The effect of cytochrome P450 metabolism on drug response, interactions, and adverse effects

Cytochrome P450 enzymes are essential for the metabolism of many medications. Although this class has more than 50 enzymes, six of them metabolize 90 percent of drugs, with the two most significant enzymes being CYP3A4 and CYP2D6. Genetic variability (polymorphism) in these enzymes may influence a patient's response to commonly prescribed drug classes, including beta blockers and antidepressants. Cytochrome P450 enzymes can be inhibited or induced by drugs, resulting in clinically significant drug-drug interactions that can cause unanticipated adverse reactions or therapeutic failures. Interactions with warfarin, antidepressants, antiepileptic drugs, and statins often involve the cytochrome P450 enzymes. Knowledge of the most important drugs metabolized by cytochrome P450 enzymes, as well as the most potent inhibiting and inducing drugs, can help minimize the possibility of adverse drug reactions and interactions. Although genotype tests can determine if a patient has a specific enzyme polymorphism, it has not been determined if routine use of these tests will improve outcomes.     

Induction or inhibition: the complexity of cytochrome P450 enzymes and their impact on drug interactions

Drug interactions are a major concern for health care professionals, the pharmaceutical manufacturing industry, the FDA, and the general public. Today we know that one of the primary causes of drug interactions is the cytochrome P450 enzymes. However, our knowledge about the complexities of these drug-metabolizing enzymes and the part they play in drug interactions is often limited. The actions and effects of P450 enzymes will be addressed in this article. Examples of induction and inhibition of P450 enzymes will illustrate how these enzymes can, in some instances, render a drug less potent and at other times more toxic. By understanding the unique actions of these enzymes, health care professionals may better anticipate, identify, explain, and manage individual responses to pharmacotherapeutic regimens.     

Interaction of cytochrome P450 3A inhibitors with P-glycoprotein

Many clinically important drug interactions occur due to inhibition of human liver cytochrome P450 3A (CYP3A) metabolism. The drug efflux pump P-glycoprotein (Pgp) can be an additional locus contributing to these drug interactions because there is overlap in drugs that are substrates for both proteins. We screened a number of CYP3A inhibitors (macrolide antibiotics, azole antifungals, and ergotpeptides) for their ability to interact with Pgp, compared with prototypical Pgp inhibitors. We used cell lines expressing human, mouse, and rat mdr1 genes. Pgp antagonism was defined by interactions of the drugs with four cell lines (LLC-PK1, L-MDR1, L-mdr1a, and L-mdr1b) using a microfluorometric calcein-AM assay and characterized for their inhibitor constant (K(i)) toward calcein-AM. The compounds were further defined for their ability to inhibit MDR1 by their effect on vinblastine accumulation into L-MDR1 cells. Representative compounds from each class of drugs were further tested as Pgp substrates, defined by the ability of human Pgp or mouse mdr1a/Pgp to transport them across a polarized kidney epithelial cell in vitro. These same compounds were administered radiolabeled in vivo to mdr1a (+/+) and (-/-) mice and the distribution of radioactivity compared. The results are summarized as follows: 1) Some drug interactions with Pgp were substrate- and/or assay-dependent. 2) Ergot alkaloids were identified as a class of MDR1/Pgp chemosensitizers. 3) The Ergot alkaloids revealed species differences in the structure-activity relationships for inhibition of Pgp. Simultaneous inhibition of Pgp by many CYP3A inhibitors contributes to human variation in the extent of drug-drug interactions.     

