Gastrointestinal absorption of carbenoxolone in the rat determined in vitro and in situ: deviations from the pH-partition hypothesis

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Abstract
The absorption of [14C] carbenoxolone from everted rat ileum in vitro and from rat stomach and ileum in situ has been examined. The rate of its mucosal to serosal transfer in vitro increases as pH increases from 5 to 8 whereas the amount bound to ileum tissue decreases with increased pH; absorption closely parallels the drug's solubility. The uptake of carbenoxolone in situ is bi-exponential and the rate constants for the two processes, have been calculated. Absorption in situ, and biliary excretion, of the drug increases with increasing pH from 5·0 to 7·4. Tissue binding to the ileum in situ is not dependent on pH except below pH 5·0 when extensive tissue accumulation of carbenoxolone occurs because of its low solubility. Tissue binding to the stomach increases markedly with decrease of pH from 7·4 to 6·5 and at pH 6·5 is 80 times greater than binding to the intestine. The rate of absorption from the stomach, at pH 6·5–7·4, was much less than that from the intestine in situ. When allowance is made for the binding of carbenoxolone to the stomach, contrary to the pH-partition hypothesis, correlation is apparent between its absorption and the amount present in the ionized form.