Recent advances in biotransformation of CNS and cardiovascular agents

Topiramate is likely to act outside of the trigeminocervical complex

BACKGROUND

To facilitate understanding the locus and mechanism of action of antimigraine preventives, we examined the effect of topiramate on trigeminocervical activation in the cat.

METHODS

Cats were anesthetized and physiologically monitored. Electrical stimulation of the superior sagittal sinus activated nociceptive trigeminovascular afferents. Extracellular recordings were made from neurons in the trigeminocervical complex.

RESULTS

Microiontophoretically delivered topiramate, applied locally at the second order synapse of the trigeminovascular system in the trigeminocervical complex, produced significant inhibition of L-glutamate-evoked firing of neurons only at the highest microiontophoretic currents (27 ± 7% at -160 nA; P  < 0.05, N  = 14 cells), but did not inhibit firing of these neurons evoked by stimulation of the craniovascular afferents (2 ± 5%, P  = 0.762, N  = 13 cells). In contrast, systemically administered topiramate (30 mg/kg intravenously) partly inhibited this firing (32 ± 10% at 15 min; F 5,35 = 3.5, P  < 0.05, N  = 8 cats). After this systemic administration, profound inhibition (70 ± 10%, P  < 0.001, N  = 7) of L-glutamate-evoked firing of cells in the trigeminocervical complex at the second order synapse of the trigeminovascular system was observed.

CONCLUSIONS

These data suggest that topiramate acts outside of the trigeminocervical complex in the cat. Determining the sites of action of preventive antimigraine treatments is crucial to developing laboratory models for the development of new therapeutics, and may vary between species.

Comparative pharmacokinetic analysis of USL255, a new once-daily extended-release formulation of topiramate

PURPOSE

To compare the pharmacokinetics of USL255, a once-daily extended-release (ER) formulation of topiramate (TPM), with Topamax (immediate-release TPM) in healthy subjects after oral dosing and evaluate the effect of food on USL255 bioavailability and pharmacokinetics.

METHODS

This randomized, single-center, open-label, cross-over design study had three dosing periods separated by 21 days of washout between treatments. Thirty-six volunteers received single doses of USL255 (200 mg) in fasted and fed conditions and two doses of Topamax (100 mg) administered 12 h apart. TPM plasma samples were analyzed by liquid chromatography-mass spectroscopy. Pharmacokinetic parameters were calculated by noncompartmental methods.

KEY FINDINGS

USL255 fasted pharmacokinetic parameters [point estimate (90% confidence interval, CI) compared to Topamax] were: relative bioavailability (F) 91.2% (84-99%), peak plasma concentration (C(max)) USL255/Topamax-ratio 59% (53-65%), time to reach C(max) (t(max)) 19.5 ± 7.2 h, accumulation ratio (R(ac)) 3.9 ± 1.2, effective half-life (t(1/2,eff)) 55.7 ± 19.9 h, terminal half-life (t(1/2,z)) 80.2 ± 14.2 h, and peak-occupancy-time (POT) 12.1 ± 4.0 h. Although the F and C(max) were unaffected by food, R(ac) and t(1/2,eff) increased to 4.9 ± 0.9, and 72.5 ± 15.4 h, respectively. In contrast to t(1/2,z,) t(1/2,eff) reflects absorption rate; therefore, USL255's t(1/2,eff) was significantly longer than Topamax's t(1/2,eff) (37.1 ± 6.5 h).

SIGNIFICANCE

Although bioequivalent to Topamax in extent of absorption, USL255 had a slower absorption rate as reflected in its lower C(max) and longer t(max), larger POT and longer t(1/2,eff), and similar R(ac) values to that of Topamax (q12 h). This relative flat plasma profile allows for once-daily dosing with diminished fluctuations in TPM plasma levels. In addition, neither USL255's peak nor extent of plasma exposure of TPM was affected by food.

