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Biomedical subjects

W D Hooper

Publications and source records attributed to W D Hooper.

107 records · Page 6Linked to original sources
PubMed1970

Outbreak of anticonvulsant intoxication in an Australian city.

An outbreak of anticonvulsant intoxication occurred in epileptic patients in Australia during 1968-9. All affected patients studied in Brisbane were taking one brand of phenytoin. In 87% of them the blood phenytoin levels were above the therapeutic range. Reduction of phenytoin dosage relieved the intoxication in all patients. The excipient in the responsible phenytoin capsules had been changed several months before the outbreak, and this change was probably related causally to the altered blood phenytoin concentrations.

Australia↗
PubMedDate not supplied

Metabolism of diazepam and related benzodiazepines by human liver microsomes.

The metabolism of diazepam has been studied in vitro using microsomal preparations from five human livers. An HPLC method was developed for the assay of diazepam, its congeners and its metabolites. Various methods for the incorporation of diazepam into the incubation medium were explored. It was shown that the use of organic solvents or small quantities of hydrochloric acid enhanced the solubility of this substrate. However all of the organic solvents tested were associated with substantial (around 50%) inhibition of metabolism of diazepam by both major pathways (N-demethylation and C3-hydroxylation). The use of hydrochloric acid gave satisfactory solubilization of diazepam, but not of pinazepam, prazepam or halazepam. Detailed metabolic studies were conducted only for diazepam, using neither hydrochloric acid nor organic solvents in the incubation medium. Formation of N-desmethyl-diazepam increased approximately linearly with diazepam concentration to 200 microM, and did not show saturation. Formation of temazepam gave a curved profile over the same range of diazepam concentrations, suggestive of a sigmoidal relationship. Michaelis-Menten parameters could not be determined for either reaction, but intrinsic clearances for N-demethylation varied over a 6-fold range. Diazepam N-demethylation was apparently promoted by the inclusion of temazepam in the incubation medium, while C3-hydroxylation of diazepam was enhanced in the presence of N-desmethyldiazepam. Mephenytoin in the incubation mixture had no effect on diazepam metabolism by either pathway. The present studies have defined some of the methodological problems inherent in in vitro metabolic studies with benzodiazepines, and have shed further light on the metabolism of diazepam in vitro by human liver.

Adult↗
PubMedDate not supplied

Valproate-associated hepatotoxicity and its biochemical mechanisms.

Intake of the anticonvulsant drug valproic acid, or its sodium salt, has been associated with occasional instances of severe and sometimes fatal hepatotoxicity. Probably at least 80 cases have occurred worldwide. The syndrome affects perhaps 1 in 10,000 persons taking the drug, and usually develops in the early weeks or months of therapy. Most instances have involved children, usually those receiving more than 1 anticonvulsant. Multiple cases have occurred in 2 families. The typical presentation is of worsening epilepsy, increasing depression of consciousness, and progressive clinical and biochemical evidence of liver failure. The liver has sometimes shown hepatocyte necrosis, and on other occasions widespread microvesicular steatosis, while cholestatic changes have also occurred. The appearances are interpreted as consistent with a drug toxicity reaction. During the hepatotoxicity increased amounts of unsaturated metabolites of valproate, notably 4-en-valproate, have been found in blood and urine. In 4 cases there has been evidence of impaired beta-oxidation of valproate with, in 1 case, accumulation of isomers of valproate glucuronide caused by intramolecular rearrangement of the conjugate. There are molecular structural similarities between 4-en-valproate and 2 known hepatotoxins (4-en-pentanoate and methylenecyclopropylacetic acid, the latter being responsible for hypoglycin poisoning). There are also clinical and histopathological similarities between valproate hepatotoxicity and both hypoglycin poisoning and certain spontaneous disorders of isoleucine metabolism (one pathway of valproate metabolism is analogous to oxidative degradation of isoleucine). Unsaturated metabolites of valproate, in particular 4-en-valproate, may contribute to the hepatotoxicity of the drug. However, since the hepatotoxicity appears to involve an element of idiosyncrasy, the primary defect in some cases may be an inherited or acquired deficiency in the drug's beta-oxidation. This defect may divert valproate metabolism towards omega-oxidation, with increased formation of the toxin 4-en-valproate, but may also allow increased formation of a toxic metabolite derived from isoleucine, since beta-oxidation of isoleucine derivatives will also be impaired.

