Pharmacokinetics and drug interactions with zonisamide

Polypharmacy is a widely employed treatment strategy in epilepsy, particularly for individuals with poorly controlled seizures. Drug combinations should be carefully considered to minimize the potential for unfavorable interactions. Older-generation antiepileptic drugs (AEDs) are well known for their pharmacokinetic interaction potential, which generally results from alterations in the metabolism of concomitant drugs due to effects on the cytochrome P450 (CYP) and uridine glucuronyl transferase enzyme systems. Newer agents, such as zonisamide, are less likely to cause adverse drug interactions. A series of interaction studies has revealed zonisamide to be without effect on the steady-state pharmacokinetics of carbamazepine, phenytoin, sodium valproate, or lamotrigine. However, zonisamide is principally inactivate by CY3A4-dependent reduction. Consequently, carbamazepine, phenytoin, and phenobarbital all increase its clearance, an interaction that may necessitate a dosage increase, but which will also permit more rapid attainment of steady-state zonisamide concentrations. Otherwise, zonisamide is essentially devoid of clinically significant interactions with other AEDs, oral contraceptives and, indeed, all other classes of therapeutic agents investigated to date. As a result, it is reasonable to conclude that zonisamide has a favorable pharmacokinetic profile and that it may be a useful and uncomplicated agent when employed as adjunctive therapy in refractory epilepsy.     

Testosterone Metabolism In Rat Brain Is Differentially Enhanced By Phenytoin-Inducible Cytochrome P450 Isoforms

Many cytochrome P450 (P450) isoforms are selectively inducible by xenobiotics, e.g. pharmaceuticals like the anti-epileptic drug phenytoin. Some of these P450 enzymes are involved in the metabolism of gonadal hormones and are of great importance, especially in early brain development. In this study, the hydroxylation of testosterone by rat brain microsomes from control and phenytoin-induced animals was examined by use of high performance liquid chromatography (HPLC) provided with a photodiode array detector (PDA). In control rats, testosterone is converted by cytochrome(s) P450 to 6alpha-hydroxytestosterone (OHT) as the main metabolite and 6beta-OHT as well as androstenedione as minor metabolites. After phenytoin treatment, brain microsomes showed a strong increase of testosterone metabolism to 2alpha-, 6beta-, 16alpha-, 16beta-OHT and androstenedione, whereby 16alpha-OHT was the main degradation product. These metabolites indicated the action of isoforms of the P450 subfamilies CYP2B, CYP2C and CYP3A. Inhibition experiments with antibodies against CYP2B1/2 and with the CYP2B specific inhibitor orphenadrine indicated the occurrence of members of this subfamily which are known to catalyse the oxidation of testosterone to 16alpha-OHT, 16beta-OHT and androstenedione. Western blots revealed the phenytoin-inducible expression of CYP2B1 and the constitutive expression of CYP3A. The latter is involved in the 6beta-hydroxylation of testosterone which was found correspondingly in control microsomes. Distinct CYP2C isoforms involved in the hydroxylation of testosterone in phenytoin-induced microsomes are not yet identified. The highly increased testosterone metabolism by phenytoin-dependent induction of specific cytochrome P450 isoforms in adult rat brain illustrates the potential influence of exogenous substances on internal regulative and metabolic pathways in the brain.     

Cytochrome P-450 activities in human and rat brain microsomes

The role of cytochrome P450 in the metabolism of dextromethorphan, amitriptyline, midazolam, S-mephenytoin, citalopram, fluoxetine and sertraline was investigated in rat and human brain microsomes. Depending on the parameters, the limit of quantification using gas chromatography-mass spectrometry methods was between 1.6 and 20 pmol per incubation, which generally contained 1500 microg protein. Amitriptyline was shown to be demethylated to nortriptyline by both rat and human microsomes. Inhibition studies using ketoconazole, furafylline, sulfaphenazole, omeprazole and quinidine suggested that CYP3A4 is the isoform responsible for this reaction whereas CYP1A2, CYP2C9, CYP2C19 and CYP2D6 do not seem to be involved. This result was confirmed by using a monoclonal antibody against CYP3A4. Dextromethorphan was metabolized to dextrorphan in rat brain microsomes and was inhibited by quinidine and by a polyclonal antibody against CYP2D6. Only the addition of exogenous reductase allowed the measurement of this activity in human brain microsomes. Metabolites of the other substrates could not be detected, possibly due to an insufficiently sensitive method. It is concluded that cytochrome P450 activity in the brain is very low, but that psychotropic drugs could undergo a local cerebral metabolism which could have pharmacological and/or toxicological consequences.     

