Gastrointestinal behavior of itraconazole in humans – Part 2: The effect of intraluminal dilution on the performance of a cyclodextrin-based solution
Hydroxypropyl-β-cyclodextrin (HP-β-CD) is known to enable absorption of the lipophilic drug itraconazole. Since the interaction between HP-β-CD and itraconazole is characterized by a non-lineair, AP-type phase-solubility diagram, the present study aimed to investigate the influence of intraluminal dilution (water intake) on the behavior and performance of an orally administered cyclodextrin-based solution of itraconazole. Subsequently, the in vivo behavior was simulated by combining in vitro dilution with permeation assessment. After the administration of a Sporanox® solution to healthy volunteers with or without a glass of water, gastrointestinal and systemic concentrations of itraconazole were simultaneously monitored. Independently of the intake of water, no gastric precipitation of itraconazole was observed. After transfer to the duodenum, precipitation occurred and was more pronounced in the condition with water, resulting in a 7.6-fold reduction in duodenal AUC0-3h compared to the condition without water. Nevertheless, plasma concentration-time profiles did not demonstrate any significant differences in AUC0-8h, Cmax and tmax. Application of freshly aspirated intestinal fluids on Caco-2 cells clearly confirmed that higher intestinal itraconazole concentrations after intake of Sporanox® without water do not generate a substantially increased itraconazole uptake. A two-stage in vitro dilution test was combined with a permeation compartment to capture this solubility-permeability interplay. In conclusion, this work demonstrates that variations in intraluminal dilution may have a drastic impact on the gastrointestinal behavior of lipophilic drugs in the presence of cyclodextrins. In the case of an AP-type interaction with cyclodextrins, the trade-off between solubility and permeability may be affected.