Intestinal fluids dictate the intraluminal environment, and therefore, they substantially affect the absorption of orally taken drugs. The characterization of human intestinal fluids (HIF) and the design of simulated intestinal fluids (SIF) mainly focus on composition, not necessarily taking into account the ultrastructure of HIF. Colloidal structures in HIF and SIF can enhance the solubilizing capacity for lipophilic drugs while decreasing the bioaccessible fraction. As such, colloids present in HIF play a crucial role and require an in-depth characterization. Therefore, the present study pursued a comprehensive characterization of the ultrastructure of fasted and fed state HIF, focusing on (i) intersubject variability in relation to composition and (ii) differences between the ultrastructure of HIF and SIF. Individual as well as pooled HIF were collected from human volunteers near the ligament of Treitz and compositionally characterized previously. A HIF population pool (20 healthy volunteers) for both fasted (FaHIF) and fed state (FeHIF) was compared to current SIF, as well as selected HIF from different individuals. The selected individual HIF represented the full spectrum of compositional characteristics. Three complementary electron microscopy techniques, cryo-TEM (transmission electron microscopy), negative stain TEM, and cryo-SEM (scanning electron microscopy), were employed to provide a comprehensive view of the colloidal structures in HIF and SIF. The use of complementary EM techniques provided a unique insight into the ultrastructure of HIF, including their native conformation. These characterizations showed that FaHIF and FaSSIF (fasted state simulated intestinal fluids) only consist of (mixed)-micelles with minimal intersubject variability. Ultrastructures in FeSSIF (fed state simulated intestinal fluids) and FeSSIF-v2 are not representative of the colloids in FeHIF since SIF lack (multi)-lamellar vesicles and lipid droplets. Furthermore, the images demonstrated significant intersubject variability in the ultrastructure of FeHIF, which may contribute to variable absorption of lipophilic drugs.
Keywords: colloidal structures; cryo-SEM; cryo-TEM; human intestinal fluids; micelles; vesicles.