Verapamil and drug metabolism by the cytochrome P450 isoform CYP1A2

Pharmacokinetic Boosting of Calcineurin Inhibitors in Transplantation: Pros, Cons, and Perspectives

ABSTRACT

The concept of pharmacokinetic (PK) boosting of calcineurin inhibitors (CNI) emerged after the FDA approval of cyclosporine-A. Several studies followed, and the proof of concept was well established by the late 1990s. This also continued for the next blockbuster immunosuppressant, tacrolimus. The driver for such research was an endeavor to save costs, as both drugs were expensive due to patent protection. Two CYP inhibitors, ketoconazole and diltiazem, have been extensively studied in this context and continue to be prescribed off-label along with the CNI. It has been observed that using ketoconazole reduces the dose requirement of tacrolimus by about 50% and 30% with diltiazem, which is in conformity with their pharmacological actions. Off-label co-prescription of these drugs with CNI is often encountered in low and middle-income countries. The foremost reason cited is economic. This article collates the evidence from the clinical studies that evaluate the PK-boosting effects of CNI and also reviews the gaps in the current evidence base. The current knowledge prevents the transplant community from making meaningful inferences about the risks and benefits of such strategies. Although the PK-boosting strategy can lead to serious adverse events, emerging evidence suggests that it may be advantageous for individuals with high CNI dose requirements. Hence, PK boosting may be an unmet need in the therapeutics of CNI. Nevertheless, there are several unanswered questions surrounding such use, and therefore, this merits testing in well-designed clinical studies. Moreover, drugs with better safer profiles and a history of successful PK boosting may be considered for evaluation with CNI.

Drug-Drug Interactions Involving Intestinal and Hepatic CYP1A Enzymes

Cytochrome P450 (CYP) 1A enzymes are considerably expressed in the human intestine and liver and involved in the biotransformation of about 10% of marketed drugs. Despite this doubtless clinical relevance, CYP1A1 and CYP1A2 are still somewhat underestimated in terms of unwanted side effects and drug–drug interactions of their respective substrates. In contrast to this, many frequently prescribed drugs that are subjected to extensive CYP1A-mediated metabolism show a narrow therapeutic index and serious adverse drug reactions. Consequently, those drugs are vulnerable to any kind of inhibition or induction in the expression and function of CYP1A. However, available in vitro data are not necessarily predictive for the occurrence of clinically relevant drug–drug interactions. Thus, this review aims to provide an up-to-date summary on the expression, regulation, function, and drug–drug interactions of CYP1A enzymes in humans.

Oleanolic Acid-Mediated Inhibition of Pregnane X Receptor and Constitutive Androstane Receptor Attenuates Rifampin-Isoniazid Cytotoxicity

Interactions between transcriptional inducers of cytochrome P450 (CYP450) and pharmacological agents might decrease drug efficacy and induce side effects. Such interactions could be prevented using an antagonist of the pregnane X receptor (PXR) and constitutive androstane receptor (CAR). Here, we aimed to determine the antagonistic effect of oleanolic acid (OA) on PXR and CAR. OA attenuated the promoter activities, expressions, and enzyme catalytic activities of CYP3A4 and CYP2B6 mediated by rifampin (RIF) and CITCO. Moreover, OA displayed species specificity for rodent PXR. Interaction of coregulators with PXR and transcriptional complexes on the CYP3A4 promoter was disrupted by OA. Additionally, OA reversed the cytotoxic effects of isoniazid induced by RIF. These data demonstrate that OA inhibited the transactivation of PXR and CAR, reduced the expression and function of CYP3A4 and CYP2B6, and may therefore serve as an effective agent for reducing probability adverse interactions between transcriptional inducers of CYP450 and therapeutic drugs.

