Absorption of 2',3'-dideoxyinosine from lower gastrointestinal tract in rats and kinetic evidence of different absorption rates in colon and rectum.

This study explored the rectal route of administration for 2',3'-dideoxyinosine (ddI). Rats were given a rectal infusion of nonradiolabeled ddI (200 mg/kg in 0.7 mL saline) over 35 min along with an intravenous (iv) bolus injection of [8-(3)H]ddI (20 microCi, equivalent to 2.1 micrograms), which was used to calculate the absolute rectal bioavailability of ddI. Maximal plasma concentrations of rectally administered unlabeled ddI were 5.4 +/- 2.2 micrograms/mL and were reached at the end of the infusion. The rectal bioavailability averaged 15.6 +/- 4.4% (n = 9). The second aim of this study was to examine the kinetics of ddI absorption from the colorectal region. Analyses of the absorption rate-time profiles by the Loo-Riegelman and deconvolution methods showed biphasic absorption: a rapid phase during infusion and a slow phase postinfusion. These profiles were inconsistent with a mammillary model with absorption from a single site with one apparent rate constant. The model which gave the best fit for infusion and postinfusion data consisted of two different sites (colon and rectum) with different apparent absorption rate constants. The two sites were connected by a first-order transfer of drug solution from rectum to colon. The apparent absorption rate constant in the rectum was 39-fold higher than that in the colon. In conclusion, these results show absorption of ddI from the colorectal region and suggest the rectal route as an alternative to the oral route. The data further suggest different absorption sites in the colorectal region, with a more rapid absorption in the rectum than in the colon.