Pharmacological preconditioning of ischemic heart disease by low-dose dipyridamole

Revisiting the Evidence for Dipyridamole in Reducing Restenosis: A Systematic Review and Meta-analysis

ABSTRACT

Atherosclerosis remains a leading cause of morbidity and mortality, with revascularization remaining a cornerstone of management. Conventional revascularization modalities remain challenged by target vessel reocclusion-an event driven by mechanical, thrombotic, and proliferative processes. Despite considerable advancements, restenosis remains the focus of ongoing research. Adjunctive agents, including dipyridamole, offer a multitude of effects that may improve vascular homeostasis. We sought to quantify the potential therapeutic impact of dipyridamole on vascular occlusion. We performed a literature search (EMBASE and MEDLINE) examining studies that encompassed 3 areas: (1) one of the designated medical therapies applied in (2) the setting of a vascular intervention with (3) an outcome including vascular occlusion rates and/or quantification of neointimal proliferation/restenosis. The primary outcome was vascular occlusion rates. The secondary outcome was the degree of restenosis by neointimal quantification. Both human and animal studies were included in this translational analysis. There were 6,839 articles screened, from which 73 studies were included, encompassing 16,146 vessels followed up for a mean of 327.3 days (range 7-3650 days). Preclinical studies demonstrate that dipyridamole results in reduced vascular occlusion rates {24.9% vs. 48.8%, risk ratio 0.53 [95% confidence interval (CI) 0.40-0.70], I2 = 39%, P < 0.00001}, owing to diminished neointimal proliferation [standardized mean differences -1.13 (95% CI -1.74 to -0.53), I2 = 91%, P = 0.0002]. Clinical studies similarly demonstrated reduced occlusion rates with dipyridamole therapy [23.5% vs. 31.0%, risk ratio 0.77 (95% CI 0.67-0.88), I2 = 84%, P < 0.0001]. Dipyridamole may improve post-intervention vascular patency and mitigate restenosis. Dedicated studies are warranted to delineate its role as an adjunctive agent after revascularization.

Performance of Plasma Adenosine as a Biomarker for Predicting Cardiovascular Risk

Adenosine boasts promising preclinical and clinical data supporting a vital role in modulating vascular homeostasis. Its widespread use as a diagnostic and therapeutic agent have been limited by its short half-life and complex biology, though adenosine-modulators have shown promise in improving vascular healing. Moreover, circulating adenosine has shown promise in predicting cardiovascular (CV) events. We sought to delineate whether circulating plasma adenosine levels predict CV events in patients undergoing invasive assessment for coronary artery disease. Patients undergoing invasive angiography had clinical data prospectively recorded in the Cardiovascular and Percutaneous ClInical TriALs (CAPITAL) revascularization registry and blood samples collected in the CAPITAL Biobank from which adenosine levels were quantified. Tertile-based analysis was used to assess prediction of major adverse cardiovascular events (MACE; composite of death, myocardial infarction, unplanned revascularization, and cerebrovascular accident). Secondary analyses included MACE subgroups, clinical subgroups and adenosine levels. There were 1,815 patients undergoing angiography who had blood collected with adenosine quantified in 1,323. Of those quantified, 51.0% were revascularized and 7.3% experienced MACE in 12 months of follow-up. Tertile-based analysis failed to demonstrate any stratification of MACE rates (log rank, P = 0.83), when comparing low-to-middle (hazard ratio (HR) 1.10, 95% confidence interval (CI) 0.68-1.78, P = 0.70) or low-to-high adenosine tertiles (HR 0.95, 95% CI 0.56-1.57, P = 0.84). In adjusted analysis, adenosine similarly failed to predict MACE. Finally, adenosine did not predict outcomes in patients with acute coronary syndrome nor in those revascularized or treated medically. Plasma adenosine levels do not predict subsequent CV outcomes or aid in patient risk stratification.

