Preferential inhibition of CYP1A enzymes in hepatic microsomes by mexiletine

CYP1A inhibitors: Recent progress, current challenges, and future perspectives

Mammalian cytochrome P450 1A (CYP1A) are key phase I xenobiotic-metabolizing enzymes that play a distinctive role in metabolic activation or metabolic clearance of a variety of procarcinogens, drugs, and endogenous substances. Human CYP1A subfamily contains two members (hCYP1A1 and hCYP1A2), which are known to catalyze the oxidative activation of some environmental procarcinogens into carcinogenic species. Increasing evidence has demonstrated that CYP1A inhibitor therapies are promising strategies for cancer chemoprevention or overcoming CYP1A-associated drug toxicity and resistance. Herein, we reviewed recent advances in the discovery and characterization of hCYP1A inhibitors, from the discovery approaches to structural features and biomedical applications of hCYP1A inhibitors. The inhibition potentials, inhibition modes, and inhibition constants of all reported hCYP1A inhibitors are comprehensively summarized. Meanwhile, the structural features and structure-activity relationships of different classes of hCYP1A1 and hCYP1A2 inhibitors are analyzed and discussed in depth. Furthermore, the major challenges and future directions for this field are presented and highlighted. Collectively, the information and knowledge presented here will strongly facilitate the researchers to discover and develop more efficacious CYP1A inhibitors for specific purposes, such as chemo-preventive agents or as tool molecules in hCYP1A-related fundamental studies.

Smoking cessation and duloxetine toxicity: A case report

OBJECTIVE

Psychiatric medications are among the drugs affected by cigarette smoke, and because of the high prevalence of smoking in the psychiatric community, psychiatric patients are particularly at risk when entering the smoke-free environment of a hospital.

CASE SUMMARY

Polycyclic aromatic hydrocarbons present in cigarette smoke are well-known inducers of cytochrome P450 (CYP) enzymes. CYP1A2 induction by cigarette smoking increases metabolic clearance of substrates of this enzyme; smoking cessation results in a decrease in CYP1A2 activity, which causes an increase in serum substrate levels. Although these effects are well documented, the effect of smoking cessation remains a hidden danger, in part, because computerized drug interaction surveillance systems are not designed to alert clinicians when an offending agent is discontinued.

PRACTICE IMPLICATION

To raise awareness and demonstrate the importance of a thorough assessment of smoking status, we present a case in which the temporal relationship of smoking cessation to the emergence of nausea, vomiting, and tachycardia 3 days later (day 3); the development of myoclonic jerks by day 5; and the resolution of symptoms on reduction of duloxetine dosage on day 44 implicates duloxetine toxicity in a patient with ischemic cardiomyopathy awaiting implantation of a left ventricular assist device.

Cytochrome P450 family 1 inhibitors and structure-activity relationships

With the widespread use of O-alkoxyresorufin dealkylation assays since the 1990s, thousands of inhibitors of cytochrome P450 family 1 enzymes (P450s 1A1, 1A2, and 1B1) have been identified and studied. Generally, planar polycyclic molecules such as polycyclic aromatic hydrocarbons, stilbenoids, and flavonoids are considered to potentially be effective inhibitors of these enzymes, however, the details of the structure-activity relationships and selectivity of these inhibitors are still ambiguous. In this review, we thoroughly discuss the selectivity of many representative P450 family 1 inhibitors reported in the past 20 years through a meta-analysis.

Effects of mexiletine, a CYP1A2 inhibitor, on tizanidine pharmacokinetics and pharmacodynamics

