Effects of cimetidine on carbamazepine auto- and hetero-induction in man

The actions of the H2-blocker cimetidine on the toxicity and biotransformation of the phosphorothioate insecticide parathion

Parathion, like most organophosphorus insecticides currently in use, must undergo cytochrome P450(P450)-dependent activation in order to exert its acute mammalian toxicity (cholinergic crisis). Since P450 isoforms play such an important role in mediating the toxicity of parathion and related insecticides, factors which significantly alter P450 activities, such as exposure to certain xenobiotics, can also be expected to affect the toxicity of these potentially hazardous insecticides. Cimetidine is a H2-histamine antagonist that has been shown to inhibit several P450-isoforms. In addition, administration of cimetidine has been reported to result in clinically significant pharmacokinetic interactions with a wide variety of drugs. In the present study coexposure to cimetidine and parathion resulted in a moderate increase in the toxicity of this pesticide. However, coexposure to cimetidine and paraoxon did not alter the toxicity of the organophosphate, indicating that cimetidine likely affected P450-dependent formation of paraoxon from parathion. In vitro incubations of mouse hepatic microsomes demonstrated that, in addition to reducing the velocity of P450-dependent metabolism of parathion, cimetidine increased the proportion of paraoxon formed (activation). and decreased the proportion of p-nitrophenol formed (detoxification). Since parathion is not eliminated significantly by other routes in the mouse, the bulk of parathion in vivo was metabolized by P450 (although more slowly) in the presence of cimetidine, leading to a greater amount of paraoxon produced, and therefore greater toxicity. Incubations with individual P450 isoforms suggested that cimetidine could act by inhibition of P450 isoforms that detoxify parathion to a greater degree than cimetidine-resistant isoforms, and/or cimetidine could alter the proportions of detoxification versus activation of certain individual isoforms.

Clinically significant pharmacokinetic drug interactions with carbamazepine. An update

Carbamazepine is one of the most commonly prescribed antiepileptic drugs and is also used in the treatment of trigeminal neuralgia and psychiatric disorders, particularly bipolar depression. Because of its widespread and long term use, carbamazepine is frequently prescribed in combination with other drugs, leading to the possibility of drug interactions. The most important interactions affecting carbamazepine pharmacokinetics are those resulting in induction or inhibition of its metabolism. Phenytoin, phenobarbital (phenobarbitone) and primidone accelerate the elimination of carbamazepine, probably by stimulating cytochrome P450 (CYP) 3A4, and reduce plasma carbamazepine concentrations to a clinically important extent. Inhibition of carbamazepine metabolism and elevation of plasma carbamazepine to potentially toxic concentrations can be caused by stiripentol, remacemide, acetazolamide, macrolide antibiotics, isoniazid, metronidazole, certain antidepressants, verapamil, diltiazem, cimetidine, danazol and (dextropropoxyphene) propoxyphene. In other cases, toxic symptoms may result from elevated plasma concentrations of the active metabolite carbamazepine-10,11-epoxide, due to the inhibition of epoxide hydrolase by valproic acid (sodium valproate), valpromide, valnoctamide and progabide. Carbamazepine is a potent inducer of CYP3A4 and other oxidative enzyme system in the liver, and it may also increase glucuronyltransferase activity. This results in the acceleration of the metabolism of concurrently prescribed anticonvulsants, particularly valproic acid, clonazepam, ethosuximide, lamotrigine, topiramate, tiagabine and remacemide. The metabolism of many other drugs such as tricyclic antidepressants, antipsychotics, steroid oral contraceptives, glucocorticoids, oral anticoagulants, cyclosporin, theophylline, chemotherapeutic agents and cardiovascular drugs can also be induced, leading to a number of clinically relevant drug interactions. Interactions with carbamazepine can usually be predicted on the basis of the pharmacological properties of the combined drug, particularly with respect to its therapeutic index, site of metabolism and ability to affect specific drug metabolising isoenzymes. Avoidance of unnecessary polypharmacy, selection of alternative agents with lower interaction potential, and careful dosage adjustments based on serum drug concentration monitoring and clinical observation represent the mainstays for the minimisation of risks associated with these interactions.

Carbamazepine pharmacokinetics in obese and lean subjects

OBJECTIVE

To compare carbamazepine pharmacokinetic parameters between obese and lean subjects following the administration of a single 200-mg tablet.

DESIGN

Single-dose intervention, open study.

SETTING

Teaching university hospital.

SUBJECTS

Eighteen obese (group A) otherwise healthy subjects, referred to the metabolic outpatient clinic, and 13 healthy lean (group B) volunteers. Inclusion criterion for the obese subjects was a body mass index (BMI = weight/height2) of more than 30 kg/m2. In the obese group, mean +/- SD total body weight (TBW), BMI, and percent of ideal body weight (IBW) were 111.4 +/- 19.9 kg, 38.8 +/- 6.0 kg/m2, and 182.7% +/- 30.7%, respectively. These values were significantly greater than the respective values of 63.2 +/- 8.3 kg, 22.4 +/- 1.6 kg/m2, and 105.8% +/- 5.8% obtained in the lean group (p < 0.001).

INTERVENTION

Single-dose oral administration of carbamazepine 200-mg tablet (Teril, Taro, Israel).

OUTCOMES

Carbamazepine elimination half-life (t1/2), apparent volume of distribution (Varea/F), and its oral clearance (Clpo/F) were derived from the drug concentration-time curves.

