First-pass Disposition of (2)-6-aminocarbovir in Rats. I. Prodrug Activation May Be Limited by Access to Enzyme

Several in vitro and in situ approaches were used to determine the dominant presystemic activation site for (2)-6-aminocarbovir, (2)-carbocyclic 2*,3*-didehydro-2*,3*-dideoxy-6-deoxy-6-aminoguanosine (6AC) conversion in rats. The in vitro disappearance half-lives (mean 6 S.D.) in the cytosolic fractions obtained from homogenates of the intestine, liver, and intestinal contents were 0.4 6 0.1 (n 5 3), 12.2 6 1.1 (n 5 3), and 15.5 (n 5 1) min, respectively. An in situ vascularly perfused intestine-liver (IPIL) study was then carried out (n 5 6) to determine the relative contribution of each presystemic organ to the overall first-pass extraction of 6AC. The 6AC extraction ratios in the intestine and liver in the IPIL were found to be 0.08 6 0.02 and 0.11 6 0.03, respectively. The intestinal extraction ratio was in dramatic contrast to the in vitro results. It was postulated that vascularly delivered 6AC had limited access to the metabolic site in the intestine. A theoretical analysis suggested that the extent of intestinal wall extraction of 6AC would be underestimated by the IPIL and should be determined after oral dosing. To compare intestinal extraction ratio in the IPIL with that after an oral administration, in situ intestinal lumen perfusions (n 5 4) and intraportal infusions (n 5 3) of 6AC were conducted in two groups of rats. The lumenally administered 6AC was extracted to a much greater extent by the intestine as compared with the IPIL, which presents 6AC to the intestine by the vascular route. The extraction ratio was found to be 0.54 6 0.06, which was significantly larger than that obtained in the IPIL. (2)-Carbovir, ((2)-carbocyclic 29,39-didehydro-29,39-dideoxyguanosine, CBV) is a potent and selective anti-HIV compound (Vince et al., 1988) with a low oral bioavailability and limited brain delivery (Huang et al., 1991; Wen et al., 1995). When (2)-6-aminocarbovir ((2)-carbocyclic 29,39-didehydro-29,39-dideoxy-6-deoxy-6-aminoguanosine, 6AC)) was evaluated as a prodrug of CBV in rats, it exhibited superiority to the parent drug in increasing systemic and central nervous system exposure to CBV (Zimmerman et al., 1992; Wen et al., 1995). Much higher CBV femoral blood concentrations were observed after an oral dose of 6AC as compared with i.v. administration of 6AC (Zimmerman et al., 1992), indicating that 6AC was substantially converted to CBV in the first-pass organs after dosing. Both the liver and intestine probably contributed to the first-pass conversion of 6AC to CBV, but their relative contributions to the first-pass activation of 6AC could not be determined in these studies. Because 6AC is most likely metabolized to CBV by adenosine deaminase (ADA) (Vince and Brownell, 1990), the intestine may be the principal organ in the first-pass conversion of 6AC, as would be consistent with the tissue distribution of this enzyme (Ho et al., 1980; Chinsky et al., 1990; Winston et al., 1992). The objectives of the present studies were to investigate the relative contributions of the liver and intestine to the first-pass conversion of 6AC to CBV. Materials and Methods 6AC and CBV were synthesized at the University of Minnesota (Beers et al., 1990; Vince and Hua, 1990) and received as gifts from Dr. Robert Vince. Trichloroacetic acid was purchased from Aldrich Chemical Company (Milwaukee, WI), Dextran T-40 was purchased from Pharmacia (Piscataway, NJ), and glucose was purchased from LyphoMed Inc. (Rosemont, IL; dextrose injection, USP, 50%). All other chemicals were reagent grade or better. In Vitro Incubation of 6AC with Intestine, Liver, and Intestinal