Adenosine as a Marker and Mediator of Cardiovascular Homeostasis: A Translational Perspective

Adenosine, a purine nucleoside, is produced broadly and implicated in the homeostasis of many cells and tissues. It signals predominantly via 4 purinergic adenosine receptors (ADORs) - ADORA1, ADORA2A, ADORA2B and ADOosine signaling, both through design as specific ADOR agonists and antagonists and as offtarget effects of existing anti-platelet medications. Despite this, adenosine has yet to be firmly established as either a therapeutic or a prognostic tool in clinical medicine to date. Herein, we provide a bench-to-bedside review of adenosine biology, highlighting the key considerations for further translational development of this proRA3 in addition to non-ADOR mediated effects. Through these signaling mechanisms, adenosine exerts effects on numerous cell types crucial to maintaining vascular homeostasis, especially following vascular injury. Both in vitro and in vivo models have provided considerable insights into adenosine signaling and identified targets for therapeutic intervention. Numerous pharmacologic agents have been developed that modulate adenmising molecule.

Current therapeutic strategies to mitigate the eNOS dysfunction in ischaemic stroke

Impairment of endothelial nitric oxide synthase (eNOS) activity is implicated in the pathogenesis of endothelial dysfunction in many diseases including ischaemic stroke. The modulation of eNOS during and/or following ischaemic injury often represents a futile compensatory mechanism due to a significant decrease in nitric oxide (NO) bioavailability coupled with dramatic increases in the levels of reactive oxygen species that further neutralise NO. However, applications of a number of therapeutic agents alone or in combination have been shown to augment eNOS activity under a variety of pathological conditions by potentiating the expression and/or activity of Akt/eNOS/NO pathway components. The list of these therapeutic agents include NO donors, statins, angiotensin-converting enzyme inhibitors, calcium channel blockers, phosphodiesterase-3 inhibitors, aspirin, dipyridamole and ellagic acid. While most of these compounds exhibit anti-platelet properties and are able to up-regulate eNOS expression in endothelial cells and platelets, others suppress eNOS uncoupling and tetrahydrobiopterin (an eNOS stabiliser) oxidation. As the number of therapeutic molecules that modulate the expression and activity of eNOS increases, further detailed research is required to reveal their mode of action in preventing and/or reversing the endothelial dysfunction.

Enhanced cardioprotection against ischemia-reperfusion injury with a dipyridamole and low-dose atorvastatin combination

Atorvastatin (ATV) limits infarct size (IS) by activating Akt and ecto-5-nucleotidase, which generates adenosine. Activated Akt and adenosine activate endothelial nitric oxide synthase (eNOS). When given orally, high doses (10 mg/kg) are needed to achieve full protection. We determined whether dipyridamole (DIP), by preventing the reuptake of adenosine, has a synergistic effect with ATV in reducing myocardial IS. In this study, rats received 3-days of the following: water, ATV (2 mg·kg−1·day−1), DIP (6 mg·kg−1·day−1), or ATV + DIP. In addition, rats received 3-days of the following: aminophylline (Ami; 10 mg·kg−1·day−1) or Ami + ATV + DIP. Rats underwent 30 min of myocardial ischemia followed by 4 h of reperfusion (IS protocol), or hearts were explanted for immunoblotting. As a result, IS in the controls was 34.0 ± 2.8% of the area at risk. ATV (33.1 ± 2.1%) and DIP (30.5 ± 1.5%) did not affect IS, whereas ATV + DIP reduced IS (12.2 ± 0.5%; P < 0.001 vs. each of the other groups). There was no difference in IS between the Ami alone (48.1 ± 0.8%) and the Ami + ATV + DIP (45.8 ± 2.9%) group ( P = 0.422), suggesting that Ami completely blocked the protective effect. Myocardial adenosine level in the controls was 30.6 ± 3.6 pg/μl. ATV (51.0 ± 4.9 pg/μl) and DIP (51.5 ± 6.8 pg/μl) caused a small increase in adenosine levels, whereas ATV + DIP caused a greater increase in adenosine levels (66.4 ± 3.1 pg/μl). ATV and DIP alone did not affect myocardial Ser473 phosphorylated-Akt and Ser1177 phosphorylated-eNOS levels, whereas ATV + DIP significantly increased them. In conclusion, low-dose ATV and DIP had synergistic effects in reducing myocardial IS and activation of Akt and eNOS. This combination may have a potential benefit in augmenting the eNOS-mediated pleiotropic effects of statins.