Khellin and visnagin differentially modulate AHR signaling and downstream CYP1A activity in human liver cells

Khellin and visnagin are two furanochromones that can be frequently found in ethnomedical formulations in Asia and the Middle East. Both compounds possess anti-inflammatory and analgesic properties, therefore modern medicine uses these compounds or structurally related derivatives for treatment of vitiligo, bronchial asthma and renal colics. Despite their frequent usage, the potential toxic properties of visnagin and khellin are not well characterized up-to-now. Many natural compounds modulate the expression and activity of cytochrome P450 1A1 (CYP1A1), which is well-known to bioactivate pro-carcinogens. The expression of this enzyme is controlled by the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor and regulator of drug metabolism. Here, we investigated the influence of both furanochromones on AHR signaling in human HepG2 hepatocarcinoma cells and primary human hepatocytes. Both compounds transactivated xenobiotic response element (XRE)-driven reporter gene activity in a dose-dependent manner and induced CYP1A1 transcription in HepG2 cells and primary hepatocytes. The latter was abolished in presence of a specific AHR antagonist. CYP1A enzyme activity assays done in HepG2 cells and primary hepatocytes revealed an inhibition of enzyme activity by both furanochromones, which may become relevant regarding the metabolism of xenobiotics and co-administered therapeutic drugs. The observed induction of several other members of the AHR gene battery, whose gene products are involved in regulation of cell growth, differentiation and migration, indicates that a further toxicological characterization of visnagin and khelllin is urgently required in order to minimize potential drug-drug interactions and other toxic side-effects that may occur during therapeutic usage of these furanochromones.

Clinically significant pharmacokinetic interactions between dietary caffeine and medications

Caffeine from dietary sources (mainly coffee, tea and soft drinks) is the most frequently and widely consumed CNS stimulant in the world today. Because of its enormous popularity, the consumption of caffeine is generally thought to be safe and long term caffeine intake may be disregarded as a medical problem. However, it is clear that this compound has many of the features usually associated with a drug of abuse. Furthermore, physicians should be aware of the possible contribution of dietary caffeine to the presenting signs and symptoms of patients. The toxic effects of caffeine are extensions of their pharmacological effects. The most serious caffeine-related CNS effects include seizures and delirium. Other symptoms affecting the cardiovascular system range from moderate increases in heart rate to more severe cardiac arrhythmia. Although tolerance develops to many of the pharmacological effects of caffeine, tolerance may be overwhelmed by the nonlinear accumulation of caffeine when its metabolism becomes saturated. This might occur with high levels of consumption or as the result of a pharmacokinetic interaction between caffeine and over-the-counter or prescription medications. The polycyclic aromatic hydrocarbon-inducible cytochrome P450 (CYP) 1A2 participates in the metabolism of caffeine as well as of a number of clinically important drugs. A number of drugs, including certain selective serotonin reuptake inhibitors (particularly fluvoxamine), antiarrhythmics (mexiletine), antipsychotics (clozapine), psoralens, idrocilamide and phenylpropanolamine, bronchodilators (furafylline and theophylline) and quinolones (enoxacin), have been reported to be potent inhibitors of this isoenzyme. This has important clinical implications, since drugs that are metabolised by, or bind to, the same CYP enzyme have a high potential for pharmacokinetic interactions due to inhibition of drug metabolism. Thus, pharmacokinetic interactions at the CYP1A2 enzyme level may cause toxic effects during concomitant administration of caffeine and certain drugs used for cardiovascular, CNS (an excessive dietary intake of caffeine has also been observed in psychiatric patients), gastrointestinal, infectious, respiratory and skin disorders. Unless a lack of interaction has already been demonstrated for the potentially interacting drug, dietary caffeine intake should be considered when planning, or assessing response to, drug therapy. Some of the reported interactions of caffeine, irrespective of clinical relevance, might inadvertently cause athletes to exceed the urinary caffeine concentration limit set by sports authorities at 12 mg/L. Finally, caffeine is a useful and reliable probe drug for the assessment of CYP1A2 activity, which is of considerable interest for metabolic studies in human populations.

Cytochrome P450 isoforms involved in metabolism of the enantiomers of verapamil and norverapamil

AIMS

The present study was conducted to evaluate metabolism of the enantiomers of verapamil and norverapamil using a broad range of cytochrome P450 isoforms and measure the kinetic parameters of these processes.

