Hepatic enzyme induction and leucocyte delta-aminolaevulinic acid synthase activity: studies with carbamazepine

Guide to drug porphyrogenicity prediction and drug prescription in the acute porphyrias

AIMS

This paper addresses two common problems in the care of carriers of acute porphyria: the choice of safe drugs for pharmacotherapy and the strategy to apply when potentially unsafe drugs cannot be avoided.

METHODS AND RESULTS

A technique is presented for prediction of risk that a certain drug may activate the disease in a gene carrier for acute porphyria. It is based on a model explaining the clinical manifestations as a result of the acute overloading of a deficient enzyme within the hepatic heme biosynthetic chain. The capacity of the drug for induction of the rate-limiting enzyme in heme biosynthesis, e.g. housekeeping 5-aminolevulinate synthase (ALAS1), is assessed by critical appraisal of reports of the outcomes of clinical use of the drug, and by theoretical criteria. The assessment occurs within the frame of a flow-scheme employing variables of increasing specificity, i.e. endocrine properties of the drug, structure and metabolism pointing to affinity to cytochrome P450, hepatic load in therapeutic use, recognized affinity to major CYP species, capacity for CYP-induction or irreversible inhibition, and capacity to activate or modulate the transduction mechanisms of nuclear receptors affecting ALAS1-gene transcription. It is proposed that in the absence of a safer alternative, an urgently needed drug not should be withheld on the grounds of potential porphyrogenicity. After risk-benefit analysis it should be prescribed, but individualized preventive measures adapted to patient vulnerability may be needed.

CONCLUSIONS

About 1000 therapeutic drugs categorized with regard to porphyrogenicity by the technique proposed are presented on the internet (http://www.drugs-porphyria.org).

Induction of endogenous pathways by antiepileptics and clinical implications

The aim of this study was to review modifications of the endogenous pathways (e.g. enzyme elevations, normal body constituent depletion or higher formation/excretion of endogenous metabolites) which could be ascribed to enzyme induction by antiepileptic drugs (AEDs). Information on older (e.g. phenobarbital, phenytoin and carbamazepine) and newer drugs (where information is available) is discussed together with clinical implications. The enzymes involved in the endogenous pathways and induced by the AEDs will not be limited to the hepatic microsomal enzymes; extrahepatic enzymes and/or enzymes present in other subcellular fractions will also be discussed, if pertinent. The induction of endogenous pathways by AEDs has been taken into account in the past, but much less emphasis has been given compared with the extensive literature on induction by AEDs of the metabolism of concomitantly administered drugs, either of the same or of different classes. Not all of the endogenous pathways examined and induced by AEDs appear to result in serious clinical consequences (e.g. induction of hepatic ALP, increased excretion of d-glucaric acid or of 6 beta-hydroxycortisol). In some cases, induction of some pathways (e.g. increase of high-density lipoprotein cholesterol or of conjugated bilirubin) might even be a beneficial side-effect, however enzyme induction is considered rather a detrimental aspect for an AED, as induction is generally a broad and a non-specific phenomenon. The new AEDs have generally less induction potential than the older agents. Yet some (felbamate, topiramate, oxcarbazepine and lamotrigine) have the potential for inducing enzymes, whereas others (levetiracetam, gabapentin and vigabatrin) appear to be completely devoid of enzyme inducing characteristics, at least as far as the enzymes investigated are concerned.

Antiepileptic drug therapy and sexual function in men with epilepsy

PURPOSE

To study the effects of antiepileptic drugs (AEDs) on sex hormone levels and sexual activity in a group of men attending a hospital-based epilepsy clinic.

METHODS

One hundred eighteen men being treated with AED therapy, 32 with epilepsy but not receiving AEDs, and 34 controls were recruited. All subjects were aged 18-65 years. Blood (20 ml) was removed for hormone assays, after which each subject completed a validated questionnaire [Sexuality Experience Scores (Frenken and Vennix, 1981)] aimed at exploring the individuals' sexual activity and attitudes to sexual morality.