Lack of interaction between cimetidine and carbamazepine

In order to investigate a possible interaction between cimetidine and carbamazepine (CBZ) 7 otherwise healthy epileptic patients on a long-term monotherapy with CBZ were given cimetidine (1 g daily) for 7 days. No significant alterations in steady-state plasma concentration of CBZ and the 10,11-epoxide metabolite (CBZ-E) were demonstrated.     

Modulation of androgen and estrogen receptor expression by antiepileptic drugs and steroids in hippocampus of patients with temporal lobe epilepsy

Purpose:   Many of the antiepileptic drugs (AED) used in therapy of temporal lobe epilepsy (TLE) are known as cytochrome P450 (CYP, P450) inducers. These AEDs are thought to modulate androgen and estrogen pathways in hippocampus, and therefore cause mental and reproductive disorders found in TLE patients. In the present study, we analyzed expression of androgen receptor (AR), estrogen receptor α (ERα), and CYP3A in the hippocampus of TLE patients and in murine hippocampal cell line HN25.1.
Methods:   Patients and cell lines had been treated with P450-inducing or noninducing AEDs, or with prednisolone, applied to prevent oedema formation prior to neurosurgical resection of the epileptic hippocampus. Human patient samples were analyzed by immunohistochemical approach, the HN25.1 cell line by quantitative RT-PCR, CAT reporter gene assay, and immunoblot.
Results:   In both, humans and cell lines, the expression of testosterone metabolising CYP3A4 (human) or CYP3A11 (mouse) and AR was up-regulated when P450-inducing AEDs and/or prednisolone had been applied. AR responsive CAT reporter gene assay indicated an increase of AR-signalling after treatment of the HN25.1 cells with the P450-inducers phenytoin and carbamazepine. ERα expression was increased only by the P450-inducing AEDs, but not by prednisolone, which indicates that pathways different from CYP3A4/11 led to ERα enhancement.
Discussion:   We conclude that P450-inducing AEDs influence AR expression and signalling in hippocampus most likely via CYP3A4/11-induction. The HN25.1 cell line holds promise to investigate the correlation between drug application and AR regulation, and to specifically address issues that are relevant to human TLE patients.     

Effects of two different extracts of St. John's wort and some of their constituents on cytochrome P450 activities in rat liver microsomes

We examined two commercially available St. John's wort extracts and some of their constituents for their potential in inducing rat cytochrome P450 (CYP450) enzyme activities after oral administration. None of the extracts or pure constituents tested enhanced the hepatic cytochrome content or the activity of cytochrome P450 isozymes in rat liver microsomes. Our results demonstrated that the reported interactions between St. John's Wort and various other drugs are not mediated by CYP 450 isoforms present in rat liver.     

中国汉族癫（癎）患者中CYP3A4＊1G基因多态性与卡马西平药物浓度的相关性研究

目的 观察CYP3A4＊ 1G基因多态性与中国汉族癫（癎）患者卡马西平稳态药物浓度的相关性.方法 检测177例中国汉族癫（癎）患者卡马西平单药治疗的稳态药物浓度,采用LDR-PCR测序分型检测CYP3A4＊ 1 G基因多态性.结果 CYP3A4＊ 1G基因AA型的卡马西平标准化浓度为（0.23±0.065） μg·mL-1,AG型为（0.25±0.065） μg·mL-1,GG型为（0.28±0.074）μg·mL-1.3种基因型间标准化血药浓度差异有统计学意义（P＜0.01）,且AA型低于GG型（P＜0.01）,AG型低于GG型（P＜0.05）,AA型低于AG型（P＜0.01）,差异均有统计学意义.结论 CYP3A4＊ 1G基因型影响中国汉族癫（癎）患者的卡马西平药物浓度,可能是卡马西平药物浓度个体差异的影响因素.     

