CE: Cytochrome P450 Enzymes

CE: Cytochrome P450 Enzymes

On-Line Continuing Education for Pharmacists
An educational series sponsored by Pfizer

Cytochrome P450 Enzymes By Daniel S. Streetman, Pharm.D.

ACPE Program Number 207-000-99-001-H01
December 1998 (Expires December 1, 2001

EDUCATIONAL OBJECTIVES
Upon completion of this article, the pharmacist should be able to:
1. Describe the importance of the cytochrome P450 enzymes in drug metabolism.
2. Define the function of the cyto-chrome P450 enzymes.
3. List drugs and foods that can inhibit or induce cytochrome P450 enzymes.
4. Describe the effects of enzyme induction or inhibition on metabolism of enzyme substrates.
5. List commonly encountered substrates of individual cytochrome P450 enzymes.

INTRODUCTION
First linked to drug metabolism in the 1970s, the cytochrome P450 enzymes have become one of the hottest topics in the world of pharmacy. The term "cytochrome P450" refers to a group of enzymes that are responsible f 727v2114h or, among other things, the metabolism of many of the drugs in use today. As knowledge about these enzymes grew, their involvement in drug interactions attracted significant attention. This was highlighted by the interaction of terfenadine (Seldane) with erythromycin and ketoconazole (Nizoral) that resulted in potentially fatal cardiac arrhythmias. Since then, several new examples of drug interactions involving cytochrome P450 enzymes have received attention. For example, the withdrawal of mibefradil (Posicor) from the U.S. market was, in part, due to its potential to inhibit some of the cytochrome P450 enzymes. Another example is the recently strengthened warnings in the labeling of cisapride (Propulsid), which stemmed from the potential of that agent to cause potentially fatal cardiac arrhythmias when combined with certain cytochrome P450 enzyme inhibitors.

As the number of new medications increases, pharmacists have been forced to bear much of the burden for detecting and preventing these potentially serious drug interactions. Unfortunately, few pharmacists (and even fewer physicians) were ever taught much, if anything, about the cytochrome P450 enzymes during their training. As a result, many practitioners are forced to rely on computer systems and drug information handbooks to detect these interactions. However, given the rapid pace at which new drugs are introduced and at which new interactions are detected, these sources rapidly become outdated, putting both patients and practitioners at risk.

The solution to this problem lies with understanding the basics of the cytochrome P450 enzymes. With the knowledge of how these enzymes work and of their role in drug interactions, pharmacists can predict interactions that are likely to occur. This article will discuss the cytochrome P450 enzymes and will provide the foundation for pharmacists to predict and prevent drug interactions involving the cytochrome P450 enzymes.

WHAT ARE THE CYTOCHROMES P450?
The cytochromes P450 (CYP) are a group of related enzymes that are found in nearly all animal species. In humans there are more than 20 different CYP enzymes, but only six (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A) account for the metabolism of nearly all clinically useful medications. Although the enzymes are somewhat related and share some general characteristics, each one is unique and has a distinct role. As a result, it is often important to be specific when discussing the cytochrome P450 enzymes, particularly in the case of drug interactions.

The individual cytochrome P450 enzymes are grouped together into families and subfamilies based on how similar one enzyme is to another. Often when referring to specific enzymes, the term "cytochrome P450" is abbreviated as CYP. Following the CYP designation will be a number identifying the family, a letter identifying the subfamily, and another number identifying the individual enzyme. For example, CYP2C9 is a cytochrome P450 enzyme, belonging to family 2 and subfamily 2C. This means that this enzyme is very closely related to CYP2C19 (same family and subfamily), somewhat related to CYP2D6 (same family), but not closely related to CYP1A2 (different family).

Location
The cytochrome P450 enzymes are primarily located in the liver, where most metabolism of drugs and other chemical compounds occurs. Certain cytochrome P450 enzymes are also found elsewhere in the body. Table 1(below) lists examples of some of the CYPs that can also be found outside of the liver and their location. The location of some enzymes may be important for the metabolism of some medications. For example, CYP3A4 is the major CYP located in the intestines and is responsible for the first-pass metabolism of many widely used medications.

