OPTIMIZATION OF THE LOCAL INHIBITION OF INTESTINAL ADENOSINE-DEAMINASE (ADA) BY ERYTHRO-9-(2-HYDROXY-3-NONYL)ADENINE - ENHANCED ORAL DELIVERY OF AN ADA-ACTIVATED PRODRUG FOR ANTI-HIV THERAPY

Previous in situ perfusion studies in rat ileal segments have demonstr ated that high concentrations (>40 mu g/mL) of erythro-9-(2-hydroxy-3- nonyl)adenine (EHNA), a semitight binding inhibitor of adenosine deami nase (ADA), are effective in completely inhibiting the intestinal meta bolism of 6-chloro-2',3'-dideoxypurine (6-Cl-ddP), an ADA activated pr odrug of the anti-HIV agent 2',3'-dideoxyinosine (ddl) designed for im proved targeting to the central nervous system. However, the intestina l absorption of EHNA results in complete inhibition of the ADA activit y in the mesenteric blood draining the isolated intestinal segment bei ng perifused and may lead to complete inhibition of ADA present in the systemic circulation and other sites, an unacceptable outcome since b ioconversion in the target tissue is required for prodrug efficacy. Th is study examines the feasibility of locally inhibiting ADA present in the intestinal wall using EHNA to increase the intestinal absorption of 6-Cl-ddP. Transport experiments conducted in isolated ileal segment s from mesenteric cannulated rats using perfusate containing prodrug a nd various concentrations of EHNA demonstrated that a 0.1 mu g/mL loga rithmic mean lumenal concentration of EHNA was effective in increasing the intestinal bioavailability of Cl-ddP to >90%. Intestinal uptake p arameters for EHNA and pharmacokinetic parameters generated in vivo in chronically catheterized rats given intravenous infusions ranging fro m 12.5 to 310 mu g/kg/min were used to demonstrate that <10% of system ic ADA would be inhibited at steady state using the optimal perfusate concentration of EHNA. Thus, in continuous perfusions it is possible t o increase the intestinal bioavailability of 6-Cl-ddP to >90% with min imal (<10%) inhibition of systemic ADA. Local inhibition of enzymes ma y be an effective strategy to increase the oral bioavailability of tis sue enzyme-activated prodrugs or other drugs which may also be substra tes for intestinal enzymes.