Cytochrome p450 phenotyping/genotyping in patients receiving antipsychotics: useful aid to prescribing?

Potential inhibition of cytochrome P450 3A4 by propofol in human primary hepatocytes

AIM: Hepatic cytochrome P450 isoenzymes constitute a superfamily of hemoproteins that play a major role in the metabolism of endogenous compounds and in the detoxification of xenobiotic molecules. P450 3A4 is one of the most important forms in human being, and mediates the metabolism of around 70% of therapeutic drugs and endogenous compounds. Propofol, a widely used intravenous anesthetic drug, is known to inhibit cytochrome P450 activities in isolated rat hepatocytes. The goal of this study was to evaluate the potential efficacy of propofol on P450 3A4 in a dose-dependent manner to understand its drug-drug interaction.

METHODS: Hepatocytes were isolated from liver specimens from hepatic angioma patients undergone hepatic surgery. Primary incubated hepatocytes were treated with 0, 0.01, 0.05, 0.1, 0.5, and 1.0 mM propofol for 24 hours. P450 3A4 activity was measured with Nash’s colorimetry. The protein expression was assessed by Western blot analysis.

RESULTS: A dose-dependent inhibitory effect of propofol was observed in cytochrome P450 3A4 activity. A minimal dosage of propofol (0.01 mM) induced a significant inhibition of P450 3A4 activity, although its regular dosages (0.01-0.1 mM) showed no inhibitory effect on the cellular protein expression of P450 3A4.

CONCLUSION: Propofol may be a potential CYP3A4 inhibitor as this anesthetic can inhibit isoenzyme activity significantly and reduce the metabolic rate of CYP3A4 substrates. This inhibition occurs at post-expression level, and concentration of propofol used clinically does not affect CYP3A4 protein expression. propofol may thus induce drug interaction of cytochrome P450 3A4 activity at the dosage used clinically.

Thioridazine steady-state plasma concentrations are influenced by tobacco smoking and CYP2D6, but not by the CYP2C9 genotype

BACKGROUND

Approximately 7% of Caucasians have genetically impaired activity of the cytochrome P450 enzyme CYP2D6 and are classified as poor metabolizers (PM). The disposition of thioridazine has been related to the CYP2D6 phenotype. The present study aimed to evaluate the influence of CYP2D6 and CYP2C9 genotypes, and tobacco smoking on steady-state thioridazine plasma levels.

METHODS

Seventy-six Caucasian psychiatric patients receiving thioridazine monotherapy were studied. Debrisoquine metabolic ratio (MR) and steady-state plasma levels of thioridazine and its metabolites, mesoridazine and sulforidazine, as well as CYP2D6 (in 74 patients) and CYP2C9 (in 63 patients) genotypes were determined.

RESULTS

The median dose-corrected, steady-state plasma concentrations (C/D) of thioridazine were related to the number of functional CYP2D6 alleles ( P<0.01), being 15.2, 7.2, 4.0, 4.2 nmol/l per milligram in subjects with no, one, two, and three or more functional CYP2D6 genes, respectively. No significant differences were found in the C/Ds of mesoridazine or sulforidazine. No relationship was found between CYP2C9 genotype and plasma levels of thioridazine or its metabolites. The median C/D of thioridazine was significantly ( P<0.001) lower in smokers (4.0 nmol/l per milligram, range: 1.0-15.5; n=58) than in nonsmokers (7.4 nmol/l per milligram, range: 2.8-23.6; n=18). Also, the C/Ds of mesoridazine and sulforidazine were lower in smokers ( P<0.01). The plasma thioridazine/mesoridazine ratio significantly correlated with the debrisoquine MR ( r(2)=0.30, P<0.001).

CONCLUSION

The results show that the plasma concentrations of thioridazine and its metabolites are influenced by tobacco smoking and the CYP2D6 genotype, and support the dose-dependent inhibition of CYP2D6 by thioridazine. CYP2C9 does not play an important role in thioridazine metabolism.

Cytochrome P450 in vitro reaction phenotyping: a re-evaluation of approaches used for P450 isoform identification

Marker substrates, chemical inhibitors, and inhibitory antibodies are important tools for the identification of cytochrome P450 (P450) isoform responsible for the metabolism of therapeutic agents in vitro. In view of the versatile and nonspecific nature of P450 enzymes, many of the marker substrates and chemical inhibitors used for P450 in vitro reaction phenotyping are isoform selective but not specific. Recently, the use of marker substrate and chemical inhibitors in CYP2D6 in vitro reaction phenotyping was questioned by Granvil et al. (2002). In comparison of a panel of 15 recombinant P450 enzymes, they found that in addition to CYP2D6, CYP1A1 is also capable of catalyzing the formation of 4-hydroxylated metabolite of debrisoquine and that the intrinsic clearance of debrisoquine by CYP2D6-mediated 4-hydroxylation is only about twice that by CYP1A1. In their study, they have also demonstrated that quinidine inhibits both CYP2D6- and CYP1A1-mediated debrisoquine 4-hydroxylation. In view of these important findings, we have reevaluated various approaches used to identify P450 isoform(s) responsible for the metabolism of therapeutic agents. While acknowledging the value of inhibitory antibodies in P450-phenotyping studies, it is our opinion that in well conducted in vitro experiments, isoform-selective chemical inhibitors can also provide valuable and reliable information. Hopefully, future efforts may produce even better P450 isoform-selective marker substrates and inhibitors.