Cytochrome P-450 activities in human and rat brain microsomes

The role of cytochrome P450 in the metabolism of dextromethorphan, amitriptyline, midazolam, S-mephenytoin, citalopram, fluoxetine and sertraline was investigated in rat and human brain microsomes. Depending on the parameters, the limit of quantification using gas chromatography–mass spectrometry methods was between 1.6 and 20 pmol per incubation, which generally contained 1500 μg protein. Amitriptyline was shown to be demethylated to nortriptyline by both rat and human microsomes. Inhibition studies using ketoconazole, furafylline, sulfaphenazole, omeprazole and quinidine suggested that CYP3A4 is the isoform responsible for this reaction whereas CYP1A2, CYP2C9, CYP2C19 and CYP2D6 do not seem to be involved. This result was confirmed by using a monoclonal antibody against CYP3A4. Dextromethorphan was metabolized to dextrorphan in rat brain microsomes and was inhibited by quinidine and by a polyclonal antibody against CYP2D6. Only the addition of exogenous reductase allowed the measurement of this activity in human brain microsomes. Metabolites of the other substrates could not be detected, possibly due to an insufficiently sensitive method. It is concluded that cytochrome P450 activity in the brain is very low, but that psychotropic drugs could undergo a local cerebral metabolism which could have pharmacological and/or toxicological consequences.     

Disposition of Carbamazepine and Phenytoin in Pregnancy

Summary: Free and total plasma concentrations of phenytoin (PHT) and carbamazepine (CBZ) and its active metabolite carbamazepine-10,11-epoxide (CBZ-E) were determined in a prospective study of 86 pregnant epileptic women. The pharmacokinetics of PHT and CBZ during the three trimesters were compared with kinetics at least 10 weeks postpartum. Plasma clearance and unbound CBZ clearance were slightly decreased during the last trimester. Total and free plasma CBZ-E concentrations did not change significantly during pregnancy. Plasma PHT clearance, on the other hand, increased from the first trimester. A less pronounced increase was observed for clearance of unbound PHT; the increase was statistically significant only during the third trimester.     

中国汉族癫患者CYP3A5*3基因多态性与卡马西平药物浓度的相关性研究

目的:观察CYP3A5*3基因多态性与中国汉族癫患者卡马西平稳态药物浓度的相关性。方法:检测采用177例卡马西平单药治疗的中国汉族癫患者的稳态药物浓度,LDR-PCR测序分型检测CYP3A5*3基因多态性。结果:CYP3A5*3基因AA型的卡马西平标准化浓度为(0.23±0.066)μg.mL-1,AG型为(0.24±0.053)μg.mL-1,GG型为(0.31±0.073)μg.mL-1。3种基因型间标准化血药浓度差异有统计学意义(P<0.000 1),且AA型低于GG型(P<0.000 1),AG型低于GG型(P<0.000 1),差异均有统计学意义。AA型虽有低于AG型趋势,但差异无统计学意义(P=0.087)。CYP3A5*3各基因型间血药浓度、剂量校正浓度间差异与上述结果类似。结论:CYP3A5*3基因型影响中国汉族癫患者的卡马西平药物浓度,是造成卡马西平药物浓度个体差异的可能因素。     

Modulation of cytochrome P450 by inflammation in astrocytes.

Activation of systemic host defense mechanisms results in the down-regulation of cytochrome P450 enzymes in the liver. This occurs for various induced and constitutive isoforms of cytochrome P450 in response to cytokines such as IFNs, IL-1, IL-6, and TNF-alpha, which are produced during infection. Although the levels of cytochrome P450 in brain regions are low, the enzymes are regionally distributed and may play a critical role in the activation or degradation of drugs and chemicals in localized areas. If activation of the immune response in the CNS by LPS modulates the activity of cytochrome P450 forms in the brain, this may alter normal metabolic pathways or contribute to drug or chemical toxicity. This hypothesis was addressed by examining the effect of LPS on a major cytochrome P450 form in isolated astrocytes obtained from newborn rats. These cells were shown to express CYP1A1/2 when induced by dibenz[a, h]anthracene (DBA) as determined by enzyme activity, immunohistochemistry, and Western blotting. The treatment of these cells with LPS significantly attenuated the activity of these enzymes but had no effect on CYP1A1/2 protein levels as determined by Western blotting. The lack of effect by detoxified LPS indicated the requirement of the lipid A region on LPS to stimulate this response. Pentoxifylline (PNTX) prevented the LPS evoked decrease in CYP1A1/2 activity suggesting that cytokine release was a required component of this effect in astrocytes. These results indicate that stimulation of the immune response by LPS in isolated astrocytes decreases CYP1A1/2 activity. The release of cytokines is implicated in this effect and thought to participate in the functional inhibition of the enzyme as no effect on CYP1A1/2 protein levels was observed.     

