Phenobarbital increases monkey in vivo nicotine disposition and induces liver and brain CYP2B6 protein

Cytochrome P450 enzymes and metabolism of drugs and neurotoxins within the mammalian brain

Cytochrome P450 enzymes (CYPs) that metabolize xenobiotics are expressed and active in the brain. These CYPs contribute to the metabolism of many centrally acting compounds, including clinically used drugs, drugs of abuse, and neurotoxins. Although CYP levels are lower in the brain than in the liver, they may influence central substrate and metabolite concentrations, which could alter resulting centrally-mediated responses to these compounds. Additionally, xenobiotic metabolizing CYPs are highly variable due to genetic polymorphisms and regulation by endogenous and xenobiotic molecules. In the brain, these CYPs are sensitive to xenobiotic induction. As a result, CYPs in the brain vary widely, including among humans, and this CYP variation may influence central metabolism and resulting response to centrally acting compounds. It has been demonstrated, using experimental manipulation of CYP activity in vivo selectively within the brain, that CYP metabolism in the brain alters central substrate and metabolite concentrations, as well as drug response and neurotoxic effects. This suggests that variability in xenobiotic metabolizing CYPs in the human brain may meaningfully contribute to individual differences in response to, and effects of, centrally acting drugs and neurotoxins. This chapter will provide an overview of CYP expression in the brain, endogenous- and xenobiotic-mediated CYP regulation, and the functional impact of CYP-mediated metabolism of drugs and neurotoxins in the brain, with a focus on experimental approaches in mice, rats, and non-human primates, and a discussion regarding the potential role of xenobiotic metabolizing CYPs in the human brain.

Use of a physiologically-based pharmacokinetic model to explore the potential disparity in nicotine disposition between adult and adolescent nonhuman primates

The widespread use and high abuse liability of tobacco products has received considerable public health attention, in particular for youth, who are vulnerable to nicotine addiction. In this study, adult and adolescent squirrel monkeys were used to evaluate age-related metabolism and pharmacokinetics of nicotine after intravenous administration. A physiologically-based pharmacokinetic (PBPK) model was created to characterize the pharmacokinetic behaviors of nicotine and its metabolites, cotinine, trans-3'-hydroxycotinine (3'-OH cotinine), and trans-3'-hydroxycotinine glucuronide (3'-OH cotinine glucuronide) for both adult and adolescent squirrel monkeys. The PBPK nicotine model was first calibrated for adult squirrel monkeys utilizing in vitro nicotine metabolic data, plasma concentration-time profiles and cumulative urinary excretion data for nicotine and metabolites. Further model refinement was conducted when the calibrated adult model was scaled to the adolescents, because adolescents appeared to clear nicotine and cotinine more rapidly relative to adults. More specifically, the resultant model parameters representing systemic clearance of nicotine and cotinine for adolescent monkeys were approximately two- to three-fold of the adult values on a per body weight basis. The nonhuman primate PBPK model in general captured experimental observations that were used for both model calibration and evaluation, with acceptable performance metrics for precision and bias. The model also identified differences in nicotine pharmacokinetics between adolescent and adult nonhuman primates which might also be present in humans.

In vitro metabolism of magnolol and honokiol in rat liver microsomes and their interactions with seven cytochrome P substrates

RATIONALE

Magnolol and honokiol are the main active components of Magnolia officinalis Rehd. et Wils. The study of their interactions with liver microsomes is very important for the clinical safety of M. officinalis Rehd. et Wils.

METHODS

The main metabolites of magnolol and honokiol in rat liver microsomes were investigated using ultrahigh-performance liquid chromatography/mass spectrometry and their possible structures were identified. In addition, cytochrome P450 (CYP450) isoenzymes of the major rat metabolites of magnolol and honokiol were identified using a specific inhibitor.

