Effect of phenobarbitone on folic acid metabolism in the rat

Anticonvulsant drugs have well-known relationships with folic acid, often leading, in epileptic patients on long term treatment to signs of folic acid deficiency and to haematological complications. The findings of this work, carried out on rats, show strong interference of phenobarbitone with folic acid metabolism, particularly with pteroylpolyglutamate synthesis in liver. This is proved by lower polyglutamate content, lower 3H folate incorporation in polyglutamate forms and in protein-bound folates, and also by lower blood radioactivity, higher urinary excretion of 3H folate metabolites and by higher 3H folate concentration in kidneys.     

Effects of phenobarbitone and phenytoin on folate catabolism in the rat

The effects of phenobarbitone and phenytoin on the catabolism of oral [2-14C] and [3',5',7,9-3H] folic acid were investigated. Normal rats were found to excrete an excess of 3H-labelled compounds into the urine and 14C-labelled compounds into the faeces. Phenytoin abolished this urinary 3H imbalance and also delayed and prolonged the overall excretion of radioactive material. Phenobarbitone appeared to increase the amounts of urinary scission products in the first 24 hr but over the 0-72 hr period both anticonvulsants decreased folate polyglutamate catabolism. As the anticonvulsants used in these experiments decreased folate catabolism in the rat it is unlikely that the megaloblastic anaemia caused by chronic anticonvulsant therapy is due to induction of the enzymes responsible for folate breakdown in vivo.     

Effects of hepatic microsomal enzyme inducers of the endogenous substrates vitamin D3 and folate in rat.

A procedure for the estimation of possible effects of potential inducers of hepatic microsomal enzymes on vitamin D₃ metabolism has been devised. Together with methods for the determination of plasma folate and urinary formimino glutamic acid levels, it has been applied to a 43 day study in rats. Two anticonvulsants, phenobarbitone and pheneturide. were employed as inducers. Although both compounds had variable yet positive effects on enzyme induction, only the former had a significant action on vitamin D₃ metabolism and plasma folate levels. These results are discussed in relationship to results from previous work carried out, predominantly in man.     

Pteroylpolyglutamate pool modifications in rat liver after chronic administration of phenobarbitone and valproate.

Chronic intraperitoneal administration of low doses of phenobarbitone and valproate caused different alterations in hepatic percentage distribution of pteroylpolyglutamate derivatives without modification of total folate content. Phenobarbitone treatment caused a significant decrease of the percentage content of reduced unsubstituted and methylene-substituted derivatives, while valproate produced an increase of the percentage content of methenyl-, formyl- and formimino-substituted derivatives and a concomitant percent increase of hexaglutamates. The modified ratios of various pteroylpolyglutamates, both in phenobarbitone- and in valproate-treated animals, probably contribute to influencing the partitioning of the one-carbon pool through the various areas of one-carbon metabolism.     

The time course of [3H]PteGlu uptake and its incorporation into pteroylpolyglutamates in liver of phenobarbitone treated rats

The uptake of an injected dose of [3H]PteGlu and its incorporation into folate forms in the liver of control and phenobarbitone-treated rats were investigated at a number of time intervals from 1 to 24 h. At 1 h the hepatic uptake of the label was quite similar in both groups of animals and radioactivity was incorporated only in pteroylmonoglutamates. At 6 h label was present also in pteroylpolyglutamate forms and radioactivity values were similar in the two groups. At longer time periods the radioactivity was lower in treated animals than in control ones. The whole data show that phenobarbitone on the one hand does not modify the transport of the vitamin; on the other, it does interfere with "pteroylpolyglutamate synthetase activity".     

Factors influencing plasma phenobarbitone levels in epileptic patients.

1 Various statistical techniques were used to study the effects of age, sex and concurrent therapy with other anticonvulsants on the relation between plasma phenobarbitone levels and doses of (i) phenobarbtione, (ii) methylphenobarbitone or (iii) primidone, in epileptic patients. 2 Methylphenobarbitone and primidone are converted to phenobarbitone in the body. The mean doses of phenobarbitone, methylphenobarbitone and primidone which produced the same plasma phenobarbitone level (15 microgram/ml) were, respectively, 1.75,2.75 and 7.75 mg kg-1 day-1. 3 For both phenobarbitone and methylphenobarbitone dose requirement to achieve a given plasma phenobarbitone level fell progressively with age. Sex influenced the relation between plasma phenobarbitone level and phenobarbitone or methylphenobarbitone dose. Interactions were detected between primidone and both phenytoin and carbamazepine. 4 In individual patients, within the limits of dosage studied, the relation between plasma phenobarbitone level and drug dose was not rectilinear if phenobarbitone itself was taken, but was rectilinear if methylphenobarbitone was taken.     