RESULTS

Carbamazepine Varea/F and t1/2 were significantly greater in group A than in group B (98.4 +/- 26.9 vs. 60.7 +/- 8.5 L, respectively, p < 0.001; and 59.4 +/- 14.7 vs. 31.0 +/- 5.0 h, respectively, p < 0.001), but its Clpo/F was reduced only slightly in obese as compared with lean subjects (19.8 +/- 5.2 vs. 23.0 +/- 4.6 mL/min, respectively, p = 0.07). Correction for IBW yielded similar results for Varea/F and t1/2, but Clpo/F per kg of IBW was significantly smaller in the obese than in the lean subjects (0.32 +/- 0.07 vs. 0.39 +/- 0.06 mL/min/kg of IBW, respectively, p < 0.02). Linear correlations were observed between Varea/F and TBW for both group A (r = 0.92, p < 0.001) and group B (r = 0.77, p < 0.002).

CONCLUSIONS

In comparison with lean subjects, carbamazepine Varea/F is significantly greater in obese subjects and its t1/2 is markedly prolonged. The minor nonsignificant effect of obesity on carbamazepine Clpo/F suggests that in obese subjects the carbamazepine daily dose should be based on IBW, not on TBW.

The kinetics of the auto-induction of ifosfamide metabolism during continuous infusion

It has often been reported that the oxazaphosphorines ifosfamide and cyclophosphamide induce their own metabolism. This phenomenon was studied in 21 paediatric patients over 35 courses of therapy. All patients received 9 gm-2 of ifosfamide as a continuous infusion over 72 h. Plasma concentrations of parent drug and of the major metabolite in plasma, 3-dechloroethylifosfamide (3DC) were determined, using a quantitative thin-layer chromatography (TLC) technique. A one-compartment model was fitted simultaneously to both ifosfamide and 3DC data. The model included a time-dependent clearance term, increasing asymptotically from an initial value to a final induced clearance and characterised by a first-order rate constant. A time lag, before induction of clearance began, was determined empirically. Metabolite kinetics were characterised by an elimination rate constant for the metabolite and a composite parameter comprising a formation clearance, proportional to the time-dependent clearance of parent drug, divided by the volume of distribution of the metabolite. Thus, the parameters to estimate were the volume of distribution of parent drug (V), initial clearance (Cli), final clearance (Cls), the rate constant for changing clearance (Kc), the elimination rate constant for the metabolite (Km) and Vm/fm, the metabolite volume of distribution divided by the fractional clearance to 3DC. The model of drug and metabolite kinetics produced a good fit to the data in 22 of 31 courses. In a further 4 courses an auto-inductive model for parent drug alone could be used. In the remaining courses, auto-induction could be demonstrated, but there were insufficient data to fit the model. For some patients this was due to a long time lag (up to 54 h) relative to the infusion time. The time lag varied from 6 to 54 (median, 12)h and values for the other parameters were Cli, 3.27 +/- 2.52 lh-1 m-2, Cls, 7.50 +/- 3.03 lh-1 m-2, V, 22.0 +/- 11.0 1 m-2, Kc, 0.086 +/- 0.074 h-1; Km, 0.159 +/- 0.077 h-1 and Vm/fm, 104 +/- 82 1m-2. The values of Kc correspond to a half-life of change in clearance ranging from 2 to 157 h, although for the majority of the patients the half-life was less than 7 h and a new steady-state level was achieved during the 72 h infusion period. This model provides insight into the time course of enzyme induction during ifosfamide administration, which may continue for up to 10 days in some protocols.(ABSTRACT TRUNCATED AT 400 WORDS)

Vigabatrin and lamotrigine in refractory epilepsy

Epilepsy arises from an imbalance of inhibitory and excitatory influences in the brain. Vigabatrin (VIG) decreases the breakdown of the inhibitory neurotransmitter gamma-aminobutyric acid, whereas lamotrigine (LTG) reduces presynaptic excitatory amino acid release. 22 patients with refractory epilepsy, treated with an anticonvulsant regimen containing VIG, entered a balanced, double blind, placebo controlled, crossover trial of additional LTG. Treatment periods of 12 weeks (25 mg, 50 mg, 100 mg LTG twice daily for four weeks at each dose, and matched placebo) were followed by wash out intervals of four weeks. 14 of the 20 patients completing the study improved, resulting in a significant fall in seizure days and numbers. Analysis of seizure type confirmed a beneficial effect on partial and secondary generalised tonic-clonic seizures. At the highest LTG dose (200 mg daily) there was a median fall of 37% in seizure count with nine (45%) patients reporting > 50% reduction. Three of these patients were seizure free during this month of treatment. Side effects were minimal throughout the study. Concentrations of other antiepileptic drugs, including those of carbamazepine 10,11-epoxide, were not modified by LTG. This study suggests a substantial efficacy for a regimen containing VIG and LTG. Combinations of drugs with complementary modes of action may provide a rational pharmacological approach to the management of refractory epilepsy.

A double-blind, placebo-controlled interaction study between oxcarbazepine and carbamazepine, sodium valproate and phenytoin in epileptic patients

1. The effect of carbamazepine (CBZ), sodium valproate (VPA) and phenytoin (PHT) on the pharmacokinetics of oxcarbazepine (OXC) was explored in three groups of 12 epileptic patients taking one of these drug as monotherapy. 2. Each patient took a single 600 mg dose of OXC followed 7 days later by 3 weeks' treatment with OXC 300 mg thrice daily and matched placebo in random order. 3. Seven untreated patients, acting as controls, were prescribed the single OXC dose and 3 weeks' active treatment only. 4. In those patients completing the study, the area under the concentration-time curve (AUC) at steady-state for hydroxycarbazepine (OHCZ), the active metabolite of OXC, was significantly lower in the CBZ-treated group than in controls (P < 0.05). 5. No other differences in AUCs or elimination half-lives for OHCZ were found between treated and untreated patients following single or multiple OXC dosing. 6. Median AUCs of CBZ, VPA and PHT during a dosage interval did not differ significantly after treatment with OXC and placebo. 7. Ten patients completing the study complained of side-effects during treatment with OXC compared with one taking placebo (P < 0.01). 8. There were no important changes in cognitive function testing during administration of OXC compared with placebo. 9. Standard doses of OXC can be given as add-on therapy in epileptic patients receiving CBZ, VPA or PHT without producing a clinically relevant pharmacokinetic interaction.

