Pharmacokinetic interactions with antiepileptic drugs

Ketogenic Diet in Neonates with Drug-Resistant Epilepsy: Efficacy and Side Effects-A Single Center's Initial Experience

BACKGROUND

For patients with pharmacoresistant epilepsy, a therapeutic option is ketogenic diet. Currently, data on young infants are scarce, particularly during hospitalization in the neonatal intensive care unit (NICU).

OBJECTIVE

The aim of the present study was to evaluate the short-term (3-month) efficacy and side effects of ketogenic diet in infants with "drugs-resistant" epilepsy treated during NICU stay.

METHODS

This retrospective study included infants aged under 2 months started on ketogenic diet during NICU hospitalization to treat drug-resistant epilepsy from April 2018 to November 2022.

RESULTS

Thirteen term-born infants were included, three (23.1%) of whom were excluded because they did not respond to the ketogenic diet. Finally, we included 10 infants. Six (60%) patients took three antiepileptics before starting the ketogenic diet, while four (40%) took more drugs. Diet had a good response in four (40%) patients. In four patients, the ketogenic diet was suspended because of the onset of serious side effects. The emetic levels of sodium, potassium, and chlorine, pH, and onset of diarrhea, constipation, and gastroesophageal reflux showed significant differences. Ketonuria was higher and blood pH lower in the group that took more than three drugs than in the group taking fewer than three drugs.

CONCLUSION

The ketogenic diet is efficacious and safe in infants, but the early and aggressive management of adverse reactions is important to improve the safety and effectiveness of the ketogenic treatment.

An audit of therapeutic drug monitoring services of anticonvulsants at a tertiary care hospital in India

BACKGROUND

Therapeutic drug monitoring (TDM) is an important adjunct to the treatment of epilepsy. However, few studies have actually correlated plasma levels of antiepileptic drugs (AEDs) with treatment response. The present audit aimed to study (i) the association between seizure control and number of AEDs, plasma AED concentration, and concomitant use of antitubercular drugs; (ii) the pattern of indications for TDM requisitions; and (iii) the association between referral for toxicity and plasma AED concentration.

METHODS

This observational and retrospective study was carried out to analyze the TDM data of patients referred between January 2008 and December 2011. As per the International League Against Epilepsy Task Force 2009, patients were categorized into responders and nonresponders. Plasma AED levels were interpreted as below, within, and above the reference range.

RESULTS

Of 3206 TDM requisitions, 67% were monotherapy and 33% were 2 or more AEDs. Only 8% were responders as against 92% nonresponders. Of 95 patients on concomitant antituberculosis treatment, 72 were nonresponders, with odds ratio (95% confidence interval) 3.71 [2.19 to 6.23]. Breakthrough seizure (37%) was the most common indication followed by suspected toxicity and routine monitoring in 22% each and suspected nonadherence in 11% of the total requests. In 52% of patients, plasma levels were below the reference range, and they were equally distributed amongst responders and nonresponders. Among patients referred for suspected phenytoin toxicity, only 59% (50.6 to 67.8) had plasma concentrations above the reference range.

CONCLUSIONS

TDM continues to remain an important tool to support dose individualization when the patient is receiving multiple AEDs or other drugs such as antitubercular medicines, to assess compliance, and to monitor and treat toxicity.

Comparison of the overnight metyrapone and glucagon stimulation tests in the assessment of secondary hypoadrenalism

OBJECTIVE

The insulin tolerance test (ITT) is contraindicated in a proportion of patients with suspected ACTH deficiency. The aim of this study was to investigate the diagnostic accuracy of the glucagon stress test (GST) compared with the overnight metyrapone test (OMT) in patients with contraindications to ITT.

DESIGN

This was a prospective comparison of the GST to the OMT in patients with suspected ACTH deficiency and contraindications to the ITT. The OMT was used as the standard for comparison. The study was conducted at two tertiary referral centres for pituitary disease.

PATIENTS

Seventy-eight patients underwent contemporaneous OMT and GST of whom 61 had sufficient suppression of cortisol during the OMT to be included in the comparison. Forty had suffered traumatic brain injury, 36 had organic pituitary disorders and two were classified as 'other'.

MEASUREMENTS

ACTH sufficiency was defined as 0800h 11-deoxycortisol ≥ 200 nmol/l on OMT and peak cortisol ≥ 440 nmol/l on GST, as per local reference ranges.

RESULTS

There was significant discrepancy between the proportion of patients diagnosed with ACTH deficiency using the OMT (39%) and GST (89%). From our data, a GST peak cortisol cut-off of ≥350 nm provides the combination of optimal sensitivity (71%) and specificity (57%), compared with a higher sensitivity (88%) but poor specificity (11%) using a cut-off of ≥440 nm.

CONCLUSIONS

The GST should be used with caution as a diagnostic test of ACTH reserve. The OMT should be used in preference to the GST to assess the hypothalamic pituitary adrenal axis where ITT is contraindicated.

Carbamazepine alone and in combination with doxycycline attenuates isoproterenol-induced cardiac hypertrophy

β-adrenergic signaling is involved in the development of cardiac hypertrophy (CH), justifying the use of β-blockers as a therapy to minimize and postpone the consequences of this disease. Evidence suggests that adenylate cyclase, a downstream effector of the β-adrenergic pathway, might be a therapeutic target. We examined the effects of the anti-epileptic drug carbamazepine (CBZ), an inhibitor of adenylate cyclase. In a murine cardiac hypertrophy model, carbamazepine significantly attenuates isoproteronol (ISO)-induced cardiac hypertrophy. Carbamazepine also has an effect in transverse aortic banding induced cardiac hypertrophy (TAB) (P=0.07). When carbamazepine was given in combination with the antibiotic doxycycline (DOX), which inhibits matrix metalloproteinases (MMPs), therapeutic outcome measured by heart weight-to-body weight and heart weight-to-tibia length ratios was improved compared to either drug alone. Additionally, the combination therapy resulted in an increase in the survival rate over a 56-day period compared to that of untreated mice with cardiac hypertrophy or either drug used alone. Moreover, in support of a role for carbamaze -pine as a β-adrenergic antagonist via cAMP inhibition, a lower heart rate and a lower level of the activated phosphorylated form of the cAMP Response Element-Binding (CREB) were observed in heart extracts from mice treated with carbamazepine. Gene expression analysis identified 19 genes whose expression is significantly altered in treated animals and might be responsible for the added benefit provided by the combination therapy. These results suggest that carbamazepine acts as a β-adrenergic antagonist. Carbamazepine and doxycycline are approved by the US Food and Drug Administration (FDA) as drugs that might complement medications for cardiac hypertrophy or serve as an alternative therapy to traditional β-blockers. Furthermore, these agents reproducibly impact the expression of genes that may serve as additional therapeutic targets in the management of cardiac hypertrophy.