Efficacy and safety of adjunctive zonisamide therapy for refractory partial seizures

An approach to the selection of appropriate antiepileptic drugs (AEDs) for inclusion in polytherapy is to take into account both the efficacy of a drug and also its mechanism of action and pharmacokinetic profile. The AED zonisamide is licensed in Europe and the USA for use as adjunctive therapy in adult patients with partial onset epilepsy. Four pivotal clinical studies in patients with refractory partial seizures demonstrated that zonisamide as an add-on was most effective at doses of >or=300 mg/day, with responder rates (>or=50% reduction from baseline in seizure frequency) ranging from 28 to 47% for all seizures. In addition, zonisamide has a unique combination of multiple mechanisms of action that are potentially complementary with concomitant AEDs. Zonisamide has no clinically relevant effects on the pharmacokinetics of other commonly used AEDs, however, co-administration with cytochrome P450 3A4 (CYP3A4) inducers or inhibitors may change zonisamide's pharmacokinetic profile. Zonisamide is well tolerated with the majority of adverse events being mild-to-moderate and generally manageable. The tolerability of zonisamide has also been shown to improve with slower drug titration and duration of drug treatment. These characteristics suggest that zonisamide may be suitable as a key adjunct in rational polytherapy.     

Lack of interaction of buprenorphine with flunitrazepam metabolism

OBJECTIVE: The authors' goal was to determine if the reported clinical adverse interaction of flunitrazepam and buprenorphine was caused by inhibition of drug metabolism. METHOD: Inhibition of flunitrazepam metabolism by buprenorphine and norbuprenorphine were determined in three human liver microsome preparations carrying the CYP2C19*1/*1 allele. Omeprazole metabolism mediated by CYP2C19 and CYP3A4 was used as a control reaction. Apparent K(i) values were determined. RESULTS: Norbuprenorphine did not inhibit the metabolism of flunitrazepam or omeprazole. Buprenorphine inhibited the formation of CYP3A4-mediated pathways of 3-hydroxyflunitrazepam and omeprazole sulfone formation (K(i) 118 and 16 microM) in human liver microsomes. Corresponding values were 38 and 90 microM in cDNA-expressed CYP3A4 microsomes. Projected in vivo inhibition of CYP3A4-mediated metabolism of flunitrazepam by buprenorphine is 0. 1%-2.5%. Estimated inhibition of buprenorphine N-dealkylation by flunitrazepam in vivo is 0.08%. CONCLUSIONS: The clinical interaction of flunitrazepam and buprenorphine is likely based on a pharmacodynamic mechanism.     

Zonisamide in Epilepsy: A Pilot Study

We compared zonisamide monotherapy (12 weeks) to carbamazepine monotherapy (12 weeks) after phenytoin baseline monotherapy (8 weeks) in an open crossover pilot study of eight adults with uncontrolled partial seizures. Zonisamide had definite antiepileptic activity in five subjects. In two of these, response to zonisamide was superior to that to either phenytoin or carbamazepine. A third subject became seizure free on zonisamide, but had to be withdrawn after 18 days because of mild Stevens-Johnson syndrome. The other three subjects were withdrawn from the study because of drug toxicity, manifested mainly by impaired higher mental function and increased seizures. The best response to zonisamide was at doses approximating 6 mg/kg/day, with plasma levels of 20-30 mg/L. Plasma levels of greater than 30 mg/L usually were associated with toxicity. The pharmacokinetics of zonisamide are complex and nonlinear, with steady-state plasma levels being approximately three times higher than those predicted from a single-dose study.     