Double dummy comparison between once and twice daily dosing with modified-release carbamazepine in epileptic patients

1. Fourteen patients with refractory epilepsy on a twice daily regimen of modified-release carbamazepine (CBZ-MR. Tegretol Retard. Ciba Pharmaceuticals) completed a balanced, double-blind, double dummy, random order, crossover comparison of 8 weeks treatment with once (o.d.) and twice daily (b.d.) dosing. In order to obtain a profile of serum CBZ concentrations over 24 h on once daily dosing, patients were randomised to taking it in the morning (o.d. a.m.) or evening (o.d. p.m.) for 4 weeks. Each treatment was taken with a placebo of the other and total tablet numbers were matched. Blood sampling was undertaken 0, 2, 4, 6, 8, 10, 12 and 24 h after the morning tablets at the end of each 4 week treatment period. 2. Overall, trough serum drug concentrations (Cmin) were lower with once daily dosing (Cmin: b.d. 7.5 mg l-1, o.d. 6.5 mg l-1, P < 0.05, 95% CI of the difference -1.3 to -0.1), but no significant differences were found in average (Cav) or peak (Cmax) concentrations, AUC values or fluctuations in CBZ concentrations. 3. Pharmacokinetic parameters for CBZ 10.11 epoxide, the active metabolite of CBZ did not differ significantly between the dosage schedules. 4. Seizure control was similar during once and twice daily dosing with CBZ-MR (median seizures/month (range): b.d. 1 (0-14.5), o.d. 0.5 (0-11), NS, 95% CI of the difference -1.8 to + 0.25). 5. There were no differences in psychomotor performance between the treatment periods. 6. More patients (n = 11) preferred treatment (P < 0.05) with once daily than twice daily dosing (n = 3) with CBZ-MR. 7. Once daily dosing with CBZ-MR should be possible in the majority of patients receiving the drug as monotherapy.

Adjuvant vigabatrin in refractory epilepsy: a ceiling to effective dosage in individual patients?

A double-blind, randomized, cross-over study of additional vigabatrin (gamma-vinyl-GABA, VGB, 1.0 g twice daily for 6 weeks, followed by 1.5 g twice daily for 6 weeks) and matched placebo was undertaken in 24 patients with refractory epilepsy. Nineteen completed the trial satisfactorily. Fewer seizure days were reported during VGB treatment [placebo 41, VGB 23, p < 0.05, 95% confidence interval (CI) -1.5 to -14]. An overall reduction in median seizure numbers failed to reach statistical significance (n = 19; placebo 52, VGB 32, NS, 95% CI -18 to +24). Subgroup analysis, however, showed a significant reduction in partial seizures (n = 17) with 2 g VGB daily (placebo 22, VGB 13, p < 0.05, 95% CI -0.5 to -16.5), but not with higher dosage (placebo 28, VGB 22, NS, 95% CI -18 to +11). A deterioration in control of partial seizures as compared with the equivalent placebo phase was observed when patients were changed from 2 to 3 g/day VGB (2 g VGB 13, 3 g VGB 22, p = 0.05, 95% CI 0 to +20). Loss of efficacy was noted in 3 patients, and seizure control worsened slightly in 5 others. One previously resistant patient developed a therapeutic response, and 2 other patients reported an additional useful reduction in seizures. In the remaining 8 patients, seizure frequency did not change. VGB did not appear to benefit tonic-clonic seizures. Serum VGB concentrations were higher during treatment with 3 g (15.5 +/- 8.9 mg/L) daily than with 2 g (13.5 +/- 11.2 mg/L). No important alterations were noted in the concentrations of concomitantly administered antiepileptic drugs (AEDs) throughout the trial. VGB is useful adjuvant therapy for treatment of partial seizures. There may be a ceiling to effective dosage. This demands individual dose titration for each patient.

Variability and clinical relevance of the interaction between sodium valproate and carbamazepine in epileptic patients

Twenty-four epileptic patients (16 females, 8 males; aged 13-62 years) were studied before and after the addition of sodium valproate (VPA) 500 mg twice daily for 5 days. All had been established previously on carbamazepine (CBZ) as monotherapy (300-1600 mg daily in divided doses). Sixteen of these patients undertook a battery of cognitive function tests before and after VPA introduction. VPA had no effect on total or free CBZ concentrations. However, median concentrations of the active metabolite, CBZ 10,11 epoxide (CBZ-E), were significantly increased (CBZ-E before VPA 1.3 mg/l, after VPA 2.1 mg/l, P less than 0.01). The median rise was 25%, although the extent of the interaction ranged from a 25% decrease to an increase of 123% in CBZ-E concentrations. This was related to the marked inter-individual variation in circulating VPA (mean 25-69 mg/l), as CBZ-E concentrations correlated significantly with total (r = 0.5, P less than 0.05, 95% CI 0 to +0.08) and free (r = 0.7, P less than 0.001, 95% CI +0.09 to +0.25) VPA levels in individual patients. Although uncontrolled, no deterioration in performance of any of the cognitive function tests was observed following the addition of VPA. This study does not support immediate clinical relevance for this drug interaction between VPA and CBZ.