RESULTS

This study suggests that the CYP2E1 subtype is responsible for the oxidation of magnolol and honokiol terminal double bonds to epoxy metabolites. CYP3A4 appears to be the major subtype responsible for further hydrolytic metabolism, while CYP1A2 may promote decarboxylation of the metabolites. CYP2A6 may be the main subtype responsible for the hydrogenation of magnolol (p < 0.05).

CONCLUSIONS

This study demonstrated that different CYP450 enzyme isoforms showed different activities in the in vitro metabolism of magnolol and honokiol in rat liver microsomes. It has certain practical applications in that we should pay attention to drug-drug interactions in clinical medications and also to drug-enzyme interactions.

Neuropsychometric correlates of efavirenz pharmacokinetics and pharmacogenetics following a single oral dose

AIMS

To determine pharmacokinetic and pharmacogenomic correlates of efavirenz central nervous system (CNS) side effects following a single dose.

METHODS

Thirty-four healthy HIV-negative African Americans were administered a 600 mg dose of efavirenz. Blood samples for pharmacokinetics were drawn serially from 0 to 12 h post-dose. Neuropsychometric testing with drowsiness visual analogue scale, grooved pegboard and letter digit substitution tests was done the day prior to dosing and at 1, 2, 3, 4 and 6 h post-dose. Subjective CNS symptoms were assessed at 6 h post-dose. Composite CYP2B6 516/983 genotype was determined.

RESULTS

Pharmacokinetic indices reflecting increased plasma efavirenz exposure were associated with slower non-dominant hand grooved pegboard task completion (Cmax , P1 h  = 0.01, P2 h  = 0.05, P3 h  = 0.03, P4 h  = 0.01; AUC, P1 h  = 0.04; clearance P1 h  = 0.05, P2 h  = 0.02, P6 h  = 0.01). In a repeated measures model analysis that adjusted timing of neuropsychometric testing for timing of peak drug concentration, clearance (P < 0.001), AUC(0.312 h) (P = 0.001) and Cmax (P = 0.008) were associated with non-dominant grooved pegboard test performance. CYP2B6 genotype trended to correlate with non-dominant hand grooved pegboard at 4 and 6 h (P = 0.07 and 0.06). Decreased drowsiness at 6 h was associated with higher Cmax (P = 0.02).

CONCLUSIONS

Following a single dose of efavirenz, an association between pharmacokinetics and neuropsychometric performance was discernable. A weaker association between genotype and neurocognitive test performance is likely mediated by effect of genotype on plasma clearance. Strategies that lower Cmax during initial dosing may decrease CNS side effects.

Identification and characterization of CYP2B6 cDNA in cynomolgus macaques (Macaca fascicularis)

Cytochrome P450 2B6 (CYP2B6), an important drug-metabolizing enzyme, is involved in the metabolism of prescribed drugs in humans. Despite its importance, cDNA for a CYP2B6 ortholog has not been identified and characterized in cynomolgus macaques, which are frequently used in preclinical studies. In this study, cDNA highly homologous to human CYP2B6 was cloned from the cynomolgus macaque liver. This cDNA contained an open reading frame of 491 amino acids and functional domains characteristic for CYP protein, such as substrate recognition sites and a heme-binding region. Cynomolgus CYP2B6 was expressed predominantly in the liver with some extra-hepatic expression among 10 tissues. Moreover, cynomolgus CYP2B6 revealed activities toward testosterone 16beta-hydroxylation and bupropion hydroxylation. These results suggest that cynomolgus CYP2B6 has a functional role in the liver.

Molecular genetics of nicotine metabolism

The molecular genetics of nicotine metabolism involves multiple polymorphic catalytic enzymes. Variation in metabolic pathways results in nicotine disposition kinetics that differ between individuals and ethnic groups. Twin studies indicate that a large part of this variance is genetic in origin, although environmental influences also contribute. The primary aim of this chapter is to review the current knowledge regarding the genetic variability in the enzymes that metabolize nicotine in humans. The focus is on describing the genetic polymorphisms that exist in cytochromes P450 (CYPs), aldehyde oxidase 1 (AOX1), UDP-glucuronosyltransferases (UGTs), and flavin-containing monooxygenase 3 (FMO3). Genetic studies have demonstrated that polymorphisms in CYP2A6, the primary enzyme responsible for nicotine breakdown, make a sizable contribution to the wide range of nicotine metabolic capacity observed in humans. Thus, special attention will be given to CYP2A6, because slower nicotine metabolism requires less frequent self-administration, and accordingly influences smoking behaviors. In addition, the molecular genetics of nicotine metabolism in nonhuman primates, mice, and rats will be reviewed briefly.

