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Vol. 286, Issue 2, 611-618, August 1998
Department of Cardiovascular Discovery (NW4), Rhône-Poulenc Rorer, Collegeville, Pennsylvania
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Abstract |
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Abstract
Introduction
Materials & Methods
Results
Discussion
References
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This study examined the cardioprotective effects and pharmacology of the novel adenosine A1/A2 receptor agonist ([1S-[1a,2b,3b,4a(S*)]]-4-[7-[[2-(3-chloro-2-thienyl)-1-methylpropyl]amino]-3H-imidazo[4,5-b] pyridyl-3-yl] cyclopentane carboxamide) (AMP 579), in a model of myocardial infarction. Experiments were performed in pentobarbital-anesthetized pigs in which myocardial infarction was induced by a 40-min occlusion of the left anterior descending coronary artery, followed by 3 hr of reperfusion. This procedure resulted in approximately 20% of the left ventricle being made ischemic in all test groups. In untreated animals, an infarct size equal to 56 ± 5% of the ischemic area was observed. Preconditioning, with two cycles of 5 min of ischemia followed by 10-min reperfusion, resulted in a 70% reduction in infarct size (17 ± 5%) relative to risk area. Administration of AMP 579 30 min before ischemia (3 µg/kg i.v. followed by 0.3 µg/kg/min i.v. through 1 hr of reperfusion) did not change blood pressure, HR or coronary blood flow but resulted in marked cardioprotection: a 98% reduction in infarct size (1 ± 1%) relative to risk area. Moreover, whereas approximately 90% of control pigs suffered ventricular fibrillation during ischemia, no fibrillation was observed in animals treated with AMP 579. Further experiments determined the effects of AMP 579 when administered 30 min after the onset of myocardial ischemia, 10 min before reperfusion. Two doses were studied: a low hemodynamically silent dose (3 µg/kg + 0.3 µg/kg/min through 1 hr of reperfusion) and a 10-fold higher dose that did cause reductions in blood pressure and HR. Both doses of AMP 579 produced a comparable cardioprotective effect, reducing infarct size to approximately 50% of that observed in control animals. The cardioprotective effect of AMP 579 was a consequence of adenosine receptor stimulation, because it was completely inhibited by pretreatment with the specific adenosine receptor antagonist CGS 15943 (1 mg/kg i.v.). However, the selective A1 receptor agonist GR 79236 (3 µg/kg + 0.3 µg/kg/min i.v.) did not reduce infarct size, which suggests that under these experimental conditions, stimulation of adenosine A2 receptors is important for the cardioprotective effect of AMP 579. The adenosine-regulating agent acadesine (5 mg/kg + 0.5 mg/kg/min i.v.) also failed to reduce infarct size. In conclusion, the novel adenosine A1/A2 receptor agonist AMP 579 produces marked cardioprotection whether administered before myocardial ischemia or reperfusion. Cardioprotection is not dependent on changes in afterload or myocardial oxygen demand and is a consequence of adenosine receptor stimulation. The pharmacological profile of AMP 579 in this model is consistent with its potential utility in the treatment of acute myocardial infarction.
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Introduction |
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Abstract
Introduction
Materials & Methods
Results
Discussion
References
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It
is estimated that at least 1.5 million people in the United States
alone suffer an acute myocardial infarction each year. The introduction
of treatment strategies to restore blood flow to the affected regions
of myocardium, either with thrombolytics or angioplasty, has increased
salvage of myocardial tissue and markedly improved short-term mortality
rates for patients who have suffered an infarction. Nevertheless,
subsequent cardiac remodeling and dilation, the development of heart
failure and long-term mortality remain significant problems (Van de
Werf, 1995
) that are closely linked to the size of the initial
infarction (Miller et al., 1995; Chareonthaitawee et
al., 1995). Thus there remains a significant unmet need for
adjunctive cardioprotective drugs that can be used during the acute
phase of an infarction to reduce infarct size and consequently improve
outcome.
