A knockout for drug metabolism
by Randall C Willis  |  Email the author

AMSTERDAM—The CYP3A family of enzymes are responsible for the metabolism of most modern drugs in the pharmaceutical armamentarium and therefore have a significant impact on both the bioavailability and systemic clearance of drugs in patients. To truly understand how big an impact, researchers at the Netherlands Cancer Institute, JJPRD, TNO Bioscience, and Slotervaart Hospital generated knockouts of endogenous CYP3A genes in mice and then introduced human CYP3A.
As they described in the Journal of Clinical Investigation, the researchers used blots, microarrays, and immunohistochemistry to verify that they had truly knocked out the mouse CYP3A genes. Interestingly, the complete knockout of CYP3A did not seem to have any impact on mouse viability, nor did it markedly alter the expression of other genes.
They then introduced human CYP3A4 cDNA such that the transgenic mice produced the enzyme in the intestine or liver. They found that only microsomes comprised of the CYP3A4-transgenic tissues metabolized cancer drug docetaxel in vitro. Similarly, when docetaxel was injected, it was metabolized by mice expressing the gene in the liver but not the intestine, whereas oral docetaxel was metabolized by the intestinal gene and not the liver gene. Furthermore, the researchers found that mouse devoid of CYP3A were significantly more sensitive to docetaxel toxicity.
As for the impact of this research on drug development, the authors state: "We expect that the CYP3A-knockout mice and the mice with transgenic human CYP3A4 expression in liver or intestine, in the CYP3A-knockout background, will provide excellent tools to study the impact of CYP3A on [drug toxicity, efficacy, and drug-drug and drug-food interaction risks]."