Download Article PDF/Slides| The XEN of Drug Metabolism and Disposition | Top of page |
One example cited by Dr. Flexner was hepatic enzyme induction, a process that involves the upregulation of enzymes involved in drug metabolism. It wasn’t clear how a single drug could turn on multiple elimination pathways. Nor was it clear why inducers of drug metabolizing enzymes—such as cytochrome P450 3A4 (CYP3A4), the most common enzyme involved in drug metabolism—were also upregulating expression of drug-transport proteins, such as P-glycoprotein.
Over the past decade, a great deal has been learned about the regulation of CYP3A4 expression. For many years, all that was known about enzyme-inducing substrate drugs—such as phenobarbitol, rifampin, efavirenz (Sustiva), and ritonavir (Norvir)—was that they were increasing the transcription of an elusive gene encoding CYP3A4. It wasn’t until seven or eight years ago that pharmacologists discovered two proteins in the nucleus of cells associated with increased transcription of the CYP3A4 gene: the pregnane X receptor (PXR) and the retinoid X receptor (RXR).
Simply put, PXR is an “orphan” nuclear receptor that upregulates transcription of downstream genes encoding drug-metabolizing enzymes, after it binds to substrate drugs. PXR then meshes with RXR to form a heterodimer that binds to an enhancer sequence in DNA, resulting in increased transcription, increased enzyme production, and increased enzyme activity.
More recently, pharmacologists identified yet another piece of the puzzle: the xenobiotic response element (XRE), a genetic locus on the 5’-flanking region of the CYP3A4 gene. And it is this enhancer—triggered by the PXR/RXR heterodimer—that upregulates the transcription of the CYP3A4 gene, resulting in increased production and activity of the CYP3A4 isoenzyme (see Figure 1). Most interestingly, it turns out that XRE is involved in a lot more than hepatic induction of CYP3A4.
“We know that phenobarbitol, rifampin, efavirenz, and ritonavir don’t just turn on CYP3A4,” Dr. Flexner said. “They can turn on a number of drug-metabolizing enzymes. They also turn on transport proteins that affect the absorption and elimination of drugs and the intracellular concentrations of drugs. We’ve come to realize that XRE is responsible for much of this.”
Aside from its role in increasing transcription of the CYP3A4 gene, XRE is also responsible for upregulating the activity of the CYP2D6 gene. XRE also increases transcription of the uridine diphosphate glucuronosyl-S-transferase (udpgt) gene, responsible for the glucuronidation of zidovudine (Retrovir) and abacavir (Ziagen). It also increases activity of P-glycoprotein—formerly known as the multi-drug resistance-1 (MDR-1) protein—responsible for pumping a variety of drugs out of the interior of cells. Studies have also concluded that XRE upregulates organic anion transporting polypeptide (OATP) genes, thus tying together an entire family of enzymes and transporters involved in the intestinal, hepatocellular, and renal uptake and elimination of exogenous compounds with a variety of divergent chemical structures. These compounds are all xenobiotics, from the Greek words xenos, meaning foreign, and biotiko, meaning organic or living.
“Now we know that one drug can turn on multiple enzymes and transport proteins, via its effect on XRE,” Dr. Flexner commented. “Interestingly, these regulatory networks are not unique to humans. They’re highly conserved in mammals and, even in insects, similar systems can be found. Metabolizing enzymes, drug excretion, and transport genes are really part of an ancient network designed to detoxify and eliminate ingested substances.” And this, Dr. Flexner explained, is the xenobiotic elimination network (XEN).
Obviously, these biochemical systems did not evolve simply to eliminate drugs from the body. XEN is characterized by coordinated induction of enzymes and transporters, along with redundancy of elimination pathways. This system, Dr. Flexner pointed out, evolved to prevent vertebrates and invertebrates from being poisoned by organic and inorganic toxic matter. “The fact is, our bodies can’t tell the difference between ritonavir, saccharin, strychnine, or a mushroom toxin. This is important for us to remember when we develop new antiretrovirals: our bodies work very hard to get rid of them before they can do what we pharmacologists would like them to do.”
The effects of XRE on metabolism and drug transport are variable. CYP3A4 is the most heavily influenced gene, more so than CYP2D6 and UDPGT, and significantly more so than P-glycoprotein and OATP. As an example, rifampin induces a tenfold increase in CYP3A4, compared to a twofold increase in P-glycoprotein or OATP. As explained by Dr. Flexner, CYP3A4 is one of the most promiscuous enzymes in biochemistry. It affects more than 10,000 different substrates and is the workhorse of detoxification in humans. And it’s not just the classic P450 isonenzymes and transporters that are involved in the elimination of xenobiotics. Certain vitamins, bile acids, and other substances play a role in activating a variety of possible drug-elimination pathways.
| Herbal Medicines and XRE | Top of page |
St. John’s wort contains at least one ingredient, hyperforin, that binds to and activates PXR. This, Dr. Flexner surmised, may explain why it is a CYP3A4 inducer. “P450 and P-glycoprotein inducers are likely to be common in herbal products, given the multitude of chemical ingredients and the evolutionary design of the XEN,” he pointed out. “These have very important implications for clinicians. It is important to know what patients are taking besides prescription drugs.”
Earlier this year, long-awaited in vivo data involving the effects of Echinacea on cytochrome P450 isoenzymes were published (Gorski, 2004). The effects of Echinacea purpurea root on CYP activity was assessed using the CYP probe drugs caffeine (CYP1A2), tolbutamide (CYP2C9), dextromethorphan (CYP2D6), and midazolam (hepatic and intestinal CYP3A4). Six male and six female healthy study volunteers completed the two-period, open-label, fixed-schedule study. Caffeine, tolbutamide, dextromethorphan, and oral and intravenous midazolam were administered before and after a short course of echinacea (400 mg four times a day for eight days) to determine in vivo CYP activities.
Echinacea administration increased the systemic clearance of intravenously administered midazolam by 34% and reduced the midazolam AUC by 23%, indicating induction of hepatic CYP3A4. Conversely, Echinacea did not affect the clearance of orally administered midazolam, although the oral bioavailability of midazolam after Echinacea dosing was significantly increased.
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