Effects of Long-Term Oral Dipyridamole Treatment on Coronary Microcirculatory Function in Patients With Chronic Stable Angina: A Substudy of the Persantine In Stable Angina (PISA) Study

AIMS

A meta-analysis of 13 randomized placebo-controlled trials demonstrated a benefit for dipyridamole therapy, particularly with longer duration of treatment. Although the mechanism of this effect is not well understood, dipyridamole increases endogenous tissue adenosine, which may have a beneficial effect on myocardial perfusion. Therefore, we measured the effects of dipyridamole on myocardial blood flow (MBF) and coronary flow reserve (CFR) by using positron emission tomography and H2O in patients with coronary artery disease.

METHODS

Forty-four patients with angiographically documented coronary artery disease were double-blind randomized to either oral dipyridamole [200 milligrams (mg) twice daily (bd)] or placebo as add-on to conventional antianginal treatment for 24 weeks. MBF was measured at rest and during dobutamine stress at baseline and study completion for the region subtended by the most severe coronary artery stenosis (Isc) and remote myocardium subtended by arteries with minimal or no disease (Rem). CFR was calculated as MBF-peak/MBF-rest.

RESULTS

Thirty-five patients completed the study. Isc MBF-rest decreased in patients receiving dipyridamole (0.10 mL/minute/g; P = 0.03) and increased in the placebo group (0.16 mL/minute/g; P = 0.01) during the 24-week study. No significant change in MBF-peak was demonstrated in either group. Consequently, Isc-CFR increased significantly in patients receiving dipyridamole (1.65 +/- 0.47 vs 1.83 +/- 0.67; P < 0.05). By contrast, Isc-CFR decreased significantly in those receiving placebo (1.74 +/- 0.44 versus 1.38 +/- 0.46; P < 0.03). No change was seen in Rem-CFR territories.

CONCLUSIONS

At the end of treatment, a reduction in baseline MBF but no significant changes in hyperemic MBF were observed in ischemic myocardial territories, and therefore the significance of the observed improvement in CFR remains unclear.

Extracellular adenosine activates AMP-dependent protein kinase (AMPK)

Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a heterotrimeric complex that senses intracellular energy status and exerts rapid regulation on energy-demanding and -consuming metabolic pathways. Although alterations in the intracellular adenosine nucleotide pool are traditionally assumed to be the consequence of changes in energy metabolism, in this study we have addressed the question of whether extracellular adenosine contributes to AMPK regulation. In the intestinal rat epithelial cell line IEC-6, addition of adenosine rapidly increases AMP intracellular concentrations and upregulates alpha1AMPK, thus promoting phosphorylation of its downstream target acetyl-CoA carboxylase (ACC). The effect of adenosine on AMPK signaling is completely blocked by transducing IEC-6 cells with an adenoviral vector expressing a mutated alpha1 subunit, resulting in a dominant-negative effect on endogenous AMPK activity. These effects are blocked by 5'-iodotubercidine (5'-ITU), an inhibitor of adenosine kinase. Moreover, inhibition of adenosine transport through the concentrative adenosine plasma membrane transporter CNT2 with formycin B results in the blockade of adenosine-mediated AMPK signaling. Extracellular adenosine is equally able to activate AMPK and promote ACC phosphorylation in liver parenchymal cell models in a manner that is also inhibited by 5'-ITU. In summary, this study shows that adenosine, when added at physiological concentrations, activates AMPK and promotes ACC phosphorylation. Adenosine must be transported and phosphorylated to exert its action. Thus, nucleoside transporters might be novel players in the complex regulation of AMPK and energy metabolism.

Blockade of nucleoside transport is required for delivery of intraarterial adenosine into the interstitium: relevance to therapeutic preconditioning in humans

BACKGROUND

Adenosine, a known mediator of preconditioning, has been infused into the coronary circulation to induce therapeutic preconditioning, eg, in preparation for angioplasty. However, results have been disappointing. We tested the hypothesis that endothelial nucleoside transporter acts as a barrier impeding the delivery of intravascular adenosine into the underlying myocardium and that this can be overcome with dipyridamole, a nucleoside transporter blocker.