Function
The cytochrome P450 enzymes have many important functions besides their ability to metabolize drugs. Other important functions include the metabolism of environmental toxins, dietary components, and various endogenous substances (i.e., steroids, prostaglandins, etc.). The goal of drug metabolism is to make the drug more water soluble so that it can be excreted by the kidneys. Usually, cytochrome P450-mediated phase I reactions inactivate the drug or compound, but sometimes the actions of the CYP enzymes make the drug or compound more active. Examples of this include losartan, which is activated by CYP2C9 and CYP3A to its active form, and acetaminophen, which is converted to a hepatotoxic metabolite (NAPQI) by CYP1A2 and CYP2E1.

Table 1. Location of Select CYP Enzymes

INDIVIDUAL CYTOCHROME P450 ENZYMES
In order to completely understand and predict cytochrome P450 drug interactions, it is important to distinguish between the different enzymes because each enzyme is affected differently.

Cytochrome P450 1A2 (CYP1A2
About 15 percent of clinically used medications are metabolized by CYP1A2. Most notable among these are theophylline, caffeine, (R)-warfarin, and several of the antidepressants and antipsychotics. Several things can alter the activity of CYP1A2. The hydrocarbons found in cigarette smoke, charbroiled foods, and cruciferous vegetables (such as broccoli, cauliflower, etc.) are capable of inducing this enzyme, but do not induce the activity of others. Several medications also effect CYP1A2 activity. Some of the antiseizure medications and rifampin induce CYP1A2 as well as several other enzymes. Omeprazole (Prilosec) and ritonavir (Norvir) also induce CYP1A2, but inhibit one or more other enzymes. The most significant inhibitors of CYP1A2 include select fluoroquinolones and fluvoxamine (Luvox).

Cytochrome P450 2C9 (CYP2C9
CYP2C9 is responsible for the metabolism of several common medications including many of the nonsteroidal anti-inflammatory drugs (NSAIDs), phenytoin, and (S)-warfarin. The rifamycins (rifampin, rifabutin, etc.) are consistent inducers of CYP2C9 activity. The barbiturates, carbamazepine, and ethanol also appear to be significant, but less consistent, CYP2C9 inducers. Amiodarone (Cordarone), fluvastatin (Lescol), and fluconazole (Diflucan) are only a few of the many potent inhibitors of this enzyme activity. Up to 23 percent of Caucasians and 2 percent of African Americans may have one or more altered CYP2C9 genes, possibly contributing to abnormally decreased enzyme activity in these individuals.

Cytochrome P450 2C19 (CYP2C19
Medications metabolized by CYP2C19 include several benzodiazepines, the new antidepressant citalopram (Celexa), many of the tricyclic antidepressants (TCAs), omeprazole (Prilosec), and lansoprazole (Prevacid). CYP2C19 is also responsible for the activation of the antimalarial drug proguanil. Rifampin induces CYP2C19 activity, and fluvoxamine (Luvox), fluoxetine (Prozac), and ticlopidine (Ticlid) inhibit this activity. Genetics and race play a very significant role in CYP2C19 activity. Two to 5 percent of Caucasians and 20 percent of Asians lack this enzyme entirely. One particular situation where this may be important is in patients who are taking omeprazole. Since omeprazole can induce CYP1A2 and inhibit CYP3A, these effects will be much more pronounced due to increased omeprazole concentrations in individuals who lack the CYP2C19 enzyme.

Cytochrome P450 2D6 (CYP2D6
CYP2D6 comprises a relatively small but significant percentage of the total cytochrome P450 in the liver. Only 2 to 6 percent of total liver cytochrome P450 is CYP2D6, but nearly 25 percent of clinically useful medications are metabolized by this enzyme. In particular, many TCAs, antiarrhythmics, and beta blockers are metabolized by CYP2D6. In addition, CYP2D6 is responsible for the conversion of codeine to morphine, accounting for the majority of its analgesic effects. Unlike other CYP enzymes, there are no known inducers of this activity, except pregnancy. Several medications are known to inhibit CYP2D6, the most potent of which include quinidine, paroxetine (Paxil), and fluoxetine (Prozac). Genetic factors play a particularly significant role in determining CYP2D6 activity. Approximately 5 to 10 percent of Caucasians and 1 to 3 percent of African Americans and Chinese lack this enzyme. Such individuals are at risk for increased toxicity from medications that are metabolized by CYP2D6.