Animals↗
PubMedDate not supplied

The elimination of phenytoin in man.

1. Plasma phenytoin (diphenylhydantoin) levels after different drug doses were correlated with urinary 5-(p-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) excretions in four subjects. 2. In three of four subjects the proportion of the phenytoin dose that was excreted as p-HPPH. In the fourth, p-HPPH output remained proportionate to dose of phenytoin until elimination of the drug fell below its input. 3. Plasma p-HPPH levels were measured in two subjects; the data suggested that the renal excretion of p-HPPH was not rate-limited. 4. In three of four subjects, there was the possibility that alternative pathways for eliminating phenytoin may have developed as drug doses increased and the capacity for forming p-HPPH became saturated. 5. Overall phenytoin elimination appeared to approach saturation at concentrations of the drug encountered therapeutically. When Michaelis-Menten kinetics were applied to data for phenytoin elimination in twenty-one adults and fifteen children, the mean apparent Km value for the adults corresponded to a plasma drug concentration of 5-8 mug/ml, and in the children to 5-3 mug/ml. The mean Vmax values in the two groups were, respectively 8-1 mg/kg per day and 12-5 mg/kg per day.

Adolescent↗
PubMedDate not supplied

The comparative bioavailability of carbamazepine in 100 mg and 200 mg tablets.

1. The bioavailabilities of carbamazepine in 100 mg and 200 mg tablets have been compared in a cross-over study of six subjects after two 600 mg doses of the drug, the different preparations being taken at 3 week intervals. 2. Areas under the plasma level curves, absorption rate constants and times to achieve peak plasma levels showed little difference between the two preparations. These findings suggest similar rates and extents of bioavailability of carbamazepine in the two preparations. 3. Calculated mean absorption and elimination parameters for carbamazepine were as follows: kabs = 0.1081 h-1, (s.d. = 0.0289); Tmax = 23.39 h, (s.d. = 8.66); k = 0.0191 h-1, (s.d. = 0.0033); VD = 0.989 1/kg, (s.d. = 0.159); and clearance = 0.0185 1/kg h, (s.d. = 0.0015).

Adult↗
PubMedDate not supplied

Rearrangement of valproate glucuronide in a patient with drug-associated hepatobiliary and renal dysfunction.

Formation of beta-glucuronidase-resistant "glucuronides" of valproic acid (VPA) by intramolecular rearrangement of biosynthetic valproate glucuronide in vivo was investigated in a patient diagnosed with VPA-associated hepatobiliary and renal dysfunction. Plasma elimination half-life of VPA following cessation of the drug was 13.9 h. At the time of the toxicity, the concentration of conjugated VPA in plasma was very high (36-54% of nonconjugated VPA levels) relative to that in normal patients (2.9%). The fraction of conjugated VPA resistant to beta-glucuronidase hydrolysis was 0.28-0.47 in plasma and 0.15-0.42 in urine. The corresponding fraction in urine from normal patients receiving VPA therapy was 0.044. The evidence was consistent with retarded elimination of biosynthetic VPA glucuronide caused by renal and hepatobiliary dysfunction. Consequent prolongation of circulation of VPA glucuronide at the slightly alkaline pH of blood would permit extensive intramolecular rearrangement which is known to be pH-, temperature-, and time-dependent. The biological consequences of the presence of such beta-glucuronidase-resistant conjugated VPA in vivo are largely unknown.

Adolescent↗
PubMedDate not supplied

Stereoselective metabolism and pharmacokinetics of racemic methylphenobarbital in humans.

