ABSTRACT:The Ministry of Health and Welfare, Japan banned coadministration of carbapenems, such as panipenem/betamipron (PAPM), meropenem (MEPM), and valproic acid (VPA) because clinical reports have indicated that the coadministration caused seizures in epileptic patients due to lowered plasma levels of VPA. In this study, we have clarified the mechanism of the drug-drug interaction using PAPM In vitro experiments using monkey liver slices suggested that the apparent synthetic rate of VPA glucuronide (VPA-G) increased in the presence of carbapenems. However, no such increase was observed in the experiment using monkey liver microsomes. Although no increase of uridine 5-diphosphate Dglucuronic acid was found in monkey liver slices in the presence of carbapenems, potent inhibitory activity of carbapenems for the hydrolysis of VPA-G was found in monkey and rat liver homogenate. In vivo hydrolysis of VPA-G was clearly shown by the existence of VPA in plasma after dosing of VPA-G to rats, and its inhibition by carbapenems was also clearly shown by the negligible levels of VPA in rat plasma after coadministration of carbapenems and VPA-G. These results clearly indicate one of the important causes of drug interaction as follows: carbapenems would inhibit the hydrolytic enzyme, which is involved in the hydrolysis of VPA-G to VPA, resulting in a decrease of plasma concentration of VPA.The increased incidence of convulsions or epileptic fits due to interaction between carbapenem antibiotics, such as panipenem/betamipron (PAPM) and meropenem (MEPM), and the antiepileptic valproic acid (VPA) was first reported clinically in 1997 (Nagai et al., 1997). Both PAPM and MEPM reduced the plasma concentration of concomitantly administered VPA, which resulted in an insufficient concentration to prevent epileptic fit. This led to prohibition of the concomitant use of carbapenem antibiotics and VPA being added to the Information on Adverse Reactions to Drugs from the Ministry of Welfare in Japan in 1996. To clarify the mechanism of this drug interaction, many studies have been conducted, but the mechanism is still not clear. (Fig. 1), is a carbapenem antibiotic newly developed by Shionogi Co. Ltd. Because its structure is similar to that of MEPM, there is a possibility of a similar drug interaction; we therefore studied the drug interactions between carbapenems, including S-4661 and VPA, and their mechanism, using in vivo and in vitro methods with monkeys and rats.
Materials and MethodsChemicals. VPA (sodium valproate) was supplied by Sigma-Aldrich (St. Louis, MO). [Carbonyl-14 C]-VPA (55 mCi/mmol, 99% pure by thin-layer chromatography) was supplied by Muromachi Chemical Co. (Tokyo, Japan). Doripenem (S-4661) was synthesized by Shionogi Research Laboratories. PAPM was supplied by Sankyo Co. Ltd. (Tokyo, Japan). MEPM was supplied by Sumitomo Pharmaceutical Co. (Osaka, Japan). All other chemicals were of analytical grade.14 C-VPA-G was extracted with methanol from SEP-PAK C18 in which urine and bile, obtained after intravenous administ...