Comparative pharmacokinetics and metabolism of levetiracetam, a new anti-epileptic agent, in mouse, rat, rabbit and dog

1. The pharmacokinetics and metabolism of C-14-levetiracetam, a new anti-epileptic agent, in mouse, rat, rabbit and dog after a single oral dose were investigated. Moreover, the in vitro hydrolysis of levetiracetam to its major carboxylic metabolite by rat tissue homogenates was investigated to identify tissues involved in the production of the metabolite. Data are also presented on the induction of the enzyme(s) involved in levetiracetam hydrolysis in the rat. 2. Levetiracetam was rapidly and almost completely absorbed. The unchanged drug accounted for a very high percentage of plasma radioactivity. Levetiracetam did not bind to plasma proteins. Although brain radioactivity concentrations were lower than those of whole blood at early time points, brain-to-blood ratios increased over time. The predominant route of elimination of total C-14 was excretion via urine, accounting for about 81, 93, 87 and 89% of the dose in the mouse, rat, rabbit and dog, respectively. Consequently, levetiracetam was poorly metabolized. It was submitted in vivo to hydrolysis and/or oxidation. Hydrolysis of the amide function of levetiracetam produced the corresponding acid. However, levetiracetam could also be oxidized at positions 3 and 4 of the 2-oxopyrrolidine ring. Finally, the compound and the corresponding acid metabolite could be oxidized at position 5 of the 2-oxopyrrolidine ring and then hydrolysed with the opening of the ring. 3. All the investigated rat tissues (liver, kidney, lung, brain, small intestine mucosa) had the potential to produce the acid metabolite. By contrast, the acid was undetectable following incubation of levetiracetam with buffer alone or heat-denaturated liver fractions. 4. No marked species or sex differences were observed in the absorption, disposition and metabolism of levetiracetam. 5. The hydrolysis of levetiracetam is carried out by an enzymatic process characterized by a broad tissue distribution. In the rat, the enzyme system hydrolysing levetiracetam is not induced by phenobarbital, at least under the experimental conditions used herein, whereas the enzyme system(s) involved in the other metabolic pathways is induced.