ABSORPTION AND INTESTINAL METABOLISM OF PURINE DIDEOXYNUCLEOSIDES ANDAN ADENOSINE DEAMINASE-ACTIVATED PRODRUG OF 2',3'-DIDEOXYINOSINE IN THE MESENTERIC VEIN CANNULATED RAT ILEUM

This study investigates the mechanisms of absorption and the role of i ntestinally localized purine salvage pathway enzymes on the ileal avai labilities of 2',3'-dideoxyinosine (ddl), a substrate for purine nucle oside phosphorylase (PNP); 2'-fluoro-2',3'-dideoxyinosine (F-ddl), a n on-PNP substrate; and 6-chloro-2',3'-dideoxypurine (6-Cl-ddP), an aden osine deaminase (ADA) activated prodrug of ddl. The potential for incr easing the intestinal availability of 6-Cl-ddP through the use of ADA inhibitors, namely, 2'-deoxycoformycin (DCF) and erythro-9-(2-hydroxy- 3-nonyl)adenine (EHNA), is also explored. Drug permeability coefficien ts across the intestinal epithelium were determined in in situ perfusi ons in the mesenteric Vein cannulated rat ileum based on both drug app earance in blood (P-blood) and disappearance from the lumen (P-lumen) and their paracellular and transcellular components were estimated by comparison to the permeabilities of two paracellular markers, mannitol and urea. Values of P-blood for ddl were determined to be (1.1 +/- 0. 3) x 10(-6) cm/s, in close agreement with the value of (1.0 +/- 0.3) x 10(-6) cm/s Obtained for F-ddl, a PNP resistant analogue of ddl havin g virtually the same molecular size and lipophilicity as ddl. This ind icates that PNP may not play an important role in the low intestinal a bsorption of ddl. The P-blood for 6-Cl-ddP, (19 +/- 2) x 10(-6) cm/s, was 4.5-fold lower than P-lumen, (84 +/- 12) x 10(-6) cm/s, which mean s that 77 +/- 6% of 6-Cl-ddP was metabolized during its intestinal tra nsport, thus qualitatively accounting for the low oral bioavailability (7%) of 6-Cl-ddP observed in vivo in rats. Extensive intracellular me tabolism of 6-Cl-ddP by ADA was confirmed by the high concentrations o f ddl found both in the intestinal lumen and blood during 6-Cl-ddP per fusions and by a rate of ddl appearance in blood which was approximate to 10-fold higher than ddl controls. Co-perfusion of the potent, hydr ophilic ADA inhibitor DCF (K-i = 0.001 - 0.05 nM) with 6-Cl-ddP led to only partial inhibition of intestinal ADA, while complete inhibition was obtained using the less potent but more lipophilic inhibitor EHNA (K-i = 1-20 nM). Hence, EHNA may be used to improve intestinal absorpt ion of 6-Cl-ddP in vivo.