Introduction

Response to drugs varies considerably from person to person, and their clinical outcomes, ranging from treatment failure to adverse drug reactions, can be largely attributed to drug metabolism. The role of cytochrome P450 (CYP) has been vastly studied for years regarding its influence in drug therapy. Predominantly operating within hepatocytes, their principal function is to metabolize hosts of xenobiotics and clearance of potentially toxic compounds. While paramount in many aspects of human biology and essential life processes, its most significant feature rests in the field of pharmacokinetic research and drug metabolism; this is the primary area with which drug developers and researchers concern themselves as different drugs can potentially affect the activity of CYP enzymes, or conversely, be affected by CYP activity,  leading to unforeseen clinical outcomes. Understanding mechanisms underlying a drug’s action on these enzymes is essential for patients to receive adequate therapy, especially in conjunction with other CYP metabolized drugs. Other features of CYP enzymes rest in their ability to synthesize and break down hormones, fat-soluble vitamin metabolism, fatty acid regulation, and clearance of various toxic endogenous and exogenous compounds.[1]

Fundamentals

Cytochrome p450 is a superfamily of membrane-bound hemoprotein isozymes with distinct classifications. While present in most body tissues, CYP enzymes predominantly occupy the liver, intestines, and kidneys, with their highest concentration in the liver. Of the total 57 isozymes discovered to date, 6 of these are responsible for  90% of drug metabolism. These six isozymes include CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4.[2] Various drugs work on different isozymes, and determining which isozymes are affected is critical in drug development.[3]

Of most importance, figuring out which drugs are inducers or inhibitors of these enzymes is crucial. Most drugs undergo deactivation, either directly or by facilitated excretion, through metabolism. Typically, if a drug acts as a CYP inducer, then that causes concurrently administered drugs targeting the same CYP isozyme to be metabolized faster and excreted. Under these circumstances, plasma levels of a specific drug may not reach a threshold value of benefit if it is cleared more rapidly by the body, leading to treatment failure. On the contrary, if a drug acts as a CYP inhibitor, then that causes other drugs to accumulate to toxic levels where overdoses and side effects may occur. These scenarios do not hold out to be true with prodrugs, which receives further review below.

Additionally, patients can fall into separate groups based on their inherited rate of drug metabolism. These groups are known as extensive, intermediate, poor, or ultrarapid metabolizers. Each of these scenarios presents its own issues that clinicians must be aware of as they can significantly impact a patient’s response to drug therapy.[4]

Issues of Concern

Due to their role in detoxifying potentially toxigenic xenobiotics and widespread nature throughout numerous organ systems, CYP enzymes are subject to a myriad of potential reactions and serve as a backbone in clinical research. The various effects of medications and other compounds on CYP enzymes are key in drug development to determine their safety and efficacy in the general public. Certain drugs are known inhibitors and inducers of specific CYP enzymes and require careful monitoring in patients taking multiple agents metabolized by the same subfamily. Two isozymes, CYP3A4 and CYP2D6, make up the bulk of drug metabolism, and drugs that interact with these enzymes should, therefore, merit closer evaluation and monitoring.[2] 

Most medications can still be administered despite this issue, barring any potential comorbidities, such as cirrhosis or viral hepatitis, that can alter the baseline activity of these enzymes. This is due to their intrinsic capacity to catalyze multiple substrates simultaneously at different sites. However, if patients develop any signs of significant dysfunction, it is essential to look at a detailed patient history, including medications and a review of associated adverse effects.  In cases of drug toxicity due to CYP inhibition, presenting symptoms would display signs of overdose. These instances can typically be treated by withholding the causative agent until plasma levels of the drug stabilize, or in more urgent scenarios, antidotes may work for rapid reversal. In cases of treatment failure due to CYP induction, treatment goals would not be adequately met and would warrant an adjustment in medication dosage or change in medication altogether. 

Prodrugs represent a different issue. By definition, prodrugs are compounds that are not pharmacologically active and require metabolic conversion to their active metabolite or bioactivation. For example, codeine, a commonly prescribed medication known for its analgesic and cough suppressant properties, is a prodrug metabolized via CYP2D6 to its active byproduct, morphine. Poor metabolizers will exhibit little to no signs of benefit from treatment due to their inherited inability to induce bioactivation. However, ultrarapid metabolizers will accumulate much higher levels of morphine, potentially leading to respiratory depression and even death. Therefore, precautions are necessary regarding both a patient’s genotype variance and the pharmacologic properties of a specific drug.[4]