RESULTS

Men taking carbamazepine (CBZ) only had significantly higher mean sex hormone-binding globulin (SHBG) levels than the control group. The CBZ group also had a significantly lower mean DHEAS concentration than the control, untreated, and sodium valproate (VPA) monotherapy groups. The phenytoin monotherapy group (PHT) had a significantly higher mean SHBG than both the control and untreated groups, and had a significantly higher mean total testosterone (TT) value than the control untreated, CBZ, and VPA groups, and a significantly lower mean DHEAS than the controls, untreated, and VPA groups. Men receiving more than one AED had significantly higher mean SHBG concentrations compared with control, untreated, and VPA groups. In addition, the polytherapy group's mean TT was significantly higher than the control and VPA groups, although its mean DHEAS concentration was lower than the control, untreated, and VPA groups. There were no significant differences between the study groups in mean FT, Budrostenedione (AND), or estradiol levels. But the CBZ, PHT, and polytherapy groups had significantly lower mean free and rogen index (FAI) than the controls. The CBZ group had a lower mean FAI than the VPA group. The polytherapy group had a lower FAI than the untreated group. Sexuality Experience Scores (SES) showed that those men receiving AEDs embraced a stricter sexual morality than the controls and untreated, and expressed greater satisfaction with their marriages than the control and untreated groups.

CONCLUSIONS

Seizure type did not affect SES scores. Multiple regression showed men who had received further education were less accepting of strict sexual morality.

Optimisation of antiepileptic drug therapy. The importance of serum drug concentration monitoring

The ability to measure the serum concentrations of antiepileptic drugs, and the widespread use of this procedure, has markedly improved the treatment given to patients with epilepsy during the past 3 decades. The monitoring of antiepileptic drug concentrations in serum is necessary for the optimal drug therapy of seizures, because the therapeutic and toxic effects of these drugs are better related to serum concentration than to administered dosage. Monitoring appeared to have a major impact on improving the effectiveness and safety of antiepileptic drug therapy. The age-related variability of pharmacokinetic parameters may also require the individualisation of therapy, with subsequent re-evaluation as the person grows older. Monitoring serum concentrations of antiepileptic drugs may help to optimise the dose. A drug concentration, however, can only be regarded as a guide around which to alter the dosage according to the patient's clinical condition. Serum drug concentration monitoring is particularly useful to ensure compliance and in helping to manage combinations of antiepileptic drugs that invariably interact. The addition or deletion of other antiepileptic drugs may change dosage requirements. Therefore, routine monitoring of antiepileptic drug serum concentrations would be extremely useful, especially in the paediatric population, and in patients who require associated antiepileptic medication.

Early stage autoinduction of carbamazepine metabolism in humans

Six healthy young adult male volunteers were given two 600 mg (2540 mu moles) oral doses of carbamazepine (CBZ) 5 days apart. Serial concentrations of CBZ and its 10,11-epoxy (CBZ-epoxide) and 10,11-dihydro-10,11-trans-dihydroxy (CBZ-diol) metabolites in plasma, and daily excretions of these substances and the 2-hydroxy (2-OH-CBZ), 3-hydroxy (3-OH-CBZ) and 9-hydroxymethyl-10-carbamoylacridan (acridan) metabolites in urine were followed for 5 days after each dose. Pharmacokinetic analysis showed that autoinduction of CBZ metabolism was present within 6-10 days of the initial drug dose. The mean oral clearance of CBZ increased from 1.48 to 1.74 l.h-1 (difference 0.26 l.h-1, 95% confidence interval 0.11 to 0.41 l.h-1) and the mean percentage urinary recovery of the amount of CBZ eliminated increased from 41.8% to 44.6% (difference 2.8%, 95% confidence interval 0.5 to 5%) between the two studies 5 days apart. The data for daily clearance to metabolite and the time-courses of the plasma CBZ-epoxide to CBZ and CBZ-diol to CBZ concentration ratios suggested that autoinduction had begun by the second day after CBZ intake, and involved not only the epoxide-diol pathway but, to a lesser extent, the oxidations to phenolic derivatives.

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.