Oxcarbazepine does not interact with cimetidine in healthy volunteers

When cimetidine (CIM) is administered together with the anti-epileptic drug carbamazepine (CBZ), a drug interaction may cause a rise in plasma concentrations of CBZ, which can result in CBZ-related toxic symptoms. The aim of this cross-over study was to investigate whether CIM influences the disposition and kinetics of the new anti-epileptic oxcarbazepine (OXC) and its metabolites. In 8 healthy volunteers there was no difference in AUC, Cmax or tmax when OXC was administered either with or without CIM. The results of this study suggest that in the treatment of epilepsy OXC offers an important advantage over the established anti-epileptics, especially when concomitant therapy with CIM is required.

Rapid tolerance to acute psychomotor impairment with carbamazepine in epileptic patients

Thirteen patients with newly diagnosed epilepsy performed a battery of psychomotor function tests before and during the first 12 weeks of carbamazepine (CBZ) therapy. Movement time (P less than 0.025), total choice reaction time (P less than 0.025), finger tapping rate (P less than 0.005) and number cancellation (P less than 0.05) were all significantly impaired after 1 week's treatment, but had returned to baseline by 4 weeks. Mean (+/- s.d.) serum CBZ concentrations and those of its active metabolite CBZ 10,11 epoxide (CBZ-E) were comparable at 1 (CBZ: 8.5 +/- 2.1 mg l-1, CBZ-E: 1.1 +/- 0.68 mg l-1) and 4 weeks (CBZ: 8.1 +/- 4.4 mg l-1, CBZ-E: 0.93 +/- 0.27 mg l-1). These results suggest the rapid development of tolerance to the acute deleterious psychomotor effects of CBZ.

Monotherapy with conventional and controlled-release carbamazepine: a double-blind, double-dummy comparison in epileptic patients

1. Twenty-one epileptic patients completed a double-blind, double-dummy, random order, crossover comparison of conventional carbamazepine (CBZ, Tegretol, Ciba-Geigy) with a new controlled-release formulation (CBZ-CR, Tegretol Retard). All participants were stabilised on maximally tolerated doses of CBZ as monotherapy (one twice daily, twelve three times daily, eight four times daily). Each preparation was taken with a matched placebo of the other for 4 weeks. 2. Peak serum CBZ concentrations (mean +/- s.e. mean) were lower (CBZ 11.4 +/- 0.4 mg l-1; CBZ-CR 10.4 +/- 0.5 mg l-1; P less than 0.01) and times to peak longer (CBZ 3.6 +/- 0.5 h, CBZ-CR 5.2 +/- 0.7 h, P less than 0.01) during CBZ-CR treatment. Mean CBZ concentrations, however, were also slightly reduced with the new formulation (CBZ 9.9 +/- 0.3 mg l-1; CBZ-CR 9.1 +/- 0.5 mg l-1, P less than 0.05) and this was associated with greater seizure frequency (CBZ 2.8 +/- 1.2, CBZ-CR 3.8 +/- 0.9; P less than 0.05) during the CBZ-CR treatment phase. 3. Diurnal fluctuations (CBZ 41 +/- 3%, CBZ-CR 28 +/- 2%, P less than 0.01) and variations (CBZ 53 +/- 5%, CBZ-CR 33 +/- 3%; P less than 0.01) in serum CBZ concentration were substantially less with CBZ-CR and were similar to those calculated during a 6 or 8 hourly dosage interval with conventional CBZ (fluctuation 33 +/- 3%, variation 42 +/- 5%).(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of enzyme induction with oxcarbazepine (600 mg daily) in healthy subjects

1. Oxcarbazepine (OXC), the 10-keto analogue of carbamazepine (CBZ), has similar anticonvulsant efficacy and possibly improved patient tolerability. Unlike CBZ, it is metabolised by reduction and may not induce hepatic monooxygenase enzymes. 2. Serum concentrations of OXC and its active metabolite 10-OH-carbazepine (10-OH-CZ) were followed after a single 300 mg dose and during and after 300 mg OXC twice daily for 29 doses in eight healthy male volunteers. 3. Antipyrine metabolism, urinary 6-beta-hydroxycortisol excretion, sex hormone binding globulin (SHBG) levels and circulating androgens were measured as indices of hepatic enzyme induction before, during and after treatment with OXC. 4. Elimination half-lives (mean +/- s.e. mean) of 10-OH-CZ were unaltered by 2 weeks' therapy with OXC (before 11.3 +/- 1.1 h; after 13.9 +/- 3 h). Trough plasma concentrations of 10-OH-CZ at steady-state (31 +/- 2.2 mumol l-1) were higher than predicted (16.5 +/- 4 mumol l-1). 5. Antipyrine metabolism, urinary 6-beta-hydroxycortisol excretion, SHBG levels and circulating androgens were unaltered by treatment with OXC. 6. OXC (600 mg daily) does not induce hepatic monooxygenase enzymes and so is likely to have more predictable dose-concentration relationships and to produce fewer physiological and pharmacological interactions than CBZ.

Clinical pharmacokinetics of drugs used in the treatment of gastrointestinal diseases (Part II)

Part I of this article, which appeared in the previous issue of the Journal, covered the following agents: histamine H2-receptor antagonists (cimetidine, ranitidine, famotidine, nizatidine); muscarinic-M1-receptor antagonists (pirenzepine); proton pump inhibitors (omeprazole); site-protective agents (colloidal bismuth subcitrate, sucralfate); antacids and prostaglandin analogues; antiemetics and prokinetics (metoclopramide, domperidone, cisapride); and antispasmodics. In Part II, we consider the anti-inflammatory salicylates, nonspecific antidiarrhoeal agents, laxatives and cathartics.

