Rat brain adenosine deaminase after 2'-deoxycoformycin administration: biochemical properties and evidence for reduced enzyme levels detected by 2'-[3H]deoxycoformycin ligand binding

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 demonstrated that high concentrations (>40 microg/mL) of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), a semitight binding inhibitor of adenosine deaminase (ADA), are effective in completely inhibiting the intestinal metabolism of 6-chloro-2',3'-dideoxypurine (6-Cl-ddP), an ADA activated prodrug of the anti-HIV agent 2', 3'-dideoxyinosine (ddI) designed for improved targeting to the central nervous system. However, the intestinal absorption of EHNA results in complete inhibition of the ADA activity in the mesenteric blood draining the isolated intestinal segment being perfused and may lead to complete inhibition of ADA present in the systemic circulation and other sites, an unacceptable outcome since bioconversion in the target tissue is required for prodrug efficacy. This 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 segments from mesenteric cannulated rats using perfusate containing prodrug and various concentrations of EHNA demonstrated that a 0.1 microg/mL logarithmic mean lumenal concentration of EHNA was effective in increasing the intestinal bioavailability of Cl-ddP to > 90%. Intestinal uptake parameters for EHNA and pharmacokinetic parameters generated in vivo in chronically catheterized rats given intravenous infusions ranging from 12.5 to 310 microg/kg/min were used to demonstrate that <10% of systemic ADA would be inhibited at steady state using the optimal perfusate concentration of EHNA. Thus, in continuous perfusions it is possible to increase the intestinal bioavailability of 6-Cl-ddP to >90% with minimal (<10%) inhibition of systemic ADA. Local inhibition of enzymes may be an effective strategy to increase the oral bioavailability of tissue enzyme-activated prodrugs or other drugs which may also be substrates for intestinal enzymes.

Absorption and intestinal metabolism of purine dideoxynucleosides and an 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 intestinally localized purine salvage pathway enzymes on the ileal availabilities of 2',3'-dideoxyinosine (ddI), a substrate for purine nucleoside phosphorylase (PNP); 2'-fluoro-2',3'-dideoxyinosine (F-ddI), a non-PNP substrate; and 6-chloro-2',3'-dideoxypurine (6-Cl-ddP), an adenosine deaminase (ADA) activated prodrug of ddI. The potential for increasing 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 coefficients across the intestinal epithelium were determined in in situ perfusions in the mesenteric vein cannulated rat ileum based on both drug appearance in blood (Pblood) and disappearance from the lumen (Plumen) and their paracellular and transcellular components were estimated by comparison to the permeabilities of two paracellular markers, mannitol and urea. Values of Pblood for ddI 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-ddI, a PNP resistant analogue of ddI having virtually the same molecular size and lipophilicity as ddI. This indicates that PNP may not play an important role in the low intestinal absorption of ddI. The Pblood for 6-Cl-ddP, (19 +/- 2) x 10(-6) cm/s, was 4.5-fold lower than Plumen, (84 +/- 12) x 10(-6) cm/s, which means that 77 +/- 6% of 6-Cl-ddP was metabolized during its intestinal transport, thus qualitatively accounting for the low oral bioavailability (7%) of 6-Cl-ddP observed in vivo in rats. Extensive intracellular metabolism of 6-Cl-ddP by ADA was confirmed by the high concentrations of ddI found both in the intestinal lumen and blood during 6-Cl-ddP perfusions and by a rate of ddI appearance in blood which was approximately 10-fold higher than ddI controls. Co-perfusion of the potent, hydrophilic ADA inhibitor DCF (Ki = 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 (Ki = 1-20 nM). Hence, EHNA may be used to improve intestinal absorption of 6-Cl-ddP in vivo.

