Influence of monocaprin on the permeability of a diacidic drug BTA-243 across Caco-2 cell monolayers and everted gut sacs.

This study explores the potential of the monoglyceride monocaprin as an enhancer of the epithelial permeability of the beta(3)-adrenoceptor agonist BTA-243, as an approach to improving the bioavailability of this drug. The permeabilities of BTA-243 and mannitol (paracellular marker) in Caco-2 cell monolayer and everted gut sac models in aqueous buffer (pH 6.8) in the presence of 1.3 and 2.0 mM monocaprin were compared with control (monocaprin-free) solutions over a period of 1 h. The transepithelial electrical resistance (TEER) of the Caco-2 cell monolayers was measured at regular time intervals throughout the experiment and after a recovery period of 30 h. Toxicological damage to the biological models associated with exposure to monocaprin was assessed by scanning electron microscopy and by the measurement of lactate dehydrogenase (LDH) release from everted gut sacs. The permeability of BTA-243 in epithelial monolayers was enhanced in the presence of 1.3 and 2.0 mM monocaprin. Measurements of TEER and mannitol permeability showed partial recovery of barrier properties after a 30 h period following exposure to 1.3 mM monocaprin. No structural damage was evident in these monolayers. Enhancement of Caco-2 permeability to BTA-243 by 2.0 mM monocaprin was significantly greater than by 1.3 mM but was irreversible; monolayers failed to recover their barrier properties after 30 h and changes in their gross morphology were observed. The mucosal to serosal transfer of BTA-243 in everted gut sac was enhanced but to a lesser extent than in the Caco-2 model. LDH release from everted gut sacs exposed to monocaprin was significantly less than that after exposure to Triton X-100, a nonionic surfactant known to cause membrane disruption.

Physicochemical and biopharmaceutical characterization of BTA-243, a diacidic drug with low oral bioavailability.

This investigation has examined possible causes of the poor bioavailability of the beta(3)-adrenoceptor agonist BTA-243. The aqueous solubility of BTA-243 is pH dependent with a solubility minimum at pH 1.5. However, the dissolution rate of the disodium salt of BTA-243 is similar at both pH 2.0 and 7.4 indicating that dissolution rate is unlikely to be the controlling factor in the absorption of BTA-243. The apparent permeability coefficient of BTA-243 across Caco-2 monolayers at pH 6 was lower than that of mannitol and therefore the epithelial permeability of the molecule in vivo is predicted to be very low and potentially bioavailability limiting. Apparent permeability coefficients were not dependent on BTA-243 concentration over the concentration range 0.5 to 12 mM, indicating that epithelial transport is unlikely to occur via a saturable mechanism. They were of similar magnitude in both directions across the monolayers, indicative of no significant effluxing of BTA-243 by components of the cell membrane. Apparent octanol/water distribution coefficients increased with decrease of pH between 2 and 6; the relatively low values at pH 4 and 6 suggest that the limited intestinal absorption predicted in vivo will occur predominantly via paracellular passive diffusion. Everted gut sac experiments performed at pH 2.0 and 6.8 suggest that at pH 2.0 a significant proportion of the BTA-243 transport occurs via the transcellular route confirming that the ionization state of the BTA-243 molecule influences the route and rate of epithelial permeability.