PubMed Health⌕ Search

Biomedical subjects

A Richens

Publications and source records attributed to A Richens.

At least 37 records · Page 2Linked to original sources
PubMed1978

Is phenytoin metabolism dose-dependent by enzyme saturation or by feedback inhibition?

The suggestion from animal experiments that phenytoin metabolism may be dose-dependent in man due to feedback inhibition by the major metabolite, 5-(p-hydroxyphenyl)-5-phenylhydantoin, was examined in 3 normal subjects by measuring phenytoin clearance during an intravenous infusion of the metabolite and during a control infusion of solvent. Clearance was measured using both carbon-labeled and unlabeled phenytoin. The infusion of metabolite did not produce any consistent change of phenytoin clearance, suggesting that feedback inhibition does not occur in man.

Adult↗
PubMed1978

Plasma protein binding interaction between phenytoin and valproic acid in vitro.

1 Valproic acid or phenytoin were added to fresh human serum in varying concentrations and their binding characteristics determined by the method of Scatchard (1949). 2 Changes in serum albumin binding were investigated for phenytoin in the presence of 280, 560, 1050 and 2100 mumol l-1 valproic acid, and for valproic acid in the presence of 40, 120, 280 and 480 mumol l-1 phenytoin. 3 Phenytoin appeared to bind to a single site on the albumin molecule and could be competitively displaced from this site by concentrations of valproic acid above 280 mumol l-1. 4 At high concentrations of valproic acid, the affinity of phenytoin for albumin was greatly decreased but the number of available binding sites was increased from one to four. 5 Valproic acid was bound to two high affinity and five low affinity binding sites but the latter were not detectable at valproic acid concentrations below 2100 mumol l-1. 6 Phenytoin displaced valproic acid from its high affinity binding sites, although this was statistically significant only at a concentration of 480 mumol l-1 phenytoin.

Binding Sites↗
PubMed1978

Water intoxication in epileptic patients receiving carbamazepine.

Plasma sodium and osmolality were determined in 80 adult epileptic patients receiving chronic treatment with carbamazepine and in 50 control patients treated with other anticonvulsant drugs. Mean plasma osmolality was significantly lower in the carbamazepine-treated patients but mean plasma sodium did not differ in the two groups. Hyponatraemia was found in five of the carbamazine-treated patients and hypo-osmolality in six. None of the control patients had hyponatraemia and only one had a borderline low osmolality. Three of the 13 patients receiving carbamazepine alone were hyponatraemic. Plasma sodium concentration correlated negatively with both daily carbamazepine dose and serum carbamazepine level. Free water clearance after an oral water load was determined in six patients on carbamazepine alone and in six normal subjects not receiving drug therapy. The capacity of some of the patients to excrete the water load was found to be grossly impaired.

Adolescent↗
PubMed1978

Ioglycamide (Biligram) studies in man--plasma binding, renal and biliary excretion studies in jaundiced and anicteric patients.

When five patients with varying degrees of hepatic impairment and a T-tube in situ were given intravenous ioglycamide at a rate of 2 mg/kg/min for two hours the mean biliary excretion in the first two hours was only 3.2% of the administered dose. In contrast, in five T-tube patients with relatively normal liver function the mean biliary excretion over the same time interval was 20.6%. The mean plasma concentration of ioglycamide achieved at the end of a two-hour intravenous infusion at 2 mg/kg/min was 1427 +/- 187 microgram/ml in six anicteric patients and 1262 +/- 82 in six jaundiced patients. Despite these very similar plasma levels the 24-hour urinary excretion of ioglycamide was 42.3 +/- 3.8% of the administered dose in the patients with jaundice compared with only 18.1 +/- 2.4% in the anicteric group. These differences probably reflect the fact that the percentage of unbound contrast agent in the plasma of the jaundiced group (11.9 +/- 1.9%) was significantly higher than that of the anicteric group (6.4 +/- 0.9%). It is suggested that bilirubin and possibly other substances in the plasma are competing with ioglycamide for binding sites on albumin. These factors need to be borne in mind when performing intravenous cholangiograms on jaundiced patients.