Interlaboratory variability in the quantification of new generation antiepileptic drugs based on external quality assessment data

PURPOSE

To assess interlaboratory variability in the determination of serum levels of new antiepileptic drugs (AEDs).

METHODS

Lyophilised serum samples containing clinically relevant concentrations of felbamate (FBM), gabapentin (GBP), lamotrigine (LTG), the monohydroxy derivative of oxcarbazepine (OCBZ; MHD), tiagabine (TGB), topiramate (TPM), and vigabatrin (VGB) were distributed monthly among 70 laboratories participating in the international Heathcontrol External Quality Assessment Scheme (EQAS). Assay results returned over a 15-month period were evaluated for precision and accuracy.

RESULTS

The most frequently measured compound was LTG (65), followed by MHD (39), GBP (19), TPM (18), VGB (15), FBM (16), and TGB (8). High-performance liquid chromatography was the most commonly used assay technique for all drugs except for TPM, for which two thirds of laboratories used a commercial immunoassay. For all assay methods combined, precision was <11% for MHD, FBM, TPM, and LTG, close to 15% for GBP and VGB, and as high as 54% for TGB (p < 0.001). Mean accuracy values were <10% for all drugs other than TGB, for which measured values were on average 13.9% higher than spiked values, with a high variability around the mean (45%). No differences in precision and accuracy were found between methods, except for TPM, for which gas chromatography showed poorer accuracy compared with immunoassay and gas chromatography-mass spectrometry.

CONCLUSIONS

With the notable exception of TGB, interlaboratory variability in the determination of new AEDs was comparable to that reported with older-generation agents. Poor assay performance is related more to individual operators than to the intrinsic characteristics of the method applied. Participation in an EQAS scheme is recommended to ensure adequate control of assay variability in therapeutic drug monitoring.

The importance of drug interactions in epilepsy therapy

Long-term antiepileptic drug (AED) therapy is the reality for the majority of patients diagnosed with epilepsy. One AED will usually be sufficient to control seizures effectively, but a significant proportion of patients will need to receive a multiple AED regimen. Furthermore, polytherapy may be necessary for the treatment of concomitant disease. The fact that over-the-counter drugs and nutritional supplements are increasingly being self-administered by patients also must be considered. Therefore the probability of patients with epilepsy experiencing drug interactions is high, particularly with the traditional AEDs, which are highly prone to drug interactions. Physicians prescribing AEDs to patients with epilepsy must, therefore, be aware of the potential for drug interactions and the effects (pharmacokinetic and pharmacodynamic) that can occur both during combination therapy and on drug discontinuation. Although pharmacokinetic interactions are numerous and well described, pharmacodynamic interactions are few and usually concluded by default. Perhaps the most clinically significant pharmacodynamic interaction is that of lamotrigine (LTG) and valproic acid (VPA); these drugs exhibit synergistic efficacy when coadministered in patients with refractory partial and generalised seizures. Hepatic metabolism is often the target for pharmacokinetic drug interactions, and enzyme-inducing drugs such as phenytoin (PHT), phenobarbitone (PB), and carbamazepine (CBZ) will readily enhance the metabolism of other AEDs [e.g., LTG, topiramate (TPM), and tiagabine (TGB)]. The enzyme-inducing AEDs also enhance the metabolism of many other drugs (e.g., oral contraceptives, antidepressants, and warfarin) so that therapeutic efficacy of coadministered drugs is lost unless the dosage is increased. VPA inhibits the metabolism of PB and LTG, resulting in an elevation in the plasma concentrations of the inhibited drugs and consequently an increased risk of toxicity. The inhibition of the metabolism of CBZ by VPA results in an elevation of the metabolite CBZ-epoxide, which also increases the risk of toxicity. Other examples include the inhibition of PHT and CBZ metabolism by cimetidine and CBZ metabolism by erythromycin. In recent years, a more rational approach has been taken with regard to metabolic drug interactions because of our enhanced understanding of the cytochrome P450 system that is responsible for the metabolism of many drugs, including AEDs. The review briefly discusses the mechanisms of drug interactions and then proceeds to highlight some of the more clinically relevant drug interactions between AEDs and between AEDs and non-AEDs. Understanding the fundamental principles that contribute to a drug interaction may help the physician to better anticipate a drug interaction and allow a graded and planned therapeutic response and, therefore, help to enhance the management of patients with epilepsy who may require treatment with polytherapy regimens.

Interactions between anticonvulsant and psychoactive drugs

This review considers the relevance of pharmacokinetic interactions between antiepileptic drugs (AEDs) and psychoactive drugs in the treatment of mood disorders in patients with epilepsy. The determination of plasma levels of some of these drugs (mainly the AEDs) has enabled clinicians to evaluate the kinetic modifications during the course of such combined therapies and to adjusting the dosages in cases of subtherapeutic or toxic levels. In general, phenobarbital, phenytoin, and carbamazepine stimulate the catabolic degradation of tricyclic antidepressants (TCAs), and TCAs have an inhibitory effect on the elimination of AEDs. The newer antidepressants that selectively inhibit the reuptake of serotonin (SSRIs), although in different fashions for the different substances (fluoxetine, fluvoxamine, paroxetine) may cause an increase of plasma AED levels through inhibition of the isoenzyme P450 2D6. Similarly, antipsychotics (APs) are more rapidly metabolized when AEDs are co-administered, whereas AED metabolism is scarcely influenced by AP. Finally, plasma levels of tranquilizers are lowered by AED co-therapy. As the concomitant administration of AED and psychoactive drugs becomes increasingly used for treatment of mood disorders in patients with or without epilepsy, therapeutic drug monitoring may be useful in designing correct and rational therapy.

Pharmacodynamic interaction between phenytoin and sodium valproate changes seizure thresholds and pattern

1. In this study we used cortical stimulation to assess the effects of phenytoin (PHT), sodium valproate (VPA), and their interaction on total motor seizure and on the constituent elements of the seizure. 2. PHT (40 mg kg(-1)) was administered as an intravenous bolus infusion to animals receiving either a continuous infusion of VPA or saline. VPA plasma concentration was maintained at levels that produced no detectable anticonvulsant effect. 3. Analysis of ictal components (eyes closure, jerk, gasp, forelimb, clonus, and hindlimb tonus) and their durations revealed both qualitative and quantitative differences in drug effects. 4. The anticonvulsant effect is represented by the increase in the duration of the stimulation required to reach a given seizure threshold. PHT significantly increased the duration of the stimulation and of the motor seizure. This increase was greatly enhanced by VPA. In addition, ictal component analysis revealed that the combination of PHT and VPA causes the reduction of a specific seizure component (JERK). 5. Neither the free fraction of PHT nor the biophase equilibration kinetics changes in the presence of VPA. It is concluded that the synergism may be due to a pharmacodynamic rather than a pharmacokinetic interaction.