Reactions of phenyl substituted barbiturates in concentrated sulfuric acid (author's transl)]

Reactions of Phenyl Substituted Barbiturates in Concentrated Sulfuric Acid Three reactions of phenobarbitone in concentrated sulfuric acid were investigated. (1) With sodium nitrite in concentrated sulfuric acid phenobarbitone forms an orange EDA-complex (Ph. Eur.). (2) With an excess of formaldehyde in 83, 7% sulfuric acid phenobarbitone gives a ruby colour (DAB 7) which is due to the formation of carbenium ions. The final product of the reaction is a yellow substance with methylene- and oxymethylene bridges. (3) The reaction of phenobarbitone with piperonal in sulfuric acid (DAB 7-DDR) is in reality a reaction with formaldehyde, which is formed as an intermediate, Only 1/13 mole of formaldehyde is formed per mole of phenobarbitone. The green colour is due to a quinone structure without methylene- or oxymethylene bridges. In this reaction piperonal can be replaced by hexamethylentetramine or trioxane.     

Changes in primidone/phenobarbitone ratio during pregnancy and the puerperium

Plasma concentrations of primidone and its metabolite phenobarbitone were monitored in 9 pregnant epileptic patients treated with primidone (and in 3 cases other antiepileptic drugs) given at constant doses throughout pregnancy and the puerperium. Phenobarbitone plasma concentrations were monitored in another 6 patients given phenobarbitone itself. A trend towards increasing primidone plasma concentrations during the second quarter of pregnancy was evident in all patients, with a concomitant significant decrease in primidone-derived phenobarbitone plasma concentrations. A trend towards a lowering of plasma concentrations of phenobarbitone administered as such was confirmed. These results suggest the usefulness of a careful monitoring of primidone and primadone-derived phenobarbitone during pregnancy and the puerperium. Discrepancies of findings with primidone and phenobarbitone are discussed in view of the possible mechanism involved.     

4-Methylpyrazole alters phenobarbitone hypnotic concentrations in rats

The liver alcohol dehydrogenase inhibitor, 4-methylpyrazole, has been tested for its ability to change the hypnotic concentrations of phenobarbitone (phenobarbital) in rats. Following a single dose of 1 mmol kg-1 i.v., administered 60 min before phenobarbitone, 4-methylpyrazole shortened the onset time and reduced the dose of phenobarbitone required to produce loss of righting reflex. Consistent with this, phenobarbitone concentrations in serum (both total and free), brain and in cerebrospinal fluid at onset of hypnosis were about half in 4-methylpyrazole compared with saline-treated rats. These results suggest that acute 4-methylpyrazole pretreatment increases the central nervous system sensitivity to phenobarbitone and presumably other barbiturates; an effect apparently distinct from its inhibition of liver alcohol dehydrogenase.     

Evaluation of a pharmacokinetic interaction between remacemide hydrochloride and phenobarbitone in healthy males

AIMS: To determine whether there is a pharmacokinetic interaction between the antiepileptic drugs remacemide and phenobarbitone. METHODS: In a group of 12 healthy adult male volunteers, the single dose and steady-state kinetics of remacemide were each determined twice, once in the absence and once in the presence of phenobarbitone. The effect of 7 days remacemide intake on initial steady-state plasma phenobarbitone concentrations was also investigated. RESULTS: Apparent remacemide clearance (CL/F) and elimination half-life values were unchanged after 7 days intake of the drug in the absence of phenobarbitone (1.25 +/- 0.32 vs 1.18 +/- 0.22 l kg(-1) h(-1) and 3.29 +/- 0.68 vs 3.62 +/- 0.85 h, respectively). Concomitant administration of remacemide with phenobarbitone resulted in an increase in the estimated CL/F of remacemide (1.25 +/- 0.32 vs 2.09 +/-0.53 l kg-1 h-1), and a decreased remacemide half-life (3.29 +/- 0.68 vs 2.69 +/- 0.33 h). The elimination of the desglycinyl metabolite of remacemide also appeared to be increased after the phenobarbitone intake (half-life 14.72 +/- 2.82 vs 9.61 +/- 5.51 h, AUC 1532 +/- 258 vs 533 +/- 281 ng ml(-1) h). Mean plasma phenobarbitone concentrations rose after 7 days of continuing remacemide intake (12.67 +/- 1.31 vs 13.86 +/- 1.81 microgram ml(-1)). CONCLUSIONS: Phenobarbitone induced the metabolism of remacemide and that of its desglycinyl metabolite. Remacemide did not induce its own metabolism, but had a modest inhibitory effect on the clearance of phenobarbitone.     