Transgenic mice with a hypomorphic NADPH-cytochrome P450 reductase gene: effects on development, reproduction, and microsomal cytochrome P450

A mouse model with a hypomorphic NADPH-cytochrome P450 reductase (Cpr) gene (designated Cpr(low) allele) was generated and characterized in this study. The Cpr gene in these mice was disrupted by the insertion of a neo gene in intron 15, which led to 74 to 95% decreases in CPR expression in all tissues examined, including olfactory mucosa, adrenal gland, brain, testis, ovary, lung, kidney, liver, and heart. In the liver, a pattern of pericentral distribution of CPR protein was preserved in the Cpr(low/low) mice, despite an overall reduction in CPR expression. Genotype distribution in F2 pups indicated limited embryonic lethality associated with the Cpr(low) allele, a finding that confirms the role of CPR-dependent enzymes in development. Adult male homozygotes had decreased body weight and decreased heart, lung, and kidney weights, whereas homozygous Cpr(low) females, which had increased serum testosterone and progesterone and decreased copulatory activities, were infertile. Furthermore, adult Cpr(low/low) mice had decreased plasma cholesterol, and some mice developed mild centrilobular hepatic lipidosis. In addition, despite apparently compensatory increases in total microsomal cytochrome P450 content in the liver and kidney, the decreases in CPR expression were accompanied by reductions in systemic clearance of pentobarbital, as well as in hepatic microsomal metabolism of acetaminophen and testosterone. These phenotypes illustrate the potential impact of a globally decreased CPR activity in human adults, and this novel knock-in mouse model provides a unique opportunity for further explorations of the in vivo roles of CPR and CPR-dependent enzymes.     

Suppression of cytochrome P450 3A protein levels by proteasome inhibitors

We have previously reported that CYP3A cross-links with polyubiquitinated proteins in microsomes from nicardipine-treated rats in a process that is distinct from classical polyubiquitination. To further examine the role of the proteasome in CYP3A degradation, we investigated the effects of proteasome inhibitors lactacystin, MG132, proteasome inhibitor 1, and hemin in primary cultures of rat and human hepatocytes. With the exception of hemin, these agents increased the total pool of ubiquitinated proteins in microsomes isolated from rat hepatocytes, indicating that lactacystin, MG132, and proteasome inhibitor 1 effectively inhibited the proteasome in these cells. All four agents caused a reduction in the amount of the major approximately 55-kDa CYP3A band, opposite to what would be expected if the ubiquitin-proteasome pathway degraded CYP3A. Only hemin treatment caused an increase in high molecular mass (HMM) CYP3A bands. Because hemin treatment did not alter levels of ubiquitin in CYP3A immunoprecipitates, the HMM CYP3A bands formed in response to hemin treatment clearly were not due to proteasome inhibition. Rather, because hemin treatment also caused an increase in HMM CYP3A in the detergent-insoluble fraction of the 10,000g pellet, the HMM CYP3A seems to represent a large protein complex that is unlikely to primarily represent ubiquitination.     

Treatment of type 2 diabetes in primary health care: a drug utilization study

BACKGROUND: Metabolic control in type 2 diabetes depends on patient adherence to therapy. Quantitative consumption data do not supply information regarding the appropriate use of medicine. Drug utilization studies are useful to identify treatment adherence problems and, thus, design interventions to improve drug use. OBJECTIVE: To describe the treatment and outcome in terms of degree of metabolic control in these patients, assess the agreement between the doses of antidiabetic drugs reported by the patient and those written in the medical record, and describe the drug utilization characteristics in relation to the standards of care. METHODS: This drug utilization study was conducted in primary care centers in Barcelona, Spain. Consecutive patients with type 2 diabetes attending a follow-up visit were interviewed regarding lifestyle, diet, glycosylated hemoglobin level, and treatment. Concordance between the information obtained through medical records and patient interviews was analyzed. RESULTS: Metabolic control was deficient in 73 (40%), acceptable in 50 (28%), and good in 58 (32%) of the 181 patients with available information. For 38 (19%) patients, there was disagreement between the dose of the antidiabetic drug reported by the patient and that written in the medical record. For 83 (41%) patients, the treatment in the medical history was in agreement with published standards. CONCLUSIONS: Identification of disagreement between standards of care and clinical practice in type 2 diabetes management is the basis to improve drug utilization and achieve better metabolic control in these patients.     