Acadesine and lipopolysaccharide-evoked pulmonary dysfunction after resuscitation from traumatic shock

BACKGROUND

We have reported that the purine precursor acadesine (AICAR) improved the microcirculation, repleted adenosine triphosphate, and attenuated local and lung neutrophil infiltration after intestinal reperfusion and that it quickly improved systemic hemodynamics after resuscitation from hemorrhagic shock. This study evaluated the therapeutic potential of AICAR after fluid resuscitated trauma.

METHODS

Anesthetized (fentanyl) mongrel pigs were subjected to tissue injury plus hemorrhage and randomized to receive resuscitation fluids comprised of shed blood plus either lactated Ringer's solution (LR) or AICAR (1 or 10 mg/kg bolus + 0.5 mg/kg/min x 30 min). Thereafter either LR or AICAR (1 or 10 mg/kg) was administered at 12-hour intervals for 72 hours. In a smaller series (n = 7) a single bolus (0.5 mg/kg) of the adenosine deaminase inhibitor deoxycoformycin was administered at the time of resuscitation. After 72 hours, and endotoxin challenge (0.5 microgram/kg, lipopolysaccharide [LPS]) was administered.

RESULTS

At 1 mg/kg (n = 9), AICAR had no obvious effect versus LR (n = 31). At 10 mg/kg AICAR (n = 11), the fluid required to stabilize hemodynamics after trauma was higher (66 +/- 5 versus 52 +/- 3 ml/kg/hr, p = 0.014), but there were fewer deaths 3 days after trauma versus LR (0 of 11 versus 4 of 31, p = 0.210), fewer deaths within 5 hours after LPS administration (3 of 11 versus 16 of 27, p = 0.074), and a longer survival time after LPS administration (4.5 +/- 0.3 versus 3.9 +/- 0.2 hr, p = 0.054). Deoxycoformycin had similar salutary effects on survival after LPS administration. LPS increased protein permeability of pulmonary capillaries, increased peak inspiratory pressures on constant tidal volume, increased dead space ventilation, and caused progressive arterial desaturation on 0.65 FiO2 (all p < 0.05). This pulmonary dysfunction was associated with a compensatory increase in cardiac output, decrease in systemic vascular resistance, increase in O2 consumption, and rise in plasma cortisol level (all p < 0.05). All these changes were blunted or eliminated with 10 mg/kg AICAR. Hematocrit and systemic pressures were maintained relatively constant after LPS administration with fluid resuscitation, but less was required with AICAR versus LR (40 +/- 8 versus 83 +/- 14 ml/kg/hr, p = 0.023). AICAR caused a concentration-related reduction in CD18 expression on LPS-stimulated neutrophils in vitro, but there was no effect versus LR on circulating leukocyte counts in vivo and no effect of AICAR on LPS-stimulated production of tumor necrosis factor in vitro or in vivo.

CONCLUSIONS

1. AICAR reduced the pulmonary dysfunction associated with posttrauma endotoxemia but had no effect on circulating leukocytes, so its mechanism could be related to an adenosine-mediated improvement in peripheral perfusion or O2 use. 2. AICAR is a generic compound that is safe and apparently efficacious in human beings, so AICAR prophylaxis could be cost-effectively administered to trauma patients.

Failure of 2'-deoxycoformycin to protect against transient forebrain ischemia in rat

We determined whether 2'-deoxycoformycin (DCF), a potent highly specific inhibitor of adenosine deaminase (ADA), protected against transient forebrain ischemic neuronal injury in rat. Anesthetized male Sprague-Dawley rats received i.p. injections of either saline, 0.5 mg/kg or 5 mg/kg DCF 2 h before undergoing a 10-min forebrain ischemic insult induced by bilateral carotid artery occlusion with concomitant hypotension. Rat brain sections taken 7 days post-ischemia showed damage mostly in the CA1 region of the hippocampus. Quantification of neuronal injury showed no significant differences between saline- or DCF-treated rats. These results indicate that, contrary to previous reports, DCF does not protect against the neuronal damage that follows forebrain ischemia in rat.