Relationship between haloperidol plasma concentration, debrisoquine metabolic ratio, CYP2D6 and CYP2C9 genotypes in psychiatric patients

BACKGROUND: Around seven percent of Caucasians are poor metabolizers of cytochrome P450, CYP2D6 due to genetically impaired activity of the enzyme. Haloperidol in vitro and in vivo inhibits the activity of CYP2D6 and also the involvement of the enzyme in haloperidol metabolism has been reported. The present study was aimed to evaluate the possible inhibition of CYP2D6 during haloperidol treatment, and to determine the effect of CYP2D6 and CYP2C9 genotypes on the plasma concentration of haloperidol. METHODS: Thirty Caucasian psychiatric patients under haloperidol monotherapy were studied. CYP2D6 activity was evaluated by the debrisoquine metabolic ratio (MR), subjects with MR > 12.6 were named as poor metabolizers. Haloperidol plasma concentration was determined by high performance liquid chromatography. RESULTS: The number of patients with debrisoquine MR > 12.6 was higher than the expected comparing to healthy volunteers (13 % vs. 6.6 %, respectively). Debrisoquine MR was correlated with the dose of haloperidol (r = 0.40, p < 0.05), and also with the plasma concentration (r = 0.58, p < 0.001). Additionally, three patients comedicated with inhibitors of CYP2D6 were studied, all of them had a debrisoquine MR > 12.6, however only one was genetically poor metabolizer of CYP2D6. CYP2D6 and CYP2C9 genotypes were not related to the dose or plasma concentration of haloperidol. CONCLUSIONS: The present data support the dose-dependent inhibitory effect of haloperidol on CYP2D6, and the influence of this enzyme activity on haloperidol plasma concentration under steady-state conditions. The inhibitory effect of haloperidol on CYP2D6 enzyme activity may result in drug interactions and unexpected high plasma concentrations when drugs metabolized by the same enzyme are given concomitantly with haloperidol.     

Carbamazepine-Valnoctamide Interaction in Epileptic Patients: In Vitro/In Vivo Correlation

Six patients stabilized with carbamazepine (CBZ) therapy received an 8-day “add-on” supplement of valnoctamide (VCD), a tranquilizer available over the counter (OTC) in several European countries that exhibits promising anticonvulsant activity in animal models. During VCD intake, serum levels of the active CBZ metabolite, carbamazepine-10,ll-epoxide (CBZ-E), increased fivefold from 1.5 ± 0.7 μg/ml at baseline to 7.4 ± 4.4 μg/ml after 4 days of VCD therapy and 7.7 ± 3.1 ^g/ml after 7 days of VCD therapy (means ± SD, p < 0.01). In 4 patients, the increase in serum CBZ-E levels was associated with clinical signs of CBZ intoxication. CBZ-E levels returned to baseline after VCD therapy was discontinued. Serum CBZ levels remained stable throughout the study. The interaction observed in this study is similar to that described in patients treated with CBZ and valpromide (VPD, an isomer of VCD). In a mechanistic study, therapeutic concentrations of VCD inhibited hydrolysis of styrene oxide in human liver mi-crosome preparations. Thus, VCD is a potent inhibitor of microsomal epoxide hydrolase (IC₅₀ 15 μM). There was a striking similarity between in vitro and in vivo inhibition potencies. In this study, VCD clearance was higher in epileptic patients (treated with CBZ) than in healthy subjects.     

Lack of Interaction of Gabapentin with Carbamazepine or Valproate

Summary: Gabapentin (GBP) studies were conducted in patients with epilepsy receiving carbamazepine (CBZ, n= 12) or valproate (VPA, n = 14) monotherapy. The effects of GBP coadministration on steady-state CBZ or VPA concentrations and of these antiepileptic drugs (AEDs) on GBP pharmacokinetics were investigated. GBP (400 mg) was coadministered every 8 h for 3% days with CBZ or for 5 1/3 days with VPA. GBP was well tolerated. Mean steady-state plasma CBZ/CBZ-10, ll-epoxide (CBZ-E) and serum VPA concentrations before, during, and after GBP administration were not significantly different. Mean steady-state GBP pharmacokinetic parameters during CBZ or VPA coadministration were similar to steady-state parameters reported in healthy subjects. Thus, no pharmacokinetic interaction exists between CBZ or VPA and GBP. No dosage adjustment is necessary when GBP and CBZ or VPA are coadministered.     

