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 obtain enhanced oral delivery of poorly membrane permeable compounds. The present study was conducted in order to assess the influence of intestinal metabolism on transepithelial flux of prednisolone prodrugs using the Caco-2 system. In addition, distribution of esterase activity along the GI tract was evaluated using homogenates of scraped intestinal mucosa from various parts of small intestine and colon of rat and pig. Prednisolone acetate (lipophilic prodrug) and prednisolone hemisuccinate (hydrophilic prodrug) were selected as model compounds for transport 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 metabolism 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-nitrophenyl acetate as a substrate showed an important interspecies difference, rat intestine possessing much higher activity than pig intestine, and a gradual decrease of esterase activity along the GI tract for the two species tested. Esterase activity in Caco-2 monolayers was twice as high as observed in colon of rat and pig, but much lower than activities 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 of hydrophilic compounds to the colon, thus preventing significant accumulation of the parent compound inside the mucosal cells. (C) 1998 Elsevier Science B.V. All rights reserved.