The anxiolytic and analgesic properties of fenobam, a potent mGlu5 receptor antagonist, in relation to the impairment of learning

Fenobam [N-(3-chlorophenyl)-N'-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea] was suggested to possess anxiolytic actions 30 years ago. Hoffmann-La Roche researchers recently reported that it is a selective and potent mGlu5 receptor antagonist, acting as a negative allosteric modulator. In the present study, we show that fenobam readily penetrates to the brain, reaching concentrations over 600 nM, clearly above the affinity for mGluR5 receptors. Fenobam (at 10, 30, and 100 mg/kg) did not affect horizontal locomotor activity in the open field test. Anxiolytic-like activity in the context freezing test was seen at 30 mg/kg, while fenobam was not active in the elevated plus maze test at the tested concentrations. Fenobam had antinociceptive actions in the formalin test at 10 and 30 mg/kg, but failed to attenuate mechanical allodynia in the chronic constriction injury model. Impairment of learning was revealed in the passive avoidance test at 30 mg/kg. Fenobam also impaired performance in both the Morris water maze and in the contextual fear conditioning test at the doses of 30 and 10 mg/kg, respectively. Prepulse inhibition, used as a model of psychomimetic activity, was not affected by fenobam at doses of up to 60 mg/kg. Our results indicate that the beneficial effects of fenobam occur in a similar dose range as the potential side-effects.

Distribution and metabolism of the antipsychotic agent mazapertine succinate in rats

The pharmacokinetics and drug disposition of 14C 1-[3-[[4-[2-(1-methylethoxy)phenyl]-1-piperazinyl]methyl]benzoy]piperidine succinate (RWJ-37796, mazapertine, Mz) have been investigated in male and female Sprague-Dawley rats. Approximately 93% of the orally administered radioactive dose (30 mg/kg) was recovered after 7 days. Fecal elimination accounted for approximately 63% of the dose while urine accounted for 30%. The rate of elimination of 14C Mz was rapid with 81% of the total fecal and 94% of the total urinary radioactivity being excreted within 24 h. There were no significant gender differences in the overall excretion pattern. The maximal plasma concentration of Mz and total radioactivity occurred at 0.5h after dosing and plasma concentrations were consistently higher in female rats. The Mz concentration declined rapidly in plasma with a terminal half-life<2 h. The total radioactive dose in plasma displayed a considerably longer terminal half-life of 9-13 h. Mz and a total of 15 metabolites were isolated and identified in these samples. Unchanged Mz accounted for <5% of the radioactive dose in excreta samples and <8% of the sample in plasma (0-24 h). Metabolites were formed by phenyl hydroxylation, piperidyl oxidation, O-dealkylation, N-dephenylation, oxidative N-debenzylation and glucuronide conjugation.

A Comparative Study of the Effect of Carbamazepine and Valproic Acid on the Pharmacokinetics and Metabolic Profile of Topiramate at Steady State in Patients with Epilepsy

PURPOSE

To compare the influence of enzyme-inducing comedication and valproic acid (VPA) on topiramate (TPM) pharmacokinetics and metabolism at steady state.

METHODS

Three groups were assessed: (a) patients receiving TPM mostly alone (control group, n =13); (b) patients receiving TPM with carbamazepine (CBZ; n = 13); and (c) patients receiving TPM with VPA (n = 12). TPM and its metabolites were assayed in plasma and urine by liquid chromatography-mass spectrometry (LC-MS).

RESULTS

No significant differences were found in TPM oral (CL/F) and renal (CL(r)) clearance between the VPA group and the control group. Mean TPM CL/F and CL(r) were higher in the CBZ group than in controls (2.1 vs. 1.2 L/h and 1.1 vs. 0.6L/h, respectively; p < 0.05). In all groups, the urinary recovery of unchanged TPM was extensive and accounted for 42-52% of the dose (p > 0.05). Urinary recovery of 2,3-O-des-isopropylidene-TPM (2,3-diol-TPM) accounted for 3.5% of the dose in controls, 2.2% in the VPA group (p > 0.05), and 13% in the CBZ group (p < 0.05). The recovery of 10-hydroxy-TPM (10-OH-TPM) was twofold higher in the CBZ group than in controls, but it accounted for only <2% of the dose. The plasma concentrations of TPM metabolites were severalfold lower than those of the parent drug.

CONCLUSIONS

Renal excretion remains a major route of TPM elimination, even in the presence of enzyme induction. The twofold increase in TPM-CL/F in patients taking CBZ can be ascribed, at least in part, to stimulation of the oxidative pathways leading to formation of 2,3-diol-TPM and 10-OH-TPM. VPA was not found to have any clinically significant influence on TPM pharmacokinetic and metabolic profiles.

Pharmacokinetic and metabolic investigation of topiramate disposition in healthy subjects in the absence and in the presence of enzyme induction by carbamazepine

PURPOSE

To characterize the metabolic profile of topiramate (TPM) in humans and to assess the influence of enzyme induction by carbamazepine (CBZ) on the pharmacokinetics and metabolic profile of TPM.