Aged↗
PubMed1977

Evaluation of a new immunoassay for determination of phenytoin and phenobarbital: results of a European collaborative control study.

The performance of a new enzyme multiplied immunoassay technique (EMIT) was compared with other current methods, namely gas-liquid chromatography and thin-layer chromatography, for the determination of phenytoin and phenobarbital in serum. Forty-three serum samples were sent as unknowns to the participating laboratories for determination. The precision of repeated determinations was very similar for EMIT and chromatography. There was good agreement among gas chromatography, thin-layer chromatography, and EMIT results. Interlaboratory variability was lower for EMIT determinations of both antiepileptic drugs. The rapid analysis of small samples, made possible by the EMIT system, could have beneficial effects on the treatment of epilepsies.

Chromatography, Gas↗
PubMed1977

Interactions with antiepileptic drugs.

Most of the currently available antiepileptic drugs have a low therapeutic ratio and therefore a drug interaction causing elevation of the serum level of one of these compounds can readily lead to drug intoxication. Phenytoin, in particular, is vulnerable because its metabolism is dose-related and at therapeutic serum levels the enzyme system involved in its degradation is easily inhibited by concurrent drug administration. As multiple drug therapy has traditionally been practised in the management of epilepsy, clinically important interactions are common. Furthermore, most of the drugs used in the treatment of major epilepsies are potent inducers of hepatic microsomal enzymes and can therefore stimulate the metabolism of concurrently-administered drugs to such an extent that they may be rendered ineffective. The use of one drug alone is recommended, where possible, in the management of epilepsy.

Acetaminophen↗
PubMed1977

Interlaboratory variability of valproic acid determinations.

19 pooled plasma specimens were sent as unknowns to 13 participating research laboratories. The interlaboratory variability between the results was very high. Only 4 out of 13 laboratories had 6--12% of their results within the 95% confidence limit for each sample. The precision of repeated determinations was fairly good. 6 out of 10 participating laboratories had a coefficient of variation of less then 5%. The reproducibility and the agreement between the different procedures for quantitative analysis of dipropylacetate is similar to that reported for other major antiepileptic drugs.

Laboratories↗
PubMed1975

Controlled trial of sodium valproate in severe epilepsy.

In a double-blind crossover trial sodium valproate or placebo was added to the existing anticonvulsant treatment of 20 patients with chronic uncontrolled epilepsy. Sodium valproate 1200 mg/day significantly reduced the frequency of both tonic-clonic and minor seizures in these patients. Only mild and transient side effects occurred (drowsiness, ataxia, and nausea), and these may have been due to the effect of adding sodium valproate to existing phenobarbitone or phenytoin treatment. Further controlled trials are needed to assess more fully the efficacy of this drug in various types of epilepsy.

Adult↗
PubMed1975

Brain concentrations of phenytoin, phenobarbitone and primidone in epileptic patients.

Plasma, brain, lumbar CSF, skeletal muscle, skin and bone concentrations of phenytoin, phenobarbitone and primidone have been measured in specimens from patients undergoing temporal lobectomy for chronic epilepsy. A good correlation was found between the plasma and brain concentrations of each drug. Similarly, a good correlation was found between the plasma and CSF concentrations of each drug. Assuming that CSF is an ultrafiltrate of plasma, the percentage of phenytoin, phenobarbitone and primidone which was unbound in plasma was 10-14%, 43% and 81% respectively. Skeletal muscle concentrations of phenytoin and phenobarbitone and the skin concentration of phenytoin, also correlated with the plasma concentrations, but the remaining tissues did not give significant correlations.

Adolescent↗
PubMed1975

Serum-phenytoin levels in management of epilepsy.

Steady-state serum-phenytoin levels were measured in five patients whose phenytoin dosage was changed three or more times for therapeutic purposes. Data from four of these patients and from fifteen others have been used to construct a nomogram which permits the clinician, given a single, accurate, steady-state phenytoin level, to adjust the dosage to achieve the desirable therapeutic concentration of 60 or 80 mumol per litre (15 or 20 mug. per ml.).

Dose-Response Relationship, Drug↗