Inhibition of platelet function during in vivo infusion of isosorbide mononitrates: relationship between plasma drug concentration and hemodynamic effects

Effect of early treatment with anti-hypertensive drugs on short and long-term mortality in patients with an acute cardiovascular event

BACKGROUND

Acute cardiovascular events represent a therapeutic challenge. Blood pressure lowering drugs are commonly used and recommended in the early phase of these settings. This review analyses randomized controlled trial (RCT) evidence for this approach.

OBJECTIVES

To determine the effect of immediate and short-term administration of anti-hypertensive drugs on all-cause mortality, total non-fatal serious adverse events (SAE) and blood pressure, in patients with an acute cardiovascular event, regardless of blood pressure at the time of enrollment.

SEARCH STRATEGY

MEDLINE, EMBASE, and Cochrane clinical trial register from Jan 1966 to February 2009 were searched. Reference lists of articles were also browsed. In case of missing information from retrieved articles, authors were contacted.

SELECTION CRITERIA

Randomized controlled trials (RCTs) comparing anti-hypertensive drug with placebo or no treatment administered to patients within 24 hours of the onset of an acute cardiovascular event.

DATA COLLECTION AND ANALYSIS

Two reviewers independently extracted data and assessed risk of bias. Fixed effects model with 95% confidence intervals (CI) were used. Sensitivity analyses were also conducted.

MAIN RESULTS

Sixty-five RCTs (N=166,206) were included, evaluating four classes of anti-hypertensive drugs: ACE inhibitors (12 trials), beta-blockers (20), calcium channel blockers (18) and nitrates (18). Acute stroke was studied in 6 trials (all involving CCBs). Acute myocardial infarction was studied in 59 trials. In the latter setting immediate nitrate treatment (within 24 hours) reduced all-cause mortality during the first 2 days (RR 0.81, 95%CI [0.74,0.89], p<0.0001). No further benefit was observed with nitrate therapy beyond this point. ACE inhibitors did not reduce mortality at 2 days (RR 0.91,95%CI [0.82, 1.00]), but did after 10 days (RR 0.93, 95%CI [0.87,0.98] p=0.01). No other blood pressure lowering drug administered as an immediate treatment or short-term treatment produced a statistical significant mortality reduction at 2, 10 or >/=30 days. There was not enough data studying acute stroke, and there were no RCTs evaluating other acute cardiovascular events.

AUTHORS' CONCLUSIONS

Nitrates reduce mortality (4-8 deaths prevented per 1000) at 2 days when administered within 24 hours of symptom onset of an acute myocardial infarction. No mortality benefit was seen when treatment continued beyond 48 hours. Mortality benefit of immediate treatment with ACE inhibitors post MI at 2 days did not reach statistical significance but the effect was significant at 10 days (2-4 deaths prevented per 1000). There is good evidence for lack of a mortality benefit with immediate or short-term treatment with beta-blockers and calcium channel blockers for acute myocardial infarction.

Pharmacological characterization of 2NTX-99 [4-methoxy-N1-(4-trans-nitrooxycyclohexyl)-N3-(3-pyridinylmethyl)-1,3-benzenedicarboxamide], a potential antiatherothrombotic agent with antithromboxane and nitric oxide donor activity in platelet and vascular preparations