Pharmacokinetic optimisation of anticonvulsant therapy

Changing attitudes towards the use of antiepileptic drugs have led to an emphasis on monotherapy with serum concentration measurement coupled with standard, weight-adjusted starting and maintenance regimens to guide initial therapy and subsequent dosage titration. Currently, the established anticonvulsants are carbamazepine, valproic acid (sodium valproate) and phenytoin. Phenobarbital is now less commonly prescribed due to its propensity to produce sedation and impair cognitive function. The value of pharmacokinetic optimisation with valproic acid is limited by its wide therapeutic index, large fluctuations in the concentration-time profile and concentration-dependent protein binding. Thus, although serum concentrations are often measured, they are rarely subjected to pharmacokinetic interpretation. Carbamazepine has a flatter concentration-time profile than valproic acid. Its target range is more clearly defined and it undergoes autoinduction of metabolism and interacts with other drugs. Pharmacokinetic principles can, therefore, be more readily applied to carbamazepine, although, in general, a simple clinical approach to its use is usually satisfactory. Phenytoin has required the greatest pharmacokinetic input due to its nonlinear pharmacokinetics and narrow target range. Many different graphical methods, equations and computer programs have been reported, some of which demand 2 steady-state, dose-concentration pairs; others function satisfactorily with only 1. Recent attempts have been made to interpret non-steady-state data. In addition, a number of workers have demonstrated the value of altering the population parameter estimates to account for ethnic differences. A pharmacokinetic approach can also be used to tailor the use of phenytoin in the treatment of status epilepticus. Dosage alterations may be needed for specific patient groups. In particular, children generally require higher dosages on a weight-for-weight basis than adults, while equivalently lower dosages should be given to neonates. Most anticonvulsants are principally cleared by hepatic mechanisms, so dosage adjustment is not usually required in renal disease, although care must be taken in interpreting serum concentrations because of changes in protein binding. Close monitoring is required in the elderly and patients with hepatic impairment, while increased dosages may be needed in critically ill patients and during pregnancy. Pharmacokinetic principles can be used in the treatment of treat self-poisoning with anticonvulsants. There are few data available on the pharmacokinetics of vigabatrin, lamotrigine, oxcarbazepine and gabapentin in patients. Due to its mode of action in binding irreversibly to its target enzyme, serum concentration monitoring of vigabatrin plays no role in optimising therapy. The value of applying pharmacokinetic principles with the other 3 drugs remains to be investigated.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.

Halothane reductive metabolism in an adult surgical population

The reductive metabolism of halothane was studied in 34 adult patients undergoing routine surgery. Reductive biotransformation of halothane was more extensive in females than males and was also enhanced in two patients treated preoperatively with phenytoin, an enzyme-inducing drug. Tobacco, ethanol and the patient's age, body weight and previous exposure to halothane did not influence reductive metabolism of halothane.

Elevation of carbamazepine plasma levels by diltiazem in rabbits: a potentially important drug interaction

The effect of diltiazem on the plasma level of carbamazepine (CBZ) was investigated in rabbits. The animals were given either CBZ alone or in combination with diltiazem and plasma samples were collected at different time intervals. The concentration of CBZ was detected using an HPLC method. Diltiazem significantly increased the area under the curve (AUC), the maximum plasma concentration (Cmax), and the elimination half-life (t1/2) of CBZ (p less than 0.05). These results suggest that a potentially harmful drug-drug interaction may occur if CBZ and diltiazem are administered concurrently.

The effect of carbamazepine on valproic acid disposition in adult volunteers

1. Pharmacokinetic parameters for valproic acid (VPA) were determined before and following 2 weeks of carbamazepine (CBZ) administration in five healthy male volunteers. Mean VPA dosage was 16.4 mg kg-1 day-1. CBZ dosage was started at 100 mg twice daily and increased after 1 week to a total daily dose of 300 mg. 2. After CBZ administration, mean VPA plasma clearance increased from 0.90 +/- 0.18 s.d. to 1.26 +/- 0.24 l h-1 (P less than 0.05) as did clearance of free VPA (20.8 +/- 7.6 to 37.0 +/- 13.6 l h-1). Mean VPA elimination rate constant increased from 0.051 +/- 0.011 to 0.067 +/- 0.011 h-1 (P less than 0.05) after CBZ administration. 3. Mean area under the serum concentration vs time curve decreased from 675.0 +/- 130.5 to 475.7 +/- 75.7 mg l-1 h (P less than 0.05) after CBZ administration. Mean serum VPA half-life decreased from 14.0 +/- 2.4 to 10.6 +/- 1.4 h (P less than 0.05). Mean serum VPA trough concentrations decreased from 44.0 +/- 16.7 to 27.0 +/- 10.4 micrograms ml-1 (P less than 0.05). 4. A significant change was not observed in the mean VPA volume of distribution after CBZ coadministration suggesting that enzyme induction rather than a competition for plasma protein binding sites was involved in this interaction. 5. Despite the increased clearance of VPA, the urinary recovery of VPA or conjugate did not increase after CBZ administration.

