Pharmacokinetics and pharmacodynamics of valproate analogues in rats. I. Spiro[4.6]undecane-2-carboxylic acid

Uncertainty factors for chemical risk assessment: interspecies differences in glucuronidation

For the risk assessment of effects other than cancer, a safe daily intake in humans is generally derived from a surrogate threshold dose (e.g. NOAEL) in an animal species to which an uncertainty factor of 100 is usually applied. This 100-fold is to allow for possible interspecies (10-fold) and interindividual (10-fold) differences in response to a toxicant, and incorporates toxicodynamic and toxicokinetic aspects of variability. The current study determined the magnitude of the interspecies differences in the internal dose of compounds for which glucuronidation is the major pathway of metabolism in either humans or in the test species. The results showed that there are major interspecies differences in the nature of the biological processes which influence the internal dose, including the route of metabolism, the extent of presystemic metabolism and enterohepatic recirculation. The work presented does not support the refinement of the interspecies toxicokinetic default to species- and pathway-specific values, but demonstrates the necessity for risk assessments to be carried out using quantitative chemical-specific data which define the fundamental processes which will influence the internal dose of a chemical (toxicokinetics), or the interaction of toxicant with its target site (toxicodynamics).

Valproic acid and ethosuximide slow the onset of maximal dentate activation in the rat hippocampus

Valproic acid and ethosuximide are commonly used to treat absence seizures, but their mechanisms of action have not been clearly defined. This is partly due to testing in different experimental models and different species. We re-examined the effect of valproic acid and ethosuximide in an experimental model of reverberatory seizures in the hippocampus of the urethane-anesthetized rat, maximal dentate activation. Valproic acid, at 450 and 600 mg/kg i.p., caused an increase in the time to onset of seizure discharges, but only the changes with 600 mg/kg reached statistical significance. Both doses of valproic acid shortened the seizure duration. Serum levels of valproic acid were determined and it appears that in this system at least 200-400 micrograms/ml are needed to produce an effect on seizure onset and duration. Ethosuximide, at 600 mg/kg i.p., delayed the onset of the seizure discharge and blocked the increase in seizure duration. Both ethosuximide and valproic acid delayed the onset of the seizure discharge in this model suggesting that these drugs may help to determine the mechanisms behind seizure initiation in the hippocampal-parahippocampal circuits.

Pharmacokinetics and pharmacodynamics of valproate analogues in rats. IV. Anticonvulsant action and neurotoxicity of octanoic acid, cyclohexanecarboxylic acid, and 1-methyl-1-cyclohexanecarboxylic acid

We examined the pharmacodynamics of valproate (VPA) and three structural analogues, octanoic acid (OA), cyclohexanecarboxylic acid (CCA), and 1-methyl-1-cyclohexanecarboxylic acid (MCCA) in rats. A pentylenetetrazol (PTZ) infusion seizure model was used to determine threshold convulsive doses of PTZ; the increase in PTZ threshold dose after administration of test compound was taken as an index of anticonvulsant activity. Each of the compounds investigated antagonized PTZ-induced seizures, with MCCA evidencing the highest potency. Both CCA and MCCA appeared to have an approximate twofold advantage relative to VPA in protective index (i.e., the ratio of concentrations that produce toxicity to concentrations that produce anticonvulsant effect), based on a rotorod assay of neurotoxicity. Examination of the time course of PTZ antagonism indicated that there was significant dissociation between pharmacokinetics and pharmacodynamics of VPA, with a marked delay in production of maximal anticonvulsant activity. In contrast, only a slight delay in production of maximal protection against PTZ-induced seizures was observed for MCCA, and no delay was evident for CCA. The data indicate that the dynamics of anticonvulsant action differ between these low-molecular-weight carboxylic acids despite their similar chemical structures.

Pharmacokinetics and pharmacodynamics of valproate analogs in rats. II. Pharmacokinetics of octanoic acid, cyclohexanecarboxylic acid, and 1-methyl-1-cyclohexanecarboxylic acid

The pharmacokinetics of valproic acid (VPA) and three structural analogs, octanoic acid (OA), cyclohexanecarboxylic acid (CCA), and 1-methyl-1-cyclohexanecarboxylic acid (MCCA), were examined in female Sprague-Dawley rats. All four carboxylic acids evidenced dose-dependent disposition. A dose-related decrease in total body clearance was observed for each test compound, suggesting the presence of saturable elimination processes. Furthermore, the apparent volume of distribution for these compounds was, with the exception of CCA, dose-dependent, indicating that binding to proteins in serum and/or tissues may be saturable. Both VPA and MCCA exhibited enterohepatic recirculation, although the degree of recirculation appeared to be dose- and compound-dependent. Significant quantities of both VPA and MCCA were excreted in the urine as base-labile conjugates, presumably representing glucuronides. In contrast, OA and CCA were not excreted in the urine as base-labile conjugates and did not evidence enterohepatic recirculation. CCA displayed apparent Michaelis-Menten kinetics, although the calculated Km was dose-dependent. The results suggest that relatively minor changes in chemical structure have a marked influence on the metabolism and disposition of low molecular weight carboxylic acids.

Effects of valproate in a model nervous system (buccal ganglia of Helix pomatia): I. Antiepileptic actions

Cellular actions of valproate (VPA) were studied using intracellular recordings of identified neuronal individuals in the buccal ganglia of Helix pomatia. Under nonepileptic conditions, VPA induced (a) a hyperpolarization, (b) slight changes in action potentials (AP), and (c) an increase in membrane resistance. Under epileptic conditions (i.e., during application of an epileptogenic drug), extracellular application of VPA decreased frequency of occurrence of epileptic depolarizations (early effect) and led to a decay in paroxysmal depolarizations (late effect). Intracellular injection of VPA could block epileptic activity in the treated neuron immediately. A metabolite of VPA (trans-2-en VPA) mainly lacked the late effect (decay in epileptic depolarizations) obtained with VPA. Results suggest that the early antiepileptic effect is exerted from the extracellular side of the neuronal membrane and that the late effect results from intracellular actions of VPA being delayed by slow access to an intracellular site.