Pharmacokinetic interactions of antiepileptic drugs

Preliminary study of the association between the elimination parameters of phenytoin and phenobarbital

BACKGROUND

Therapeutic drug monitoring is essential for both phenytoin and phenobarbital therapy given their narrow therapeutic indexes. Nevertheless, the measurement of either phenytoin or phenobarbital concentrations might not be available in some rural hospitals. Information assisting individualized phenytoin and phenobarbital combination therapy is important. This study's objective was to determine the relationship between the maximum rate of metabolism of phenytoin (Vmax) and phenobarbital clearance (CLPB), which can serve as a guide to individualized drug therapy.

METHODS

Data on phenytoin and phenobarbital concentrations of 19 epileptic patients concurrently receiving both drugs were obtained from medical records. Phenytoin and phenobarbital pharmacokinetic parameters were studied at steady-state conditions. The relationship between the elimination parameters of both drugs was determined using simple linear regression.

RESULTS

A high correlation coefficient between Vmax and CLPB was found [r=0.744; p<0.001 for Vmax (mg/kg/day) vs. CLPB (L/kg/day)]. Such a relatively strong linear relationship between the elimination parameters of both drugs indicates that Vmax might be predicted from CLPB and vice versa.

CONCLUSIONS

Regression equations were established for estimating Vmax from CLPB, and vice versa in patients treated with combination of phenytoin and phenobarbital. These proposed equations can be of use in aiding individualized drug therapy.

Central nervous system activity of the proanthocyanidin-rich fraction obtained from Croton celtidifolius in rats

Objectives

The aim of the present study was to evaluate the possible neurobehavioural effects in rats of the proanthocyanidin-rich fraction (PRF) isolated from the bark of Croton celtidifolius (Euphorbiaceae).

Methods

Adult Wistar rats were treated with the PRF (0.3–30 mg/kg) and evaluated in different behavioural paradigms classically used for the screening of drugs with psychoactive effects.

Key findings

Acute intraperitoneal (i.p.) administration of PRF decreased spontaneous locomotor activity (open field arena and activity cage), enhanced the duration of ethyl ether-induced hypnosis, increased the latency to the first convulsion induced by pentylenetetrazole (60 mg/kg, i.p.) and attenuated apomorphine-induced (0.5 mg/kg, i.p.) stereotyped behaviour. In lower doses, PRF (0.3 or 3 mg/kg, i.p.) increased the frequency of open arm entries in the elevated plus-maze test.

Conclusions

The present findings suggest that the systemic administration of PRF induces a wide spectrum of behavioural alterations in rats, consistent with the putative existence of hypnosedative, anticonvulsant and anxiolytic compounds.

Genetic polymorphism of cytochrome P450 2C9 in diphenylhydantoin-induced cutaneous adverse drug reactions

BACKGROUND

Apart from allergic mechanisms, a lack or mutation of metabolic enzymes may cause adverse drug reactions. Patch testing has rarely been useful in cutaneous adverse drug reactions (CADRs) induced by diphenylhydantoin (DPH). Genetic polymorphisms leading to altered metabolic processes of cytochrome P(450) (CYP) 2C9, a main metabolic enzyme for DPH, may be the pathological mechanism for certain cases of DPH-induced CADRs.

OBJECTIVE

To examine the effects of an altered CYP2C9 variant, CYP2C9*3, on DPH-induced CADRs.

METHODS

Ten patients with DPH-induced CADRs were examined for CYP2C9 genetic polymorphisms. The results were compared with non-exposed controls and 39 neurological patients without DPH-induced CADRs despite exposure to DPH. The patients with DPH-induced CADRs were also patch tested with anti-epileptic drugs and the results were compared with 40 DPH-exposed and 58 non-exposed controls.

RESULTS

A heterozygous CYP2C9*3 variant was found in three of the 10 DPH-induced CADR patients. The crude odds ratios (OR) of the patients compared with those of exposed and non-exposed controls were 167 and 71, respectively. Only one neurological patient, who had never taken DPH, showed the variant in both exposed (P=0.007) and non-exposed (P=0.001) controls. Positive patch-test results were displayed in three of the ten DPH-induced patients, but the patients with positive patch-test reactions to DPH differed from those with the CYP2C9*3 polymorphism. No patients and controls displayed a CYP2C9*2 variant.

CONCLUSION

A CYP2C9*3 variant could play a role in the proportion of patients with DPH-induced CADRs that differ from patients with DPH-induced CADRs showing positive patch-test results.

Oral antifungal drug interactions

The recent approval of itraconazole and terbafine for the treatment of onychomycosis has launched a new era in the therapeutic management of this previously resistant form of dermatomycoses. These agents represent safe and effective treatments. The clinician should, however, become knowledgeable with the potential drug interactions that are discussed in this article.

Antiepileptic drug selection and adverse effects: an overview

In choosing an antiepileptic drug, not only efficacy but also potential adverse effects have to be considered. Adverse effects that have to be taken into account include acute and chronic systemic toxicity, cognitive side effects, and teratogenesis. Acute toxicity may be dose-related, allergic or an idiosyncratic reaction. Chronic toxicity may involve the nervous system or other organs. In determining the role of new antiepileptic drugs such as lamotrigine, vigabatrin, felbamate, and gabapentin a proper evaluation of both efficacy and adverse effects is required.

Medical management of recurrent seizures in dogs and cats

The problem of recurrent seizures is a common and challenging one in veterinary medical practice. The pathophysiology and pharmacologic suppression of focal seizure activity have been studied extensively in basic research settings, yet little is known of the genesis, modulation, and termination of generalized seizures, the most common form of seizures noted to occur in companion animals. Knowledge concerning the pharmacokinetic fate of anticonvulsant drugs currently used in veterinary medicine is adequate, though prospective clinical studies of the efficacy of these drugs in the treatment of various types of seizures are lacking. This study will review the available literature regarding the pharmacology, use, and side effects of anticonvulsant drugs currently available for control of recurrent seizures in companion animals. Alternative anticonvulsant drugs will also be described.

[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.

Pharmacokinetics of free and total sodium valproate in adolescents and young adults during maintenance therapy

The pharmacokinetics of total and free valproic acid (VPA) in plasma and whole blood was investigated in seven adolescents and young adults (mean age 17.3 years) during a dosage interval at steady state. The concentration curves of VPA in whole blood after an oral morning dose (mean 8.2 mg/kg body wt.) closely followed those in plasma but at a reduced level. The apparent volume of distribution (Vd) of total VPA was 0.150-0.197 l/kg body wt. and of free VPA 0.911-1.58 l/kg body wt., which indicates considerable distribution of unbound VPA as well as drug binding to extravascular proteins. The terminal half-life of free VPA (6.4-6.7 h) was significantly shorter (P less than 0.05) than the half-life of total VPA (10.4-11.9 h). The binding of VPA in plasma was concentration dependent and fluctuated considerably within the individual dosage intervals. Concentrations of unbound VPA in plasma water of whole blood varied to a corresponding degree, since distribution to blood cells was low (mean 2.2%). It is concluded that there are substantial differences in the pharmacokinetics of free and total VPA. This may contribute to the well-known poor correlation between dose, plasma concentrations and effect of VPA.