The stereoselectivity of the metabolism and pharmacokinetics of methylphenobarbital (MPB) was studied in six healthy adult male volunteers given single oral doses of the racemic drug. All of the volunteers were phenotypically extensive metabolizers of the drug. The R- and S-enantiomers of MPB were analyzed in plasma by an enantioselective HPLC method, and the enantiomers of the 4-hydroxy-MPB metabolite in urine by a similar procedure. The (R)-MPB was extensively hydroxylated, with an average of 49.56% of that enantiomer being recovered in urine as (R)-4-hydroxy-MPB. Only 7.16% of the (S)-MPB was converted to the corresponding hydroxy metabolite. The extensive hydroxylation of (R)-MPB resulted in rapid elimination of this enantiomer, with a terminal plasma half-life of 7.52 +/- 1.70 (SD) hr. The (S)-MPB, the only recognized metabolites of which were (S)-4-hydroxy-MPB and phenobarbital (PB), was eliminated very slowly [t1/2, 69.78 +/- 14.77 (SD) hr]. The oral clearance of (R)-MPB (0.470 +/- 0.184 (SD) liters/hr/kg) was much higher than that of (S)-MPB [0.017 +/- 0.001 (SD) liters/hr/kg]. The extreme differences in metabolic fate and pharmacokinetics of the enantiomers of MPB are interesting. Most of the circulating PB seemed to be derived from (S)-MPB. In other respects the pharmacodynamic implications of these pharmacokinetic differences are unclear, because the relative anticonvulsant potencies of the enantiomers of MPB are unknown.

Administration, Oral↗
PubMedDate not supplied

Impaired biliary elimination of beta-glucuronidase-resistant "glucuronides" of valproic acid after intravenous administration in the rat. Evidence for oxidative metabolism of the resistant isomers.

A major metabolite of valproic acid (VPA) is its glucuronic acid conjugate (VPA-G). The disposition of VPA-G was compared with that of its intramolecularly rearranged, beta-glucuronidase-resistant isomers (collectively called VPA-G-R) after iv bolus administration to pentobarbitone-anesthetized rats. VPA-G was eliminated from blood more rapidly than VPA-G-R. After administration of dose A (predominantly VPA-G) and dose B (predominantly VPA-G-R) to rats with catheterized bladders and bile ducts, total conjugated VPA in blood declined from 110 micrograms of VPA/ml at 2 min to 1.1 micrograms/ml at 1 and 3 hr, respectively. A role for systemic hydrolysis of VPA-G was demonstrated by blood concentrations of free VPA which increased until 30 min. A minor role for systemic hydrolysis of VPA-G-R may be possible but cannot be proved from the current data. Urinary excretion (57 and 56% of doses A and B, respectively, in 3 hr) was greater than biliary excretion (32 and 10% of the doses, respectively, in 3 hr). The lower biliary elimination of VPA-G-R may be caused in part by impaired transport from blood to hepatocytes and/or hepatocytes to bile, but a role for phase I metabolism of the VPA moiety of VPA-G-R was demonstrated by recovery of 4.4% of dose B as 4-hydroxy-VPA. This latter mechanism was less (or not) applicable to VPA-G since only 0.4% of dose A was recovered as 4-hydroxy-VPA. Other VPA metabolites measured were quantitatively less important. These results were consistent in rats where either or both of the urinary and biliary elimination routes were surgically blocked.

Animals↗
PubMedDate not supplied

Qualitative and quantitative studies of methylphenobarbital metabolism in man.

The accumulation of methylphenobarbital (MPB) and phenobarbital (PB) in plasma in two volunteers who were given continuous once-daily oral doses of MPB for 3 weeks was demonstrated by use of a selected ion-monitoring GC/MS assay. It was shown that the PB concentration exceeded the MPB concentration in plasma after about day 4, and that both barbiturates achieved plateau concentrations after about 2 weeks. GC/MS studies on the urine of these volunteers permitted the identification of several new metabolites of MPB. These included 5-ethyl-5-(4-hydroxyphenyl)-1-methylbarbituric acid (p-OH-MPB), and the 3-O-methylcatechols of both MPB and PB. The meta-isomers of hydroxy-MPB and hydroxy-PB were identified in urine extracts, but were shown to be methodological artifacts. Quantitative studies, with use of a HPLC assay, were carried out for p-OH-MPB, PB and p-OH-PB in urine, and it was shown that these three substances collectively accounted for some 50% of the administered dose (greater than 30% as p-OH-MPB).

Administration, Oral↗
PubMedDate not supplied

Identification of p-hydroxyprimidone as a minor metabolite of primidone in rat and man.