A double-blind, placebo-controlled study on the effect of vigabatrin on in vivo parameters of hepatic microsomal enzyme induction and on the kinetics of steroid oral contraceptives in healthy female volunteers

PURPOSE

This study was conducted to determine whether vigabatrin affects in vivo indices of hepatic microsomal enzyme activity and the pharmacokinetics of steroid oral contraceptives in healthy subjects.

METHODS

Under double-blind conditions, 13 female healthy volunteers received, in random order and with a washout interval of > or = 4 weeks, two oral 4-week treatments with vigabatrin (VGB) (maintenance dosage, 3,000 mg daily) and placebo, respectively. The clearance and half-life of antipyrine (a broad marker of drug oxidation capacity), the urinary excretion of 6-beta-hydroxycortisol (a selective marker of cytochrome CYP3A-mediated oxidation), and the activity of serum gamma-glutamyltransferase (a nonspecific index of microsomal enzyme activity) were determined after 3 weeks of each treatment. The single-dose kinetics of a combined oral contraceptive containing 30 micrograms ethinyl estradiol and 150 micrograms levonorgestrel were also determined after 3 weeks of treatment by specific radioimmunologic assays.

RESULTS

VGB treatment had no influence on antipyrine clearance (28 +/- 5.6 vs. 30 +/- 4.5 ml/h/kg on placebo), antipyrine half-life (15.5 +/- 3.5 vs. 14.1 +/- 2.1 h), urinary 6-beta-hydroxycortisol excretion (488 +/- 164 vs. 470 +/- 228 nmol/ day), 6-beta-hydroxycortisol-to-cortisol concentration ratio (6.8 +/- 3.1 vs. 6.1 +/- 3.1) and serum gamma-glutamyltransferase activity (12 +/- 3 vs. 11 +/- 3 IU/L). No difference in pharmacokinetic parameters between VGB and placebo sessions were found for ethinyl estradiol (half-life, 12.5 +/- 3.2 vs. 13.9 +/- 3.2 h; AUC, 874 +/- 301 vs. 939 +/- 272 ng/ L/h) and levonorgestrel (half-life, 17.7 +/- 5.2 vs. 23.1 +/- 9.8 h; AUC, 27.5 +/- 9.6 vs. 30.0 +/- 12.0 micrograms/L/h). Two subjects, however, showed a 50 and a 39% reduction in ethinyl estradiol AUC during VGB treatment.

CONCLUSIONS

At therapeutic dosages, VGB did not modify in vivo indices of hepatic microsomal enzyme activity and did not interfere significantly with the CYP3A-mediated metabolism of ethinyl estradiol and levonorgestrel. Based on these data, VGB is unlikely to affect consistently the efficacy of steroid oral contraceptives or interact pharmacokinetically with drugs that are eliminated mainly by oxidative pathways, particularly those involving cytochrome CYP3A.

Evaluation of pharmacokinetic interaction between bromfenac and phenytoin in healthy males

An open-label, nonrandomized, multiple-dose, inpatient study was conducted in healthy male volunteers to compare the pharmacokinetics of bromfenac and phenytoin when the drugs are given individually and concomitantly. Twelve men received multiple oral doses of bromfenac for 4 days and then oral phenytoin for up to 14 days followed by concomitant administration of bromfenac and phenytoin for 8 days. Concomitant administration of the two drugs caused an approximate 40% decrease in the mean peak plasma concentration (Cmax) and the interdose area under the concentration-time curve (AUC) of bromfenac. The oral clearance (Clpo) of bromfenac doubled and the volume of distribution increased by 77%. For phenytoin, the mean peak serum concentration and the AUC increased by 9% and 11%, respectively, in the presence of bromfenac. The only change in unbound phenytoin was a 16% increase in the AUC. Although statistically significant, the changes in the pharmacokinetic parameters of phenytoin and unbound phenytoin were small. Adjustments in the dose of phenytoin should not be required during concomitant administration of bromfenac, although each patient's clinical status should be evaluated individually.

Triazolam is ineffective in patients taking rifampin

BACKGROUND

Triazolam is metabolized predominantly by cytochrome P450 3A4 (CYP3A4). Rifampin (rifampicin) is a potent inducer of CYP3A4 and it is known to markedly reduce plasma concentrations and effects of drugs such as midazolam. The possible interaction between rifampin and triazolam was examined in this study.

METHODS

The pharmacokinetics and pharmacodynamics of triazolam were investigated in a randomized, double-blind crossover study with two phases. Ten young healthy volunteers took either 600 mg rifampin once daily or placebo for 5 days. On the sixth day, 0.5 mg triazolam was administered orally. Timed blood samples were collected and the effects of triazolam were measured with five psychomotor tests for 10 hours.

RESULTS

The area under the plasma triazolam concentration-time curve in the rifampin phase was only 5.1% of that in the placebo phase (0.74 +/- 0.14 versus 14.8 +/- 1.0 ng.hr/ml [mean +/- SEM; p < 0.001]). Rifampin pretreatment decreased the maximum plasma concentration of triazolam to 12.4% of the control value (i.e., from 2.9 +/- 0.2 to 0.36 +/- 0.06 ng/ml [p < 0.001]) and the elimination half-life from 2.8 +/- 0.1 to 1.3 +/- 0.1 hours (p < 0.001). All psychomotor tests showed markedly reduced effects (p < 0.01) of triazolam after rifampin pretreatment.

CONCLUSIONS

Triazolam is ineffective during rifampin treatment. This is most likely due to increased metabolism of triazolam after induction of CYP3A4 in the gut wall and liver by rifampin. It is advisable to use hypnotic agents that are not metabolized by CYP3A4 during treatment with rifampin or other potent inducers of CYP3A4.

Ciprofloxacin decreases plasma phenytoin concentrations in the rat

The pharmacokinetic interaction between phenytoin and ciprofloxacin was studied in rats. One group of animals was given phenytoin (20 mg/kg, p.o.) as a single daily dose for 7 days. In another group of animals, the same protocol was followed except that ciprofloxacin was given in two equal doses (15 mg/kg, i.p., each) on days 5, 6 and 7. On day 8, phenytoin blood sampling was performed at 0.083, 0.25, 0.5, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0 and 12.0 h. Similarly, urine samples were collected at 4, 6 and 8 h following drug administration from the same animals that had received either phenytoin alone or phenytoin together with ciprofloxacin. The concentrations of phenytoin in the plasma and urine were measured using an HPLC method. Ciprofloxacin significantly (P < 0.05) reduced the area under the curve (AUC), the maximum plasma concentration (Cmax) and the elimination half-life (t1/2) of phenytoin. Additionally, ciprofloxacin increased phenytoin concentrations in urine at 4, 6 and 8 h. These results show that ciprofloxacin decreases the plasma levels of phenytoin when the two drugs are given concurrently.