In 1986, Murry et al. demonstrated that hearts possess an
endogenous defensive mechanism whereby prior exposure to brief periods of ischemia protects the heart from the injury induced by a subsequent prolonged ischemic insult. Since the first description of this phenomenon, which has been termed "ischemic preconditioning," a
large number of studies have focused on determining the mechanism responsible (Downey, 1992; Parratt, 1994). Early evidence indicated that endogenous adenosine was the mediator of preconditioning in rabbit
(Liu et al., 1991) and dog (Grover et al., 1992;
Auchampach and Gross, 1993) models of myocardial ischemia/reperfusion
injury. These same studies also demonstrated that the adenosine
receptor involved was the A1 subtype. Subsequently, the
cardioprotective effects of A1 receptor activation before
ischemia have been shown in all species evaluated. Thus an agonist with
high affinity for the A1 receptor should be an effective
cardioprotective agent. However, for increased clinical relevance to
acute myocardial infarction, it is important that a drug also exert
cardioprotective effects when administered after the onset of ischemia,
before and during reperfusion. This is a much more rigorous test,
because ischemia-induced tissue damage is already well established at the point where drug is administered. A number of studies have shown
that exogenous adenosine administered at or immediately before
reperfusion can reduce infarct size in animal models of acute
myocardial infarction. The first of these studies was reported by
Olafsson et al. in 1987. Interestingly, the selective
A2 receptor agonist CGS21680 has been reported to be
cardioprotective when administered before reperfusion (Schlack et
al., 1993), and it has been suggested that the cardioprotective
effects of adenosine at reperfusion are mediated predominantly by
activation of the A2 subtype receptor (Zhao et
al., 1994).
Because both adenosine A1 and A2 receptor stimulation have been reported to attenuate myocardial damage caused by ischemia and reperfusion, we postulated that an agent with high affinity for these receptors should be an effective cardioprotective agent. The aim of this study was to examine the cardioprotective profile and pharmacology of AMP 579 (fig. 1), a novel adenosine agonist with high affinity for the A1 (Ki = 5 nM) and A2a (Ki = 56 nM) receptor subtypes (unpublished observations, manuscript submitted). To give the results more relevance to the pharmacological treatment of acute myocardial infarction, we assessed the cardioprotective properties of AMP 579 both when the compound was administered as a prophylactic treatment before ischemia and when it was administered immediately before reperfusion.
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Materials and Methods |
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Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Surgical preparation and instrumentation. Yucatan mini-pigs (16-41 kg) of both sexes were sedated with 4 to 5 mg/kg of telazol (tiletamine HCl and zolazepam HCl, Fort Dodge Laboratories, Inc., Fort Dodge, Iowa) i.m. and anesthetized with 25 mg/kg of sodium pentobarbital (Anpro Pharmaceutical, Arcadia, CA) injected into the dorsal ear vein. An endotracheal tube was inserted via a tracheotomy, and each animal was connected to a mechanical respirator (Harvard Apparatus) and artificially ventilated (10-12 strokes/min, 10-12 ml/kg tidal volume, room air supplemented with 100% O2). The left external jugular vein was cannulated with a polyethylene catheter (PE-205, Becton Dickinson, Parsippany, NJ) for continuous administration of pentobarbital (0.12-0.15 mg/kg/min). After a right femoral cut-down, polyethylene catheters were also introduced into the femoral artery and vein for measurement of aortic BP and infusion of fluids (0.9% saline, 5 ml/hr) and test compounds, respectively. A pig-tailed microtip catheter (Millar Instruments, Houston, TX) was inserted through the right internal carotid artery, and the tip was placed in the left ventricle for measurement of LVP and dP/dt. Arterial blood gas samples were measured using a Corning model 168 pH Blood Gas System (Ciba-Corning, Medfield, MA), and respirator settings were adjusted accordingly.