Urine specimens from rats and humans who received single doses of primidone (PRM) have been investigated by GC/MS procedures. In addition to the previously documented metabolites of PRM, a small chromatographic peak was encountered which had a mass spectrum suggesting a hydroxy-PRM derivative. Synthesis of p-hydroxy-PRM from PRM was effected; the para isomer was separated from unwanted isomers by preparative HPLC. The PMR spectrum of the synthetic compound proved the position of the hydroxy substituent to be para. This compound had identical GC retention time and an almost identical mass spectrum with that obtained in the urinary extracts. Para-hydroxy-PRM was therefore confirmed as a new, minor metabolite of PRM in rat and man.

Adult↗
PubMedDate not supplied

pH-dependent rearrangement of the biosynthetic ester glucuronide of valproic acid to beta-glucuronidase-resistant forms.

A major metabolite of the antiepileptic drug valproic acid (VPA) in animals and man is the glucuronic acid conjugate, which is cleaved by incubation with beta-glucuronidase (specific for 1-O-substituted-beta-D-glucopyranosiduronic acids) or hydrolysis in strong acid or alkali. Previous studies revealed that an often substantial proportion of the alkali-labile conjugated VPA in stored urine or bile samples was not hydrolyzed by beta-glucuronidase, suggesting the presence of nonglucuronide conjugates. In the present study, bile from a NaVPA-treated rat was preincubated at 37 degrees C for 3 hr at pH values from -0.8 to 12.9, and then analyzed for nonconjugated VPA, VPA released by hydrolysis with beta-glucuronidase, and VPA released by hydrolysis with alkali. At pH 3-7, all alkali-labile conjugated VPA remained susceptible to beta-glucuronidase hydrolysis, whereas at pH 0-3 and 7-11, a proportion became resistant to the enzyme. GLC and GC/MS analysis of trimethylsilyl derivatives of the intact conjugates revealed the appearance of seven additional peaks, adjacent to the biosynthetic ester glucuronide, of which six were structural isomers and one was a dehydrated species. The data were consistent with acid- and base-catalyzed intramolecular acyl migration of the valproate moiety away from the C-1 position, with subsequent processes of ring-opening, mutarotation, and lactonization yielding structural isomers and lactones which were not substrates for beta-glucuronidase. It was further shown that these rearrangements are time- and temperature-dependent. Consequently, sample handling and storage conditions of ester glucuronides prior to analysis are of prime importance, since hydrolysis with beta-glucuronidase is frequently used for identification and quantification of glucuronides.

Animals↗
PubMedDate not supplied

Stereoselective hydroxylation of tacrine in rats and humans.

An enantiospecific method was developed for assessing the stereochemistry of tacrine (9-amino-1,2,3,4-tetrahydroacridine monohydrochloride monohydrate; THA) metabolism to 1-hydroxytacrine (1-OH-THA) in humans and rats. In addition, limited metabolic studies with human liver microsomal preparations were conducted, and the stereochemistry of rac-1-OH-THA disposition was also examined. The analytical method incorporates an achiral normal phase separation and isolation of 1-OH-THA, followed by a chromatographic step using chiral normal-phase chromatography to resolve the enantiomers of 1-OH-THA. The achiral method was applied to quantitation of total 1-OH-THA in human urine specimens collected for 24 hr following administration of a single 40 mg oral dose of tacrine to 15 healthy elderly volunteers. Total 1-OH-THA accounted for approximately 5% of the administered dose. THA and 2-OH-THA were also quantitated and found to comprise < 1% and approximately 2% of the administered dose, respectively. 4-OH-THA was not detectable. The dextrorotatory (+)-isomer comprised approximately 94% of the 1-OH-THA recovered in urine. In vitro studies utilizing human liver microsomes found enantioselective formation of the (+)-isomer (approximately 90%), whereas incubations with rac-1-OH-THA showed residual substrate to be racemic. The method was also applied to determination of the enantiomeric composition of 1-OH-THA in the urine of rats given a single oral 16 mg/kg dose of THA. The percentage of 1-OH-THA excreted in urine as the (+)-isomer was 94%.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