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.

Effect of the removal of individual antiepileptic drugs on antipyrine kinetics, in patients taking polytherapy

1. Antipyrine (AP) clearance, half-life and volume of distribution were determined in 52 patients, taking one or more antiepileptic drug (AED), before and 4 weeks after the complete removal of phenytoin (PHT, n = 20), carbamazepine (CBZ, n = 15) and sodium valproate (VPA, n = 17). 2. PHT removal was associated with a mean 13% fall in AP clearance and a mean 16% increase in AP half-life, in patients who were also taking CBZ with or without barbiturates. There was no significant difference between patients who did, and did not, take barbiturates, in addition to CBZ. 3. CBZ removal was associated with a mean 45% fall in AP clearance and a mean 69% increase in AP half-life, if there was no inducing AED comedication, but had no effect on AP clearance and half-life if PHT and/or barbiturates were also being taken. 4. Removal of VPA had no effect on AP clearance or half-life. 5. The removal of PHT, CBZ and VPA had no significant effect on AP volume of distribution. 6. PHT appears to be a more powerful inducer of hepatic enzyme activity, as measured by the AP test, than is CBZ.

The effects of chronic carbamazepine treatment of haem biosynthesis in man and rat

The anticonvulsant drug carbamazepine has been reported to produce a condition clinically and biochemically similar to acute intermittent porphyria (AIP). We have determined the effect of chronic carbamazepine treatment on the activities of the enzymes of haem biosynthesis in circulating blood cells and on the urinary excretion of porphyrins and their precursors in 53 epileptic patients receiving monotherapy and in 42 age- and sex-matched controls. In the patients the mean activity of leucocyte 5-aminolaevulinic acid (ALA) synthase, the rate-limiting enzyme of the pathway, was 218% of control values (p less than 0.001) and ALA-dehydratase activity was reduced by 37% (p less than 0.001). Circulating carbamazepine concentrations correlated negatively with ALA dehydratase (rs = -0.45; p less than 0.01). Porphobilinogen deaminase and uroporphyrinogen decarboxylase appeared unaffected by carbamazepine treatment. Significant quantitative increases in the urinary excretion of porphobilinogen and total porphyrins (both p less than 0.05) accompanied the changes in enzyme activity. Similar dose-dependent effects on ALA synthase and ALA dehydratase were shown to occur in rats treated for 5 days with 3 different doses of carbamazepine. These findings further support the porphyrinogenicity of carbamazepine, but the pattern of enzyme alteration differs from that found in AIP.

[Hepatic mono-oxygenases]

Most drugs and xenobiotics are lipid-soluble compounds that need to be transformed into more polar water-soluble molecules by a system of hepatic monoxygenases in order to be excreted by the kidney and the liver. This system is also called cytochrome P-450. It is found in animals, as well as plants. It is located in the cellular endoplasmic reticulum of numerous tissues, but it is most active in the liver. It is made up of several isoenzymes differing from one another by the structure of their apoproteins, their immunological characteristics and their affinity for various substrates. Cytochrome P-450 has great variability, being influenced by exogenous factors (drug intake, ionizing radiation, stress, diet) and individual endogenous factors (age, sex, genetic factors). Several non specific tests exploring the system are available. They include: direct investigations carried out on liver biopsies, which are seldom used in clinical practice, and indirect investigations, such as the measurement of the clearance of exogenous substances, of urinary metabolites of endogenous substances and of specific enzymes. Induction and inhibition of microsomal activity are of the utmost interest to the clinician in various fields such as toxicology, carcinogenesis, drug interactions or drug habituation, metabolic regulations and maintenance of body homeostasis. Seven classes of enzyme inducers have been defined, but the exact mechanism of this has only been identified for two of them (the barbiturate and polycyclic hydrocarbon groups). Several drugs have been identified as enzyme inhibitors, the best known to the anaesthesiologist being macrolide antibiotics, imidazole derivatives, cimetidine, chloramphenicol and isonazide.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

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.