The effect of clobazam on steady state plasma concentrations of carbamazepine and its metabolites

Steady state metabolite/parent drug plasma ratios were measured in 15 epileptic patients on carbamazepine (CBZ) monotherapy and in seven patients treated with CBZ and clobazam (CLB). CBZ plasma concentrations did not differ between the two groups but patients also treated with CLB exhibited higher concentrations of CBZ-10,11-epoxide (CBZ-E) and trans-10,11-dihydro-10,11-dihydroxy-CBZ (CBZ-T). Ratios between all of the metabolites of CBZ and the parent compound were higher in patients on polytherapy but the ratio between metabolites was not different. CLB comedication causes a moderate increase (about 1.5-fold) in CBZ metabolism, probably by inducing its epoxidation.

Psychopharmacology in patients with hepatic and gastrointestinal disease

Psychotropic drugs often need to be prescribed to patients who also have pre-existing gastrointestinal (GI) and/or hepatic disease. This paper addresses the effect of GI and hepatic disease on the pharmacokinetics of psychotropic drugs, the effect of psychotropic drugs on pre-existing GI and hepatic diseases, the adverse GI and hepatic effects of psychotropic medications, the effects of GI medications on mental status, and the potential drug interactions between commonly prescribed GI medications and psychotropic drugs. Drug selection and dosage modification based on these considerations should allow safe and effective psychotropic treatment for patients with pre-existing GI and/or hepatic disease.

Attenuation of low dose phenobarbital induction of hepatic microsomal aminopyrine N-demethylase activity in rats by cimetidine

Cimetidine, a substituted imidazole, is an inhibitor of hepatic cytochrome P-450-mediated drug metabolism in rats and humans. We investigated the effect of cimetidine on phenobarbital induction of hepatic microsomal aminopyrine N-demethylase activity in the rat. Phenobarbital induction of aminopyrine N-demethylase was log-linear in the range of 1-6 mg/kg/day and the ED50 was approximately 3 mg/kg/day. Cimetidine 75 mg/kg (four times a day) attenuated the induction of aminopyrine N-demethylase activity by 58% in low dose (3 mg/kg/day) but not in high dose (40 mg/kg/day) phenobarbital treated rats. This result could not be explained by residual inhibition of enzyme activity by cimetidine and suggests that cimetidine affects the induction of hepatic cytochrome P-450 by low dose phenobarbital.

The effects of cimetidine on the oxidative metabolism of estradiol

Cimetidine, a histamine H2-receptor antagonist widely used to treat peptic ulceration, is known to cause gynecomastia and sexual dysfunction in some men. Since cimetidine inhibits the cytochrome P-450-dependent biotransformation of numerous drugs, we investigated the possibility that it might also inhibit the cytochrome P-450--dependent metabolism of estradiol. Radiometric analysis of urine and serum samples from nine normal male volunteers showed that the extent of 2-hydroxylation of estradiol was significantly reduced from a mean (+/- SEM) of 31.7 +/- 2.3 percent to 19.7 +/- 2.3 percent (P less than 0.0001) after two weeks of oral treatment with cimetidine (800 mg twice a day); the 16 alpha-hydroxylation of estradiol was unaffected. At the same time, the urinary excretion of 2-hydroxyestrone decreased by approximately 25 percent (P less than 0.0002), and the serum concentration of estradiol increased by approximately 20 percent (P less than 0.04). The mean percentage of estradiol 2-hydroxylation was also rapidly reduced, from 36.8 +/- 4.4 percent to 24.5 +/- 3.4 percent in six men after one week of oral cimetidine at a lower dosage (400 mg twice a day; P less than 0.0006). In a separate study of seven men, ranitidine, a second-generation H2-receptor antagonist, was found to have no effect on the 2-hydroxylation of estradiol. This study demonstrates that the administration of cimetidine to men decreases the 2-hydroxylation of estradiol and results in an increase in the serum estradiol concentration. This mechanism may help to account for the signs and symptoms of estrogen excess reported with the long-term use of cimetidine.

A double-blind comparison of conventional and controlled-release carbamazepine in healthy subjects

1. Eight healthy subjects took part in a balanced, double-blind, crossover comparison of conventional carbamazepine (Tegretol, Ciba-Geigy Ltd, CBZ-C) and a novel controlled-release formulation (Tegretol CR Divitabs, Ciba-Geigy Ltd; CBZ-CR). An initial single dose of either preparation was followed 1 week later by a 2 week course of 200 mg twice daily. 2. Following the single dose, CBZ-CR produced a concentration plateau from 6-56 h at 50-60% of the CBZ-CR peak. 3. After 2 weeks' treatment, CBZ daytime levels measured as area under the concentration-time curve over a dosage interval were 7% lower with CBZ-CR, but this difference was not statistically significant. 4. CBZ-CR showed less diurnal fluctuation (12%) of CBZ than CBZ-C (24%; P less than 0.025) with less rapid changes in concentration (P less than 0.02). 5. Diurnal fluctuation of free CBZ and of CBZ 10,11 epoxide, the active metabolite, did not differ significantly between the two preparations. 6. Auto-induction of CBZ metabolism resulted from the administration of both formulations. The mean elimination half-life was 23 h (CBZ-C) and 25 h (CBZ-CR) after dose 29 compared with a base-line value of 37 h (both P less than 0.02). Antipyrine metabolism was also induced to a similar extent in both legs of the study (P less than 0.01). 7. No significant alteration in psychomotor function was demonstrated with either preparation. 8. CBZ-CR fulfils the criteria for a controlled-release preparation with comparable apparent bioavailability to CBZ-C. Further pharmacokinetic and, more importantly, pharmacodynamic studies are required in epileptic patients to confirm a clinical advantage over the currently available formulation.