METHODS

Twelve healthy subjects received a single oral dose of TPM (200 mg) on two randomized occasions. On one occasion, TPM was administered alone, and on the other, it was given on day 18 of a 24-day treatment with CBZ (maintenance dosage, 600 mg/day). Blood and urine samples were collected for > or = 72 h after dosing. TPM and its metabolites were assayed in plasma and urine by a specific liquid chromatography-mass spectroscopy (LC-MS) method.

RESULTS

Mean TPM oral clearance (CL/F) increased from 1.2 L/h (control) to 2.2 L/h after CBZ treatment. Mean TPM half-life decreased from 29 h to 19 h. TPM was excreted extensively in urine both under noninduced (56%) and CBZ-induced conditions (40%). 2,3-O-Des-isopropylidene-TPM (2,3-diol-TPM) was identified as the most prominent urinary metabolite, with a recovery accounting for 3.2% and 7.9% of the TPM dose under noninduced and induced conditions, respectively. Corresponding recovery values for 10-hydroxy-TPM (10-OH-TPM) were 1.2% and 1.8%, respectively. The control AUC(metabolite)/AUC(drug) ratio for 2,3-diol-TPM and 10-OH-TPM were 1.5% and 0.6%, and they increased by threefold and twofold, respectively, after CBZ treatment.

CONCLUSIONS

TPM remains appreciably excreted unchanged in urine (41%) under CBZ-induced conditions, even though TPM CL/F increased by twofold. Although 2,3-diol-TPM and 10-OH-TPM were measured in unconjugated form, the significant increases in their AUC and urinary excretion are consistent with the twofold increase in TPM clearance.

Pharmacokinetic interactions of topiramate

Topiramate is a new antiepileptic drug (AED) that has been approved worldwide (in more than 80 countries) for the treatment of various kinds of epilepsy. It is currently being evaluated for its effect in various neurological and psychiatric disorders. The pharmacokinetics of topiramate are characterised by linear pharmacokinetics over the dose range 100-800 mg, low oral clearance (22-36 mL/min), which, in monotherapy, is predominantly through renal excretion (renal clearance 10-20 mL/min), and a long half-life (19-25 hours), which is reduced when coadministered with inducing AEDs such as phenytoin, phenobarbital and carbamazepine. The absolute bioavailability, or oral availability, of topiramate is 81-95% and is not affected by food. Although topiramate is not extensively metabolised when administered in monotherapy (fraction metabolised approximately 20%), its metabolism is induced during polytherapy with carbamazepine and phenytoin, and, consequently, its fraction metabolised increases. During concomitant treatment with topiramate and carbamazepine or phenytoin, the (oral) clearance of topiramate increases 2-fold and its half-life becomes shorter by approximately 50%, which may require topiramate dosage adjustment when phenytoin or carbamazepine therapy is added or discontinued. From a pharmacokinetic standpoint, topiramate is a unique example of a drug that, because of its major renal elimination component, is not subject to drug interaction due to enzyme inhibition, but nevertheless is susceptible to clinically relevant drug interactions due to induction of its metabolism. Unlike old AEDs such as phenytoin and carbamazepine, topiramate is a mild inducer and, currently, the only interaction observed as a result of induction by topiramate is that with ethinylestradiol. Topiramate only increases the oral clearance of ethinylestradiol in an oral contraceptive at high dosages (>200 mg/day). Because of this dose-dependency, possible interactions between topiramate and oral contraceptives should be assessed according to the topiramate dosage utilised. This paper provides a critical review of the pharmacokinetic interactions of topiramate with old and new AEDs, an oral contraceptive, and the CNS-active drugs lithium, haloperidol, amitriptyline, risperidone, sumatriptan, propranolol and dihydroergotamine. At a daily dosage of 200 mg, topiramate exhibited no or little (with lithium, propranolol and the amitriptyline metabolite nortriptyline) pharmacokinetic interactions with these drugs. The results of many of these drug interaction studies with topiramate have not been published before, and are presented and discussed for the first time in this article.

Topiramate and lamotrigine pharmacokinetics during repetitive monotherapy and combination therapy in epilepsy patients

PURPOSE

To determine at steady state (in the same group of patients): (a) the pharmacokinetics (PK) of lamotrigine (LTG) with LTG monotherapy, (b) the PK of LTG concomitantly administered with topiramate (TPM) at three escalating TPM doses (100, 200, and 400 mg/day), (c) the PK of TPM at three escalating TPM doses while receiving fixed-dose LTG therapy, and (d) the PK of TPM with TPM monotherapy.

