Dr. Richard Kim is leading the VUMC research.
Tailored drugs on the horizon: VUMC study
Responses to medications are as individual as the patients taking the drugs. Given equivalent doses of a medicine, some patients will have blood concentrations too low to be effective; others will experience high drug levels and toxicity.
One of the reasons, said Dr. Richard B. Kim, associate professor of Medicine and Pharmacology, is the variable activity of a group of enzymes that metabolize — chemically alter — drugs.
“We are trying to understand why some people have lots of drug-metabolizing enzyme activity and other people don’t,” Kim said. He and colleagues reported recently in Nature Medicine that a protein called HNF4 regulates the expression of a common drug-metabolizing enzyme. The finding adds another layer to the complex regulation of drug-metabolizing enzymes and could be a source of inter-individual variability in drug response.
“If we can understand why patients respond to drugs differently, we will be able to tailor prescriptions so that they are more effective and less toxic,” Kim said.
The current work focuses on the expression of the P450 metabolic enzyme named CYP3A4, an enzyme thought to be involved in the metabolism of nearly 50 percent of all currently prescribed drugs, Kim said. CYP3A4 is a target of drug-induced expression, that is, some drugs cause levels of CYP3A4 to rise, which can spell trouble for other medicines that a patient might be taking.
“Anything that induces CYP3A4 lowers the blood levels of other medicines that are metabolized by CYP3A4, like cyclosporine and oral contraceptive agents, and then the patient has a lack of drug efficacy,” Kim said.
In searching for the mechanism of this CYP3A4 induction, other investigators had discovered that nuclear receptors — in particular two called PXR and CAR — participate in the phenomenon. When they studied CYP3A4 induction in a cell-based system, Kim and colleagues quickly realized that PXR or CAR alone were not enough.
“We could see the activation of expression, with just PXR or CAR, in liver-derived cells,” Kim said, “but not in non-liver cells.” Liver cells, the investigators deduced, must have another factor that participates in the CYP3A4 induction.
Using a set of molecular pharmacology and gene expression tools, the researchers found that a different nuclear receptor called HNF4 seemed to be the factor that non-liver cells were missing. Addition of HNF4 to non-liver cells activated CYP3A4 to the same levels as in liver cells.
“We were really surprised that we could get gene activation in non-liver cells that was equivalent to liver cells just by having this one factor (HNF4) plus PXR,” Kim said.
The investigators extended their cell studies to whole animals using a novel in vivo imaging technique. They introduced their gene expression constructs into mice and measured glowing light in living mice to study induced expression.
“The livers of the animals responded faithfully to what we saw in vitro, clearly demonstrating that HNF4 is involved in the activation of CYP3A4,” Kim said.
The investigators also studied CYP3A4 expression and drug-induced activation in mice with genetic defects in the HNF4 gene. When HNF4 was not functional, the basal levels of CYP3A4 were reduced, and drugs that induce CYP3A4 expression were not fully effective, Kim said. “It all fit together nicely.”
Kim said the studies raise new issues about drug-mediated induction of genes and in addition, shed light on more fundamental questions of tissue-specific gene expression. Because a single factor could account for liver-specific activation of CYP3A4, Kim is optimistic that tissue-specific gene expression may not be “as hopelessly complex as we previously thought.”
A limited number of nuclear receptor pairs, Kim said, may be sufficient to confer tissue-specific activation of genes. The presence of HNF4, along with PXR or CAR, in the liver and intestines, then, would explain why CYP3A4 expression is induced in these tissues.
The studies suggest that investigators seeking to understand variable drug responses pay attention to HNF4, Kim said. “We think that drugs that interact with HNF4 in a deleterious or unexpected way could cause a multitude of problems related to the expression of drug-metabolizing enzymes like CYP3A4.”
Collaborators on the Nature Medicine studies include Rommel G. Tirona, Wooin Lee, Brenda F. Leake, Lu-Bin Lan, Cynthia Brimer Cline, Vishal Lamba, Fereshteh Parviz, Stephen A. Duncan, Yusuke Inoue, Frank J. Gonzalez, and Erin G. Schuetz. The work was supported by the National Institutes of Health.