An effect of phenobarbitone on griseofulvin metabolism in the rat

Prior administration of phenobarbitone to male and female rats dosed orally or intravenously with griseofulvin caused a fall in blood levels of the antibiotic. The effect of a single oral dose of phenobarbitone was significant after 12 hr and maximal between 12 and 48 hr, and it lasted for at least 96 hr; it was more pronounced when the barbiturate was administered repeatedly. Liver slices from animals dosed with phenobarbitone metabolized griseofulvin more rapidly than did those from undosed animals. The possible relevance of these findings to the clinical use of griseofulvin is discussed.     

Lack of pharmacokinetic interaction between retigabine and phenobarbitone at steady-state in healthy subjects

AIMS: To evaluate potential pharmacokinetic interactions between phenobarbitone and retigabine, a new antiepileptic drug. METHODS: Fifteen healthy men received 200 mg of retigabine on day 1. On days 4-32, phenobarbitone 90 mg was administered at 22.00 h. On days 26-32, increasing doses of retigabine were given to achieve a final dose of 200 mg every 8 h on day 32. The pharmacokinetics of retigabine were determined on days 1 and 32, and those for phenobarbitone on days 25 and 31. RESULTS: After administration of a single 200 mg dose, retigabine was rapidly absorbed and eliminated with a mean terminal half-life of 6.7 h, a mean AUC of 3936 ng x ml(-1) x h and a mean apparent clearance of 0.76 l x h(-1) x kg(-1). Similar exposure to the partially active acetylated metabolite (AWD21-360) of retigabine was observed. After administration of phenobarbitone dosed to steady-state, the pharmacokinetics of retigabine at steady-state were similar (AUC of 4433 ng x ml(-1) x h and t1/2 of 8.5 h) to those of retigabine alone. The AUC of phenobarbitone was 298 mg x l(-1) x h when administered alone and 311 mg x ml(-1) x h after retigabine administration. The geometric mean ratios and 90% confidence intervals of the AUC were 1.11 (0.97, 1.28) for retigabine, 1.01 (0.88, 1.06) for AWD21-360 and 1.04 (0.96, 1.11) for phenobarbitone. Individual and combined treatments were generally well tolerated. One subject was withdrawn from the study on day 10 due to severe abdominal pain. Headache was the most commonly reported adverse event. No clinically relevant changes were observed in the electrocardiograms, vital signs or laboratory measurements. CONCLUSIONS: There was no pharmacokinetic interaction between retigabine and phenobarbitone in healthy subjects. No dosage adjustment is likely to be necessary when retigabine and phenobarbitone are coadministered to patients.     

Phenytoin: an inhibitor and inducer of primidone metabolism in an epileptic patient.

The interaction between primidone and phenytoin was studied in an epileptic patient treated with primidone only and primidone plus phenytoin for 3 months. Plasma and urine levels of drugs and metabolites were monitored daily by GC and GC-MS. The addition of phenytoin to the regimen increased steady-state plasma levels of phenobarbitone and phenylethylmalonamide (PEMA), metabolites of primidone, and decreased levels of primidone and unconjugated p-hydroxyphenobarbitone (p-OHPB), a metabolite of phenobarbitone. After withdrawal of phenytoin, plasma phenobarbitone and primidone levels slowly returned to previous steady-state levels, PEMA rapidly decreased to lower levels than before, and p-OHPB levels rose rapidly. Urinary excretion of primidone and its metabolites paralleled the changes in their plasma levels after the addition of phenytoin but the percentage of unconjugated p-OHPB in urine was unchanged during the course of the study. In conclusion phenytoin initially induces the conversion of primidone to PEMA and phenobarbitone, although each to a different extent, but it appears to inhibit the hydroxylation of phenobarbitone. Thus, two apparently contradictory phenomena seem to be involved in the primidone-phenytoin interaction. The net effect is an enhanced increase in plasma phenobarbitone levels.     

