advisor for Celsis In Vitro Technologies.
1. J. Sahi, K.K. Khan KK, and CB Black, Drug Metab Lett 3: 176-83, (2008).
2. J.M. Hutzler et al., Drug
Meta Disposition 2:267-75 (2011).