Concentrations and effects of oral midazolam are greatly reduced in patients treated with carbamazepine or phenytoin

Midazolam is a short-acting benzodiazepine which is used as an oral hypnotic agent in several countries. We studied the pharmacokinetic and pharmacodynamic aspects of an oral 15-mg dose of midazolam in 6 patients with epilepsy who are also taking carbamazepine (CBZ) or phenytoin (PHT). We compared results with those obtained in 7 noninduced control subjects. Plasma concentrations and effects of midazolam were measured for 10 h. In patients with epilepsy, the area under the plasma concentration-time curve (AUC) of midazolam (mean +/- SEM) was only 5.7% (0.60 +/- 0.16 vs. 10.5 +/- 0.6 microgram x min/ml), and the peak midazolam concentration was 7.4% (5.2 +/- 1.2 vs. 70.4 +/- 9.0 ng/ml) of its value in control subjects (p < 0.001). The elimination half-life (t l/2) of midazolam was 1.3 +/- 0.2 h in patients and 3.1 +/- 0.1 h in controls (p < 0.001). The low plasma midazolam concentrations in the patient group were associated with reduced pharmacodynamic effects as compared with control subjects [e.g., the Critical Flicker Fusion Test (CFFT), p < 0.05]. Induction of CYP3A (cytochrome P-450IIIA) enzymes by CBZ and PHT is the most likely explanation of the great difference in the pharmacokinetic and pharmacodynamic profiles of oral midazolam in the two groups.

Adverse effects of established and new antiepileptic drugs: an attempted comparison

Seizures are but one aspect of the negative impact epilepsy has on patients' lives. Adverse effects of antiepileptic treatment may affect the patient's quality of life to an even greater extent than the occurrence of seizures. Adverse effects of antiepileptic drugs (AEDs) are common, and because the differences in efficacy are often marginal, adverse effects may be the most important factor in choosing the best AED for the patient. The search for more efficient and less toxic agents is constantly ongoing. Current evidence suggests that the new generation of AEDs is as efficient as the established AEDs and exhibits fewer adverse effects, but the scientific evidence from randomised clinical trials comparing established and new AEDs with each other is still pending.

Lack of interaction of gabapentin with carbamazepine or valproate

Gabapentin (GBP) studies were conducted in patients with epilepsy receiving carbamazepine (CBZ, n = 12) or valproate (VPA, n = 14) monotherapy. The effects of GBP coadministration on steady-state CBZ or VPA concentrations and of these antiepileptic drugs (AEDs) on GBP pharmacokinetics were investigated. GBP (400 mg) was coadministered every 8 h for 3 1/3 days with CBZ or for 5 1/3 days with VPA. GBP was well tolerated. Mean steady-state plasma CBZ/CBZ-10,11-epoxide (CBZ-E) and serum VPA concentrations before, during, and after GBP administration were not significantly different. Mean steady-state GBP pharmacokinetic parameters during CBZ or VPA coadministration were similar to steady-state parameters reported in healthy subjects. Thus, no pharmacokinetic interaction exists between CBZ or VPA and GBP. No dosage adjustment is necessary when GBP and CBZ or VPA are coadministered.

Vigabatrin does not affect the intestinal absorption of phenytoin in rat duodeno-jejunal loops in situ

The antiepileptic drug vigabatrin (GVG) is known to decrease significantly the serum concentration of concurrently administered phenytoin (PHT) in epileptic patients. To assess a possible mechanism for this interaction, the effect of GVG on the intestinal absorption of PHT was investigated by means of circulation experiments in an in situ rat duodeno-jejunal loop. GVG did not affect the rate of disappearance of PHT from the loop perfusing medium, providing evidence against occurrence of GVG-induced impairment of PHT absorption.

Influence of age and co-medication on steady-state phenobarbital serum level-dose ratios in Japanese paediatric patients

The effects of age and co-medication on steady-state phenobarbital (PB) level-dose (L/D) ratios were evaluated retrospectively in 539 paediatric patients. The PB L/D ratio increased significantly with age up to 15 years. Associated anti-epileptic therapy affected the PB L/D ratio, which was significantly increased in patients on polypharmacy as compared to patients on monopharmacy. Therefore, routine monitoring of PB serum levels is extremely useful, particularly in the paediatric age group, and in patients who require associated anti-epileptic medication.

Elevation of plasma phenytoin by viloxazine in epileptic patients: a clinically significant drug interaction

The effect of viloxazine (150-300 mg daily for 21 days) on plasma phenytoin levels at steady state was examined in 10 epileptic patients stabilised on a fixed phenytoin dosage. After starting viloxazine treatment, plasma phenytoin concentrations increased by 37% on average (range 7-94%) from a mean value of 18.8 micrograms/ml at baseline to a mean value of 25.7 micrograms/ml during the last week of combined therapy. In four patients the rise in plasma phenytoin was associated with the development of signs of phenytoin toxicity. Discontinuation of viloxazine resulted in return of plasma phenytoin towards baseline values and disappearance of the clinical symptoms. The mechanism of interaction probably involves inhibition of phenytoin metabolism by viloxazine. Careful monitoring of plasma phenytoin levels is recommended in patients treated with phenytoin who need to be started on viloxazine therapy.

Differential effects of valproic acid and enzyme-inducing anticonvulsants on nimodipine pharmacokinetics in epileptic patients

1. The single dose pharmacokinetics of orally administered nimodipine (60 mg) were investigated in normal subjects and in two groups of epileptic patients receiving chronic treatment with hepatic microsomal enzyme-inducing anticonvulsants (carbamazepine, phenobarbitone or phenytoin) and sodium valproate, respectively. 2. Compared with the values found in the control group, mean areas under the plasma nimodipine concentration curve were lowered by about seven-fold (P less than 0.01) in patients taking enzyme-inducing anticonvulsants and increased by about 50% (P less than 0.05) in patients taking sodium valproate. 3. Nimodipine half-lives were shorter in enzyme-induced patients than in controls (3.9 +/- 2.0 h vs 9.1 +/- 3.4 h, means +/- s.d., P less than 0.01), but this difference could be artifactual since in the patients drug concentrations declined rapidly below the limit of assay, thus preventing identification of a possible slower terminal phase. In valproate-treated patients, half-lives (8.2 +/- 1.8 h) were similar to those found in controls.

