Exploring drug solubility in fasted human intestinal fluid aspirates: Impact of inter-individual variability, sampling site and dilution.

One of the main factors defining intestinal drug absorption is the solubility of the compound in the gastrointestinal environment. This study reports the solubility of a series of 27 commonly used acidic, neutral and basic drugs in human intestinal fluid samples collected from the duodenum or jejunum of healthy volunteers under fasted state conditions. The interindividual variability as well as the impact of factors such as pH, sampling site and bile salts on the solubility in human intestinal fluids was investigated. The solubility measurements were evaluated using a statistical experimental design. Variability in solubility across volunteers and sampling sites was highly compound-specific and appeared to be substantial for weak acids and bases and for lipophilic drugs. Both pH of the samples and the abundance of amphiphilic components were responsible for the variability observed in the solubility values obtained. The results confirm strong interindividual differences in intraluminal solubility, especially for compounds with high lipophilicity and/or compounds with a pKa value within the physiological pH range. It is important to recognize this variability in intestinal drug solubility as it may considerably influence the therapeutic outcome among patients.