Free drug concentration monitoring in clinical practice. Rationale and current status

Recent advances in techniques to determine free drug concentrations have lead to a substantial increase in the monitoring of this parameter in clinical practice. The majority of drug binding to macromolecules in serum can be accounted for by association with albumin and alpha 1-acid glycoprotein. Albumin is the primary binding protein for acidic drugs, while binding to alpha 1-acid glycoprotein is more commonly observed with basic lipophilic agents. Alterations in the concentrations of either of these macromolecules can result in significant changes in free fraction. Diseases such as cirrhosis, nephrotic syndrome and malnourishment can result in hypoalbuminaemia. Burn injury, cancer, chronic pain syndrome, myocardial infarction, inflammatory diseases and trauma are all associated with elevations in the concentration of alpha 1-acid glycoprotein. Treatment with a number of drugs has also been shown to increase alpha 1-acid glycoprotein serum concentrations. A wide variety of biological fluids have been examined for their ability to provide an estimation of free drug concentration at receptor sites. The most useful fluid for estimating free drug concentrations appears to be plasma or serum, with subsequent treatment of the sample to separate free and bound drug by an appropriate technique. The two most widely used methods are equilibrium dialysis and ultrafiltration. Of these two, ultrafiltration has the greatest utility clinically because it is rapid and relatively simple. The major difficulty associated with this method involves the binding of drug to the ultrafilters, but significant progress has been made in solving this problem. Several authors have endorsed the routine use of free drug concentration monitoring. Data examining the clinical usefulness of free drug concentration monitoring for phenytoin, carbamazepine, valproic acid, disopyramide and lignocaine (lidocaine) are reviewed. While available evidence suggests that free concentrations may correlate with clinical effects better than total drug concentrations, there are insufficient data to justify the recommendation of the routine use of free drug concentration monitoring for any of these agents at present.

Carbamazepine and ethanol elicited responses in rodents

The interaction between carbamazepine, and anticonvulsant with clinical efficacy in alcohol withdrawal syndrome, and ethanol was studied in rodents. Voluntary intake of ethanol by the rat was the behavioral performance test used to assess one aspect of such interaction. Carbamazepine, 50 mg/kg, IP, caused aversion to ethanol drinking. The drug was devoid of action on rat hepatic ethanol and acetaldehyde metabolizing enzymes, i.e., alcohol- and aldehyde dehydrogenase, and on testicular aldehyde dehydrogenase. The moderate induction of the latter by prolonged ethanol consumption was antagonized by a single dose of carbamazepine (50 mg/kg). Administration of carbamazepine, 50 mg/kg twice daily for three consecutive days, moderately inhibited mouse liver alcohol dehydrogenase in the male but not in the female mouse. This treatment did not alter endogenous mouse cardiac lactate dehydrogenase isoenzymes or hepatic aldehyde dehydrogenase in either sex. The enzymatic portion of the study suggests species and sex differences in the effects of carbamazepine studied. The reduction of voluntary drinking of ethanol by carbamazepine may have clinical implications, e.g., the extension of its use in alcohol withdrawal phase to alcohol abstinence.

Induction of cytochromes P-450 in pancreatic disease: consequence, coincidence or cause?

We have examined the pharmacokinetics of antipyrine and of theophylline--validated probes for cytochromes P-450 activities--in a series of patients with pancreatic disease. The half-life of each drug was significantly lower, and its clearance faster, in patients than in controls and this pattern was detected in the subgroups with acute pancreatitis (6), chronic pancreatitis (22), or pancreatic cancer (4). These data suggest induction of cytochromes P-450 in all forms of exocrine pancreatic disease. Enzyme induction is unlikely to be secondary to pancreatic malfunction since there was no correlation between prevailing exocrine status, as assessed by secretin-pancreozymin tests, and the half-life or clearance of either drug. The corollary is that induction of the mono-oxygenases by environmental agents, both recognised and unidentified, is a primary event in pancreatic disease. The possible relevance of this finding is discussed.