Cytochrome P450 2E1 (CYP2E1
Although CYP2E1 metabolizes a relatively small fraction of clinically used medications, this enzyme plays a significant role in the activation and inactivation of toxins. CYP2E1 metabolizes primarily small organic molecules (ethanol, carbon tetrachloride, etc.). Tables 2 and 3 list selected inducers and inhibitors of CYP2E1 activity. Researchers are currently interested in the possibility of using CYP2E1 inhibitors to prevent toxicity associated with compounds that form toxic metabolites via CYP2E1 (i.e., acetaminophen-induced liver damage).

Cytochrome P450 3A (CYP3A
CYP3A is both the most abundant and most clinically significant family of cytochrome P450 enzymes. The CYP3A family is actually composed of four major enzymes -- CYP3A3, CYP3A4, CYP3A5, and CYP3A7. CYP3A4 is the most common form of the CYP3A enzymes found in adults and is the form implicated in most drug interactions. However, since these enzymes are so closely related (most are 97 percent similar), they are often referred to collectively by the subfamily name, CYP3A. Up to 60 percent of the liver's total cytochrome P450 is CYP3A, and nearly 50 percent of all clinically used medications are metabolized by CYP3A. This explains much of the reason that so many important drug interactions involve this enzyme. In addition to the many medications it metabolizes, CYP3A is responsible for the metabolism of most of the body's endogenous steroids. Another particularly important consideration is the fact that CYP3A is also located in the small intestine and is responsible for the majority of first-pass metabolism. This is important because increases or decreases in first-pass metabolism can have the effect of administering a much smaller or larger dose than usual. Notable inducers of CYP3A include the glucocorticoids, rifampin, carbamazepine, phenobarbital, and phenytoin. Among the significant CYP3A inhibitors are grapefruit juice, erythromycin, ketoconazole, clarithromycin, and verapamil.

Enzyme Polymorphism
The term "polymorphism" is used to describe a genetic trait that is present in the population in at least two forms. The genetic trait referred to here is the activity of a particular cytochrome P450 enzyme. While most of the enzymes are present in all individuals, some people may completely lack one or more particular enzymes. Enzymes that are known to be polymorphic include CYP2C19 and CYP2D6. Individuals without CYP2C19 and/or CYP2D6 are at risk for more frequent and more severe adverse effects because of decreased elimination of drugs metabolized by that particular enzyme. These individuals are also at risk for decreased response to drugs requiring activation by CYP2C19 or CYP2D6.

Table 2. Select Common Cytochrome P450 Enzyme Inducers

Table 3. Select Common Cytochrome P450 Enzyme Inhibitors

CYTOCHROME P450: IMPORTANT GENERAL CONSIDERATIONS
1. Some drugs are metabolized by more than one cytochrome P450 enzyme.

• Often this means that adding an enzyme inhibitor will result in less of an effect since the drug has a second metabolic pathway. An example of this is propranolol, which is metabolized by several cytochrome P450 enzymes, including CYP2D6. So if a patient was taking propranolol and was then started on quinidine (a potent CYP2D6 inhibitor), there may be an increase in the propranolol serum concentration, but it will more than likely be only a minimal increase with little, if any, clinical significance.
• Occasionally, however, effects on any of the metabolic pathways may have significant clinical consequences. This is the case most often with drugs with a very narrow margin of safety.

Drugs that are administered as a mixture of multiple isomers [R(+) and S(-)] often behave as, and are metabolized as, two separate drugs.

• The most important example of this is warfarin, which is administered as a mixture of (R)-warfarin and (S)-warfarin. The body handles these different isomers as different drugs. The (R)-warfarin isomer is metabolized in the liver by many cytochrome P450 enzymes, including CYP1A2 and CYP3A. On the other hand, (S)-warfarin is only metabolized by CYP2C9. Since the (S)-warfarin isomer is about five times as potent as the (R)-warfarin isomer, this is a very important consideration.

Drugs that inhibit a certain cytochrome P450 enzyme are not necessarily metabolized by that enzyme, and by the same token, drugs that are metabolized by an enzyme do not necessarily inhibit that enzyme.

• Enzyme inhibitors may interact with an enzyme in such a way as to block the activity of that enzyme without being metabolized by the enzyme. An example of this is quinidine, which is a potent CYP2D6 inhibitor. Quinidine inhibits CYP2D6 activity but is metabolized only by CYP3A.
• Also, drugs that are metabolized by an enzyme do not necessarily inhibit the activity of that enzyme. There is always the potential for competitive inhibition of an enzyme when dealing with two or more drugs metabolized by the same enzyme; however, any such inhibition is nearly always very minor and of little clinical significance at concentrations seen with therapeutic doses. This is why drugs like nifedipine (Procardia) and simvastatin (Zocor) do not interact with each other, even though both are metabolized by CYP3A.

