Cardiovascular effects of nucleoside analogs

Potential for modulation of platelet function via adenosine receptors during inflammation

Traditionally, platelets are known to play an important role in haemostasis and thrombosis; however, they serve also as important modulators of inflammation and immunity. Platelets secrete adhesion molecules and cytokines, interact with leukocytes and endothelium, and express toll-like receptors involved in a direct interaction with pathogens. Platelets express A2A and A2B subtypes of receptors for adenosine. The activation of these receptors leads to an increase in cAMP concentration in the cytoplasm, thereby resulting in inhibited secretion of pro-inflammatory mediators and reduced cell activation. Therefore, platelet adenosine receptors could be a potential target for inhibiting platelet activation and thus down-regulating inflammation or immunity. The biological effects of adenosine are short-lasting, because the compound is rapidly metabolized; hence, its lability has triggered efforts to synthesize new, longer-lasting adenosine analogues. In this article, we have reviewed the literature regarding the pharmacological potential of adenosine and other agonists of A2A and A2B receptors to affect platelet function during inflammation. LINKED ARTICLES: This article is part of a themed issue on Platelet purinergic receptor and non-thrombotic disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.4/issuetoc.

A new class of pyrimidine nucleosides: inhibitors of hepatitis B and C viruses

Hepatitis B virus (HBV) and hepatitis C virus (HCV) infections are major health threats worldwide leading to liver cirrhosis, liver cancer and mortality. Herein, we report a new category of dideoxy pyrimidine nucleosides possessing a 4'-carboxyl (or carboxymethyl) function (7-9, 13, 16, 17), which are discovered as potential antiviral agents. For the first time, these nucleosides are recognized to be inhibitors of HBV and/or HCV replication. Among 4'-carboxy compounds, 3',4'-didehydrothymidine (16) was most effective against DHBV, HBV and HCV. Modification of the 4'-position in compound 7 from a carboxyl to carboxymethyl group (17) did not affect the anti-HBV activity but greatly increased the anti-HCV activity. Importantly, 17 yielded synergistic antiviral effect when combined with ribavirin without toxicity. The activity exhibited by a single agent towards both hepatitis viruses and no detectable in vitro cytotoxicity make this new class of compounds of interest.

Highlights on the development of A(2A) adenosine receptor agonists and antagonists

Although significant progress has been made in the past few decades demonstrating that adenosine modulates a variety of physiological and pathophysiological processes through the interaction with four subtypes of a family of cell-surface G-protein-coupled receptors, clinical evaluation of some adenosine receptor ligands has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors, low brain penetration (which is important for the targeting of CNS diseases), short half-life of compounds, or a lack of effects, in some cases perhaps due to receptor desensitization or to low receptor density in the targeted tissue. Currently, three A(2A) adenosine receptor agonists have begun phase III studies. Two of them are therapeutically evaluated as pharmacologic stress agents and the third proved to be effective in the treatment of acute spinal cord injury (SCI), while avoiding the adverse effects of steroid agents. On the other hand, the great interest in the field of A(2A) adenosine receptor antagonists is related to their application in neurodegenerative disorders, in particular, Parkinson's disease, and some of them are currently in various stages of evaluation. This review presents an update of medicinal chemistry and molecular recognition of A(2A) adenosine receptor agonists and antagonists, and stresses the strong need for more selective ligands at the A(2A) human subtype.