The influence of carbamazepine on the heme biosynthetic pathway

Carbamazepine, a drug which is widely used in neurological diseases, has a porphyrogenic effect in chick embryo liver cells in culture. It increased the concentration of cellular porphyrins by 80-fold and delta-aminolevulinate synthase activity by 4-fold. The increase in the accumulation of porphyrins preceded that of ALAS activity. Measurements of the activities of aminolevulinate dehydrase, porphobilinogen deaminase, and uroporphyrinogen decarboxylase showed that C inhibits UROD up to nearly 50% and PBGD activity up to 20%, but does not affect the activity of ALAD. The pattern of accumulation of porphyrins, mainly uro- and heptacarboxylporphyrin, is compatible with an inhibition of UROD. We may, therefore, conclude that the porphyrogenic effect of C in monolayers of chick embryo liver cells is the result of its inhibitory effect on the activity of UROD.

Carbamazepine substitution in severe partial epilepsy: implication of autoinduction of metabolism

Established partial seizures are often refractory to treatment and many patients receive polypharmacy. An attempt was made to improve seizure control with the substitution of carbamazepine (CBZ) for existing treatment in 7 consecutive unremitting cases of partial epilepsy referred by their physicians as 'intractable'. This produced a significant improvement in control of partial (P less than 0.02) and secondary generalized (P less than 0.01) seizures, with 5 patients experiencing a 50% or greater reduction in seizure frequency. A single patient suffered a generalized seizure during the period of changeover. In 3 cases auto-induction of CBZ metabolism resulted in temporary loss of seizure control which was restored by an increase in dose. A policy of planned substitution of CBZ in partial epilepsy previously regarded as intractable may be successful in selected patients. The possible deleterious effect of CBZ auto-induction should be anticipated.

Therapeutic drug monitoring--the need for audit?

The experience of therapeutic drug monitoring (TDM) of seven drugs as part of the normal biochemistry services over the period April 1982-83 is reported. Reagent costs alone for these assays exceeded pounds 15,000. Of 1,841 digoxin estimations, 56 per cent fell within the therapeutic range whereas 25 per cent were potentially toxic. Fifty per cent of 968 lithium measurements were lower than 4 mg L-1, placing many of these patients at risk of treatment failure. Only 29 per cent and 32 per cent of 369 theophylline and 440 phenytoin concentrations respectively were within their well-established ranges. In only around 20 per cent of requests for theophylline and phenytoin assay was sufficient clinical information supplied to the laboratory. In the majority of patients with theophylline or phenytoin concentrations outwith the therapeutic range, further analysis was not requested and so optimisation of dosage cannot be assumed to have occurred. The wide therapeutic ranges of carbamazepine and phenobarbitone ensured that most patients attained acceptable concentrations. Sodium valproate analysis was requested on 160 occasions despite the poor correlation between the concentration of this drug and its therapeutic and toxic effects. TDM can be an expensive exercise which must be subjected to rigorous cost-benefit analysis. Requests should be made on a customised drug assay form and interpretative advice on individual patient problems made available. Recommendations for the organisation, daily running and clinical supervision of TDM are made.

The disposition of bupropion and its metabolites in healthy male volunteers after single and multiple doses

The pharmacokinetics of bupropion and 3 of its basic metabolites were determined in 8 young, healthy, male volunteers after single and multiple oral doses of bupropion. Plasma profiles were obtained: 1) after a single 100 mg oral dose of bupropion hydrochloride, 2) following administration of 100 mg 8-hourly for 14 days and 3) again after a single 100 mg dose 14 days later. Plasma concentrations of the parent drug and metabolites were determined by high-performance liquid chromatography. Saliva secretion and pupil diameters were measured, subjective assessments of sleep made using visual analogue scales and side effects, blood counts and biochemistry were monitored. After the first dose mean elimination half lives (t1/2) of bupropion, and metabolites I and II were 8, 19 and 19 h respectively. On repeated administration there was little accumulation of the parent drug and no evidence for induction of its own metabolism. Accumulation of I was consistent with its rate of elimination after single doses while that of II was greater than predicted with prolongation of t1/2 to 35 h. Metabolite III was barely detectable after single doses but its accumulation on multiple dosing was consistent with its long half life (35 h) determined on occasion 2. Saliva secretion was significantly reduced during the multiple dosing period but there were no complaints of dry mouth. Subjective assessments of sleep were not significantly altered though one subject reported vivid dreams. There were no other adverse reactions.