Phenytoin-bupropion interaction: effect on plasma phenytoin concentration in the rat

The effect of coadministration of bupropion (50 mg kg-1, p.o.) on the disposition profile of phenytoin has been studied in the rat. Plasma phenytoin concentration was measured serially for 10 h by HPLC. Bupropion had little or no effect on the pharmacokinetic parameters of an acutely administered dose of phenytoin. Following multiple doses of phenytoin however (i.e. steady state) the coadministration of bupropion resulted in significant increases in the elimination half-life (t 1/2), the area under the plasma concentration-time curve (AUC) and the time to maximum plasma concentration (tmax). Allowing for the limitations of single dose studies, these results point to a possible pharmacokinetic interaction between bupropion and phenytoin--the clinical significance of which needs to be assessed.

Correlation of free phenytoin to serum albumin in cancer patients

The objectives of this study were to compare the total and free phenytoin serum concentrations of cancer patients with hypoalbuminemia with those of cancer patients with normal serum albumin and to correlate the percentage of free phenytoin with the albumin concentration. A total of 22 patients were studied, 13 with normal albumin concentration and 9 with low albumin. The mean free phenytoin in the normal albumin group was 9.9 (+/- 1.3) percent and 17.6 (+/- 4.6) percent in the low albumin group. With the groups combined, the mean free phenytoin was 13.1 percent (range 8.3-22.2) with the albumin range of 20-45 g/L. There was a significant negative correlation (r = -0.9, p less than 0.001) between the percentage of free phenytoin and the measured serum albumin. In cancer patients, the serum albumin concentration appears to be the key factor that determines the percentage of free phenytoin. In cancer patients with low serum albumin concentration, the total and free phenytoin concentration should be measured for adequate assessment of phenytoin therapy.

The disposition of primidone in elderly patients

1. The pharmacokinetics and metabolism of primidone at steady-state were studied in 10 elderly patients aged 70-81 years and eight control subjects aged 18-26 years. 2. Primidone half-lives and clearance values (mean +/- s.d.) were similar in the elderly and in the young (12.1 +/- 4.6 vs 14.7 +/- 3.5 h and 34.8 +/- 9.0 vs 33.2 +/- 7.2 ml h-1 kg-1 respectively. 3. The serum concentrations of the metabolites phenylethylmalonamide (PEMA) and phenobarbitone relative to those of parent drug were higher in the elderly than in the young, the difference being significant (P less than 0.01) in the case of PEMA. 4. The renal clearances of primidone, phenobarbitone and PEMA were moderately decreased in the elderly but this reduction was statistically significant only for PEMA. Elderly patients excreted a reduced proportion of unchanged primidone and an increased proportion of PEMA in urine. 5. Ageing is associated with a greater accumulation of PEMA, which is unlikely to have a major clinical significance.

Lack of effect of piroxicam on theophylline clearance in healthy volunteers

This study was conducted to determine if piroxicam alters the clearance or metabolism of theophylline in healthy adults. Six male volunteers were given aminophylline 6 mg/kg iv before and after piroxicam treatment. Piroxicam 20 mg/d po was administered for seven days. Serum samples were collected before and for 24 hours after each aminophylline dose. Theophylline serum concentrations were determined using the Abbott TDx automated immunoassay system. Theophylline and its 1,3-dimethyluric acid metabolite were measured in 24-hour urine collections. Serum theophylline concentration versus time curves were plotted and pharmacokinetic parameters determined for each subject. None of the derived parameters differed as a result of piroxicam exposure. Theophylline metabolites, clearance, half-life, and volume of distribution were compared during the baseline and piroxicam phases and were found to be similar. It was concluded that piroxicam does not alter the clearance, distribution, volume, or metabolism of theophylline in healthy adult volunteers.

Phenytoin 4-hydroxylation by rabbit liver P450IIC3 and identification of orthologs in human liver microsomes

The ability of rabbit liver microsomes to 4-hydroxylate phenytoin to 5-(4-hydroxyphenyl)-5-phenylhydantoin was studied. No significant difference was observed between the capacity of control and rifampicin, phenobarbital, acetone, 2,3,7,8-tetrachlorodibenzo-p-dioxin and phenytoin induced rabbit liver microsomes to 4-hydroxylate phenytoin. In reconstitution experiments using six purified rabbit cytochromes P450 isozymes, only P450IIC3 was capable of 4-hydroxylating phenytoin whereas P450IA1, P450IA2, P450IIB4, P450IIIA6, and P450IVB1 were inactive. Further, anti-P450IIC3 IgG completely inhibited phenytoin 4-hydroxylase activity in rabbit liver microsomes. The above data suggest a major role for the constitutive isozyme P450IIC3 in phenytoin 4-hydroxylase activity in rabbit liver. In human liver microsomes P450IIC3 IgG inhibited phenytoin 4-hydroxylase activity by 66%, suggesting that an ortholog to rabbit P450IIC3 is in part responsible for this activity in man.

Problems of drug surveillance--an international view

The importance of Post Marketing Surveillance (PMS) is stressed, both to determine the therapeutic value of the drug and its adverse effects revealed in a large population of patients. A survey of such adverse effects of anti-epileptic drugs is provided: metabolic, dose-related and idiosyncratic. Problems of monotherapy and of drug interactions are discussed, as well as questions of central nervous system toxicity and of teratogenicity.

Decreased phenytoin levels in patients receiving chemotherapy

PURPOSE AND METHODS

Seizures and unexpectedly low phenytoin levels prompted a retrospective review of phenytoin doses and serum levels in patients with primary brain tumors following the administration of cisplatin and carmustine (BCNU) chemotherapy.

RESULTS

All patients who received three or more cycles of this chemotherapeutic regimen required an increase in their maintenance phenytoin dose to maintain therapeutic phenytoin levels. The average increase in the daily phenytoin dose was 41% (range, 20% to 100%). In addition, 17 of 26 (65%) assessable chemotherapy cycles were accompanied by a greater than 20% decrease in phenytoin levels or an increase in phenytoin requirements. Significant changes in phenytoin levels occurred as early as two days after administration of chemotherapy and were seen exclusively in treatment cycles that contained cisplatin. Five additional cases were found in the literature in which serum phenytoin concentrations decreased after the administration of antineoplastic agents.

CONCLUSIONS

These observations are important to physicians treating patients who are receiving phenytoin. Serious consequences can be avoided by expecting changes in phenytoin dosage requirements after the administration of chemotherapy, monitoring serum levels frequently, and making appropriate adjustments in phenytoin dosages. Prospective evaluations are needed to confirm these observations, to define the chemotherapeutic agents that predispose patients to these alterations in phenytoin's pharmacology, and to determine if drugs other than phenytoin are affected by the administration of chemotherapy.