Clinically significant pharmacokinetic drug interactions with psychoactive drugs: antidepressants and antipsychotics and the cytochrome P450 system

Psychotherapeutic drugs (antipsychotics and antidepressants) are widely used for treating anxiety. Many psychotherapeutic drugs are metabolized mainly by cytochrome P450 (CYP)2C19 and CYP2D6, and are often administered with other drugs. Therefore, it is necessary to be careful when coadministering psychotherapeutic drugs whose metabolism might be inhibited by other drugs. In particular, selective serotonin reuptake inhibitors (SSRIs) inhibit the metabolism of psychotherapeutic drugs mediated by CYP2C19 and CYP2D6. It is useful to phenotype CYP2C19 and CYP2D6 (extensive metabolizers or poor metabolizers) before giving such medication. Knowledge of substrates, inhibitors and inducers of CYP isoenzymes may help clinicians to anticipate and avoid psychotherapeutic drug interactions and improve rational prescribing practices. In addition, genotyping for these drugs may be also useful in preventing side-effects.     

Clinically important pharmacokinetic drug-drug interactions: role of cytochrome P450 enzymes

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues and many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In the future, it may help to identify individuals at greatest risk of drug interactions and adverse events.     

Antipsychotic-induced priapism in an HIV patient: a cytochrome P450-mediated drug interaction

Background

With upwards of 48% of human immunodeficiency virus (HIV)-infected persons having a probable psychiatric disorder, the possibility of cross-class drug interactions causing adverse effects or fatalities exists.

Aims

This report discusses an emergent case of low-flow priapism caused by an interaction between a previously prescribed combination protease inhibitor (PI) and newly added antipsychotic medications.

Methods

A 50-year-old HIV-positive man on highly active antiretroviral therapy (HAART), including the combination PI, lopinavir/ritonavir (Kaletra^®), experienced an episode of priapism hours after beginning two new antipsychotic medications. Quetiapine (Seroquel^®) and perphenazine (Trilafon^®) were added to treat a diagnosed schizoaffective disorder.

Results

The patient presented to the emergency department complaining of a constant, painful erection lasting approximately 42 h. Treatment with intracavernous ephedrine, irrigation, and aspiration helped achieve detumescence.

Conclusion

This case displays the immediate and detrimental effects due to the addition of antipsychotic medications to previously altered cytochrome P450 (CYP450) enzyme levels. The inhibition of CYP450 enzymes 3A4 and 2D6 by the combination PI, lopinavir/ritonavir, was likely the major culprit in causing greater than expected free levels of perphenazine and quetiapine resulting in priapism.     

Treatment of the elderly with 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors: focus on drug interactions

With the aging of the population, death from coronary heart disease (CHD) and stroke has become more prevalent. Cardiovascular disease (CVD) risk factors, such as hypertension, obesity, and diabetes mellitus increase with age as well. Recent secondary-prevention studies have established the positive effect of statins in decreasing the risk of CHD mortality through the lowering of cholesterol. Statins have an excellent safety record, at least with users under age 65, and provide a cheaper alternative to more costly medical options. The most serious side effect associated with their use is myopathy, which is infrequent. Drug interactions have been found with drugs that compete for the same CYP450 isoenzymes as statins. Several drugs have been shown to significantly inhibit the CYP3A4 pathway; in combination with statins such as lovastatin, simvastatin, atorvastatin, and cerivastatin, they have been shown to elevate serum concentrations of these statins, or may increase the risk of myopathy. Alternatively, other drugs can inhibit the CYP2C9 pathway and may elevate serum concentration of fluvastatin. Due to the number of medications the elderly receive, an understanding of the various metabolic pathways is of vital importance to minimize the potential for drug interactions. The elderly population, while at high risk for CVD, is currently undertreated. Statins can effectively lower low-density lipoprotein cholesterol levels and lessen the risk of CVD for this population.     

Orally given melatonin may serve as a probe drug for cytochrome P450 1A2 activity in vivo: a pilot study.