Thromboxane (TX) A(2), prostacyclin (PGI(2)), and nitric oxide (NO) regulate platelet function and interaction with the vessel wall. Inhibition of TXA(2), implemented synthesis of PGI(2), and supply of exogenous NO may afford therapeutic benefit. 2NTX-99 [4-methoxy-N(1)-(4-trans-nitrooxycyclohexyl)-N(3)-(3-pyridinylmethyl)-1,3-benzenedicarboxamide], a new chemical entity related to picotamide, showed antithromboxane activity and NO donor properties. 2NTX-99 relaxed rabbit aortic rings precontracted with norepinephrine or U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy-prosta-5Z,13E-dien-1-oic acid; EC(50), 7.9 and 17.1 microM, respectively), an effect abolished by 10 microM 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ). 2NTX-99 inhibited arachidonic acid (AA)-induced washed platelet aggregation (EC(50), 9.8 microM) and TXB(2) formation (-71% at 10 microM), and its potency increased in the presence of aortic rings (EC(50), 1.4 microM). In whole rabbit aorta incubated with homologous platelets, AA caused contraction and TXA(2) formation, reduced by 2NTX-99 (10-40 microM): contraction, -28 and -47%, TXA(2) formation, -37 and -75.4%, respectively, with concomitant increase in PGI(2). 2NTX-99 (20-40 microM) inhibited U46619-induced aggregation in rabbit platelet-rich plasma (PRP) (-74 +/- 6.7 and -96 +/- 2.4%, respectively) and inhibited collagen-induced aggregation in human PRP (-48.2 +/- 10 and -79.2 +/- 6%), whereas ozagrel was ineffective. In human embryonic kidney 293 cells transfected with the TXA(2) receptor isophorm alpha receptor, 2NTX-99 did not compete with the ligand, [(3)H]SQ29,548 ([(3)H][1S-[1alpha,2beta(5Z),3beta,4alpha]]-7-[3-[[2-(phenylamino)-carbonyl]hydrazino]methyl]-7-oxabicyclo[2,2,1]-hept-2-yl]-5-heptanoic acid), or prevent inositol phosphate accumulation. After oral administration (50-250 mg/kg), 2NTX-99 inhibited TXA(2) production in rat clotting blood (-71 and -91%); at 250 mg/kg, an area under the curve, 0 to 16 h, of 149.5 h/microg/ml and a t(1/2) of 6 h were calculated, with a C(max) value of 31.8 +/- 8.2 microg/ml. An excellent correlation between plasma concentrations and TXA(2) inhibition occurs. 2NTX-99 controls platelet function and vessel wall interaction by multifactorial mechanisms and possesses therapeutic potential.

Modulation of prostaglandin biosynthesis by nitric oxide and nitric oxide donors

The biosynthesis and release of nitric oxide (NO) and prostaglandins (PGs) share a number of similarities. Two major forms of nitric-oxide synthase (NOS) and cyclooxygenase (COX) enzymes have been identified to date. Under normal circumstances, the constitutive isoforms of these enzymes (constitutive NOS and COX-1) are found in virtually all organs. Their presence accounts for the regulation of several important physiological effects (e.g. antiplatelet activity, vasodilation, and cytoprotection). On the other hand, in inflammatory setting, the inducible isoforms of these enzymes (inducible NOS and COX-2) are detected in a variety of cells, resulting in the production of large amounts of proinflammatory and cytotoxic NO and PGs. The release of NO and PGs by the inducible isoforms of NOS and COX has been associated with the pathological roles of these mediators in disease states as evidenced by the use of selective inhibitors. An important link between the NOS and COX pathways was made in 1993 by Salvemini and coworkers when they demonstrated that the enhanced release of PGs, which follows inflammatory mechanisms, was nearly entirely driven by NO. Such studies raised the possibility that COX enzymes represent important endogenous "receptor" targets for modulating the multifaceted roles of NO. Since then, numerous papers have been published extending the observation across various cellular systems and animal models of disease. Furthermore, other studies have highlighted the importance of such interaction in physiology as well as in the mechanism of action of drugs such as organic nitrates. More importantly, mechanistic studies of how NO switches on/off the PG/COX pathway have been undertaken and additional pathways through which NO modulates prostaglandin production unraveled. On the other hand, NO donors conjugated with COX inhibitors have recently found new interest in the understanding of NO/COX reciprocal interaction and potential clinical use. The purpose of this article is to cover the advances which have occurred over the years, and in particular, to summarize experimental data that outline how the discovery that NO modulates prostaglandin production has impacted and extended our understanding of these two systems in physiopathological events.

The effect of nitric oxide on platelets when delivered to the cardiopulmonary bypass circuit

Nitric oxide (NO) decreases platelet adhesion to foreign surfaces in the in vitro models of cardiopulmonary bypass (CPB). We hypothesized that NO, delivered into the membrane oxygenator (MO), would exert a platelet-sparing effect after CPB. Forty-seven patients scheduled for coronary artery surgery were randomized to either a NO group, in which NO (100 ppm) was delivered into the MO, or a control group, in which CPB was conducted without NO. Platelet numbers, platelet aggregation response to 2.5-20 microM adenosine diphosphate, and beta-thromboglobulin levels were measured after induction of anesthesia, after 1 h on CPB and 2 h after the end of CPB. Met-hemoglobin levels were measured during CPB. The amount of blood products administered and chest tube drainage were measured in the first postoperative 18 h. NO delivered into the MO for up to 180 min did not increase met-hemoglobin levels above 4%. NO inhibited the platelet aggregation response to 2.5 microM ADP during CPB, otherwise NO had no other detectable effect on the aggregation responses or the levels of beta-thromboglobulin. Platelet numbers were not significantly altered by NO. NO did not alter the use of blood products or chest tube drainage. In conclusion, this study suggests that NO delivered into the MO of the CPB circuit does not significantly alter platelet aggregation and numbers, and does not affect bleeding.