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

The effect of cimetidine (CMT; 400 mg twice daily) and matching placebo on the enzyme-inducing properties of carbamazepine (CBZ; 200 mg at night for 15 days) was studied in seven healthy male volunteers. CMT alone had no significant effect on antipyrine kinetics, urinary 6 beta-hydroxycortisol excretion or leucocyte delta-aminolaevulinic acid synthase (ALA.S) activity. CBZ increased leucocyte ALA.S activity by 204% following 1 week's treatment (P less than 0.001). Thereafter, ALA.S activity fell despite continued CBZ administration. Concomitant CMT did not influence this response. Antipyrine clearance and urinary 6 beta-hydroxycortisol excretion were both increased by CBZ after 2 weeks' treatment (P less than 0.01). CMT blocked CBZ induction of antipyrine metabolism but the rise in urinary 6 beta-hydroxycortisol excretion was unaffected. Plasma CBZ concentrations 10, 14 and 18 h following the 8th and 15th doses were higher when CMT was taken concurrently (P less than 0.05). CBZ half-life fell by 36% and clearance rose by 29% (both P less than 0.001) with placebo and by 10% and 7% (both NS) when CMT was taken concurrently. CMT inhibits CBZ auto- and hetero-induction in man. Epileptic patients receiving CBZ chronically may be at risk of toxicity if CMT is also prescribed.

Treatment of intractable neurogenic pain with carbamazepine

Sixteen patients with previously intractable neurogenic pain were treated with carbamazepine (CBZ) for a period of six weeks. CBZ initial daily dosage of 400 mg was increased by a similar amount every second week to a maximum of 1,200 mg. Single dose kinetic studies prior to initiating CBZ therapy showed that these patients metabolised the drug similarly to healthy controls. Side effects, including rash (3) and ataxia (3), necessitated discontinuation of treatment in six patients. A further three patients withdrew because of lack of effect. Of the seven patients who completed the protocol, five showed a significant fall in pain score at all dosage increments with a maximum effect at the highest dose (p less than 0.01). In the seven patients for whom data was available at all three CBZ dosages, there was a significant correlation between sedation scores and CBZ concentrations (p less than 0.005). CBZ may be of value in the management of chronic neurogenic pain. Further controlled studies are indicated.

Effect of carbamazepine on haem biosynthesis in man

The effects of the enzyme-inducing anticonvulsant carbamazepine on haem biosynthesis in healthy male subjects is reported. A dose-dependent increase in the activity of leucocyte delta-aminolaevulinic-acid synthase, the rate-limiting enzyme of haem biosynthesis, was noted following 400 and 600 mg carbamazepine daily in the same eight subjects. This rise was maximal after 1 week's treatment (400 mg: 509% +/- 285 of baseline; 600 mg: 1062% +/- 170 of baseline; P less than 0.01). Values fell from this peak during the second week despite continuing carbamazepine administration. This pattern was confirmed in a further six subjects taking carbamazepine 400 mg daily for 3 weeks in whom more frequent enzyme measurements were made. The activity of uroporphyrinogen-1-synthase in the erythrocyte fell by 10-15% during the treatment periods (P less than 0.01). Uroporphyrin and penta-, hexa- and hepta-carboxylic porphyrins appeared in the urine of all subjects during CBZ therapy. Changes in daily urinary porphyrin and precursor excretion were inconsistent. CBZ is a porphyrinogenic drug which mimics the changes in enzyme activities and urinary porphyrin ester profile found in patients with latent acute intermittent porphyria, who have a genetic deficiency in uroporphyrinogen-1-synthase activity. Leucocyte delta-aminolaevulinic-acid synthase may provide a suitable in vivo system for testing the porphyrinogenicity of drugs in man.

