Depression of drug metabolism in liver microsomes after treating rats with unsaturated fatty acids

Effect of dosing vehicle on the toxicity and metabolism of unsaturated aliphatic nitriles

The effect of dosing vehicle on toxicity and metabolism of unsaturated aliphatic nitriles was investigated in male Sprague-Dawley rats. Five unsaturated aliphatic nitriles--acrylonitrile, methacrylonitrile, allylnitrile, crotononitrile and fumaronitrile--were prepared in five different dosing vehicles--saline, corn oil, safflower oil, mineral oil, olive oil and Tween-20. Groups of six male rats were given 0.5 LD50 doses of the nitriles by gavage and they were observed for 12 It for cholinomimetic and central nervous system effects. Cyanide and glutathione levels were determined in blood and various organs at 1, 3 and 6 h after nitrile administration and thiocyanate levels were determined at 6 h after nitrile administration. The results indicate that all the vehicles studied potentiated the toxicity of all the nitriles compared to nitriles administered in saline and significantly increased their metabolism to cyanide and thiocyanate and nitrile-induced depletion of glutathione in rats. This behavior of vehicles illustrates the difficulty of identifying suitable vehicles for administration of lipophilic compounds in toxicology studies.

Plasma protein binding kinetics of valproic acid over a broad dosage range: therapeutic implications

The aim of the study was to characterize, from the relationship between total and free serum levels of valproic acid obtained over a broad dosage range (10-50 mg/kg), the parameters defining the in-vivo kinetic behaviour of the binding of valproic acid to plasma proteins, their pharmacokinetic and clinical repercussions, and their application to therapeutic drug monitoring (TDM). The study was performed in nine healthy adults (20-35 years) who were given doses of 1000 (group A), 2000 (group B) and 3000 mg (group C) of sodium valproate according to a compensated cross-over design, simultaneously determining the total and free serum levels of valproic acid over a 24-h period. The mean free fraction increases with dose, although this increase is only significant (P < 0.05) for the highest dose (3000 mg). The variation in the free fraction of valproic acid begins to become significant (P < 0.05) at a total drug concentration above 100 mg/l. The mean values of the dissociation constant (K) and binding sites (n) were 460 mumol/l and 1.79, respectively, showing a variability of 86.6 and 38.7%, respectively, and a residual variability of 13.0%. Significant differences (P < 0.05) were found for the total plasma clearance (Cl) but not for the intrinsic plasma clearance (Clu) values, despite their tendency to decrease with the dose. If TDM is to be used for valproic acid, it is the free serum levels that should be determined, especially if high doses are administered, because the total serum levels are not a true reflection of the free ones, as is the case of other anti-epileptic drugs.

Beta-oxidation of valproic acid. I. Effects of fasting and glucose in humans

The effect of fasting and glucose-infusion on valproate (VPA) disposition was investigated to determine the involvement of endogenous fatty acid (FA) beta-oxidation in the metabolism of VPA. Fifteen healthy volunteers received multiple oral doses of VPA to achieve steady state under both conditions. Depressed plasma FA concentrations in the glucose-infused state (median 51%, p less than 0.0001) were associated with lower unbound plasma VPA fractions (median 17%, p less than 0.0001). Unbound plasma VPA concentrations were notably lower in the glucose-infused state due to significantly higher (median 41%, p less than 0.0001) metabolic clearance, beta-oxidative metabolite formation clearance, representing the largest urinary dose fraction recovered, was significantly higher (median 60%, p less than 0.004) in the glucose-infused state. This finding is consistent with competition between endogenous FA and VPA for the enzymes of beta-oxidation modulated by conditions which affect FA mobilization to the site of catabolism.

Valproate increases diazepam impregnation selectively in CNS of the rat after subchronic administration

Valproate-Diazepam association is not unusual in the treatment of epilepsies. The present paper investigates in an experimental study the effect of sodium valproate (VPA) on the regional cerebral level of diazepam (DZP), and the relationship with plasma or erythrocytes amounts after subchronic administration. The VPA addition increases the DZP levels in peripheral and central compartments. The results shows a linear correlation between the drug concentration in all areas studied and the plasmatic or erythrocytic amounts during the CNS - impregnation phase. The VPA influence is greater in the CNS where the DZP impregnation is selectively increased, specially in cortex and cerebellum.