Enzyme induction by eating charcoal-grilled steak with no effect on blood lipids

1. There has been interest in the suggestion that enzyme-inducing drugs, such as anticonvulsants, may produce beneficial changes in lipoprotein levels, in particular a rise in the ratio of high density lipoprotein cholesterol to total cholesterol. 2. This controlled study observed the effects of diets of charcoal or oven-cooked beef on antipyrine clearance (a commonly used measure of drug metabolizing capacity), the apparent oral clearance of phenacetin (a measure of cytochrome P448-dependent enzyme activity) and blood lipids in 18 healthy volunteers. 3. Charcoal-cooked beef increased antipyrine clearance by an average of 20% (P less than 0.059) and increased the apparent oral clearance of phenacetin fivefold (P less than 0.01). In contrast, oven-cooked beef did not significantly alter either measure of microsomal function. Neither diet had any effects on blood lipids. 4. We conclude that the type and degree of enzyme induction achieved by this type of dietary manipulation does not produce beneficial changes in lipoprotein profiles. A previously noted rise in high density lipoprotein cholesterol levels in volunteers fed charcoal-cooked beef may have been due to the effects of charcoal formed by charring of the beef during cooking.

A review of the antiarthritic efficacy and safety of etodolac

Etodolac (Lodine, Ramodar, Ultradol), an anti-inflammatory, analgesic agent, is the first of a new class of nonsteroidal anti-inflammatory drugs (NSAIDs), the pyranocarboxylic acids. A review of the literature on numerous clinical studies showed that etodolac (200 to 600 mg/day) is effective in the treatment of osteoarthritis and rheumatoid arthritis. Etodolac has also been shown to be very well tolerated. In double-blind studies, there were no significant differences in the incidences of new patient complaints except for indigestion between etodolac-treated groups and placebo-treated groups. Gastrointestinal microbleeding associated with etodolac was comparable to that with placebo and was significantly less than that associated with other commonly used NSAIDs, such as ibuprofen, indomethacin, piroxicam, and naproxen. The results of laboratory tests, including a detailed analysis of hepatic and renal function, have revealed few abnormalities, most of which were clinically unimportant. When administered to healthy subjects, etodolac had no pharmacokinetic interactions with three other drugs that are highly bound to serum protein: warfarin, glyburide, and phenytoin.

[Appearance of EEG abnormalities in two patients during long-term treatment with B vitamins]

Two female patients, one treated for epilepsy by phenobarbital (PB) and carbamazepine (CBZ), the other suffering from neurosis and free of any drug therapy, were given group B vitamins (riboflavin and nicotinic acid respectively) and showed during the long-term administration of these vitamins 2-3 Hz generalized electro-encephalographic (EEG) abnormalities. These EEG changes were detected incidentally and were not associated with any clinical manifestation; they disappeared progressively within several weeks after discontinuation of vitamin therapy. In our opinion, it is likely that the treatment with B vitamins was entirely responsible for the EEG changes, as one of the patients did not receive any other drug, while the other did not show any significant change of PB and CBZ plasma levels during vitamin therapy. However, other cases reported will be necessary to confirm our impressions.

Pharmacokinetic profile of flunarizine after single and multiple dosing in epileptic patients receiving comedication

This preliminary clinical study describes the pharmacokinetic characteristics of flunarizine (FLN) following single and multiple dosing in epileptic patients receiving comedication. Three groups [phenytoin (PHT) only, carbamazepine (CBZ) only, and PHT plus CBZ] of four patients each were studied. Large interindividual differences, but no statistically significant differences in pharmacokinetic parameters, were observed between the three groups. Following a single dose, mean values (and ranges) for apparent clearance, volume distribution, and elimination half-life (t1/2) were 0.504 L/h/kg (0.086-1.119), 12,431 L (1,959-20,920), and 308 h (61-506), respectively. FLN had no effect on PHT or CBZ steady-state levels but PHT or CBZ appeared to induce the metabolic disposition of FLN. The effect of dose on FLN kinetics could not be evaluated in this preliminary study.

Pharmacology of LY201409, a potent benzamide anticonvulsant

LY201116 [4-amino-N-(2,6-dimethylphenyl)benzamide], an effective anticonvulsant in several animal models, is rapidly metabolized by N-acetylation in rats, mice and monkeys. In an attempt to preclude metabolic N-acetylation sterically, we investigated LY201409, an analogue possessing two methyl groups ortho to the 4-amino substituent. This structural modification successfully altered the metabolic pathway, and LY201409 displayed potent anticonvulsant activity. LY201409 antagonized maximal electroshock (MES)-induced seizures with ED50 values of 16.2 and 4.2 mg/kg after oral administration to mice and rats, respectively. The compound did not effectively antagonize seizures induced by a variety of chemical convulsants in rats, but did block pentylenetetrazol-induced seizures in mice. Thus, among the classic anticonvulsants, the profile of phenytoin most closely resembles that of LY201409. Studies conducted with the rotorod and horizontal screen assays in mice and behavioral studies in rats suggested that doses of LY201409 that produced CNS side-effects such as sedation or ataxia were well separated from the anti-MES doses. LY201409 was a potent, dose-dependent potentiator of hexobarbital-induced sleeping time in mice. Oral administration of 6.0 mg/kg led to a 372% increase in sleep time relative to control values. Although LY201409 is a potent and effective anticonvulsant, it is also one of the most potent potentiators of hexobarbital-induced sleep time yet described.

Progabide-induced changes in carbamazepine metabolism

Carbamazepine (CBZ) and carbamazepine 10,11 epoxide (CBZ-E) concentrations were measured during a safety and efficacy trial of progabide. The average CBZ and CBZ-E serum concentrations were calculated from serial measurements during placebo and active treatment periods. Significant decreases in CBZ and significant increases in CBZ-E were observed after the first dose of progabide, and these changes persisted during 3 months of active treatment. Ninety-five percent of the patients had increases in the epoxide/CBZ ratio at the end of 3 months of treatment. These changes are consistent with displacement of CBZ from protein binding sites and inhibition of CBZ-E metabolism induced by progabide and are analagous to the interaction between valproate and CBZ.

Lack of effect of erythromycin on the pharmacokinetics of single oral doses of phenytoin

The effect of erythromycin on the pharmacokinetics of phenytoin was studied in eight healthy volunteers using a balanced randomised cross-over design. Volunteers received a single oral dose of 400 mg of phenytoin sodium during each phase. During the treatment phase, the phenytoin sodium dose was administered 4.5 days after the commencement of a 7 day course of erythromycin base (250 mg every 6 h). There was no significant difference between the control and treatment phases (P greater than 0.05) with respect to the area under the plasma phenytoin concentration-time curve, the fraction of phenytoin unbound in plasma, the area under the unbound phenytoin concentration-time curve, the elimination half-life of phenytoin or the fraction of the dose excreted in urine as free and conjugated hydroxyphenylphenylhydantoin. This single dose study indicated that the intrinsic clearance of unbound phenytoin was unaffected by the concurrent administration of erythromycin.