Subacute cannabinoid treatment: anticonvulsant activity and withdrawal excitability in mice.

1 The effects of subacute treatment with cannabidiol, delta 9-tetrahydrocannabinol (delta 9-THC), phenytoin and phenobarbitone on anticonvulsant activity and on withdrawal excitability in mice were compared in three electrically induced seizure-threshold tests. 2 In the maximal electroshock-threshold test, subacute treatment did not alter the anticonvulsant activity of cannabidiol, phenytoin or phenobarbitone, but tolerance developed to delta 9-THC. 3 In the 60 Hz electroshock-threshold test, the activity of delta 9-THC and cannabidiol did not change, but tolerance developed to phenobarbitone, and there was an increase in sensitivity to phenytoin. 4 In the 6 Hz electroshock-threshold test, there was an increase in sensitivity to both delta 9-THC and cannabidiol, there was tolerance to phenobarbitone, while the activity of phenytoin did not change. 5 Although tolerance developed in some of the seizure-threshold tests to delta 9-THC and phenobarbitone, tolerance to cannabidiol and phenytoin did not develop in any of the tests. 6 Hyperexcitability followed withdrawal from only delta 9-THC (6 Hz and 60 Hz electroshock-threshold tests) and phenobarbitone (maximal electroshock-threshold and 60 Hz electroshock-threshold tests). 7 The delta 9-THC withdrawal hyperexcitability suggests that the use of marihuana may jeopardize the control of seizures in epileptics.     

Muscle relaxant action of phenobarbitone in genetically spastic rats: an electromyographic study

The muscle relaxant effect of phenobarbitone was studied in genetically spastic rats which exhibit spontaneous tonic activity in the electromyogram (EMG) of the gastrocnemius muscle. Phenobarbitone, 10-30 mg/kg i.p., reduced the tonic activity in the EMG of the gastrocnemius muscle of such rats in a dose- and time-dependent manner. The GABA antagonists bicuculline, 2 mg/kg i.p., and picrotoxin, 2 and 3 mg/kg i.p., reduced the muscle relaxant effect of phenobarbitone, 20 and 30 mg/kg. The benzodiazepine receptor antagonists, Ro 15-1788, 5 mg/kg, and CGS 8216, 5 mg/kg (doses which do not affect tonic activity in the EMG), failed to alter the depressant effect of phenobarbitone 30 mg/kg, in the EMG. Beta-Carboline-3-carboxylic acid methylester (beta-CCM), 2 mg/kg i.p., while not affecting the tonic activity in the EMG, reversed the depressant effect of phenobarbitone, 30 mg/kg. Both Ro 15-1788, 5 mg/kg, and CGS 8216, 5 mg/kg, prevented the reversal of the depressant action of phenobarbitone, 30 mg/kg, produced by beta-CCM, 2 mg/kg. The results indicate that the muscle relaxant action of phenobarbitone in genetically spastic rats is mediated via GABA-related mechanisms and add further support to the hypothesis that both Ro 15-1788 and CGS 8216 are specific antagonists at benzodiazepine receptors, devoid of intrinsic activity at moderate doses. The results also suggest that reversal of the muscle relaxant action of phenobarbitone by beta-CCM is mediated via a GABA/benzodiazepine receptor/chloride ionophore complex.     

Dose-response relationships for initiation of rat liver tumours by diethylnitrosamine and promotion by phenobarbitone or alcohol

Small doses of initiators, such as the nitrosamines, are present in the diet and man is also exposed to promoters, such as phenobarbitone or alcohol. A simple two-stage model of rat hepatocarcinogenesis with a single ip dose of diethylnitrosamine (DEN) as the initiator and sodium phenobarbitone or ethanol given in the drinking-water for 12-18 months as the promoter was used to investigate dose-response relationships for initiation and promotion. Phenobarbitone given alone for 12 months had no carcinogenic effect on rat liver. Low doses of DEN, given prior to phenobarbitone promotion, resulted in the formation of hyperplastic nodules, but only the high dose of DEN (30 mg/kg) resulted in carcinoma formation. Basophilic foci showed a dose-response relationship with DEN and it is suggested that these may be more important than nodules in carcinoma formation and that they represent a heterogeneous group. Only the top concentration of phenobarbitone (1000 micrograms/ml) promoted carcinoma initiated by 30 mg DEN/kg although lower doses of phenobarbitone produce significant enzyme induction. A weak enzyme inducer, 5% ethanol, was as effective a promoter as 1000 micrograms phenobarbitone/ml. Enzyme induction and tumour formation are therefore not directly related. Commencing top-dose phenobarbitone promotion 10 months after the single dose of DEN led to tumour formation, demonstrating the persistence of DEN-initiated cells. The significance to man of the apparent thresholds for tumour initiation and promotion is discussed.     