IMPLICATIONS

Nitric oxide affects platelet function. We demonstrated that nitric oxide delivered into the gas inflow of the cardiopulmonary bypass circuit membrane oxygenator does not significantly alter platelet numbers or function.

Exogenous nitric oxide inhibits platelet activation in whole blood

We examined the effect of NO on collagen-induced whole blood aggregation and platelet activation in whole blood by using impedance aggregometry and flow cytometry. For the extracellular generation of NO, we chose sodium nitroprusside dihydrate (SNP), and as intracellular generators of NO, L-arginine and isosorbide dinitrate (ISDN). The latter two significantly inhibited whole blood aggregation, whereas SNP had no such effect. The inhibitory effect of ISDN was diminished by addition of methylene blue (MB) or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl 3-oxide (carboxyl-PTIO), and the inhibitory effect of L-arginine was diminished by addition of N(G)-monomethyl-L-arginine monoacetate (L-NMMA). Although the addition of ISDN increased the cyclic guanosine monophosphate (cGMP) level in whole blood and in the suspension of platelets and white blood cells (PLTs + WBCs), no increase was found in platelet-rich plasma (PRP). The P-selectin expression on the platelet surface in whole blood was reduced by ISDN and L-arginine. These findings suggest that the intracellular generation of NO inhibits whole blood aggregation, and this mechanism may play an important role in its antithrombotic effect in whole blood.

Different effects of calcium antagonists, nitrates, and beta-blockers on platelet function. Possible importance for the treatment of unstable angina

BACKGROUND

The three major classes of antianginal drug all inhibit platelet aggregation at high concentrations in vitro, but detecting clinically relevant effects has proved to be more difficult. We used whole-blood flow cytometry, a sensitive method that allows direct measurement of activation antigens on the surface of individual platelets in whole unfixed blood, to evaluate the effect of representative antianginal drugs on platelet function in vivo in healthy volunteers.

METHODS AND RESULTS

The effects of glyceryl trinitrate (GTN), amlodipine, and atenolol were studied in nine normal volunteers. Fibrinogen binding to activated GP IIb/IIIa and expression of P-selectin, GP Ib, and GP IIb/IIIa on the platelet surface were measured. In addition, fibrinogen binding and P-selectin expression were measured in response to ex vivo stimulation with the agonists ADP and thrombin. The three drugs had very different effects on platelets. GTN inhibited platelet fibrinogen binding and expression of P-selectin at rest and in response to agonist stimulation, whereas amlodipine enhanced P-selectin expression and atenolol increased fibrinogen binding in response to agonists. Atenolol did not block the stimulatory effects of epinephrine on ADP-induced platelet activation. GTN neutralized the proactivatory effects of amlodipine, whereas the effects of atenolol and amlodipine were not additive.

CONCLUSIONS

The three main classes of antianginal medication have different and possible clinically relevant effects on platelet behavior in vivo, nitrates causing inhibition of aggregation (fibrinogen binding) and degranulation (P-selectin expression), calcium antagonists enhancing degranulation, and beta-blockers enhancing aggregation.

Effects of organic nitrate vasodilators on platelet function before and after cardiopulmonary bypass

Various in vitro, ex vivo and in vivo tests have shown that organic nitrates attenuate platelet function. Because organic nitrates are commonly administered to patients undergoing cardiac surgery, the postoperative bleeding tendency observed in these patients might be strengthened by nitrates. Therefore, we compared the acute effects of nitroglycerin (0.5 micrograms kg-1 min-1) and isosorbide dinitrate (0.5 or 2.5 micrograms kg-1 min-1) with those of placebo on platelet function both before and after cardiopulmonary bypass in 40 patients undergoing coronary artery bypass grafting (CABG). Bleeding time, platelet retention on glass beads, i.e. platelet adhesiveness, and thromboelastograph tracings were used as indicators of platelet function. Although nitroglycerin and isosorbide dinitrate induced significant haemodynamic changes, e.g. decreases in arterial and pulmonary arterial pressure, they had no significant effects on the indices of platelet function. We conclude that, when given in haemodynamically effective doses, neither nitroglycerin nor isosorbide dinitrate have any measurable acute effect on platelet function as evaluated with on-site tests in patients undergoing CABG surgery.