Carbamazepine-risperidone interactions in patients with epilepsy

Because concomitant administration of psychoactive and antiepileptic drugs is increasing progressively in neurologic and psychiatric practice, the aim of the current study was to evaluate the pharmacokinetic interactions between risperidone (RISP) and carbamazepine (CBZ) plasma concentrations in a group of patients with epilepsy with behavioral disturbances. The authors assessed eight patients on CBZ monotherapy (CBZ extended-release capsules) at a mean dosage of 625 +/- 253 mg/day (range, 400-1,200 mg/day) for at least 1 year. RISP (1 mg in one daily dose) was added to CBZ therapy for the occurrence of behavior disturbances. CBZ blood levels were assessed before (T0), 24 hours after (T1), and 2 weeks after (T2) RISP administration. Steady-state plasma concentrations of CBZ increased from 6.67 +/- 0.41 microg/mL at baseline to 7.37 +/- 0.59 microg/mL (p < 0.01) at T1, to 7.95 +/- 0.47 microg/mL (p < 0.0001) at T2. The pharmacokinetic data suggest either a possible role of RISP in inhibiting the cytochrome P450 microsomal enzyme system (CYP)-3A4 pathway or a potential role of CYP2D6 in CBZ metabolism.     

Augmentative effects of fluvoxamine on duloxetine plasma levels in depressed patients

Duloxetine is a potent and selective inhibitor of serotonin and norepinephrine reuptake with weak activity on dopamine reuptake. Enzymes involved in duloxetine metabolism are cytochrome P450 isoenzymes (CYP) CYP1A2 and to a lesser extent CYP2D6 whereas the selective serotonin reuptake inhibitor Fluvoxamine is known to be a potent inhibitor of CYP1A2. Changes in plasma levels of duloxetine revealing pharmacokinetic interactions with fluvoxamine, clinical effects and adverse effects of adding fluvoxamine in thirteen patients with a steady-state duloxetine treatment by intraindividual comparisons were analyzed in this retrospective survey. Patients had been treated with duloxetine under steady-state conditions until fluvoxamine was added. Plasma duloxetine levels were measured at steady state of different daily doses due to lacking experience with the combination of DLX and FLX. Adding 25 mg of fluvoxamine (FLX) per day to a steady-state treatment with 30?mg of duloxetine (DLX) in 8 patients led to an average increase of duloxetine plasma levels that was 3-fold with a magnitude of 50-506%. Our findings indicate that duloxetine plasma levels can be enhanced by a potent CYP1A2 inhibition by FLX and that DLX, even in higher plasma levels, seems to be well tolerated. The use of combined treatments, however, underscores the importance of understanding pharmacokinetic interactions.     

Cellular localization and functional significance of CYP3A4 in the human epileptic brain

Purpose: Compelling evidence supports the presence of P450 enzymes (CYPs) in the central nervous system (CNS). However, little information is available on the localization and function of CYPs in the drug‐resistant epileptic brain. We have evaluated the pattern of expression of the specific enzyme CYP3A4 and studied its co‐localization with MDR1. We also determined whether an association exists between CYP3A4 expression and cell survival. Methods: Brain specimens were obtained from eight patients undergoing resection to relieve drug‐resistant seizures or to remove a cavernous angioma. Each specimen was partitioned for either immunostaining or primary culture of human endothelial cells and astrocytes. Immunostaining was performed using anti‐CYP3A4, MDR1, GFAP, or NeuN antibodies. High performance liquid chromatography–ultraviolet (HPLC‐UV) analysis was used to quantify carbamazepine (CBZ) metabolism by these cells. CYP3A4 expression was correlated to DAPI) condensation, a marker of cell viability. Human embryonic kidney (HEK) cells were transfected with 4′,6‐diamidino‐2‐phenylindole (CYP3A4 to further evaluate the link between CYP3A4 levels, CBZ metabolism, and cell viability. Key Findings: CYP3A4 was expressed by blood–brain barrier (BBB) endothelial cells and by the majority of neurons (75 ± 10%). Fluorescent immunostaining showed coexpression of CYP3A4 and MDR1 in endothelial cells and neurons. CYP3A4 expression inversely correlated with DAPI nuclear condensation. CYP3A4 overexpression in HEK cells conferred resistance to cytotoxic levels of carbamazepine. CYP3A4 levels positively correlated with the amount of CBZ metabolized. Significance: CYP3A4 brain expression is not only associated with drug metabolism but may also represent a cytoprotective mechanism. Coexpression of CYP3A4 and MDR1 may be involved in cell survival in the diseased brain.