Diclofenac hydroxylation in monkeys: Efficiency, regioselectivity, and response to inhibitors

The catalytic efficiency, regioselectivity, and response to chemical inhibitors of diclofenac (DF) hydroxylation in three Old World monkey liver microsomes (rhesus, cynomolgus, and African green monkey) are different from those determined with human liver microsomes. In contrast to the high affinity-high capacity (low Km-high Vmax) characteristics of DF 4'-hydroxylation in humans, this reaction proceeded in all monkey species with catalytic efficiencies >20-fold lower. However, DF 5-hydroxylation, a negligible reaction in human liver microsomes, was kinetically favored in monkeys mainly due to the increased Vmax values. Chemical inhibitors (reversible or mechanism-based) selective to human CYP3A4 and CYP2C9 failed to differentiate monkey orthologs involved in DF hydroxylation. Immunoinhibition studies with monoclonal antibodies against human CYPs revealed the major contribution of CYP2C and CYP3A to 4'- and to 5-hydroxylation, respectively, in rhesus and cynomolgus liver microsomes. However, in African green monkeys, in addition to CYP2C, CYP3A also appeared to be involved in 4'-hydroxylation. Further studies with recombinant rhesus and African green monkey CYP2C and CYP3A enzymes (rhesus CYP2C75, 2C74, and 3A64; African green monkey CYP2C9agm and CYP3A4agm) confirmed the major role of CYP enzymes of these two subfamilies in DF 4'- and 5-hydroxylation. Clearly, while monkey CYP2C and 3A enzymes retain the same substrate selectivity towards DF hydroxylation as their human orthologs, their altered catalytic efficiency and response to chemical inhibitors may indicate different structural features of active sites as opposed to human orthologs.

Expression, microsomal and mitochondrial activities of cytochrome P450 enzymes in brain regions from control and phenobarbital-treated rabbits

Expression and monooxygenase activity of various cytochrome P450 (CYP) enzymes along with constitutive androstane (CAR) and the pregnane X (PXR) receptors were investigated in the brain of control and phenobarbital-treated rabbits (80 mg/kg for 4 days). RT-PCR analysis, using specific primers, demonstrated that in control rabbits mRNAs of CYP 2A10, 2B4/5 and 3A6 were expressed, though to a different extent, in the liver, as well as in brain cortex, midbrain, cerebellum, striatum, hippocampus and hypothalamus, whilst CYP2A11 and 4B1 were not expressed in the hypothalamus. CAR was expressed in liver and all the brain regions examined, whereas the PXR was expressed only in liver and cortex. Real time RT-PCR analysis demonstrated that in vivo treatment with phenobarbital, in contrast with what happened in liver, did not induce the expression of CYP 2B4/5 mRNA in cortex, midbrain and cerebellum. NADPH cytochrome c reductase and some other enzymatic activities markers of CYP 2A, 2B, 3A and 4B activities were studied in liver microsomes as well as in microsomes and mitochondria of brain cortex, midbrain and cerebellum of control and phenobarbital-treated rabbits. In contrast to what was observed in liver, phenobarbital treatment did not induce the aforementioned monooxygenase activities in brain. However, we cannot exclude that a longer phenobarbital treatment may lead to a significant induction of CYP activities in brain. These findings indicated that brain CYPs, despite the presence of CAR, were resistant to phenobarbital induction, indicating a possible different regulation of these enzymes between brain and liver.