The aim of this study was to determine whether mexiletine, a CYP1A2 inhibitor, altered the pharmacokinetics and pharmacodynamics of tizanidine. The pharmacokinetics of tizanidine were examined in an open-label study in 12 healthy participants after a single dose of tizanidine (2 mg) with and without mexiletine coadministration (50 mg, 3 times as a pretreatment for a day and 2 times on the study day). Compared with tizanidine alone, mexiletine coadministration increased the peak plasma concentration (1.8 +/- 0.8 vs 5.3 +/- 1.8 ng/mL), area under the curve (4.5 +/- 2.2 vs 15.4 +/- 6.5 ng x h/mL), and the half-life (1.3 +/- 0.2 vs 1.8 +/- 0.7 h) of tizanidine, respectively (P < .05). Reduction in systolic blood pressure (-10 +/- 8 vs -24 +/- 7 mm Hg) and diastolic blood pressure (-10 +/- 7 vs -18 +/- 8 mm Hg) after tizanidine administration was also significantly enhanced by coadministration of mexiletine (P < .01). Of the 15 patients treated with tizanidine and mexiletine, 4 suffered tizanidine-induced adverse effects such as drowsiness and dry mouth in the retrospective survey. Present results suggested that coadministration of mexiletine increased blood tizanidine concentrations and enhanced tizanidine pharmacodynamics in terms of reduction in blood pressure and adverse symptoms.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Comparative molecular field analysis and QSAR on substrates binding to cytochrome P450 2D6

In this study, we utilized comparative molecular field analysis (CoMFA) to gain a better understanding of the steric and electrostatic features of the cytochrome p450 2D6 (CYP2D6) active site. The training set consists of 24 substrates with reported K(M) values from liver microsomal CYP2D6 spanning an activity range of almost three log units. The low energy conformers were fit by root mean square (RMS) to minaprine at the site of metabolism and to the protonated nitrogen. In this manner, we constructed two CoMFA models, one model with a distance constraint and another without. The model with the distance parameter (non-cross-validated R(2)=0.99) was approximately equal to the CoMFA without a distance parameter (non-cross-validated R(2)=0.98). Validation of our CoMFA was accomplished by predicting the K(M) values of 15 diverse CYP2D6 substrates not in the original training set resulting in a predictive R(2)=0.62. Finally, we also pursued correlations of pK(a) and log P with CYP2D6 substrate K(M) in an effort to investigate other physicochemical properties.

4-Hydroxylation of debrisoquine by human CYP1A1 and its inhibition by quinidine and quinine

A panel of 15 recombinant cytochromes P450 expressed in human B-lymphoblastoid cells was used to study debrisoquine 4-hydroxylation. Both CYP2D6 and CYP1A1 carried out the reaction. The apparent K(m) (micromolar) and V(max) (picomoles per minute per picomole of P450) for CYP2D6 were 12.1 and 18.2 and for CYP1A1 were 23.1 and 15.2, respectively. CYP1A1 debrisoquine 4-hydroxylase was inhibited by the CYP1A1 inhibitor alpha-naphthoflavone and the CYP1A1 substrate 7-ethoxyresorufin. Additionally and surprisingly, this reaction was also inhibited by quinidine and quinine, with respective IC(50) values of 1.38 +/- 0.10 and 3.31 +/- 0.14 microM, compared with those for CYP2D6 debrisoquine 4-hydroxylase of 0.018 +/- 0.05 and 3.75 +/- 2.07 microM, respectively. Anti-CYP1A1 monoclonal antibody (mAb) 1-7-1 abolished CYP1A1 debrisoquine hydroxylase and anti-CYP2D6 mAb 50-1-3 eradicated CYP2D6 debrisoquine 4-hydroxylase. Three further CYP2D6-specific reactions were tested: dextromethorphan O-demethylation, bufuralol 1'-hydroxylation, and sparteine dehydrogenation. The CYP2D6 specificity, judged by the CYP2D6/CYP1A1 activity ratios was 18.5, 7.0, 6.0, and 1.6 for dextromethorphan, bufuralol, sparteine, and debrisoquine, respectively. Thus, debrisoquine is not a specific CYP2D6 substrate and quinidine is not a specific CYP2D6 inhibitor. These findings have significant implications for the conduct of in vitro drug metabolism inhibition studies and underscore the fallacy of "specific chemical inhibitors" of a supergene family of enzymes that have overlapping substrate specificities. The use of highly specific mAbs in such studies is mandated. It is unclear as yet whether these findings have implications for the relationship between CYP2D6 genotype and in vivo debrisoquine 4-hydroxylase activity.