Effects of phenobarbitone and 6-methylprednisolone pretreatment on pressure/flow relations in the superior mesenteric and iliac arterial beds of the rat

The in-situ blood perfused rat superior mesenteric arterial and right iliac arterial preparations were used to investigate the effects of pretreatment for 5 days with either phenobarbitone (80 mg kg-1 daily) or 6-methylprednisolone (17 mg kg-1 daily) on regional vascular resistances. The elevation of the regression of perfusion pressure on perfusion rate in the superior mesenteric arterial bed was significantly decreased by both phenobarbitone and 6-methylprednisolone over the range of flow rates used. Neither phenobarbitone nor 6-methylprednisolone had a significant effect on hepatic portal venous pressure nor was there any significant effect on the regression of perfusion pressure on flow rate in the in-situ iliac arterial bed. It is concluded that pretreatment with either phenobarbitone or 6-methylprednisolone lowers vascular resistance in the superior mesenteric arterial by approximately 9 and 8% respectively but does not affect vascular resistance in the iliac arterial bed.     

Biliary excretion of barbiturates

1. Pentobarbitone and phenobarbitone are excreted into the bile of rats.2. The concentration of pentobarbitone and/or its metabolites in the bile is 22-fold higher than in the plasma; the concentration of phenobarbitone and/or its metabolites in bile is 10-fold higher than in the plasma.3. Probenecid decreases the excretion of the barbiturates into the bile.4. Twenty-eight per cent of a 35 mg/kg dose of pentobarbitone and 18% of a 75 mg/kg dose of phenobarbitone is excreted in the bile in 6 hours.5. Most of the pentobarbitone and phenobarbitone excreted into the bile is not in the form of the parent compound but rather as a more polar metabolite or metabolites.     

Phenobarbitone population pharmacokinetics from routine clinical data: role of patient characteristics for estimating dosing regimens.

Routine clinical pharmacokinetic data collected from patients receiving phenobarbitone have been analysed to evaluate the role of patient characteristics for estimating dosing regimens. The data were analysed using NONMEM, a computer program designed for population pharmacokinetic analysis that allows pooling of data. The pharmacokinetic model of phenobarbitone was described using a one-compartment steady-state model. The effect of a variety of developmental and demographic factors on clearance was investigated. NONMEM estimates indicated a nonlinear function of total body weight as the optimum adjustment of phenobarbitone clearance. Concomitant administration of phenobarbitone and other antiepileptic drugs showed a decrease of phenobarbitone clearance in young children. The dosing method based on clearance values obtained by NONMEM analysis allowed the prediction of the steady-state concentration as a function of maintenance dose with acceptable error for therapeutic drug monitoring.     

The effect of phenobarbitone pre-treatment on vitamin K1 disposition in the rat and rabbit

The effect of phenobarbitone enzyme induction on the pharmacokinetics of an intravenous pharmacological dose (1 mg/kg) of vitamin K1 was studied in the rabbit. Phenobarbitone pretreatment significantly (P less than 0.01) increased the plasma clearance of vitamin K1 and decreased the terminal (beta) half-life from 2.08 +/- 0.56 to 0.99 +/- 0.30 hr. However, phenobarbitone pretreatment did not alter the pharmacodynamic response to vitamin K1 measured as the increase in prothrombin complex activity, in brodifacoum-anticoagulated rabbits. In the rat, phenobarbitone enzyme induction increased the extent and rate of biliary excretion of polar vitamin K1 metabolites following intravenous administration of the vitamin. Perturbation of vitamin K1 metabolism by phenobarbitone enzyme induction is not dependent on the concentration of the vitamin. The greater hepatic elimination resulted in lower systemic blood concentrations of both vitamin K1 and the 2,3-epoxide. A similar reduction in the concentration of vitamin K1 in the blood of epileptic mothers treated with anticonvulsants such as phenobarbitone may explain the coagulation defect frequently observed in their offspring [K. R. Mountain, J. Hirsh and A.S. Gallus, Lancet ii, 265 (1970)].