Acadesine (AICA riboside) inhibits platelet aggregation in human whole blood

Inhibitors of platelet signal transduction as anti-aggregatory drugs

In the treatment and prevention of cardiovascular diseases, inhibition of platelet aggregation is of fundamental importance. Inhibition of platelet aggregation can be achieved by either inhibition of membrane receptors or by interception of signalling pathways. While receptor antagonism provides high specificity, the inhibition of platelet signal transduction is more effective. The effectiveness results from the inhibition of platelets, regardless of the cause of activation. These common pathway inhibitors are either intercepting platelet activating mechanisms or amplifying the action of endogenous platelet inhibitors. The physiological anti-aggregants are the endothelial factors NO and prostacyclin, which elevate intracellular cGMP or cAMP content, respectively. By administration of NO-releasing agents, prostacyclin analogues or other cyclic nucleotide elevating drugs the platelet anti-aggregatory action of endothelial factors can be effectively mimicked. Besides antiplatelet activity these drugs also act on vascular smooth muscle causing relaxation and therefore vasodilation, an additional beneficial effect. Inhibition of phosphodiesterases causes elevation of platelet cyclic nucleotide content and thus inhibits platelet aggregation and causes vasodilation. Another relevant target for anti-aggregatory treatment is the arachidonic acid metabolic pathway. This pathway can be intercepted by blockade of either cyclooxygenase-1 (COX-1) or thromboxane synthase. Inhibition of these enzymes may be further amplified by additional antagonism of the thromboxane receptor thus not only preventing formation of thromboxane but also activation of thromboxane receptor by thromboxane precursors, which were particularly effective in clinical trials. In vivo these precursors may be metabolised to prostacyclin in the endothelium and consequently provide additional platelet anti-aggregatory activity. A rather new target for platelet anti-aggregatory treatment is the ecto-nucleotidase CD-39 which limits the plasma level of nucleotides. While several of the novel anti-aggregatory drugs were disappointing in clinical studies combinations of drugs with different effector enzymes showed potent antithrombotic efficacy.

Adenosine and AICA-riboside fail to enhance microvascular endothelial preservation

Crystalloid cardioplegia may cause coronary microvascular endothelial dysfunction during cardiopulmonary bypass. The perioperative administration of either adenosine or AICA-riboside (acadesine, 5-aminoimidazole-4-carboxamide-1-ribofuranoside) has been associated with improved myocardial functional preservation and improved coronary blood flow after ischemic arrest. To examine if this enhanced recovery of myocardial function and perfusion may be related to improved endothelial preservation, pigs were placed on cardiopulmonary bypass and the hearts were arrested with plain cold, hyperkalemic (K+ = 25 mmol/L) crystalloid cardioplegia, or cardioplegic solution containing either 0.1 mmol/L adenosine or 50 mumol/L AICA-riboside, which causes the release of endogenous adenosine. AICA-riboside (375 mg) also was infused over 30 minutes before cardioplegia in the later group. After 1 hour of ischemic cardioplegia, hearts were reperfused for 1 hour while the pigs were weaned from cardiopulmonary bypass. Coronary arterioles (90 to 190 microns in diameter) from both subepicardial and subendocardial regions of the left ventricle were studied in vitro in a pressurized, no-flow state with videomicroscopy. After contraction of vessels by 25% to 40% of the baseline diameter, drugs were applied extraluminally. Relaxation of control arterioles to serotonin was slightly greater in vessels from the subendocardial region compared with vessels from the subepicardial region, and subendocardial arterioles were more affected by cardioplegia than were subepicardial vessels. In contrast, relaxations of control microvessels to bradykinin and the calcium ionophore A23187 were similar in the two transmural myocardial regions. Responses to bradykinin and A23187 were significantly and similarly reduced after ischemic arrest with plain crystalloid cardioplegia.(ABSTRACT TRUNCATED AT 250 WORDS)