With the advent and increasing popularity of herbal medicines and supplements, many people are now subjecting themselves to natural compounds that have not undergone rigorous scientific investigation to understand their side effect profile fully. Part of the worry behind people taking these supplements is that they have unknown effects both on the human body and in combination with prescription medications. For example, St. John’s Wort, an herbal remedy anecdotally believed to help treat depression and pseudodementia, is a known inducer of CYP3A4 and can lead to decreased plasma concentration of various prescription medicines. On the other hand, grapefruit juice is a known inhibitor of CYP3A4 and can have opposite effects. Nicotine is another compound that induces CYP1A2, and both nicotine and caffeine serve as substrates to CYP1A2, explaining a potential mechanism behind the phenomenon of increased tolerance to caffeine among smokers.[5][6]

Cellular Level

Within cells, CYP enzymes closely associate with the endoplasmic reticulum and inner mitochondrial membrane activity. The enzymes surrounding the endoplasmic reticulum further express their action to metabolize external substances such as drugs, whereas the mitochondrial enzymes focus on internal substances such as steroid hormone metabolism and fatty acid regulation. Activation of these enzymes is prompted by transcription, as evidenced by varying degrees of expression in correspondence to mRNA levels.[7]

The purpose of drug metabolism is to safely administer the active constituent(s) accordingly and transform an exogenous compound into a more hydrophilic substance. Once it is water-soluble, drugs and their metabolic byproducts travel to the kidneys, where they are then filtered and excreted from the body.[8][9] Drug metabolism can classify under three different phases in which the CYP system is responsible for phase I. Reactions that occur during this phase include oxidation, reduction, and hydrolysis - oxidation being the chief mode of metabolism.

Molecular Level

CYP alleles group as wild-type vs. variant. Persons who inherit two wild-type alleles will generally be extensive metabolizers (normal), whereas persons with two variant alleles will be poor metabolizers (defective). Those that carry one allele of each are intermediate metabolizers. There is another group known as ultrarapid metabolizers who inherit more than two wild-type alleles, which are subject to their own potential drug effects. Genetic polymorphisms are one theory regarding why certain people exhibit varying tolerance levels of a specific drug. A poor metabolizer may express signs of toxicity at drug dosages that an extensive metabolizer could tolerate.[4][6]

Outside of heritable DNA, CYP expression can undergo modification by epigenetic changes. Factors shown to play a significant role in CYP expression with varying effects on specific isozymes include age, sex, hormones, infection/inflammation, and environmental factors. Interestingly, females expressed a higher concentration of CYP enzymes, indicating one reason females are subject to metabolize drugs faster.[2]

Research in the molecular field of CYP enzymes is still developing, and the addition of the human genome project will only prove more vital as an element in personalized medicine. Still, it is a known fact that polymorphism plays a significant role in drug metabolism due to the inherited CYP activity of individuals.

Mechanism

CYP enzymes undergo a multitude of reactions, each with slightly different variations depending on the enzyme and substrate involved. As monooxygenases, CYP's primary role is the addition of an oxygen atom to the substrate. In general, the mechanism CYP exhibits in catalyzing reactions depends on a few simple steps.

1. CYP enzymes consist of an active site made of a heme-iron center. The iron compound is bound to the protein by a cysteine thiolate molecule.
2. Substrate(s) bind to the active site at the heme group, which induces a conformational change of the enzyme's active site.
3. Reductases are then responsible for electron transfer from NAD(P)H.
4. Oxygen binds to the ferrous-heme group following the reduction of iron.
5. Another electron is added via reduction, creating a Fe-O2 group known as a peroxide state.
6. The peroxide group is short-lived as it gets protonated twice to release water and another compound known as P450 Compound 1 (FeO3+). This activity allows for the acquisition of the necessary and further hydroxylated to a more excretable, hydrophilic compound. In general, the result of the mechanism appears as such:

O2 + NAD(P)H + H+ + RH → NAD(P)+ + H2O + ROH

Drugs and other compounds that already have a free polar group can skip phase I, where they will proceed directly to phase II for conjugation.[10][8][4]

Pathophysiology

As mentioned before, inflammatory states can play a determinant factor in CYP expression. Diseases of the liver, such as cirrhosis, can lead to fibrotic scarring causing damage to hepatocytes and decreased metabolic capacity. The clinical effects of cirrhosis can be further related to CYP enzymes. Due to its role in steroidogenesis and metabolism, some of the hormonal effects seen in cirrhotics, such as gynecomastia and spider angiomas, can be related to an aberrant change in CYP activity.[11] In conjunction with decreased metabolic function, patients can also exhibit decreased plasma proteins affecting drug transport mechanisms.[12]