METHODS AND RESULTS

We infused saline or adenosine (0.125 and 0.5 mg/min) into the brachial artery while monitoring forearm blood flow (FBF) and interstitial adenosine levels with microdialysis probes implanted in the flexor digitorum superficialis of the forearm in 7 healthy volunteers during intravenous administration of saline or dipyridamole (loading dose, 0.142 mg/kg per min for 5 minutes followed by 0.004 mg/kg per min). Adenosine produced near maximal forearm vasodilation, increasing FBF from 4.0+/-0.7 to 10.4+/-1.9 and 13.1+/-1.6 mL/100 mL per min for the low and high doses, respectively, but did not increase muscle dialysate adenosine concentration (from 88+/-21 to 65+/-23 and 85+/-26 nmol/L). Intravenous dipyridamole enhanced resting muscle dialysate adenosine (from 77+/-25 to 147+/-50 nmol/L), adenosine-induced increase in FBF (from 4.1+/-0.8 to 12.6+/-3 and 15.1+/-3 mL/100 mL per min for the low and high dose, respectively), and the delivery of adenosine into the interstitium (to 290+/-80 and 299+/-143 nmol/L for the low and high dose, respectively, P=0.04).

CONCLUSIONS

Intravascular adenosine is likely ineffective in inducing myocardial preconditioning because of poor interstitial delivery. This can be overcome by blocking the nucleoside transporter with dipyridamole.

Anaesthetics and cardiac preconditioning. Part II. Clinical implications

There is compelling evidence that preconditioning occurs in humans. Experimental studies with potential clinical implications as well as clinical studies evaluating ischaemic, pharmacological and anaesthetic cardiac preconditioning in the perioperative setting are reviewed. These studies reveal promising results. However, there are conflicting reports on the efficacy of preconditioning in the diseased and aged myocardium. In addition, many anaesthetics and a significant number of perioperatively administered drugs affect the activity of cardiac sarcolemmal and mitochondrial K(ATP) channels, the end-effectors of cardiac preconditioning, and thereby markedly modulate preconditioning effects in myocardial tissue. Although these modulatory effects on K(ATP) channels have been investigated almost exclusively in laboratory investigations, they may have potential implications in clinical medicine. Important questions regarding the clinical utility and applicability of perioperative cardiac preconditioning remain unresolved and need more experimental work and randomized controlled clinical trials.

Pharmacological modulation, preclinical studies, and new clinical features of myocardial ischemic preconditioning

The term "ischemic preconditioning (PC)" was first applied to canine myocardium subjected to brief episodes of ischemia and reperfusion that tolerated a more prolonged episode of ischemia better than myocardium not previously exposed to ischemia. Protective effect of myocardial ischemic PC was demonstrated in several animal species, resulting in the strongest endogenous form of protection against myocardial injury, jeopardized myocardium, infarct size, and arrhythmias other than early reperfusion. New onset angina before acute myocardial infarction, episodes of myocardial ischemia during coronary angioplasty or bypass surgery, and the "warm-up" phenomenon may represent clinical counterparts of the PC phenomenon in humans. Here, we have attempted to summarize pharmacological modulation, preclinical studies, and new clinical features of ischemic PC. To date, the pathophysiological basis of the "chemical PC" is still not well established, and "putting PC in a bottle" for clinical applications still remains a new pharmacological venture.

Clinical effects of ischemic preconditioning

A variety of experimental studies have confirmed that preconditioning the myocardium by brief periods of ischemia represents a powerful cardioprotective effect resulting in a reduction of infarct size. After 15 years of research in the experimental laboratory, some evidence shows the existence of preconditioning in human patients with coronary artery disease: repeated balloon inflations before coronary angioplasty induce preconditioning-like effects; moreover, some studies demonstrate better clinical outcome in patients with angina before acute myocardial infarction, resembling a preconditioning effect. So far, a few drugs have been identified as potential mediators of preconditioning, e.g., adenosine, adenosine receptor agonists, and adenosine triphosphate-sensitive potassium channel openers. Before coronary angioplasty and heart surgery, these preconditioning mimetics might be used to protect myocardial tissue by means of preconditioning. Further research is required before preconditioning mimetics could be used for therapy in patients with chronic myocardial ischemia. Possible antipreconditioning effects of several drugs, e.g., sulfonylurea drugs have to be considered in the treatment of patients with coronary artery disease.