Abnormal antioxidant defence in some tissues of congenitally obese mice

The concentration of lipoperoxides (estimated as thiobarbituric acid-reactive material) and some components of the antioxidant defence system have been compared in various tissues of lean and congenitally obese mice. NADPH-stimulated lipoperoxide generation in vitro was significantly higher in microsomes (microsomal fractions) prepared from obese hepatic tissue than lean. Plasma, liver and brain lipoperoxide concentration was significantly higher in obese mice. In blood derived from obese mice the concentration of non-enzymic antioxidants including caeruloplasmin and vitamin A was higher, but hepatic retinol concentration was lower in these animals. In all the tissues assayed the glutathione peroxidase activity against H2O2 was less than its activity against cumene hydroperoxide. Assayed with either substrate, glutathione peroxidase activity was significantly higher in the brain and blood of obese mice than their lean counterparts. Conversely, liver glutathione peroxidase was decreased in obese animals, representing 43% of the activity of the lean-mouse liver enzyme against H2O2 and 81% of the cumene hydroperoxide-reducing activity. The liver of obese mice had significantly less, and the kidneys more, oxidized glutathione than the corresponding tissues of lean mice. Further investigations on hepatic tissue indicated that glutathione reductase activity was lower in the obese animals, but there was no significant difference between glucose-6-phosphate dehydrogenase activity in obese and lean mice.

Interaction of selenium deficiency and fat intake in the regulation of enzymes associated with peroxide metabolism

The present study was designed to examine the effect of selenium deficiency on the activities of heme hydroperoxidase and glutathione peroxidases in the liver of male rats maintained on either a 5% or a 25% corn oil diet and treated with phenobarbital. Our results showed that although the basal levels of cytochrome P-450 and heme hydroperoxidase were unaffected by selenium deficiency, the magnitude of phenobarbital induction was impaired because of the depletion of this trace element. This effect was accentuated especially in rats with a high-fat intake. Selenium deprivation resulted in a virtual disappearance of glutathione peroxidase activity when assayed with hydrogen peroxide, because of depletion of the selenium-dependent enzyme. In contrast, only a 60% reduction in glutathione peroxidase activity was observed when assayed with cumene hydroperoxide. Phenobarbital administration was found to increase the activity of the latter only. Unlike the situation with the hemoprotein, dietary fat had no influence on either the basal or stimulated glutathione peroxidase activities, regardless of the selenium status of the animals.

Systemic interaction between valproic acid and free fatty acids in rhesus monkeys

The interaction between valproic acid (VPA) and free fatty acids (FFA) was investigated at a systemic level in eight rhesus monkeys. Intralipid emulsion (I.L.) was infused to increase plasma FFA levels. During I.L. treatment, plasma FFA levels were elevated in all cases (12 cases) with increases ranging from 38 to 525% (median, 102%). Valproate free fraction (fp) was elevated in 11 of 12 cases, with increases ranging between 7 and 100% (median, 18%). A positive correlation between the relative increase in FFA and the corresponding increase in fp of VPA was obtained. Intralipid infusion was associated with changes in the systemic clearance of VPA (an increase in five cases and a decrease in seven cases). The intrinsic clearance (Clint) of VPA was decreased in 9 of 12 cases with decreases ranging from 13 to 78% (median, 22%). Urinary excretion of valproate glucuronide was decreased in three of four monkeys during I.L. treatment. A significant positive correlation (p less than 0.05) between the relative increase in FFA levels and the decrease in Clint of VPA were also observed. The mechanism of interaction between VPA and FFA in rhesus monkey is more complex than a simple protein binding displacement and involves at least one metabolic inhibition component such as glucuronidation.

Phenytoin--valproic acid interaction in rhesus monkey

The interaction between phenytoin (PHT) and valproic acid (VPA) was investigated in the rhesus monkey. PHT was given by multiple IV bolus to reach a steady state and then continued while two successive VPA infusions were added. Plasma was assayed for total VPA and PHT as well as free PHT. The addition of VPA produced no change in total PHT levels, a 50% increase in PHT free fraction (increase in free PHT levels), and a decrease in PHT overall elimination rate constant. The increase in free fraction was also documented in a separate in vitro study. The increase in free PHT levels suggested that VPA decreased PHT intrinsic metabolic clearance. This interaction may thus involve a dual mechanism-namely protein binding displacement of PHT and inhibition of PHT metabolism. This mechanism is compatible with other studies showing that VPA is an inhibitor of drug metabolism.