Due to the presence of CYP enzymes in intestinal wall tissue, diseases of the small and large bowels can affect CYP enzymes as well. While the brush-border enzymes and other parts of the intestines play a limited role in the metabolism of drugs and other xenobiotics, their role in drug absorption is their most significant contributing feature. This activity can play a factor in altering the oral bioavailability of drugs, rendering some treatments ineffective or greater than expected, and creating greater responsibility on the clinician's part to dose the medication to provide therapeutic benefit appropriately. Grapefruit juice is an example of a natural compound that affects drug metabolism at the site of the intestinal wall in small amounts. Absorbed by the small intestine, a natural flavonoid found in grapefruits known as naringin acts locally to inhibit CYP3A4 enterocytes, potentially lead to greater plasma concentrations of 3A4-mediated drugs.[4]

Infections, both widespread systemic issues and organ-specific conditions, have been shown to decrease CYP expression as well, through cytokine-mediated downregulation.[2] Multiple interleukins, interferons, tumor necrosis factor, and other cytokines involved in the cell-signaling process of the immune system's inflammatory response are at play here, highlighting the importance and role of inflammation in various processes once again.

Clinical Significance

All individuals on drug therapy should receive an evaluation to check for any concurrent medications that may influence the activity of another drug based on their effects on CYP activity. Based on the characteristics of the drug's profile, CYP enzymes can be either induced or inhibited-marking a significant area of concern in patients due to issues with metabolism and clearance. This can cause considerable adverse effects for patients as medications may either accumulate to toxic levels or clear from the system too rapidly, leading to treatment failure. Also, clinicians must be aware of naturally occurring compounds that can alter the actions of CYP enzymes, such as grapefruit juice, nicotine-containing products, and St. John’s Wort, to name a few. Special precautions are necessary for patients who have decreased baseline activity or injury to CYP enzymes.

Additionally, genes play a role in an individual’s natural composition of CYP enzymes and determine their rate of metabolism. Looking into the future of medical advancements regarding pharmacogenomics, genetic polymorphisms of CYP alleles could take precedent in determining the proper therapy and drug dose for individual patients. With this information, we would better be able to predict drug response in patients.

Common cytochrome p450 inducers, inhibitors, and substrates of the primary isozymes mentioned in this article are listed below.

CYP1A2:

• Inhibitors: amiodarone, cimetidine, ciprofloxacin, fluvoxamine
• Inducers: carbamazepine, phenobarbital, rifampin, tobacco
• Substrates: caffeine, clozapine, theophylline

CYP2C9:

• Inhibitors: amiodarone, fluconazole, fluoxetine, metronidazole, ritonavir, trimethoprim/sulfamethoxazole
• Inducers: carbamazepine, phenobarbital, phenytoin, rifampin
• Substrates: carvedilol, celecoxib, glipizide, ibuprofen, irbesartan, losartan

CYP2C19:

• Inhibitors: fluvoxamine, isoniazid, ritonavir
• Inducers: carbamazepine, phenytoin, rifampin,
• Substrates: omeprazole, phenobarbital

CYP2D6:

• Inhibitors: bupropion, duloxetine, fluoxetine, paroxetine, quinidine, ritonavir, sertraline, terbinafine
• Inducers: none
• Substrates: amitriptyline, carvedilol, codeine, dextromethorphan, diltiazem, donepezil, haloperidol, metoprolol, nifedipine, ondansetron, oxycodone, propranolol, risperidone, tamoxifen, tramadol

CYP2E1:

• Inhibitors: none
• Inducers: ethanol, isoniazid, tobacco
• Substrates: acetaminophen, theophylline, verapamil

CYP3A4:

• Inhibitors: amiodarone, amitriptyline, aprepitant, carvedilol, chloramphenicol, cimetidine, ciprofloxacin, clarithromycin, codeine, donepezil, fluvoxamine, haloperidol, imatinib, ketoconazole, metoprolol, paroxetine, risperidone, ritonavir, tramadol, verapamil
• Inducers: carbamazepine, griseofulvin, phenobarbital, phenytoin, rifampin, St. John’s wort
• Substrates: alprazolam, amlodipine, buspirone, calcium channel blockers, caffeine, citalopram, clopidogrel, cocaine, cyclosporine, diazepam, erythromycin, estradiol, lidocaine, losartan, many chemotherapeutic drugs, montelukast, quetiapine, sertraline, sildenafil, statin drugs, tacrolimus, warfarin, zolpidem

It is crucial to keep in mind that many drugs that serve as either an inducer or inhibitor of an isozyme also act as a substrate. For the sake of eliminating redundancy, they were not mentioned twice here.[13]