METHODS

This was an open-label, sequential, single-group, dose-escalating PK study in which 13 patients with epilepsy not optimally controlled with LTG received stable-dose LTG monotherapy for 2 weeks, followed by stable-dose LTG therapy combined with escalating doses of TPM for </=16 weeks, stable-dose TPM therapy combined with tapered-dose LTG therapy for 4 weeks, and stable-dose TPM monotherapy for 2 weeks. Serial blood and urine samples were collected before and during TPM dosing, and safety data were collected throughout the study.

RESULTS

The exposure, or area under the plasma LTG concentration-time curve within a dosing interval at steady state (AUCss), did not change in the presence of TPM, with mean AUCss values ranging at each TPM dose level between 66 and 81 mg x h/L with concomitant LTG/TPM therapy compared with 77 mgxh/L with LTG monotherapy. No significant change was found in the steady-state peak (Cmax) and trough (Cmin) plasma levels of LTG in the presence and absence of TPM. The mean (+/-SD) oral clearance (CL/F) of TPM (400 mg/day) was 2.6 +/- 1.1 L/h when given alone and 2.7 +/- 0.7 L/h when given with LTG. The similarity of CL/F values also was reflected by the similar exposure (AUCss), Cmax, and Cmin values of TPM in the absence, and presence of LTG.

CONCLUSIONS

The results of this study show that no PK interaction between TPM and LTGwas observed at the doses used in this study.

Analysis of topiramate and its metabolites in plasma and urine of healthy subjects and patients with epilepsy by use of a novel liquid chromatography-mass spectrometry assay

A novel liquid chromatography-mass spectrometry (LC-MS) method was developed and validated for quantification of topiramate (TPM) and its metabolites 10-hydroxy topiramate (10-OH-TPM), 9-hydroxy topiramate (9-OH-TPM), and 4,5-O-desisopropylidene topiramate (4,5-diol-TPM) in plasma and urine. The method uses 0.5 mL of plasma or 1 mL of urine that is extracted with diethyl ether and analyzed by LC-MS. Positive ion mode detection enables tandem mass spectrometric (MS/MS) identification of the aforementioned four compounds. Calibration curves of TPM, 4,5-diol-TPM, 9-OH-TPM, and 10-OH-TPM in plasma and urine were prepared and validated over the concentration range of 0.625 to 40 microg/mL using TPM-d(12) as an internal standard. Calibration curves were linear over this concentration range for TPM and its metabolites. Accuracy and precision ranged in urine from 83% to 114% and 4% to 13% (%CV), respectively, and in plasma from 82% to 108% and 6% to 13%, respectively. The applicability of the assay was evaluated by analyzing plasma samples from a healthy subject who received a single oral dose of TPM (200 mg) and urine samples from 11 patients with epilepsy treated with TPM (daily dose between 100 to 600 mg) alone or with other antiepileptic drugs. Only TPM was detected and quantified in the plasma samples, and its concentration ranged between 0.7 and 4.3 microg/mL. The concentrations of TPM and 10-OH TPM were quantifiable in all urine samples and ranged from 20 to 300 microg/mL for TPM and from 1 to 50 microg/mL for 10-OH-TPM. The metabolites 4,5-diol-TPM and 9-OH-TPM were also detected in all urine samples, but their concentrations were quantifiable only in 4 patients. An unidentified peak in the chromatograms obtained from patients' urine was attributed to 2,3-O-desisopropylidene topiramate (2,3-diol-TPM). Due to a lack of reference material of 2,3-diol TPM and the similar MS/MS spectrum with 4,5-diol-TPM, the calibration curves of 4,5-diol-TPM were used for the quantification of its isomer 2,3-diol-TPM. Based on these determinations, the apparent 2,3-diol-TPM-to-TPM concentration ratio in patients' urine ranged from 0.05 to 0.51 and the 10-OH-TPM-to-TPM ratio ranged from 0.02 to 0.17. In conclusion, a novel LC-MS method for the assay of TPM and four of its metabolites in plasma and urine was developed. Its utilization for analysis of urine samples from patients with epilepsy showed that the method was suitable for analysis of TPM and its metabolites in clinical samples. Two quantitatively significant TPM metabolites (10-OH-TPM and 2,3-diol-TPM) and two quantitatively minor metabolites (9-OH-TPM and 4,5-diol-TPM) were detected and quantified in urine samples from patients with epilepsy.