Decreased estrogen hydroxylation in male rat liver following cimetidine treatment

Administration of cimetidine (600 mumol/kg x 5) to adult male rats resulted in 55 and 25% decreases, respectively, in estradiol 2- and 16 alpha-hydroxylation. The same treatment also decreased the activities of ethylmorphine demethylase, aryl hydrocarbon hydroxylase, aniline hydroxylase and heme oxygenase but did not inhibit the activities of 7-ethoxycoumarin de-ethylase and delta-aminolevulinic acid synthase or decrease cytochrome P-450 content. In vitro addition of cimetidine (10-300 microM) also inhibited estradiol hydroxylations, and the effect was additive in rats pretreated with cimetidine in vivo; the other enzymic activities studied were completely unaffected by in vitro addition of cimetidine. In contrast, there was no effect of cimetidine either in vivo or in vitro on any of these activities in female rats. The results point to a wide variation in the susceptibilities of different isozymes of cytochrome P-450 to inhibition by cimetidine and suggest that such differential susceptibilities are also highly dependent on the sex of the animal.

Urinary excretion of 6 beta-hydroxycortisol and the time course measurement of enzyme induction in man

The effect of enzyme induction by antipyrine, phenobarbitone and rifampicin on the time-course of urinary 6 beta-hydroxycortisol (6 beta-OHC) excretion was investigated in healthy volunteers. The drugs were given chronically for either seven or 14 days. Significant increases in 6 beta-OHC excretion were observed after 4 days administration of antipyrine (1.2 g), 13 days administration of phenobarbitone (100 mg), and only 2 days administration of rifampicin (0.6 or 1.2 g). During 14 days rifampicin administration (1.2 g) 6 beta-OHC excretion, for individual subjects, reached a maximum on Days 11-14 when excretion was significantly greater than on day 7. On stopping rifampicin, in a 7-day study, excretion decreased over the next six days, but still remained significantly elevated compared to the original control values. These studies show that measurement of urinary 6 beta-hydroxycortisol provides a simple non-invasive method with which to monitor the time-course of enzyme induction by drugs in man. However, the method cannot be used to predict clinically important drug interactions until the cytochrome P-450 enzyme responsible for cortisol 6 beta-hydroxylation has been fully characterized.

Reduced felodipine bioavailability in patients taking anticonvulsants

Felodipine is a dihydropyridine calcium antagonist, structurally related to nifedipine, which undergoes extensive first-pass hepatic metabolism and normally has an oral bioavailability of 15%. Felodipine disposition was studied in 10 patients who had microsomal enzyme induction due to chronic anticonvulsant therapy, and in 12 normal volunteers matched for age and sex. Plasma felodipine concentrations after a 5 mg oral dose were grossly reduced in the epileptic patients: the mean peak concentration was 1.6 (vs 8.9) nmol/l, and the area under the curve was only 2.0 (vs 30.0) nmol.h/l. The relative bioavailability of felodipine in the epileptic patients was thus only 6.6% of that in the normal subjects, and less than 1% of the oral dose was systemically available. Patients on anticonvulsant treatment will require substantially higher doses of felodipine to achieve plasma concentrations equivalent to those in non-induced subjects.

Ranitidine versus cimetidine. A comparison of their potential to cause clinically important drug interactions

The literature on H2-antagonist drug interactions is now extensive. The whole subject is so complicated as to make things difficult for the potential prescriber. However, it is possible to reduce most of the information contained in the literature to a few simple messages. Firstly, H2-antagonists bind to cytochrome P450 and may inhibit the metabolism of drugs eliminated by the mixed function oxygenase system. In this respect, cimetidine has a marked effect which, in most studies, has reached statistical significance. Ranitidine, on the other hand, has a much weaker effect which, even if demonstrable, is statistically non-significant. The potential benefit of ranitidine, however, has to be weighed against the relative costs of the 2 drugs. Secondly, H2-antagonists inhibit gastric acid production and may alter the rate of gastric emptying, and hence the rate of drug absorption. They may also have some effect on portal vein and hepatic artery flow. However, these effects are small and probably not clinically relevant. Thirdly, pharmacodynamic effects of H2-antagonist-drug interactions are difficult to demonstrate in planned studies, and although they are reported from time to time, adverse reactions of consequence are relatively uncommon. Fourthly, the prescriber needs to be aware that cimetidine may produce higher plasma concentrations of some drugs which have a fairly narrow therapeutic range, and this may be clinically important. Examples of drugs for which it may be undesirable to inadvertently increase plasma concentrations include warfarin, theophylline and phenytoin. Finally, for most drugs metabolised by the liver, the risk of an important interaction is small. However, if such an interaction is noted it may be helpful to refer to the other reported cases, and a number of references are included here.

Drug interactions that matter. A critical reappraisal

Drug interactions are ubiquitous but those with proven clinical relevance are much less common. Only when the combined effects of the interacting drugs are greater or less than the arithmetic sum of their individual actions can the event be considered a true interaction. This eliminates many candidate 'interactions' which in reality merely describe the summation of similar or opposing, but independent, drug effects. An appreciation of those drug interactions that really do matter can be best achieved by combining a practical knowledge of the pharmacological mechanisms involved with awareness of the most vulnerable patients (those with little reserve capacity) and the drugs associated with the greatest risk (those with a narrow therapeutic index). This review follows these guidelines and provides an account of well documented drug interactions categorised according to mechanism.

