| Research Abstract |
Mammalian carboxylesterases (CESs, EC3.1.1.1) efficiently catalyze the hydrolysis of a variety of drugs containing ester and amide bonds. CESs are localized in the endoplasmic reticulum and cytosol of a wide variety of organs. A drug containing ester is mainly hydrolyzed in the intestine and the liver after oral administration. However, the substrate specificities for CESs have not been clarified. In the present study, we demonstrated significant species differences of the hydrolytic activity in the intestine and liver. In addition, the CES activity in human intestine and liver microsomes was examined and compared to the human CES isoforms, in order to establish an approach for design of prodrugs. Furthermore, we try to develop a cell culture system to estimate the hydrolysis of a compound containing ester during intestinal absorption.
The ester derivatives of model compounds were stereoselectively converted into parent drugs in the liver and intestine of various species. In vitro hydrolytic activity of human intestine and liver was closer to those of monkey than rat, rabbit and dog. The difference of hydrolytic activity in each tissue of various animals attributes to a diversity of the CES molecule. The hydrolysis activity in human intestine was represented by the HU3 (CES2 family) activity, and that in human liver was represented by the HU1 (CES1 family) activity, in spite of the existence of a multigene family in the liver. The hydrolysis properties for each human CES isoform are important for the design of prodrugs. Furthermore, the feasibility of Caco-2 cell line, the most commonly used cell line for estimation of intestinal absorption, in estimating the hydrolysis of ester drugs during absorption was examined. However, original Caco-2 cell was found to be unsuitable for the estimation of hydrolysis in the intestine, because of the presence of both CES1 and CES2 enzymes. We are now studying the way to control CES enzymes expression in Caco-2 cells.
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