Effects of dipyridamole, soluflazine and related molecules on adenosine uptake and metabolism by isolated human red blood cells

Identification via a Parallel Hit Progression Strategy of Improved Small Molecule Inhibitors of the Malaria Purine Uptake Transporter that Inhibit Plasmodium falciparum Parasite Proliferation

Emerging resistance to current antimalarial medicines underscores the importance of identifying new drug targets and novel compounds. Malaria parasites are purine auxotrophic and import purines via the Plasmodium falciparum equilibrative nucleoside transporter type 1 (PfENT1). We previously showed that PfENT1 inhibitors block parasite proliferation in culture. Our goal was to identify additional, possibly more optimal chemical starting points for a drug discovery campaign. We performed a high throughput screen (HTS) of GlaxoSmithKline's 1.8 million compound library with a yeast-based assay to identify PfENT1 inhibitors. We used a parallel progression strategy for hit validation and expansion, with an emphasis on chemical properties in addition to potency. In one arm, the most active hits were tested for human cell toxicity; 201 had minimal toxicity. The second arm, hit expansion, used a scaffold-based substructure search with the HTS hits as templates to identify over 2000 compounds; 123 compounds had activity. Of these 324 compounds, 175 compounds inhibited proliferation of P. falciparum parasite strain 3D7 with IC₅₀ values between 0.8 and ∼180 μM. One hundred forty-two compounds inhibited PfENT1 knockout (pfent1Δ) parasite growth, indicating they also hit secondary targets. Thirty-two hits inhibited growth of 3D7 but not pfent1Δ parasites. Thus, PfENT1 inhibition was sufficient to block parasite proliferation. Therefore, PfENT1 may be a viable target for antimalarial drug development. Six compounds with novel chemical scaffolds were extensively characterized in yeast-, parasite-, and human-erythrocyte-based assays. The inhibitors showed similar potencies against drug sensitive and resistant P. falciparum strains. They represent attractive starting points for development of novel antimalarial drugs.

Recent progress and market analysis of anticoagulant drugs

This review describes epidemiology of thromboembolic disease in China and abroad, evaluates trends in the development of anticoagulant drugs, and analyzes the market situation based on large amounts of accumulated data. Specifically, we describe advances in clinical application of anticoagulants and analyze the most commonly used anticoagulants in the market systematically.

Syzygium cumini is more effective in preventing the increase of erythrocytic ADA activity than phenolic compounds under hyperglycemic conditions in vitro

Syzygium cumini (S. cumini) is a plant known for its antidiabetic properties. The aim of this study was to evaluate the effect of Sc aqueous leaf extract (ASc) on adenosine deaminase (ADA) activity in erythrocytes (RBCs) exposed to high glucose concentrations (30 mM) in vitro. We also investigated the effects of the main phenolic compounds found in ASc (gallic acid, rutin, and chlorogenic acid) and the effects of insulin, caffeine, and dipyridamole, which are substances involved in the adenosine metabolism, on ADA activity in vitro. Blood samples were obtained from healthy volunteers and a suspension of RBCs was used for the determination of ADA activity. The results showed that: (1) the effect of ASc on ADA activity was more significant than the combination of phenolic compounds; (2) insulin, caffeine, or dipyridamole prevented high glucose increase of ADA activity at doses as low as 50 μU/mL, 25 μM, and 1 μM, respectively; (3) the inhibitory effect caused by ASc on erythrocyte ADA activity remained practically the same after the combination of the extract with insulin or caffeine; (4) when RBCs were exposed to ASc plus dipyridamole, this chemical attenuated the effect of ASc on ADA activity, suggesting an antagonism or a competition with ASc by the same site of action. Therefore, ASc was more effective in preventing the increase in ADA activity than phenolic compounds, suggesting that ASc may collaborate to improve endothelial dysfunction, antioxidant, anti-inflammatory, and antithrombotic properties of adenosine by affecting its metabolism. The results of this study help to provide evidence of the empirically supported benefits of the use of S. cumini in diabetes.

Platelet activation markers, microparticles and soluble adhesion molecules are elevated in patients with arteriosclerosis obliterans: therapeutic effects by cilostazol and potentiation by dipyridamole

We evaluated the plasma concentrations of platelet activation markers, microparticles and soluble adhesion molecules in patients with arteriosclerosis obliterans (ASO) and compared the beneficial effects of cilostazol alone and combination therapy of cilostazol and dipyridamole in these patients. There was a significant elevation of CD62P, CD63, PAC-1, annexin V, platelet-derived microparticles (PDMPs), sP-selectin, sE-selectin, sICAM-1 and sVCAM-1 in the ASO patients compared with the controls. Platelet aggregation was decreased by 2 weeks of cilostazol monotherapy in the ASO patients. Adding dipyridamole to the cilostazol therapy for 2 weeks further reduced platelet aggregation. While treatment with cilostazol alone reduced levels of CD62P, CD63, PAC-1, annexin V, PDMP, and sP-selectin, the combination therapy reduced these parameters further. While sE-selectin and cell adhesion molecules did not change significantly after 2 weeks of combination therapy, they exhibited a remarkable decrease after 16 weeks of combination treatment. These findings suggest that platelets are activated in ASO patients, and cilostazol is effective to reduce platelet activation. Furthermore, dipyridamole may potentiate the beneficial effect of cilostazol in ASO patients. Combination use of both drugs may help to prevent the onset of cardiovascular complications in patients with ASO by activated platelets and PDMP.

