Valproic acid (VPA), its unsaturated metabolites and pent-4-enoate (4-PA) were studied for potential hepatotoxicity in rats. 4-PA, 4-en-VPA and 2,4-dien-VPA were potent inducers of microvesicular steatosis in young rats. Microvesicular steatosis induced by the 4-en-VPA was accompanied by ultrastructural changes characterized by myeloid bodies, lipid vacuoles and mitochondrial abnormalities. Myeloid bodies and lipid vacuoles were seen to a lesser extent in 2,4-dien-VPA and 4-PA-treated rats. VPA failed to induce discernible liver lesions in young rats even at near lethal doses of 700 mg per kg per day. The drug did, however, induce hepatic lipid accumulation in mature rats and in young rats dosed concomitantly with phenobarbital. beta-oxidation inhibition and several other biochemical alterations were observed in rats dosed with VPA, its unsaturated metabolites and 4-PA. It was suggested that beta-oxidation inhibition observed in both VPA and en-metabolite-treated rats occurred by different mechanisms. VPA inhibits by a transient sequestering of CoA while the CoA esters of some en-VPA-metabolites, particularly 4-en-VPA, inhibit specific enzyme(s) in the beta-oxidation sequences.

The metabolic routes of valproic acid (VPA) were studied by i.p. administration of the mono-unsaturated and hydroxylated metabolites to rats. Conjugation with glucuronic acid was a major metabolic route for VPA and its metabolites. Conjugation with glycine was a minor route for VPA, but was of more importance with the unsaturated metabolites. The hydroxylated metabolites, which were further oxidized to oxo-derivatives and subsequently to the dicarboxylic acids, were not metabolically dehydrated to form unsaturated metabolites. Multiple metabolic pathways, including dehydrogenation, isomerization, hydration, hydroxylation, reduction and epoxidation were inferred from the metabolites obtained after dosage of the unsaturated metabolites. Six dien-VPA metabolites were detected in VPA-treated rats, four of which are present in patients. It was concluded that 3-en-VPA and 4-en-VPA pathways, originating through dehydrogenation, are distinct from the omega- and omega-1-hydroxylation pathways. Enzyme induction from co-administration of phenobarbital caused enhancement of the minor omega-1-oxidation pathway, yet the largest effect on clearance came from increases in glucuronidation. Mitochondrial processes were unaffected, resulting in decreased contribution to the total clearance.

The role of metabolites in valproic acid (VPA)-associated hepatotoxicity was studied in rats. The most steatogenic mono-unsaturated metabolite, 4-en-VPA, caused the greatest changes in indicators of beta-oxidation inhibition (dicarboxylic aciduria, beta-hydroxybutyrate reduction); however, the biochemical effects were much less pronounced than those reported for hypoglycin. Steatosis in VPA-treated rats occurred only at nearly lethal doses. Phenobarbital induction was confirmed as a predisposing factor; however, it appeared not to greatly enhance production of 4-en-VPA or its recognized metabolites, which collectively comprised only 0.5% of the dose. Elevated oxo-VPA metabolites in serum and 2-propylglutarate in liver were associated with toxicity. Among the newly discovered minor metabolites with possible biologic effects were diols (suggesting epoxide precursors) and a series of dienes and trienes. The rarity of severe human hepatotoxicity indicates that, normally, beta-oxidation inhibition is compensated, and cellular defense mechanisms prevail over reactive metabolites. This requires adequate nutrition; on the other hand, severe glycogen depletion may promote toxicity by compromising glucuronidation, the major clearance route. Other literature comments are also supported: (i) caution is indicated for patients with various unusual congenital disorders (e.g., organic acidurias or other mitochondrial defects), and (ii) monotherapy obviates both the predisposition to toxicity and the requirement of large doses to produce therapeutic levels.

The effects of age and gender on the single and multiple dose pharmacokinetics of zileuton have been examined in a phase I nonblinded study. A total of 27 healthy volunteers were evaluable, 9 in the young group (age range 20 to 40 years; 5 males and 4 females) and 18in the elderly group (range 65 to 81 years; 9 males and 9 females). A single oral dose of zileuton 600mg was given to all volunteers on day I of the study and at 6-hour intervals from days 3 to 7.Analysis of variance showed slight but significant decreases in the mean apparent clearance of total and free drug in the healthy elderly population after a single zileuton dose, but no significant age-related differences after multiple 6-hourly doses. Similarly, zileuton peak and trough plasma concentrations, and values for half-life, volume of distribution and protein binding were not significantly affected by age after either a single dose or multiple administration. Moreover, gender effects on the pharmacokinetics were also absent after correction for bodyweight differences.From the result s of the pre sent study, it is concluded that there is no pharmacokinetic basis for alteration of zileuton dosage schedules in elderly patients.

An ultrafiltration technique or equilibrium dialysis has been used to study the in vitro human plasma protein binding of racemic zileuton, its individual enantiomers, and its pharmacologically inactive metabolite N-dehydroxyzileuton.The plasma protein binding of zileuton and N-dehydroxyzileuton over the concentration range of 0.1 to 100 mg/L averaged 93.1 ± 0.22 and 92.0 ± 0.12%, respectively. However, there appeared to be a stereoselective effect, with the R( +) enantiomer of zileuton demonstrating greater binding to plasma proteins than the S(-) enantiomer (96 vs 88%, respectively) . Zileuton was bound to both human serum albumin (40 gIL) and ell-acid glycoprotein (l g/L), although binding affinity to albumin was approximately 3-fold greater. Displacement interactions of zileuton with warfarin, salicylate, theophylline, naproxen, ibuprofen, prednisone, and terfenadine were minimal. The blood to plasma concentration ratio for zileuton and N-dehydroxyzileuton ranged from 0.65 to 0.68, indicating that these compounds were mainly distributed in the plasma.Thus, zileuton is approximately 93% bound to plasma proteins at expected therapeutic concentrations in vitro, and this figure is largely unaffected by several commonly prescribed agents with which the drug may be coadministered.