METHODS

Cytochrome P450 cDNA-expressed cells and microsomes from a P450-expressed lymphoblastoid cell line were incubated with 40 microm concentrations of R- or S-verapamil and R- or S-norverapamil and metabolite formation measured by h.p.l.c. as an initial screening. Those isoforms exhibiting substantial activity were then studied over a range of substrate concentrations (2.5-450 microm ) to estimate the kinetic parameters for metabolite formation.

RESULTS

P450s 3A4, 3A5, 2C8 and to a minor extent 2E1 were involved in the metabolism of the enantiomers of verapamil. Estimated Km values for the production of D-617 and norverapamil by P450 s 3A4 and 3A5 were similar (range=60-127 microm ) regardless of the enantiomer of verapamil studied while the Vmax estimates were also similar (range=4-8 pmol min-1 pmol-1 P450). Only nominal production of D-620 by these isoforms was noted. Interestingly, P450 2C8 readily metabolized both S- and R-verapamil to D-617, norverapamil and PR-22 with only slightly higher Km values than noted for P450s 3A4 and 3A5. However, the Vmax estimates for P450 2C8 metabolism of S- and R-verapamil were in general greater (range=8-15 pmol min-1 pmol-1 P450) than those noted for P450 s 3A4 and 3A5 with preference noted for metabolism of the S-enantiomer. Similarly, P450 s 3A4, 3A5 and 2C8 also mediated the metabolism of the enantiomers of norverapamil with minor contributions by P450 s 2D6 and 2E1. P450s 3A4 and 3A5 readily formed the D-620 metabolite with generally a lower Km and higher Vmax for S-norverapamil than for the R-enantiomer. In contrast, P450 2C8 produced both the D-620 and PR-22 metabolites from the enantiomers of norverapamil, again with stereoselective preference seen for the S-enantiomer.

CONCLUSIONS

These results confirm that P450s 3A4, 3A5 and 2C8 play a major role in verapamil metabolism and demonstrate that norverapamil can also be further metabolized by the P450s.

Cytochromes of the P450 2C subfamily are the major enzymes involved in the O-demethylation of verapamil in humans

The calcium channel blocker verapamil [2,8-bis-(3,4-dimethoxyphenyl)-6-methyl-2-isopropyl-6-azaoctanitrile+ ++] undergoes extensive biotransformation in man. We have previously demonstrated cytochrome P450 (CYP) 3A4 and 1A2 to be the enzymes responsible for verapamil N-dealkylation (formation of D-617 [2-(3,4-dimethoxyphenyl)-5-methylamino-2-isopropylvaleronitrile], and verapamil N-demethylation (formation of norverapamil [2,8-bis-(3,4-dimethoxyphenyl)-2-isopropyl-6-azaoctanitrile]), while there was no involvement of CYP3A4 and CYP1A2 in the third initial metabolic step of verapamil, which is verapamil O-demethylation. This pathway yields formation of D-703 [2-(4-hydroxy-3-methoxyphenyl)-8-(3,4-dimethoxyphenyl)-6-methyl-2-isopro pyl-6-azaoctanitrile] and D-702 [2-(3,4-dimethoxyphenyl)-8-(4-hydroxy-3-methoxyphenyl)-6-methyl-2-isopro pyl-6-azaoctanitrile]. The enzymes catalyzing verapamil O-demethylation have not been characterized so far. We have therefore identified and characterized the enzymes involved in verapamil O-demethylation in humans by using the following in vitro approaches: (I) characterization of O-demethylation kinetics in the presence of the microsomal fraction of human liver, (II) inhibition of verapamil O-demethylation by specific antibodies and selective inhibitors and (III) investigation of metabolite formation in microsomes obtained from yeast strain Saccharomyces cerevisiae W(R), that was genetically engineered for stable expression of human CYP2C8, 2C9 and 2C18. In human liver microsomes (n=4), the intrinsic clearance (CLint), as derived from the ratio of Vmax/Km, was significantly higher for O-demethylation to D-703 compared to formation of D-702 following incubation with racemic verapamil (13.9 +/- 1.0 vs 2.4 +/- 0.6 ml*min-1*g-1, mean+/-SD; p<0.05), S-verapamil (16.8 +/- 3.3 vs 2.2 +/- 1.2 ml* min-1*g-1, p<0.05) and R-verapamil (12.1 +/- 2.9 vs 3.6 +/- 1.3 ml*min-1*g-1; p<0.05), thus indicating regioselectivity of verapamil O-demethylation process. The CLint of D-703 formation in human liver microsomes showed a modest but significant degree of stereoselectivity (p<0.05) with a S/R-ratio of 1.41 +/- 0.17. Anti-LKM2 (anti-liver/kidney microsome) autoantibodies (which inhibit CYP2C9 and 2C19) and sulfaphenazole (a specific CYP2C9 inhibitor) reduced the maximum rate of formation of D-703 by 81.5 +/- 4.5% and 45%, that of D-702 by 52.7 +/- 7.5% and 72.5%, respectively. Both D-703 and D-702 were formed by stably expressed CYP2C9 and CYP2C18, whereas incubation with CYP2C8 selectively yielded D-703. In conclusion, our results show that enzymes of the CYP2C subfamily are mainly involved in verapamil O-demethylation. Verapamil therefore has the potential to interact with other drugs which inhibit or induce these enzymes.

