DRUG ABSORPTION STUDIES OF PRODRUG ESTERS USING THE CACO-2 MODEL - EVALUATION OF ESTER HYDROLYSIS AND TRANSEPITHELIAL TRANSPORT

The design of lipophilic ester prodrugs is a widely used approach to o btain enhanced oral delivery of poorly membrane permeable compounds. T he present study was conducted in order to assess the influence of int estinal metabolism on transepithelial flux of prednisolone prodrugs us ing the Caco-2 system. In addition, distribution of esterase activity along the GI tract was evaluated using homogenates of scraped intestin al mucosa from various parts of small intestine and colon of rat and p ig. Prednisolone acetate (lipophilic prodrug) and prednisolone hemisuc cinate (hydrophilic prodrug) were selected as model compounds for tran sport studies. In transport studies using prednisolone acetate (100 mu M), almost complete ester hydrolysis and an increased transepithelial flux of prednisolone were observed. Virtually no transport nor metabo lism was observed when the hemisuccinate ester was used, illustrating its poor ability to cross membranes. Incubation studies with purified carboxylesterase showed that prednisolone acetate was rapidly degraded (t(1/2) = 2.94 min), while prednisolone hemisuccinate degradation was very low. Studies on site dependency of esterase activity using p-nit rophenyl acetate as a substrate showed an important interspecies diffe rence, rat intestine possessing much higher activity than pig intestin e, and a gradual decrease of esterase activity along the GI tract for the two species tested. Esterase activity in Caco-2 monolayers was twi ce as high as observed in colon of rat and pig, but much lower than ac tivities measured in the small intestine. It can be concluded that the rat may not be a suitable choice for oral bioavailability studies of ester prodrugs; it may also be advantageous to target ester prodrugs o f hydrophilic compounds to the colon, thus preventing significant accu mulation of the parent compound inside the mucosal cells. (C) 1998 Els evier Science B.V. All rights reserved.