| Intestinal absorption is a complex process that can be further complicated by interactions with enzymes and transporters. Adenosine deaminase (ADA)-activated prodrugs have been designed to target the central nervous system for the treatment of HIV-associated dementia, but their oral absorption is very poor, due to significant metabolism by intestinal ADA. Local inhibition of intestinal ADA by erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) has been shown to effectively increase the absorption of the ADA activated prodrug, 6-chloro-2',3'-dideoxypurine riboside (6-Cl-ddP), but the potential for negative systemic side effects from the inhibitor require that the method be optimized.; The ADA activated prodrug 2'-beta-fluoro-2 ',3'-dideoxyadenosine (F-ddA) is more susceptible to ADA metabolism, and less permeable than 6-Cl-ddP, providing greater challenge for delivery. F-ddA perfusions showed significant metabolism during absorption, with higher metabolism in the jejunum, consistent with the intestinal ADA expression pattern. Oral administration with and without a low EHNA dose showed that local inhibition of intestinal ADA can significantly increase F-ddA delivery with little apparent effect on systemic ADA activity.; Previous studies indicated that carriers do not affect the intestinal dideoxynucleoside transport. However, perfusion of ddI, F-ddI, and F-ddA into the intestinal vasculature revealed significant transport asymmetry. Concentration dependence for ddI and probenecid co-administration with both ddI and F-ddI showed their transport mediated by a carrier on the basolateral membrane. Probenecid had no effect on F-ddA metabolism, or metabolite distribution, and directionality did not affect the extent of metabolism.; During F-ddA perfusions, metabolite back flux accounts for the largest fraction of the absorbed dose regardless of the direction of perfusion, and a slow rise to steady state was seen for metabolite back flux in lumen to blood, but not blood to lumen, perfusions. The traditional model for metabolism during absorption, with metabolism in a well-mixed membrane, could not describe the data generated from F-ddA perfusions. More complicated models involving serosal distribution, multiple sites of metabolism, and segregated blood flow were better able to describe the data, but limitations in software and the type of data available make it difficult to determine the best model to describe the metabolism and transport of F-ddA during absorption. |