Reduction of dosing frequency of carbamazepine with a slow-release preparation

The occurrence of side effects and epileptic seizures and the pharmacokinetics of carbamazepine (CBZ) and carbamazepine-10,11-epoxide were studied using a slow-release CBZ preparation, Neurotol slow, and a conventional CBZ preparation, Tegretol. The study was an open, randomized cross-over trial, with a 2 week study period for each preparation. Tegretol was given 3 times and Neurotol slow twice a day. The earlier CBZ dose was kept unchanged. The initial sample consisted of 24 adult epileptic patients receiving CBZ treatment of whom 20 patients were evaluable. The fluctuation in serum CBZ concentrations did not differ significantly between the 2 treatment periods, even though the interdose interval of Neurotol slow was 4 h longer than that of Tegretol. The switch-over from conventional CBZ to the slow-release formulation did not seem to alter the efficacy and side effects of CBZ. By using Neurotol slow instead of a conventional CBZ preparation, Tegretol, it is evidently possible to reduce the dosing frequency from 3 times a day to twice daily administrations.

Disopyramide pharmacokinetics and metabolism: effect of inducers

1. The disposition of orally administered disopyramide was studied in a population of smokers (n = 6) and non-smokers (n = 8) before and during phenobarbitone treatment (100 mg daily for 21 days; Cp 21st day = 13.9 +/- 2.0 micrograms ml-1). The comparative inducibility of these populations by phenobarbitone was assessed as was the inductive effect of cigarette smoking, per se. Furthermore, the determinants of the intensity of the inductive effect were examined, as well as the effect of the barbiturate on the binding of disopyramide to alpha 1-acid glycoprotein (AGP). 2. Smokers and non-smokers exhibited similar half-lives (6.48 +/- 1.49 vs 6.66 +/- 1.02 h), apparent total body clearances (0.100 +/- 0.020 vs 0.117 +/- 0.034 l h-1 kg-1), mean renal clearances (0.043 +/- 0.0093 vs 0.057 +/- 0.013 l h-1 kg-1) and apparent intrinsic metabolic clearances (0.057 +/- 0.015 vs 0.060 +/- 0.024 l h-1 kg-1) before phenobarbitone treatment. 3. Both populations responded comparably to barbiturate exposure in that apparent intrinsic metabolic clearance more than doubled. Interestingly, the magnitude of this increase was highly dependent on the observed baseline apparent intrinsic metabolic clearance, (r' = 0.81; P less than 0.001). 4. Phenobarbitone treatment of non-smokers resulted in an increase in the AUC of the active metabolite N-despropyl disopyramide (MND), but not significantly (3.8 +/- 1.6 vs 4.1 +/- 2.3 micrograms ml-1 h). Similar results were observed in smokers (3.5 +/- 1.4 vs 3.9 +/- 2.0 micrograms ml-1 h, respectively). 5. The percent of administered dose recovered in urine as disopyramide in non-smokers was significantly decreased upon phenobarbitone treatment (43 +/- 6% vs 25 +/- 5%), whereas the percent of dose recovered as MND increased significantly in this group (25 +/- 6% vs 31 +/- 5%). The population of smokers responded similarly. 6. At doses typically used to achieve hepatic microsomal enzyme induction in man, phenobarbitone treatment caused no significant change or trend towards a change in serum AGP concentrations as measured using the radial immunodiffusion method in nonsmokers (67.4 +/- 19.9 mg dl-1 vs 68.0 +/- 40.7 mg dl-1) or smokers (64.5 +/- 15.7 vs 67.9 +/- 14.9). Similarly, when AGP concentration was estimated in serum from non-smokers using a nephelometric method no effect attributable to phenobarbitone was observed (47.9 +/- 1.3 vs 47.9 +/- 16.8 mg dl-1). Consistent with this observation, disopyramide free fraction was not affected by barbiturate treatment.

The risk-benefit ratio of anticonvulsant drugs

The concepts underlying the notion of a risk-benefit ratio for anticonvulsant therapy have determined the development of the drug treatment of epilepsy over many years. The risk element in the ratio arises from the various possible physical and psychological adverse effects of anticonvulsant therapy; the benefit is derived from the capacity of therapy to prevent seizures and thus reduce the disadvantages which result from having epilepsy. The physical adverse effects of anticonvulsant therapy may involve many tissues and organs. The drugs tend to depress cerebral, cerebellar and brain stem function, and may slow peripheral nerve conduction. Prolonged intake may cause hypocalcaemia and osteoporosis, folate depletion, various haematological and immunological abnormalities, and overgrowth of subcutaneous and gingival tissues. Idiopathic reactions may involve the skin, lymph nodes, liver, pancreas, kidney and thyroid, and cause electrolyte disturbances, while maternal anticonvulsant intake during pregnancy may be associated with an increased incidence of fetal malformations. Local reactions may occur at drug administration sites, and anticonvulsants may interact pharmacokinetically and pharmacodynamically with co-administered drugs. The taking of anticonvulsants sometimes has undesirable psychological effects on both the patient and his or her family. Epilepsy itself often results in adverse psychological consequences which emanate from the uncertainty and insecurity that is imposed by the unpredictable occurrence of seizures, from the limitations epilepsy sets on the patient's lifestyle and employment prospects, and from unfavourable community attitudes towards the disorder. Contemporary anticonvulsant therapy is not fully effective in all patients, but to the extent that it can control seizures it may help alleviate these emotional burdens that are a result of epilepsy. The consensus of present day medical opinion is that, in the great majority of clinical situations, the benefits of anticonvulsant therapy outweigh the disadvantages. However, to provide optimal management for individual patients, the risk-benefit ratio of therapy must be repeatedly assessed at all stages of a patient's treatment, and therapeutic decisions taken in the light of the ratio as it applies to the individual.

Pharmacokinetic interactions of progabide with other antiepileptic drugs

The influence of progabide, a new antiepileptic drug, on the pharmacokinetic profiles of phenobarbital, phenytoin, carbamazepine, and valproic acid was evaluated in four separate studies, each including six young healthy volunteers. The pharmacokinetic parameters of the associated antiepileptic drugs were measured before and after repeated administration of progabide (600 mg t.i.d.) for 15 days. A significant reduction of the total body clearance of both phenytoin and phenobarbital and a higher Cmax value of carbamazepine epoxide and phenobarbital were observed. No modifications were noticed for the kinetic profiles of carbamazepine and valproic acid. These modifications may be of clinical relevance and suggest that, as a general rule, when progabide is added to an established treatment with phenobarbital or phenytoin or carbamazepine, an adjustment of previous posology may be necessary.

