Drug Solubility in Human Colonic Fluids and Comparison to Small Intestinal and Simulated Fluids

Colonic drug absorption is a prerequisite for a drug's suitability for colon-targeted and extended-release formulations. Since drug solubility is a key factor for uptake in the gastrointestinal tract, reliably estimating solubility in the human colon is essential for determining the feasibility of such formulation approaches. To date, our understanding of colonic drug solubility, how it compares to the proximal small intestine, and how it is linked to luminal fluid composition is limited by the scarcity of reference data available. Therefore, this study aimed to measure and compare the apparent solubility of eight drugs with varying physicochemical properties (apixaban, danoprevir, dexloxiglumide, febuxostat, fenofibrate, rofleponide, ticagrelor and tofacitinib) in pooled aspirates from the proximal human colon and small intestine, along with simulated media and buffers commonly used in solubility assessment. Additionally, the composition of the pooled luminal fluids was characterized. Whereas solubility in colonic and small intestinal fluids was comparable for most drugs, the small intestine's solubilizing capacity clearly exceeded that of the colon for the lipophilic drugs fenofibrate and ticagrelor. Extensive degradation of danoprevir was observed in both luminal fluids. Prediction of small intestinal solubility of the lipophilic compounds fenofibrate and ticagrelor was improved in fasted state simulated intestinal fluid compared to blank buffer, although the solubilizing capacity of the human fluids was only captured partially. Fasted state simulated colonic fluid solely improved the colonic solubility prediction of fenofibrate, while the prediction of ticagrelor remained outside the 2-fold prediction error threshold. Solubilities of all other drugs were predicted reasonably well in blank buffers and simulated media. The results generated in this study may serve as reference data for the validation of improved in vitro and in silico tools for colonic drug solubility prediction.