Dipyridamole enhances NO/cGMP-mediated vasodilator-stimulated phosphoprotein phosphorylation and signaling in human platelets: in vitro and in vivo/ex vivo studies

BACKGROUND AND PURPOSE

Dipyridamole and in particular dipyridamole in combination with low-dose aspirin are very effective in preventing recurrent stroke. However, the mechanism(s) underlying this dipyridamole effect have not been elucidated. Since dipyridamole inhibits the cGMP-specific phosphodiesterase type V in vitro, we hypothesized and tested whether therapeutically relevant dipyridamole concentrations enhance NO/cGMP-mediated effects in intact human platelets studied ex vivo.

METHODS

Phosphorylation of vasodilator-stimulated phosphoprotein (VASP), an established marker of NO/cGMP effects in human platelets, was quantified by phosphorylation-specific antibodies and Western blots. Serotonin secretion and thromboxane synthase activity were determined by fluorometric quantification of derivatized serotonin and synthase products, respectively.

RESULTS

Endothelium-derived factors such as NO and prostaglandin I2 are known to elevate both cGMP and cAMP levels with concomitant platelet inhibition and VASP phosphorylation. In our in vitro experiments, therapeutically relevant concentrations (3.5 micromol/L) of dipyridamole amplified only cGMP-mediated VASP phosphorylation due to the NO donor sodium nitroprusside, but not cAMP-mediated effects. Furthermore, thromboxane synthase activity and serotonin secretion, events important for initial platelet activation, were inhibited by sodium nitroprusside, an effect also enhanced by dipyridamole, demonstrating the functional relevance of these observations. Finally, the ex vivo enhancement of NO/cGMP effects was also observed with platelets obtained from healthy volunteers treated with extended-release dipyridamole.

CONCLUSIONS

Under therapeutically relevant conditions, dipyridamole enhances platelet inhibition by amplifying the signaling of the NO donor sodium nitroprusside. These data support the concept that enhancement of endothelium-dependent NO/cGMP-mediated signaling may be an important in vivo component of dipyridamole action.

How to get from antiplatelet to antithrombotic treatment

Despite undisputed clinical benefit of platelet inhibitors in an acute interventional setting, chronic antiplatelet treatment with aspirin and clopidogrel have shown a moderate reduction of approximately 18% in the prevention of myocardial infarction and stroke. More recent data on the effect of combining the two compounds leading to an irreversible blockade of cyclooxygenase as well as the adenosine diphosphate receptor showed increased protection from cardiac events in patients with unstable angina and no significant protection from stroke. This improvement did come with the price of a significant increase in major bleeding, requiring hospitalization or transfusion. Furthermore, the most potent inhibition of platelet aggregation by blocking the binding of fibrinogen to activated platelets did not further improve the clinical outcome. All major clinical investigations of the effect of chronic administration have been recently terminated owing to an overall increased mortality rate for the entire class of drugs of orally active fibrinogen receptor antagonists. This poses the question of whether measuring platelet aggregation in vitro and ex vivo can serve as an adequate surrogate parameter to select antithrombotic therapy, particularly in the chronic setting. When evaluating the importance of other endogenous antithrombotic defense systems, the test for inhibition of platelet aggregation must fail. The importance of the endogenous defense systems is easily demonstrated by in vivo studies of thrombus formation in smaller vessels. The rapid and well-balanced interaction of local prothrombotic as well as antithrombotic factors is key in keeping the process confined. By carefully shifting this balance with antiplatelet therapy in combination with therapy to amplify the endogenous antithrombotic defense systems, a far more pronounced overall antithrombotic therapeutic effect can be demonstrated. By employing more complex antithrombotic tests performed in vivo, this balance can be studied. These studies confirm the importance of the ratio of two independent antithrombotic treatments when given together, which was not reflected in in vitro tests but was found to correlate with clinical outcome data. The attention in selecting antithrombotic treatment is to be shifted from approaches only directed toward the inhibition of platelets to an antithrombotic treatment, which includes amplification of the local therapeutic effects often mediated through the vessel wall.

Adenosine deaminase: functional implications and different classes of inhibitors

Adenosine deaminase (ADA) is an enzyme of the purine metabolism which catalyzes the irreversible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine, respectively. This ubiquitous enzyme has been found in a wide variety of microorganisms, plants, and invertebrates. In addition, it is present in all mammalian cells that play a central role in the differentiation and maturation of the lymphoid system. However, despite a number of studies performed to date, the physiological role played by ADA in the different tissues is not clear. Inherited ADA deficiency causes severe combined immunodeficiency disease (ADA-SCID), in which both B-cell and T-cell development is impaired. ADA-SCID has been the first disorder to be treated by gene therapy, using polyethylene glycol-modified bovine ADA (PEG-ADA). Conversely, there are several diseases in which the level of ADA is above normal. A number of ADA inhibitors have been designed and synthesized, classified as ground-state and transition-state inhibitors. They may be used to mimic the genetic deficiency of the enzyme, in lymphoproliferative disorders or immunosuppressive therapy (i.e., in graft rejection), to potentiate the effect of antileukemic or antiviral nucleosides, and, together with adenosine kinase, to reduce breakdown of adenosine in inflammation, hypertension, and ischemic injury.