Biopharmaceutical Characterization of Grapiprant Within the BCS Framework Under Canine-Relevant Conditions: Solubility and Caco-2 Permeability Assessment

Background: The Biopharmaceutics Classification System (BCS) classifies drug substances according to their aqueous solubility and intestinal permeability; however, its application to veterinary drugs should account for species-specific gastrointestinal physiology. Grapiprant is a selective prostaglandin E2 receptor subtype 4 (EP4) antagonist approved as an oral tablet to control pain and inflammation associated with osteoarthritis in dogs, but its properties within the BCS framework remain unclear. Methods: This study evaluated the equilibrium solubility of grapiprant across pH conditions relevant to the canine gastrointestinal tract and calculated the dose number (D0) based on different gastric fluid volumes. A Caco-2 cell monolayer model was used to assess grapiprant intestinal permeability and the effects of time, concentration, pH, and efflux transporter inhibitors on its transepithelial transport. Results: Grapiprant showed relatively small changes in solubility across the tested pH range. D0 varied with pH and gastric fluid volume and approached or fell below 1 at larger fluid volumes. In the Caco-2 model, grapiprant showed generally limited apparent absorptive permeability, with permeability varying with pH. Efflux ratios were greater than 1, and verapamil reduced the efflux ratio, suggesting possible P-glycoprotein (P-gp) involvement. Conclusions: These findings suggest that both solubility and apparent intestinal permeability may limit the oral absorption of grapiprant under certain canine-relevant conditions. Accordingly, this study provides a biopharmaceutical characterization of grapiprant within the BCS framework rather than a definitive canine BCS classification, highlighting the importance of considering canine gastrointestinal conditions when interpreting its solubility and permeability. The results may support future optimization of dosing conditions and oral formulations.