Identification of P450 enzymes involved in metabolism of verapamil in humans

The calcium channel blocker verapamil[2,8-bis-(3,4-dimethoxyphenyl)-6-methyl-2-isopropyl-6- azaoctanitrile] is widely used in the treatment of hypertension, angina pectoris and cardiac arrhythmias. The drug undergoes extensive and variable hepatic metabolism in man with the major metabolic steps comprising formation of D-617 [2-(3,4-dimethoxyphenyl)-5-methylamino-2-isopropylvaleronitrile] and norverapamil [2,8-bis-(3,4-dimethoxyphenyl)-2-isopropyl-6-azaoctanitrile]. The enzymes involved in metabolism of verapamil have not been characterized so far. Identification of these enzymes would enable estimation of both interindividual variability in verapamil metabolism introduced by the respective pathway and potential for metabolic interactions. We therefore characterized the enzymes involved in formation of D-617 and norverapamil. The maximum rate of formation of D-617 and norverapamil was determined in the microsomal fraction of 21 human livers which had been previously characterized for the individual expression of various P450 enzymes (CYP1A2, CYP2C, CYP2D6, CYP2E1 and CYP3A3/4) by means of Western blotting. Specific antibodies directed against CYP3A were used to inhibit formation of D-617 and norverapamil. Finally, formation of both metabolites was investigated in microsomes obtained from yeast cells which were genetically engineered for stable expression of human P450. Formation of D-617 was correlated with the expression of CYP3A (r = 0.85; P < 0.001) and CYP1A2 (r = 0.57; P < 0.01) in the microsomal fraction of 21 human livers after incubation with racemic verapamil.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effect of grapefruit juice and its bitter principal, naringenin, on CYP1A2 dependent metabolism of caffeine in man

1. The effects of grapefruit juice and naringenin on the activity of the human cytochrome P450 isoform CYP1A2 were evaluated using caffeine as a probe substrate. 2. In vitro naringin was a potent competitive inhibitor of caffeine 3-demethylation by human liver microsomes (Ki = 7-29 microM). 3. In vivo grapefruit juice (1.2 l day-1 containing 0.5 g l-1 naringin, the glycone form of naringenin) decreased the oral clearance of caffeine by 23% (95% CI: 7%-30%) and prolonged its half-life by 31% (95% CI: 20%-44%) (n = 12). 4. We conclude that grapefruit juice and naringenin inhibit CYP1A2 activity in man. However, the small effect on caffeine clearance in vivo suggests that in general the ingestion of grapefruit juice should not cause clinically significant inhibition of the metabolism of other drugs that are substrates of CYPIA2.