Effect of sodium valproate on carbamazepine disposition and psychomotor profile in man

1 The effect of sodium valproate (VPA; 500 mg thrice daily for 7 days) and matched placebo on the disposition and psychomotor profile of a single dose of carbamazepine (CBZ; 10 mg kg-1) was studied in eight healthy male subjects using a randomised balanced crossover design. 2 VPA alone had no effect on antipyrine clearance, urinary 6 beta-hydroxycortisol excretion and a battery of psychomotor function tests after 3 days' treatment despite achieving a mean steady-state concentration (90 +/- 6 mg 1(-1)) well within the target range (50-100 mg 1(-1)) for the drug. 3 VPA pre-treatment did not alter total CBZ area under the concentration-time curve (AUC 0-59 h) but did prolong CBZ elimination half life by 12% (P less than 0.01). AUC 0-59 h for free plasma CBZ was 13% higher (P less than 0.02) and half-life of unbound CBZ 16% longer (P less than 0.02) during VPA treatment. CBZ-10,11 epoxide (CBZ-E) levels (52%) and CBZ-E/CBZ ratios (45%) were both elevated by concurrent VPA (P less than 0.05) and free CBZ fraction was increased by 7% (P less than 0.02). 4 The sole effect of VPA on the psychomotor profile of CBZ was prolongation of card sorting time (P less than 0.05), although CBZ-related side effects were reported as more severe when VPA was also taken (P less than 0.01). 5 These data suggest that VPA displaces CBZ from plasma protein binding sites and inhibits the metabolism of both the parent drug and its epoxide.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of haem arginate therapy on porphyrin metabolism and mixed function oxygenase activity in acute hepatic porphyria

The effect of haem arginate on porphyrin metabolism and haemoprotein function was studied during seven attacks of acute hepatic porphyria in 5 patients. In each attack it greatly reduced the overproduction of porphyrin precursors and repressed the overactivity of the rate-controlling enzyme of haem synthesis delta-aminolaevulinic acid (ALA) synthase measured in leucocytes. Antipyrine clearance, an index of the oxidative function of cytochromes P-450, the major group of hepatic haemoproteins, was increased during haem therapy. Thus, haem arginate not only suppresses the overproduction of haem precursors but also improves hepatic oxidative metabolism in acute porphyria.

Intradose and circadian variation in circulating carbamazepine and its epoxide in epileptic patients: a consequence of autoinduction of metabolism

Total and free carbamazepine (CBZ), and CBZ 10,11 epoxide (CBZ-E) concentrations were measured over 24 h in 19 patients receiving CBZ 400 mg b.i.d. either as monotherapy (n = 13) or combined with another anticonvulsant (n = 6). Differences in CBZ and CBZ-E disposition between day and night dosing were minor. Mean plasma CBZ concentrations were higher and CBZ-E/CBZ ratios were lower in the monotherapy patients. Variations in total and free plasma CBZ levels were comparable in the monotherapy and polypharmacy groups. Peak free and total CBZ concentrations coincided at approximately 4 h postdose. Free CBZ levels correlated significantly with total in each patient. The extent of variation in total plasma CBZ concentration during 24 h correlated significantly with antipyrine clearance in the monotherapy group. Circadian rhythms are unlikely to influence CBZ disposition to a clinically relevant extent. Measurement of peak and trough CBZ concentrations should improve the value of therapeutic drug monitoring. The diurnal variation in CBZ concentration appears related to the degree of autoinduction of metabolism and is substantial enough to warrant the development of a slow-release preparation of the drug.

Pharmacokinetic interactions of cimetidine 1987

The number of studies on drug interactions with cimetidine has increased at a rapid rate over the past 5 years, with many of the interactions being solely pharmacokinetic in origin. Very few studies have investigated the clinical relevance of such pharmacokinetic interactions by measuring pharmacodynamic responses or clinical endpoints. Apart from pharmacokinetic studies, invariably conducted in young, healthy subjects, there have been a large number of in vitro and in vivo animal studies, case reports, clinical observations and general reviews on the subject, which is tending to develop an industry of its own accord. Nevertheless, where specific mechanisms have been considered, these have undoubtedly increased our knowledge on the way in which humans eliminate xenobiotics. There is now sufficient information to predict the likelihood of a pharmacokinetic drug-drug interaction with cimetidine and to make specific clinical recommendations. Pharmacokinetic drug interactions with cimetidine occur at the sites of gastrointestinal absorption and elimination including metabolism and excretion. Cimetidine has been found to reduce the plasma concentrations of ketoconazole, indomethacin and chlorpromazine by reducing their absorption. In the case of ketoconazole the interaction was clinically important. Cimetidine does not inhibit conjugation mechanisms including glucuronidation, sulphation and acetylation, or deacetylation or ethanol dehydrogenation. It binds to the haem portion of cytochrome P-450 and is thus an inhibitor of phase I drug metabolism (i.e. hydroxylation, dealkylation). Although generally recognised as a nonspecific inhibitor of this type of metabolism, cimetidine does demonstrate some degree of specificity. To date, theophylline 8-oxidation, tolbutamide hydroxylation, ibuprofen hydroxylation, misonidazole demethylation, carbamazepine epoxidation, mexiletine oxidation and steroid hydroxylation have not been shown to be inhibited by cimetidine in humans but the metabolism of at least 30 other drugs is affected. Recent evidence indicates negligible effects of cimetidine on liver blood flow. Cimetidine reduces the renal clearance of drugs which are organic cations, by competing for active tubular secretion in the proximal tubule of the kidney, reducing the renal clearances of procainamide, ranitidine, triamterene, metformin, flecainide and the active metabolite N-acetylprocainamide. This previously unrecognised form of drug interaction with cimetidine may be clinically important for both parent drug, and metabolites which may be active.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of rifampicin on haem and bilirubin metabolism in man

