Primidone-induced embryolethality and DRL deficits in surviving offspring.

Pregnant Sprague-Dawley rats were administered primidone (PRM) by oral gavage on gestation days 8-20 in doses of 0 or 120 mg/kg. This dose did not produce body weight differences in the dams during the dosing period nor were there differences in the birth weights of the offspring. PRM was embryolethal with only 43% of drug-treated dams maintaining their pregnancies, whereas 100% of the pregnant controls produced offspring. An analysis of resorption sites in PRM-treated dams that did not deliver showed a nearly identical number of implantation sites (12.6) compared to the litter size of controls (12.8) that delivered pups. There were no overall differences in exploratory activity levels between PRM-treated and control animals. However, in the PRM-treated females there was an absence of the sexually dimorphic increase in activity seen in control females when compared to control males. The PRM-treated males showed an impairment in the acquisition of a DRL-20 (differential reinforcement of low rates) operant schedule over a 9-week acquisition period. There were no differences in the total response rates between the groups, suggesting that this is a specific learning deficit and not a performance deficit. The results of these experiments provide evidence that prenatal PRM exposure can be embryolethal and also impair behavior in the surviving rat offspring.

Developmental and behavioral effects of prenatal primidone exposure in the rat.

Pregnant Sprague-Dawley rats were administered primidone (PRM) by oral gavage on gestation days 8-17 in doses of 0.40, and 80 mg/kg. Although these doses of PRM did not produce significant differences in litter size, birth weight, mortality, date of attainment of developmental landmarks or measures of preweaning reflex and motor development, there were a number of significant differences that developed as the animals approached and entered adulthood. When tested as adults, the 80 mg/kg male rats showed a deficit in the performance of an eight-arm radial maze task. These same animals showed a significant reduction in open field activity when tested as adults. In addition, both male and female PRM-treated animals showed reduced body weights at different periods corresponding to onset of sexual maturation during development. These findings are consistent with the larger body of literature reporting on the neurobehavioral teratology of phenobarbital, including its ability to produce lesions in the hippocampus and endocrine dysfunction resulting in reproductive deficits. These results suggest that PRM produces its adverse effects as a result of its metabolism to phenobarbital, which in turn affects the limbic system.

Effect of primidone in somatic and germ cells of mice.

Primidone, an anti-convulsant drug, was tested in mice for mutagenicity in somatic cells by the micronucleus test and in germ cells by the sperm-head abnormality assay. Mice were treated orally with the drug at doses of 4.37, 8.75 and 13.11 mg/mouse. The results indicate that the drug is capable of inducing mutations both in somatic and germ cells of mice.

Mutagenicity of antiepileptic drugs. II. Phenytoin, primidone and phenobarbital.

The antiepileptic drugs, phenytoin, primidone and phenobarbital, were tested in Chinese hamster ovary (CHO) cells for their ability to induce sister-chromatid exchanges (SCEs) and structural chromosomal aberrations (CAs), with and without metabolic activation. SCEs and CAs were analyzed in the same cell population. The results are negative.

Primidone, phenobarbital, and PEMA: I. Seizure protection, neurotoxicity, and therapeutic index of individual compounds in mice.

Neurotoxicity and protection against maximal electroshock and Metrazol seizures from primidone (PRM), phenobarbital (PB), and phenylethylmalonamide (PEMA) were determined in mice for each drug separately and expressed in terms of brain concentrations. Compared with PB, PEMA was 16 times less potent against electroshock and Metrazol seizures but only 8 times less toxic. Primidone was markedly less neurotoxic than PB and equally potent against electroshock, but PRM had no effect against Metrazol or bicuculline. PRM is a relatively nontoxic anticonvulsant with a different action than PB, and PEMA is both a weak and a relatively toxic anticonvulsant.

Primidone, phenobarbital, and PEMA: II. Seizure protection, neurotoxicity, and therapeutic index of varying combinations in mice.

Neurotoxicity and protection against maximal electroshock (MES) and pentylenetrazol (Metrazol) seizures were determined in mice for various combinations of primidone (PRM), phenobarbital (PB), and phenylethylmalonamide (PEMA). The results suggest that PRM and PB together are superior to either one alone in terms of spectrum of activity and relative toxicity. The protection against Metrazol and the toxicity of PB are both potentiated by PEMA at low concentrations. PEMA also potentiates the toxicity of combined PRM plus PB, without altering their protection against MES, thus lowering their therapeutic index. We conclude that PRM and PB together have an advantage over PB alone, especially when their brain concentration ratio is at or above 1 and PEMA concentrations are low. These conditions are usually not present at steady state in patients treated with PRM.

Determination of phenobarbital and phenytoin in serum by a mechanized enzyme immunoassay (EMIT) in comparison with a gas-liquid chromatographic method.

The antiepileptic drugs phenobarbital and phenytoin were determined in serum by enzyme immunoassay (Emit, Syva Corp.) and gas-liquid chromatography. The Emit assays were mechanized by the use of an Eppendorf analyzer 5010. The precision of the Emit system was sufficient (coefficient of variation within series 6-13% and from day to day 8-15% with various calibrators and control sera). Moreover the Emit method is rapid, specific and easy to perform. The procedure requires only 10 microliter of serum per determination. A disadvantage however is the high cost of the reagents. A comparison of the results obtained by Emit and gas-liquid chromatography in a series of about 50 patients showed a good correlation between both methods (correlation coefficient r = 0.968 for phenobarbital and 0.978 for phenytoin).

Teratogenic activity and metabolism of primidone in the mouse.

Primidone, 25, 50, 100, and 150 mg/kg, was administered orally to mice of the I.C.I. strain from days 6-16 of pregnancy. The fetuses were removed by caesarian section on day 19 and examined by dissection and alizarin staining for gross structural and skeletal defects. The most common abnormalities found were palatal defects with full-length or submucosal clefts. In the controls--25, 50, 100, and 150 mg/kg groups--the incidence of palatal defects was 0/85, 16/84, 18/117, 19/102, and 17/92 fetuses, respectively. Essentially no other major or minor drug-related abnormalities were found. The metabolism of primidone in the pregnant and nonpregnant mouse was also studied and shown to be similar to that previously reported in the rat. Peak blood levels of primidone were obtained after 1 hr; they fell to very low levels by 6 hr. and were completely cleared by 24 hr. The metabolites produced, PEMA and phenobarbital, are similar to those produced in other species including man. Blood levels following single oral doses of 5 to 150 mg/kg were dose-related so that no explanation for the lack of dose-related teratogenic effect was found.

The necessity of drug level monitoring in anticonvulsant drug therapy.

A survey of 'steady-state' serum levels of anticonvulsant drugs from 221 epileptic patients at a university hospital was conducted. Serum concentrations of phenobarbital, primidone, and diphenylhydantoin were determined by a gas chromatographic method. Sixty-five percent (130) of the patients receiving diphenylhydantoin had levels below the therapeutic range of 10-20mug/ml. Subtherapeutic levels appear to be due to inadequate dosage adjustment. Only 25% (33) of the patients receiving phenobarbital had levels below the therapeutic range. Serum levels of diphenylhydantoin or phenobarbital could not be predicted from dosage. Most patients received two or more drugs. Over 10% of the patients had potentially toxic levels of anticonvulsant drugs. High levels of diphenylhydantoin were easily recognized clinically but high levels of phenobarbital were not.