In vitro identification of metabolic pathways and cytochrome P450 enzymes involved in the metabolism of etoperidone

1. In vitro studies have been carried out to investigate the metabolic pathways and identify the hepatic cytochrome P450 (CYP) enzymes involved in etoperidone (Et) metabolism. 2. Ten in vitro metabolites were profiled, quantified and tentatively identified after incubation with human hepatic S9 fractions. Et was metabolized via three metabolic pathways: (A) alkyl hydroxylation to form OH-ethyl-Et (M1); (B) phenyl hydroxylation to form OH-phenyl-Et (M2); and (C) N-dealkylation to form 1-m-chlorophenylpiperazine (mCPP, M8) and triazole propyl aldehyde (M6). Six additional metabolites were formed by further metabolism of M1, M2, M6 and M8. 3. Kinetic studies revealed that all metabolic pathways were monophasic, and the pathway leading to the formation of OH-ethyl-Et was the most efficient at eliminating the drug. On incubation with microsomes expressing individual recombinant CYPs, formation rates of M1-3 and M8 were 10-100-fold greater for CYP3A4 than that for other CYP forms. The formation of these metabolites was markedly inhibited by the CYP3A4-specific inhibitor ketoconazole, whereas other CYP-specific inhibitors did not show significant effects. In addition, the production of M1-3 and M8 was strongly correlated with CYP3A4-mediated testosterone 6beta-hydroxylase activities in 13 different human liver microsome samples. 4. Dealkylation of the major metabolite M1 to form mCPP (M8) was also investigated using microsomes containing recombinant CYP enzymes. The rate of conversion of M1 to mCPP by CYP3A4 was 503.0 +/- 3.1 pmole nmole(-1) min(-1). Metabolism of M1 to M8 by other CYP enzymes was insignificant. In addition, this metabolism in human liver microsomes was extensively inhibited by the CYP3A4 inhibitor ketoconazole, but not by other CYP-specific inhibitors. In addition, conversion of M1 to M8 was highly correlated with CYP3A4-mediated testosterone 6beta-hydroxylase activity. 5. The results strongly suggest that CYP3A4 is the predominant enzyme-metabolizing Et in humans.

Topiramate and phenytoin pharmacokinetics during repetitive monotherapy and combination therapy to epileptic patients

PURPOSE

To evaluate the potential pharmacokinetic interactions between topiramate (TPM) and phenytoin (PHT) in patients with epilepsy by studying their pharmacokinetics (PK) after monotherapy and concomitant TPM/PHT treatment.

METHODS

Twelve patients with epilepsy stabilized on PHT monotherapy were enrolled in this study, with 10 and seven patients completing the phases with 400 and 800 mg TPM daily doses, respectively. TPM was added at escalating doses, and after stabilization at the highest tolerated TPM dose, PHT doses were tapered. Serial blood and urine samples were collected for PK analysis during the monotherapy phase or the lowest PHT dose after taper and the concomitant TPM/PHT phase. Potential metabolic interaction between PHT and TPM also was studied in vitro in human liver microsomal preparations.

RESULTS

In nine of the 12 patients, PHT plasma concentrations remained stable, with a mean (+/-SD) area under the curve (AUC) ratio (combination therapy/monotherapy) of 1.13 +/- 0.17 (range, 0.89-1.23). Three patients had AUC ratios of 1.25, 1.39, and 1.55, respectively, and with the addition of TPM (800, 400, and 400 mg daily, respectively), their peak PHT plasma concentrations increased from 15 to 21 mg/L, 28 to 36 mg/L, and 27 to 41 mg/L, respectively. Human liver microsomal studies with S-mephenytoin showed that TPM partially inhibited CYP2C19 at very high concentrations of 300 microM (11% inhibition) and 900 microM (29% inhibition). Such high plasma concentrations would correspond to doses in humans that are 5 to 15 times higher than the recommended dose (200-400 mg). TPM clearance was approximately twofold higher during concomitant TPM/PHT therapy

CONCLUSIONS

This study provides evidence that the addition of TPM to PHT generally does not cause clinically significant PK interaction. PHT induces the metabolism of TPM, causing increased TPM clearance, which may require TPM dose adjustments when PHT therapy is added or is discontinued. TPM may affect PHT concentrations in a few patients because of inhibition by TPM of the CYP2C19-mediated minor metabolic pathway of PHT.