Mechanisms of action of nitrates

Glyceryl trinitrate, isosorbide dinitrate, and isosorbide-5-mononitrate are organic nitrate esters commonly used in the treatment of angina pectoris, myocardial infarction, and congestive heart failure. Organic nitrate esters have a direct relaxant effect on vascular smooth muscles, and the dilation of coronary vessels improves oxygen supply to the myocardium. The dilation of peripheral veins, and in higher doses peripheral arteries, reduces preload and afterload, and thereby lowers myocardial oxygen consumption. Inhibition of platelet aggregation is another effect that is probably of therapeutic value. Effects on the central nervous system and the myocardium have been shown but not scrutinized for therapeutic importance. Both the relaxing effect on vascular smooth muscle and the effect on platelets are considered to be due to a stimulation of soluble guanylate cyclase by nitric oxide derived from the organic nitrate ester molecule through metabolization catalyzed by enzymes such as glutathione S-transferase, cytochrome P-450, and possibly esterases. The cyclic GMP produced by the guanylate cyclase acts via cGMP-dependent protein kinase. Ultimately, through various processes, the protein kinase lowers intracellular calcium; an increased uptake to and a decreased release from intracellular stores seem to be particularly important.

Effects of an oral dose of isosorbide dinitrate on platelet function and fibrinolysis in healthy volunteers

1. A randomised double-blind placebo-controlled study was performed to investigate the effects of isosorbide dinitrate (ISDN; 20 mg orally) on various aspects of platelet function and fibrinolysis in vivo in 12 healthy volunteers. 2. Measurements were performed at rest (before and after tablet ingestion) and during platelet activation by adrenaline (0.4 nmol kg-1 min-1; 30 min infusion). 3. At rest, ISDN did not alter plasma concentrations of beta-thromboglobulin (beta TG). EC50 values for ADP induced aggregation in vitro (Born aggregometry) or ex vivo filtragometry readings. Adrenaline markedly increased platelet aggregability in vivo as measured by filtragometry and elevated levels of beta TG in plasma. ISDN treatment did not affect these responses in the group as a whole. 4. Individuals responding to ISDN with more pronounced vasodilatation at rest showed a lesser increase in aggregability during the ensuing adrenaline infusion (r = -0.66, P = 0.02) despite higher adrenaline levels during ISDN. In individuals showing a significant decrease in systolic blood pressure (n = 8) ISDN tended to attenuate the adrenaline induced increase in platelet aggregability (filtragometry; P = 0.08), despite higher plasma adrenaline and noradrenaline levels after ISDN ingestion. 5. Plasma concentrations of ISDN and its active metabolites isosorbide-5-mononitrate and isosorbide-2-mononitrate were not correlated to haemodynamic or platelet variables. 6. Fibrinolytic activity (t-PA antigen and activity, PAI-1 antigen and activity) increased similarly during the adrenaline infusion following ISDN and placebo. 7. It is concluded that ISDN may affect platelet aggregation responses to adrenaline in vivo, but only in individuals showing significant haemodynamic responses to ISDN.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversal of human platelet aggregation by low concentrations of nitroglycerin in vitro in normal subjects

The potential reversal of platelet aggregation in vitro by nitroglycerin in low concentrations was explored using both optical aggregometry and electron microscopy. Venous blood was collected from a cohort of normal volunteers (20 men and 10 women) aged 21 to 65 years. Aggregation in platelet-rich plasma was induced by adenosine diphosphate in concentrations just sufficient to maintain a steady state of aggregation, without a spontaneous disaggregation phase (3.5 to 5 microM). Administration of nitroglycerin after the induction of aggregation caused both inhibition of the primary wave of developing aggregation and marked disaggregation. This combined effect was maximal when nitroglycerin was added at 0.5 minute after the beginning of aggregation. The observed reversal of platelet aggregation by nitroglycerin was concentration-dependent. Significant effects occurred with nitroglycerin concentrations greater than or equal to 10(-8) M. Concentration associated with 50% reversal of aggregation was 1.52 +/- 0.24 (SEM) x 10(-6) M. Electron microscopy revealed that 10(-6) M nitroglycerin induced a significant reduction in both platelet clumping and morphologic changes associated with aggregation. The results of the current study suggest a beneficial antiplatelet effect of nitroglycerin in restoring homeostasis in the face of incipient platelet aggregation. The clinical use of nitroglycerin in patients with acute ischemic syndromes may rest on this action.