Changes in circulating androgens during short term carbamazepine therapy

Serum concentrations of testosterone, androstenedione, dehydroepiandrosterone sulphate (DHAS), sex hormone binding globulin (SHBG) and luteinising hormone (LH) were measured before, during and after 21 days treatment with carbamazepine (CBZ)400 mg daily in six healthy male subjects. Induction of hepatic microsomal enzyme activity was confirmed by an increase in antipyrine clearance (P less than 0.02) and a fall in circulating CBZ concentrations from the seventh to the fourteenth CBZ dose (P less than 0.05). Within 7 days of starting CBZ there was a rise in SHBG (P less than 0.05) and a fall in testosterone, free testosterone fraction, DHAS and androstenedione (P less than 0.05). Testosterone, free testosterone fraction and androstenedione levels rose towards baseline by the end of the treatment period while DHAS concentration remained low (P less than 0.05). The rise in SHBG and increased androgen catabolism is most likely to be secondary to induction of hepatic monooxygenase activity by CBZ. These changes may be implicated in the production of sexual dysfunction encountered in some epileptic patients on chronic anticonvulsant therapy.

Effect of stanozolol on delta-aminolaevulinic acid synthase and hepatic monooxygenase activity in man and rat

Stanozolol is an anabolic steroid which is used in the treatment of aplastic anaemia and has been recently advocated for the prophylaxis of vascular thrombosis. Similar steroid substances stimulate the activity of delta-aminolaevulinic acid synthase (ALA S), the rate limiting enzyme of haem biosynthesis, in rat hepatocytes and chick embryo liver cell cultures and activate acute hepatic porphyria. In the present study stanozolol (10 mg daily for 14 days) has been shown to increase significantly leucocyte ALA S activity in 9 healthy male subjects. There was a concomitant rise in urinary ALA and total porphyrin excretion but no change in antipyrine kinetics or urinary 6 B hydroxycortisol excretion. In a complementary study in male Sprague Dawley rats, stanozolol administered intraperitoneally, produced a dose-dependent increase in hepatic ALA S activity without changing hepatic cytochrome P 450 content. Stanozolol has been clearly shown to elevate ALA S activity, probably directly, and thereby, porphyrin production without affecting hepatic monooxygenase activity. This porphyrinogenic effect may be relevant to the successful treatment of aplastic anaemia with anabolic steroids. Leucocyte ALA S activity may provide a human system for the study of drug porphyrinogenicity in vivo.

Changes in circulating thyroid hormones during short-term hepatic enzyme induction with carbamazepine

The effect of short-term hepatic enzyme induction with carbamazepine (CBZ) on circulating thyroid hormone concentrations was studied in 10 healthy male subjects. CBZ 400 mg per day was given for 21 days in 6 subjects and for 14 days in a further 4. In the former group the effect of therapy on the pituitary/thyroid axis was also assessed by measuring thyroid stimulating hormone (TSH) response to thyrotrophin-releasing hormone. CBZ therapy resulted in induction of hepatic monooxygenase activity, evidenced by a fall in antipyrine half-life (11.1 +/- 0.7 to 7.6 +/- 0.7 h; p less than 0.001), and a rise in antipyrine clearance (0.72 +/- 0.06 to 0.98 +/- 0.1 ml min-1 kg-1; p less than 0.001). A significant fall in total serum thyroxine (T4) (81.9 +/- 2.9 to 75.1 +/- 2.9 nmol l-1), and triiodothyronine (T3); (1.59 +/- 0.07 to 1.37 +/- 0.05 nmol l-1) and free T4 (16.03 +/- 0.82 to 14.2 +/- 0.8 pmol l-1) was seen after CBZ therapy. (all p less than 0.05). No significant change in reverse T3 or thyroid binding globulin occurred. In the 6 subjects studied for 21 days, maximal changes were found following 14 days' treatment. Basal and stimulated TSH remained unaltered. These effects on circulating thyroid hormone concentrations are likely to be secondary to hepatic enzyme induction leading to accelerated nondeiodinative hepatic hormone disposal. The reason for the failure of pituitary TSH secretion to rise in response to the fall in circulating T4 and T3 is unclear but may have implications for chronic treatment with CBZ in epileptic patients.