BACKGROUND: Melatonin is a hormone that is metabolized by cytochrome P450 (CYP) 1A2 to its main primary metabolite 6-hydroxymelatonin. We therefore evaluated the utility of oral melatonin as a marker of hepatic CYP1A2 activity. METHODS: Twenty-five milligrams of melatonin was given at 9:30 am to 12 healthy Swedish volunteers, who had previously been phenotyped for CYP1A2 with caffeine. Melatonin and conjugated 6-hydroxymelatonin were analyzed by liquid chromatography-mass spectrometry in blood samples taken between 0.5 and 6.5 hours after drug intake. Serum concentrations of melatonin and conjugated 6-hydroxymelatonin, or their ratio at different time points, and the apparent melatonin clearance were tested for correlation with caffeine clearance. RESULTS: We found a significant correlation between apparent clearance of melatonin and caffeine clearance with a Spearman rank correlation coefficient (Rs) of 0.75 (P =.005). The melatonin concentration 1.5 hours after administration also closely correlated with the caffeine clearance (Rs = -0.62; P =.03). Inclusion of conjugated 6-hydroxymelatonin gave no closer correlations. CONCLUSION: Melatonin might be developed as an alternative to caffeine as a probe drug for CYP1A2 phenotyping.     

Influence of cytochrome P450 polymorphisms on drug therapies: pharmacogenetic, pharmacoepigenetic and clinical aspects

The polymorphic nature of the cytochrome P450 (CYP) genes affects individual drug response and adverse reactions to a great extent. This variation includes copy number variants (CNV), missense mutations, insertions and deletions, and mutations affecting gene expression and activity of mainly CYP2A6, CYP2B6, CYP2C9, CYP2C19 and CYP2D6, which have been extensively studied and well characterized. CYP1A2 and CYP3A4 expression varies significantly, and the cause has been suggested to be mainly of genetic origin but the exact molecular basis remains unknown. We present a review of the major polymorphic CYP alleles and conclude that this variability is of greatest importance for treatment with several antidepressants, antipsychotics, antiulcer drugs, anti-HIV drugs, anticoagulants, antidiabetics and the anticancer drug tamoxifen. We also present tables illustrating the relative importance of specific common CYP alleles for the extent of enzyme functionality. The field of pharmacoepigenetics has just opened, and we present recent examples wherein gene methylation influences the expression of CYP. In addition microRNA (miRNA) regulation of P450 has been described. Furthermore, this review updates the field with respect to regulatory initiatives and experience of predictive pharmacogenetic investigations in the clinics. It is concluded that the pharmacogenetic knowledge regarding CYP polymorphism now developed to a stage where it can be implemented in drug development and in clinical routine for specific drug treatments, thereby improving the drug response and reducing costs for drug treatment.     

Lack of cytochrome P450 1A2 interaction effect of ABT-761 on the pharmacokinetics of theophylline

ABT-761 is a second-generation 5-lipoxygenase inhibitor for the treatment of asthma. The effects of ABT-761 on the pharmacokinetics of theophylline were assessed in 15 adult volunteers in a phase I, multiple-dose, open-label, one-period study. Subjects received a single 400-mg dose of theophylline on days 1 and 8 and 200-mg oral doses of ABT-761 once daily beginning on day 3 and continuing through day 9. The pharmacokinetic parameters of theophylline after administration of theophylline alone and concomitantly with ABT-761 were compared using a paired t-test. No statistically significant differences were observed between the pharmacokinetic parameters of theophylline administered alone and theophylline with concomitant administration of ABT-761. The 95% confidence interval for the ratio of the mean with ABT-761 dosing to the mean for theophylline alone was 0.970 to 1.127 for area under the plasma concentration-time curve and 0.887 to 1.036 for maximal plasma concentration. The lack of an inhibition effect by ABT-761 on theophylline clearance suggested that ABT-761 may have a low affinity for the cytochrome P450 1A2 isozyme, the primary isozyme responsible for theophylline metabolism.