Haem and bilirubin metabolism was studied in seven healthy volunteers during 4 weeks treatment with rifampicin 600 mg at night. The serum unconjugated bilirubin concentration increased 2-3 fold in the first 24 h of treatment (P less than 0.01) and subsequently fell to below pretreatment values (P less than 0.05) during the third and fourth weeks of rifampicin therapy. In each subject, the activity in leucocytes of delta-aminolaevulinic acid (ALA) synthase increased and that of protoporphyrinogen (proto) oxidase decreased during the first week of therapy. The mean ALA synthase activity was most markedly increased on day 4 being seven-fold its pretreatment value (P less than 0.01), and the mean proto oxidase activity most depressed on day 2 at 50% its pretreatment value (P less than 0.02). There was increased urinary excretion of porphobilinogen (PBG) during the first week of therapy and increased excretion of porphyrins and PBG during the third week of treatment. The increase in ALA synthase activity and haem precursor excretion can be explained by the combination of increased haem demand for haemoprotein induction and partial block in haem synthesis due to reduced proto oxidase activity.

Antipyrine metabolism in acute hepatic porphyria in relapse and remission

Antipyrine kinetics following a single oral dose were obtained in porphyric patients in attack and in remission and in controls. The clearance of antipyrine was significantly lower during an acute porphyric attack (median: 0.34 ml min-1 kg-1; range: 0.1-0.71, P less than 0.05) than in patients in remission (median: 0.53 ml min-1 kg-1; range: 0.28-0.87) or controls (median: 0.52 ml min-1 kg-1; range: 0.32-0.93). There was a significant negative correlation between weight-adjusted antipyrine clearance and the urinary excretion of the porphyrin precursors, delta-aminolaevulinic acid (r = 0.86, P less than 0.001) and porphobilinogen (r = 0.82, P less than 0.002). These data suggest that the more severe the porphyric attack, the greater the impairment of hepatic monooxygenase activity.

Pharmacokinetics and pharmacodynamics of intravenous midazolam in patients with severe alcoholic cirrhosis

Midazolam kinetics and psychomotor function were studied after an intravenous dose of 0.075 mg/kg body weight in seven patients with alcoholic cirrhosis and eight control patients. Four of the seven cirrhotics died of complications of their liver disease within six months of the study. The metabolism of midazolam was significantly impaired in the cirrhotic patients (p less than 0.025). These patients also had evidence of greater sedation than the control group for up to six hours after the dose was administered (p less than 0.05). The clearance of midazolam did not correlate significantly with the serum albumin, or bilirubin, or with the kinetics of antipyrine, or indocyanine green. This study shows significant delay in the elimination of midazolam and decreased psychomotor function in patients with severe alcoholic liver disease. Caution is needed in using this drug for premedication in such patients before endoscopy.

Effect of carbamazepine on psychomotor performance in näive subjects

The effect of a single dose of carbamazepine (CBZ), 10 mg kg-1, on a battery of simple psychomotor tests was investigated in 12 healthy subjects (6 male, 6 female) in a balanced randomised double blind placebo controlled cross-over study. Psychomotor testing and blood sampling for total and free plasma CBZ, and CBZ 10, 11 epoxide concentration were performed at 10, 12, 14, 16, 18 and 34 h after oral dosing (23.00 h the previous evening). CBZ impaired i) critical flicker fusion threshold frequency at all time points up to 18 h (p less than 0.005); ii) total choice reaction time at 10 h (p less than 0.005) and 18 h (p less than 0.008); iii) card sorting at 14 h (p less than 0.001). No significant effect on finger tapping was noted. Subjects adjudged themselves more sedated on CBZ as compared to placebo at 12, 14 and 16 h (p less than 0.008). Plasma total and free CBZ concentrations (mean +/- SD) peaked at 10 h (8.8 +/- 0.2 mg 1-1) and 16 h (1.88 +/- 0.3 mg 1-1) after dosing respectively. CBZ 10, 11 epoxide values were all less than 10% of total CBZ concentrations and, therefore, were unlikely to contribute to the pharmacodynamic effect. Total choice reaction time was significantly more impaired in females (p less than 0.05) but no sex difference occurred with the other tests or CBZ concentrations at any time point. No significant correlations were found between individual total or free CBZ concentrations and corresponding test performances at each time point.(ABSTRACT TRUNCATED AT 250 WORDS)

Controlled evaluation of a supplementary dose of carbamazepine on psychomotor function in epileptic patients

The effect of an additional dose of 400 mg carbamazepine (CBZ) on a series of simple psychomotor tests was investigated in 8 patients with epilepsy receiving chronic CBZ monotherapy in a balanced randomised double-blind placebo controlled cross-over study. Psychomotor testing and blood sampling for total and free CBZ and CBZ 10,11 epoxide (CBZ-E) concentrations were performed at 10, 12, 14, 16 and 18 h after the extra dose which was administered at 23.00 h on the previous evening. The CBZ increment produced significant impairment of choice reaction recognition time from 10-16 h after the dose total choice reaction time at 12 h card sorting at 12 h sedation scoring at 12 h. No significant effect on critical flicker fusion threshold, finger tapping or simple memory testing was noted. No patient reported increased side-effects in the placebo phase while 5 noted new symptoms likely to be attributable to the additional CBZ. Areas under the concentration-time curves from 10-18 h were higher following CBZ than placebo for total and free CBZ and CBZ-E concentrations. This study has demonstrated decrements in performance of a series of simple psychomotor tests in epileptic patients receiving a supplemental CBZ dose. Patients with epilepsy who require high CBZ concentrations for optimal control of seizures may be at risk of concurrent impairment of psychomotor function. Simple objective measures of performance may help in assessing the benefit-risk ratio.