The metabolism of 14C-oxcarbazepine in man

The disposition of the new anti-epileptic agent oxcarbazepine (10,11-dihydro-10-oxo-5H-dibenz[b,f]azepine-5-carboxamide) has been studied in two healthy volunteers following an oral 400 mg dose of 14C-labelled drug. The dose was excreted almost completely in the urine (94.6 and 97.1%) within six days. Faecal excretion amounted to 4.3 and 1.9% of the dose in the two subjects. In the 0-6 days urine samples the biotransformation products have been isolated and identified. 10,11-Dihydro-10-hydroxycarbamazepine (GP 47,779) and its two diastereoisomeric O-glucuronides were found as main metabolites. Taken together, they accounted for 79% of urinary 14C. Unchanged oxcarbazepine, and its sulphate and glucuronide conjugates were isolated in smaller amounts only (13%). Other minor metabolites were the trans- and cis-isomers of 10,11-dihydro-10,11-dihydroxy-carbamazepine (approximately 4%), and a phenolic derivative of GP 47,779 (less than 1%). The biotransformation of oxcarbazepine proceeds mainly by reduction to GP 47,779, and subsequent conjugation with glucuronic acid. Reduction is stereospecific, favouring the S-configuration of GP 47,779. Direct conjugation of oxcarbazepine, in the enol form, is a minor pathway. Oxidative reactions are unimportant.

Drug-drug and drug-disease interactions with nonsteroidal anti-inflammatory drugs

Nonsteroidal anti-inflammatory drugs may cause a number of drug interactions. They can displace other drugs from serum proteins, and some can affect the metabolism or decrease the renal elimination of other drugs. In addition, they can attenuate the pharmacologic effect of other drugs, such as diuretic and antihypertensive agents, without affecting their disposition. Lastly, many disease states and aging can affect the handling of nonsteroidal anti-inflammatory drugs, mandating dose adjustment of some of these agents in certain clinical conditions. Some drugs may require more of these adjustments than others.

Treatment of hydatid disease with high oral doses of mebendazole. Long-term follow-up of plasma mebendazole levels and drug interactions

Plasma mebendazole levels were analysed retrospectively in patients treated for inoperable infections with Echinococcus multilocularis or granulosus. In 10 patients receiving mebendazole at 4 dose levels there was no relation between dose and plasma concentration. In 17 patients followed on the same dose for more than 18 months, the plasma levels varied with individual coefficients of variation ranging from 27 to 72%. The data reveal the limitations of single measurements of plasma mebendazole and emphasize the need for repeated monitoring. Coadministration of phenytoin and carbamazepine seemed to lower plasma levels, presumably as a result of enzyme induction. It was not possible appreciably to raise the mebendazole concentrations by inhibition of drug metabolizing enzymes with cimetidine.

Pharmacokinetics of the antidepressant drug viloxazine in normal subjects and in epileptic patients receiving chronic anticonvulsant treatment

In order to evaluate the influence of chronic antiepileptic drug treatment on the kinetics of the antidepressant viloxazine (VLX), six drug-free control subjects and six epileptic patients treated with one or two anticonvulsants (phenobarbital, carbamazepine or phenytoin) were given a single oral dose of VLX (200 mg). On a separate occasion, the patients were also given 200 mg VLX by IV infusion. Plasma VLX levels were determined by GLC. Following oral dosing, VLX was rapidly absorbed from the gastrointestinal tract (peak levels at 0.5-4 h); plasma level profiles showed a considerable interindividual variability but did not differ significantly between patients and controls. Terminal half-lives were 4.3 +/- 1.5 h in the patients and 4.3 +/- 1.8 h in the controls. Clearance and volume of distribution calculated after IV dosing in the patients were 124 +/- 11 ml h-1 kg-1 and 0.73 +/- 0.28 l/kg, respectively. The absolute oral availability was 85 +/- 14%. At variance with findings reported for other antidepressants, VLX kinetics do not appear to be significantly altered by concurrent treatment with enzyme-inducing antiepileptic drugs.

Interactions between anticonvulsants and other commonly prescribed drugs

Many drug interactions can be demonstrated, but only a few are so clinically significant that they necessitate adjusting drug dosages. The same drug combination may produce changes of variable extent or direction in different individuals. The reasons for this variability include genetic control of the rate and inducibility of drug metabolism, and environmental factors such as contact with chemicals. Among antimicrobial agents, chloramphenicol may cause accumulation of phenytoin (PHT) and phenobarbital (PB), and isoniazid may cause PHT, carbamazepine (CBZ), and primidone (PRM) to accumulate. Erythromycin may cause accumulation of CBZ. Among anti-ulcer agents, antacids may reduce PHT concentration while cimetidine may cause accumulation of PHT, CBZ, and diazepam (DZP). Salicylates displace strongly binding drugs such as PHT, DZP, or valproate (VPA) from the binding sites in plasma proteins, which may lead to some decline of the total plasma level with an increase in the unbound drug percentage. Conversely, anticonvulsants may influence the dosage requirements of oral anticoagulants by inducing their metabolism. Failures of oral contraceptives have been attributed to anticonvulsants in some patients. Probably the most predictable interaction that necessitates dosage adjustment is accumulation of PB caused by VPA. Intentional inhibition of PRM metabolism by nicotinamide serves as an example of attempts to utilize an interaction for improved therapeutic effect.

In vivo plasma protein binding interaction between valproic acid and naproxen

The effect of naproxen on the kinetics of free and total plasma valproic acid (VPA) was investigated in 6 normal volunteers by using a recently developed simple ultrafiltration technique associated with an immuno-enzymatic assay (Free Level System I, Syva). Each subject received a single oral dose of sodium valproate on two occasions: a) on a control day and b) during concurrent treatment with naproxen (500 mg b.i.d. for 5 consecutive days). Naproxen caused a slight but significant decrease in total plasma VPA levels but left free VPA levels essentially unchanged. The free VPA fraction increased with increasing total VPA concentrations: at equivalent values of total VPA, however, the free fraction was higher in the presence of naproxen. It is concluded that naproxen exerts a moderate displacing effect on protein bound VPA, thereby increasing the clearance of total drug but leaving essentially unchanged the clearance of free drug.

Free level monitoring of antiepileptic drugs. Clinical usefulness and case studies

The free fraction of phenytoin, carbamazepine and valproic acid shows considerable interindividual variability, especially in the presence of associated disease or drug interactions. When binding is altered, the total concentration no longer reflects the amount of pharmacologically active drug in the plasma: this may mislead the clinician into making inappropriate dosage adjustments. Measuring the free drug concentration eliminates a potential source of interpretative errors and may be preferentially used to monitor therapy in selected patients.