Characterization of adenosine receptors on rat ileum, ileal longitudinal muscle and muscularis mucosae

Adenosine receptors were studied in isolated rat ileum, ileal longitudinal muscle and muscularis mucosae, using a range of agonists and an antagonist. In the rat ileal longitudinal muscle adenosine receptor agonists relaxed the tissues. N6-cyclopentyladenosine (CPA) was more potent than 5'-N-ethylcarboxamidoadenosine (NECA) or adenosine and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (1 nM) gave a 5-fold parallel shift to the right of the concentration-response curves to both CPA and NECA corresponding to an apparent pA2 value of 9.6 suggesting that the agonists relax via adenosine A1 receptors. In the intact ileum adenosine receptor agonists also relaxed the tissue but NECA and CPA were equipotent. DPCPX (3 nM) however inhibited responses to both CPA and NECA with dose-ratios of 8 and 15.6, corresponding to pA2 values of 9.3 and 9.7, respectively. DPCPX (300 nM) gave a much greater shift to the right of the concentration-response curve to NECA with a dose-ratio of 769, corresponding to an apparent pA2 of 9.4. This suggests that the agonists are acting at adenosine A1 receptors to cause relaxation of the whole tissue. Adenosine receptor agonists contracted rat ileal muscularis mucosae with a potency order indicative of an A adenosine receptor. DPCPX (3-100 nM) antagonized responses to CPA giving a linear Schild plot with a slope close to unity and a pA2 of 8.4 suggesting an action on adenosine A1 receptors.

Effects of several 5'-carboxamide derivatives of adenosine on adenosine receptors of human platelets and rat fat cells

The effects of several 5'-carboxamide derivatives of adenosine on stimulatory (Ra) adenosine receptors of human platelets and inhibitory (Ri) adenosine receptors of rat fat cells have been compared. 5'-N-Cyclopropylcarboxamidoadenosine (CPCA) and 5'-N-ethylcarboxamidoadenosine (NECA) most potently inhibited ADP-induced aggregation of human platelets as shown by IC50-values of 0.24 and 0.34 mumol/l. 5'-N-Methylcarboxamidoadenosine (MECA; IC50 0.81 mumol/l) and 5'-N-carboxamidoadenosine (NCA; IC50 2.1 mumol/l) were less potent, whereas adenosine, 2-chloroadenosine and (-)N6-phenylisopropyladenosine [(-)PIA] exhibit IC50-values of about 1.5 mumol/l. Nearly the same rank order of potency was obtained for stimulation of adenylate cyclase activity of platelet membranes and for inhibition of [3H]NECA binding to human platelets. In order to examine the effects of the carboxamide analogues on Ri adenosine receptors of rat fat cells inhibition of lipolysis and adenylate cyclase were studied. (-)PIA was the most potent inhibitor of lipolysis as shown by an IC50 of 0.5 nmol/l followed by CPCA (IC50 1.1 nmol/l) and NECA (IC50 1.3 nmol/l), whereas MECA (IC50 17.9 nmol/l) and NCA (IC50 20.1 nmol/l) were much less potent than NECA in inhibiting lipolysis. Similar results were obtained for inhibition of adenylate cyclase activity of fat cell membranes and for competition with [3H]PIA binding to fat cell membranes. The relative potencies of the adenosine analogues at both receptor subclasses were calculated from the ratio of the IC50-values for inhibition of platelet aggregation and of lipolysis. (-)PIA showed the highest selectivity for Ri receptors as indicated by a 2,900-fold lower IC50 for the antilipolytic than for the anti-aggregatory effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism and disposition of 2',3'-di-O-nitro-adenosine-5'-(N-ethyl-carboxamide) in dogs

2',3'-Di-O-nitro-[8-3H]-adenosine-5'-(N-ethyl-carboxamide) (20 micrograms/kg) was denitrated completely within 1-3 hr in perorally and intravenously dosed dogs. Extremely rapid disappearance of the unchanged drug in serum was parallelled by the instantaneous appearance of mononitrates with 3'-mononitrate levels exceeding those of 2'-mononitrate three-fold. The mononitrates were eliminated with a half-life of 30-70 min, giving rise to the completely denitrated product, adenosine-5'-(N-ethyl-carboxamide) (NECA). The latter product was not further metabolized and was eliminated with a half-life of about 4 hr. Urinary excretion averaged 50% of the administered dose within 4 days and was represented essentially by the completely denitrated drug. Volatile 3H-label of the drug was found in serum and urine during in vivo experiments. Oral bioavailability of the drug was about 90%. In vitro studies indicated that thiols are involved in denitration and reactions are catalysed by glutathione S-transferases, which were partially purified from dog liver. Nitrate ester cleavage was more easily accomplished at the 2'-position than at the 3'-position of the drug and resulted in the liberation of inorganic nitrite. Comparison of in vitro denitration rates gave the following ranking order; 2',3'-di-O-nitro-NECA greater than isosorbide-2,5-dinitrate greater than 2'-nitro-NECA greater than 3'-nitro-NECA greater than isosorbide-2-mononitrate, while nitrate ester cleavage of isosorbide-5-mononitrate was not detectable.

