PT  - JOURNAL ARTICLE
AU  - Tomohiro Nishimura
AU  - Nobuyuki Amano
AU  - Yoshiyuki Kubo
AU  - Midori Ono
AU  - Yukio Kato
AU  - Hisashi Fujita
AU  - Yoshiaki Kimura
AU  - Akira Tsuji
TI  - Asymmetric Intestinal First-Pass Metabolism Causes Minimal Oral Bioavailability of Midazolam in Cynomolgus Monkey
AID  - 10.1124/dmd.106.013037
DP  - 2007 Aug 01
TA  - Drug Metabolism and Disposition
PG  - 1275--1284
VI  - 35
IP  - 8
4099  - http://dmd.aspetjournals.org/content/35/8/1275.short
4100  - http://dmd.aspetjournals.org/content/35/8/1275.full
SO  - Drug Metab Dispos2007 Aug 01; 35
AB  - Oral bioavailability of some drugs is substantially lower in cynomolgus monkeys than in various other species, including humans. In the present study, midazolam was used as a model drug to investigate the reason for the lower bioavailability in these monkeys. The bioavailability of midazolam after oral administration was minimal in monkeys and rats, being only 2.1 and 1.1%, respectively. In monkeys, this low bioavailability could not be explained simply in terms of a hepatic first-pass effect. To examine the roles of intestinal metabolism and transport, we evaluated apical-to-basal and basal-to-apical transport of midazolam, and the formation of metabolites in small intestinal tissues using an Ussing-type chamber. The values of mucosal extraction ratio were estimated to be 0.97, 0.93, and 0.89 during apical-to-basal transport in the upper, middle, and lower small intestine of monkeys, respectively, whereas the corresponding values for rats were close to zero, indicating that extensive metabolism of midazolam occurs, particularly in the upper region of the small intestine in monkeys, but not rats. Interestingly, formation of the metabolites was much greater during transport in the apical-to-basal direction than in the basal-to-apical direction, and this could be well explained by a mathematical model based on the assumption that extensive metabolism is associated with the uptake process of midazolam from the apical cell surface. Thus, we conclude that an asymmetric distribution of metabolic activity in the small intestine, leading to extensive metabolism during uptake from the apical cell surface, accounts for the minimal oral bioavailability of midazolam in cynomolgus monkeys. The American Society for Pharmacology and Experimental Therapeutics