Absorption and distribution of naproxen in rats orally treated with naproxen betainate sodium salt monohydrate. Comparison with naproxen.

The S-naproxen betainate sodium salt monohydrate (naproxen-betaNa, CAS 104124-26-7, Aprenin) was synthesized to improve bioavailability and tolerability of naproxen. 24 albino rats were treated with naproxen-betaNa (84 mg/kg) and 24 with S-naproxen (naproxen) (50 mg/kg) by the oral route, the doses being equimolar. The animals were sacrificed and naproxen was assayed in timed plasma samples drawn off over a 24-h period and in tissues excised 1 h after administration. Peak concentrations of naproxen proved to be higher with naproxen-betaNa than with naproxen as such. The area under the curve of naproxen concentrations observed with the two administrations overlapped as did concentrations of the drug in the lungs, myocardium and liver. Naproxen concentrations in the gastric wall after naproxen-betaNa proved to be lower than after administration of naproxen as such, which allowed the authors to assume that naproxen-betaNa has a better gastric tolerability.