Do Indomethacin Co-Crystals Interact with Intestinal Barrier Differently from their Parent Mixtures?

The therapeutic efficacy of a pharmaceutical formulation depends on its bioavailability, i.e. the absorption extent and rate of the active pharmaceutical ingredient (API) into the bloodstream following its administration. The solubility and dissolution properties of co-crystals can allow to increase the bioavailability of poorly water soluble active pharmaceutical ingredients (APIs) [1,2]. A co-crystal can be considered as a crystalline complex of two or more molecules bound together in the crystal lattice through non-covalent interactions, often including hydrogen bonding.It is currently believed that the co-crystallization strategy should not induce changes on the pharmacological profile of the APIs [2], even if it is not yet clear whether a co-crystal would be defined as a physical mixture or as a new chemical entity [1]. In order to clarify these aspects, we chose indomethacin as guest poorly aqueous soluble molecule and compared its properties with those of its co-crystals obtained with 2-hydroxy-4-methyl-pyridine (co-crystal 1), 2-methoxy-5-nitroaniline (co-crystal 2) and saccharine (co-crystal 3). In particular, we performed a systematic comparison among indomethacin, its co-crystals and their parent physical mixtures by evaluating via HPLC analysis the API dissolution profile, its ability to permeate across intestinal cell monolayers (NCM460) and its oral bioavailability in rat. The indomethacin dissolution profile in 200 mM phosphate buffer was not altered by the presence of co-crystallizing agents as physical mixtures, whereas significant increases in solubility and dissolution rate were observed for co-crystals 1 and 3. Furthermore, there was a qualitative concordance between the API dissolution patterns and the relative oral bioavailabilities in rats. The permeation studies were performed by glucose-enriched PBS (phosphate buffer saline 10 mM) as dissolution medium of the indomethacin powder, being the concentration of 200 mM phosphate buffer too high to allow the cell survival. The dissolution profiles in PBS of indomethacin showed some marked differences with respect to the patterns obtained in 200 mM phosphate buffer, attributable to the PBS relatively weak buffering power. Co-crystal 1 induced a drastic decrease of the transepithelial electrical resistance (TEER) value of NCM460 cell monolayers, whereas its parent mixture did not evidence any effect. The saccharin-indomethacin mixture induced a drastic decrease of the TEER value of monolayers, whereas its parent co-crystal 3 did not induce any effects on their integrity, being anyway able to increase the permeation of indomethacin. These opposite effects induced on a biologic system between co-crystals and their related physical mixtures appear unexpected [3], being currently believed that the co-crystallization strategy should influence the dissolution properties of an API, without inducing changes on its pharmacological profile. Taken together, these results demonstrate for the first time different effects induced by co-crystals and their parent physical mixtures on a biologic system, findings that could raise serious concerns about the use of co-crystal strategy to improve API bioavailability without performing appropriate investigations. [1] M.S. Jung, J.S. Kim, M.S. Kim, A. Alhalaweh, W. Cho, S. J. Hwang, S.P. Velaga, J. Pharm. Pharmacol. 2010, 62, 1560−1568. [2] H. G. Brittain, Cryst. Growth Des. 2012, 12, 5823−5832. [3] V. Ferretti, A. Dalpiaz, V. Bertolasi, L. Ferraro , S. Beggiato, F. Spizzo, E. Spisni, B.Pavan, Mol Pharm. 2015, 12, 1501.