Pharmacogenetics of drug metabolising enzymes: importance for personalised medicine

The number of polymorphisms identified in genes encoding drug metabolising enzymes, drug transporters, and receptors is rapidly increasing. In many cases, these genetic factors have a major impact on the pharmacokinetics and pharmacodynamics of a particular drug and thereby influence the sensitivity to such drug in an individual patient with a certain genotype. The highest impact is seen for drugs with a narrow therapeutic index, with important examples emerging from treatment with antidepressants, oral anticoagulants, and cytostatics, which are metabolised by the polymorphic enzymes cytochrome P450 2D6 (CYP2D6), cytochrome P450 2C9 (CYP2C9), and thiopurine-S-methyltransferase (TPMT), respectively. In order to apply the increasing amount of pharmacogenetic knowledge to clinical practise, specific dosage recommendations based on genotypes will have to be developed to guide the clinician, and these recommendations will have to be evaluated in prospective clinical studies. Such development will lead to a patient-tailored drug therapy which hopefully would be more efficient and will result in fewer adverse drug reactions.

Different alterations of cytochrome P450 3A4 isoform and its gene expression in livers of patients with chronic liver diseases

AIM: To determine whether parenchymal cells or hepatic cytochrome P450 protein was changed in chronic liver diseases, and to compare the difference of CYP3A4 enzyme and its gene expression between patients with hepatic cirrhosis and obstructive jaundice, and to investigate the pharmacologic significance behind this difference.

METHODS: Liver samples were obtained from patients undergoing hepatic surgery with hepatic cirrhosis (n = 6) and obstructive jaundice (n = 6) and hepatic angeioma (controls, n = 6). CYP3A4 activity and protein were determined by Nash and western bloting using specific polychonal antibody, respectively. Total hepatic RNA was extracted and CYP3A4cDNA probe was prepared according the method of random primer marking, and difference of cyp3a4 expression was compared among those patients by Northern blotting.

RESULTS: Compared to control group, the CYP3A4 activity and protein in liver tissue among patients with cirrhosis were evidently reduced. (P < 0.01) Northern blot showed the same change in its mRNA levels. In contrast, the isoenzyme and its gene expression were not changed among patients with obstructive jaundice.

CONCLUSION: Hepatic levels of P450s and its CYP3A4 isoform activity were selectively changed in different chronic liver diseases. CYP3A4 isoenzyme and its activity declined among patients with hepatic cirrhosis as expression of cyp3a4 gene was significantly reduced. Liver's ability to eliminate many clinical therateutic drug substrates would decline consequently, These findings may have practical implications for the use of drugs in patients with cirrhosis and emphasize the need to understand the metabolic fate of therapeutic compounds. Elucidation of the reasons for these different changes in hepatic CYP3A4 may provide insight into more fundamental aspects and mechanisms of imparied liver function.

Development of neuroleptic agents: pharmacogenetics and current safety issues of regulatory concern

The development of safe and effective new drug treatments for schizophrenia poses a challenging task. This class of drugs is known to be associated with a wide range of serious and troublesome safety problems that include neurological, cardiac, endocrine, and metabolic side effects. Many of these drugs have a narrow therapeutic index and generate metabolites that often have their own unique pharmacological profile different from the parent compound. These features make it imperative that the optimal dose schedules for neuroleptic drugs are carefully characterized. Many of these drugs are metabolized by cytochrome P450 enzymes, which show genetic polymorphism and a bi modal distribution within the population, A significant subset of the population cannot eliminate these drugs as effectively as the majority. This brings an added dimension of complexity in characterizing the dose and individualizing therapy. Many neuroleptic agents are proarrhythmic with an adverse effect on cardiac repolarization. They are prone to prolonging the QT interval and inducing torsade de pointes. Given the potentially fatal outcome of this ventricular tachyarrhythmia, drug development programs need to ensure that the proarrhythmic potential of any new neuroleptic agent is thoroughly explored and its proarrhythmic risk characterized. The clinical use of many of these drugs is further troubled by their high potential for drug-drug interactions. These too need to be adequately investigated during development The approval and the labeling of a new neuroleptic agent require a careful regulatory assessment of its risk/benefit ratio in comparison with the available alternatives. Their safe and effective use in routine clinical practice depends on careful attention to prescribing information, especially the contraindications, precautions, and patient-monitoring requirements.