Cytochrome P450-mediated reactions do not always inactivate compounds. Some reactions activate previously inactive compounds, and some turn active, but relatively nontoxic chemicals into toxic metabolites.

• Although most cytochrome P450 reactions inactivate compounds, some reactions may activate compounds or create toxic metabolites. This distinction is important because inhibitors of these reactions will decrease the effect of the compounds rather than increasing the effect. The same reversal would be true for inducers of such reactions.
• Examples of drugs that are activated by cytochrome P450 reactions include losartan (Cozaar), codeine, and acetaminophen. Losartan, an inactive parent compound, is metabolized by CYP2C9 and CYP3A to its active metabolite (E-3174), which exerts its antihypertensive effects. A small percentage of a dose of codeine, which has little analgesic activity itself, is converted by CYP2D6 into morphine. Individuals who are CYP2D6 poor metabolizers (who, due to polymorphism of CYP2D6, lack the enzyme completely) experience little pain relief from codeine due to the lack of morphine formation following codeine administration. If individuals consume excessive amounts of acetaminophen, if there is a defect in phase II metabolism, or if CYP2E1 activity is induced (by alcohol, for example), potentially hazardous amounts of the toxic metabolite are formed.

CYTOCHROME P450-MEDIATED DRUG INTERACTIONS
Any drug that is metabolized by one or more of the cytochrome P450 enzymes is a potential target for interactions (table 4). The role of the pharmacist is to determine if an interaction is likely to occur and if so, is that interaction likely to be clinically significant.

The first step towards applying information about the cytochrome P450 enzymes is to become familiar with the common inducers and inhibitors of the individual enzymes. Once a practitioner is familiar with the common inducers and inhibitors, that information can be used as a signal to identify medications that are likely to alter (increase or decrease) the metabolism of others.

The second step is to investigate a potential interaction. Once a possible interaction is detected (i.e., the patient brings in a new prescription for erythromycin-CYP3A inhibitor), the patient's medication profile is searched for medications that may be affected by the new agent. Information about the metabolism of a drug can be found in the package insert or in many reference texts and is generally found in the section discussing pharmacokinetics or drug metabolism.

The third step is to decide if any interactions are likely, and if so, are they clinically important. For example: a patient brings in a prescription for ciprofloxacin (Cipro). First, the pharmacist recognizes that ciprofloxacin (Cipro) is a CYP1A2 inhibitor. Second, the pharmacist reviews the patient's medications and finds that the patient is also taking theophylline, amitriptyline (Elavil), and lisinopril (Prinivil). Since theophylline and amitriptyline (Elavil) are both metabolized (at least in part) by CYP1A2, ciprofloxacin (Cipro) could inhibit their metabolism, possibly resulting in toxicity.

Table 4. Select Known Cytochrome P450 Substrates

CASE EXAMPLE
Q: A 53-year-old Caucasian woman who has been coming to your pharmacy for several years brings in a prescription for erythromycin that she received for treatment of a mild pneumonia. You see that she is currently taking metoprolol (Lopressor), hydrochlorothiazide, simvastatin (Zocor), and naproxen (Naprosyn). Are there any potential cytochrome P450-mediated drug interactions?

A: She is currently receiving three drugs metabolized by one or more cytochrome P450 enzymes (metoprolol-CYP2D6, simvastatin-CYP3A, and naproxen-CYP1A2 and CYP2C9). Since erythromycin is a well-known CYP3A inhibitor, it may increase concentrations of simvastatin by inhibiting its metabolism by CYP3A. This may place the patient at increased risk of myositis or hepatotoxicity from the simvastatin (Zocor). Although metoprolol (Lopressor) and naproxen (Naprosyn) are metabolized by cytochrome P450, they are not likely to interact with erythromycin since they are not metabolized by CYP3A.

SUMMARY
The cytochrome P450 enzymes are the most important drug-metabolizing enzymes in humans, and as such, play a major role in many drug-drug and drug-food interactions. Knowledge of the location and function of these enzymes is key to understanding their importance.

Daniel S. Streetman, PharmD, is currently a second year fellow in clinical pharmacology and pharmacogenetics at Bassett Healthcare in Cooperstown, NY.