The phenomenon and cause of the dose-dependent oral absorption of chlorothiazide in rats: Extrapolation to human data based on the body surface area concept

Abstract

The reported incomplete and dose-dependent absorption of chlorothiazide in humans was demonstrated in six rats after five oral solutions at doses of 0.93, 2.55, 9.23, 25.6, and 70.2 mg/kg. Mean 48-hr urinary recoveries of intact drug were 57.3, 50.4, 36.7, 22.8, and 15.3%, respectively. A similar degree of dose dependency in absorption was found in rat, dog, and human when the doses were related to unit body surface area (BSA) but not on unit body weight, indicating similar interspecies absorptive capacity in terms of unit BSA. This finding may be partly rationalized by marked similarities in the reported solution transit time (2–3 hr) in the small intestine as well as in the calculated gross surface area of the small intestine per unit BSA (0.163 for rat and 0.132 for human). Contrary to the previous postulation of a specific absorption site, the drug was absorbed from different regions of the GI tract with apparent 1-hr absorption rates, studied by the in situclosed-loop method, in the following rank order: jejunum (34.6%)> duodenum (32.7%)> large intestine (20.1%)>ileum (18.0%) > stomach (12.4%). Different from the commonly assumed first-order absorption process, the intestinal loop absorption was concentration-dependent, suggesting a saturable mechanism. For example, the absorption rate at 0.008 mg/mL was higher than that at 0.2mg/mL in Heal loops (61%, p.