Adenosine receptors mediating cardiac depression

Several adenosine analogs were evaluated for their effects on rate and contractility in guinea pig isolated atria. Among adenosine agonists, (-)-N-(1-methyl-2-phenylethyl) adenosine (l-phenylisopropyladenosine; l-PIA) and N-cyclohexyladenosine (CHA), decreased rate and force at nanomolar concentrations, whereas 2-chloroadenosine, N,N-dimethyladenosine (N6-dimethyladenosine) and (+)-N-(1-methyl-2-phenylethyl)adenosine (d-phenylisopropyladenosine; d-PIA) were less potent cardiac depressants. The degree and order of potency of these agonists suggest that the cardiac depressant effects of adenosine are mediated via A1-receptors. The cardiac depressant effects of CHA and l-PIA were antagonized by theophylline and 1,3-diethyl-8-phenylxanthine (DPX).

The effects of adenosine-5'-ethylcarboxamide on liver blood flow and hepatic glucose, lactate and pyruvate balances in dogs

The actions of adenosine-5'-ethylcarboxamide (744-96), a long-acting adenosine analogue, on liver, portal and intestinal balances of glucose, lactate and pyruvate and on hepatic and portal blood flow were investigated in 6 chloralose-anaesthetized mongrel dogs. 744-96 led to an increase in portal and hepatic blood flow. Glucose release by the liver and glucose uptake by the non-hepatic splanchnic area (portal balance) were markedly increased by 744-96. Hepatic lactate and pyruvate balances were reversed from uptake to release by the adenosine analogue. The changes in glucose balances compare closely to the actions of glucagon, which is known to be released by 744-96. Apart from these possible glucagon-mediated actions, a direct action of the adenosine analogue must be assume from the changes in lactate metabolism. The results of this study are indicative of a substrate-mobilising action of adenosine in addition to its well-known vasodilatory action.

Subclasses of external adenosine receptors

Cell surface adenosine receptors mediate either stimulation or inhibition of adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1], and the receptors that mediate these different responses can be discriminated with selected adenosine analogs. 5'-N-Ethylcarboxamide-adenosine is a more potent agonist at stimulatory receptors (Ra) than is N6-phenylisopropyladenosine, whereas the reverse potency order is seen with inhibitory receptors (Ri). The potency of adenosine is intermediate between the potencies of these two analogs. The relative potencies of adenosine receptor agonists are maintained in physiological responses in intact cells, such as steroidogenesis and inhibition of lipolysis. As with adrenergic receptors, subclasses of adenosine receptors differ functionally and pharmacologically.

Vasodilator activity of adenosine analogs

Various adenosine analogs were evaluated as smooth muscle vasodilators. The compounds were screened ititially using a dog hindlimb preparation. Most analogs were less potent than adenosine. The three most potent compounds were tested for coronary vasodilator effects and duration of action on the isolated rabbit heart. 5'-Deoxy-5'-chloroadenosine and 5'-deoxy-5'-bromoadenosine were equipotent with adenosine but possessed a longer duration of action 2',3',5'-Trideoxy-3',5'-dichloroadenosine, an analog lacking both the 2'- and 3'-hydroxyl groups, had significant vasodilator activity.