The heart was exposed by means of a mid-line sternotomy and suspended in a pericardial cradle. A section of the LAD was exposed just distal to its first diagonal branch and instrumented with an electromagnetic flow probe (Skalar, Delft, The Netherlands, 1.5-2.0 mm in diameter) and an occlusive snare. Needle electrodes were placed in the right foreleg and both legs for measurement of Lead II ECG. Body temperature was rigorously maintained at 37°C by the use of two thermal blankets placed both dorsally and ventrally. All pigs were given a 100 U/kg bolus injection of heparin (SoloPak Laboratories Inc., Elk Grove Village, IL) at the start of the stabilization period. MAP, HR, LVP, dP/dt, CBF and rate pressure product (RPP) were recorded throughout the experiment on an MI2 data processing system (Modular Instruments, Malvern, PA).Control pigs. Control animals (n = 8) were allowed to stabilize after surgery for 60 min. A 40-min LAD occlusion, achieved by tightening the occlusive snare, was then initiated. After occlusion, the snare was released and blood flow was restored to the ischemic area. The animal was monitored for another 3 hr, after which time the pig was given an overdose of pentobarbital. The heart was fibrillated electrically with a 600-mAmp alternating current, removed, rinsed in cold tap water, blotted dry with paper towels and weighed.
Preconditioning and effects of AMP 579 when administered before myocardial ischemia. After surgery and instrumentation, animals were allowed to stabilize for 60 min. The pigs were divided into two groups: preconditioned and AMP 579-pretreated. Preconditioned pigs (n = 6) underwent two cycles of a 5-min LAD occlusion, followed by 10 min of reperfusion (30 min total) before a 40-min ischemic period. AMP 579-pretreated pigs (n = 4) were given a loading dose of 3 µg/kg i.v. over 2 min, followed by an infusion (0.3 µg/kg/min i.v.) starting 30 min before the 40-min LAD occlusion, continuing through the ischemia period and ending 60 min into reperfusion. Hearts were removed after 3 hr of reperfusion as described for the control group.
Effects of AMP 579 when administered before myocardial
reperfusion.
Animals were divided into four groups: high-dose and
low-dose AMP 579, acadesine (AICA riboside,
[5-aminoimidazole-4-carboxamide 1-
-D-ribofuranoside],
Sigma Chemical Co., St. Louis, MO) and GR79236 (synthesized by
Rhône-Poulenc Rorer). The pigs were allowed to stabilize for 60 min after surgery and instrumentation, as were the control and
pretreated pigs. The LAD was then occluded for 40 min. Ten minutes
before reperfusion, pigs receiving high-dose AMP 579 (n = 6) were given a loading bolus of 30 µg/kg i.v. over 2 min, followed
by 3 µg/kg/min i.v. for 68 min (lasting 1 hr into the reperfusion
period). Low-dose AMP 579 (3 µg/kg bolus and 0.3 µg/kg/min i.v.)
was administered to another group of pigs (n = 6) in a
similar manner. The third group (n = 6) received the
adenosine-modulating agent acadesine (Mullane, 1993; 5 mg/kg bolus and
0.5 mg/kg/min i.v.) over the same time frame. The last group of pigs
(n = 6) received the selective A1 receptor
agonist GR79236 (Gurden et al., 1993) at the same
concentration as the low-dose AMP 579 group (3 µg/kg bolus and 0.3 µg/kg/min i.v.). Pigs were allowed to reperfuse for another 2 hr
before their hearts were removed as previously described.
) (RBI, Natick, MA) at a dose of 1 mg/kg i.v. 15 min
before reperfusion. Five of these pigs were then given low-dose AMP 579 (3 µg/kg followed by a 0.3 µg/kg/min infusion) starting 10 min
before reperfusion and continuing for 1 hr into reperfusion.
Determination of area at risk and infarct size. The heart was mounted on a Langendorff perfusion apparatus and prepared for dual perfusion. The LAD was cannulated at the site of the occluder and perfused with a 0.6% solution of triphenyltetrazolium chloride (TTC). This method stains viable myocardium bright red, whereas necrotic tissue appears pale. The heart was perfused simultaneously through the aorta with a 0.5% Evans Blue solution to delineate the noninvolved myocardium from the area at risk. Perfusion was carried out for approximately 3 min at a pressure similar to the animal's MAP at the end of the experiment. The heart was then rinsed under tap water, blotted dry and weighed. The atria and right ventricle were removed, and the left ventricle was weighed and then sliced by hand into six transverse sections approximately 7 to 10 mm thick from apex to the occluder site with an Accu-Edge trimming knife (model 4786, Miles Scientific, Elkhart, IN). Each slice was blotted dry and weighed, as was the portion of the heart proximal to the occluder. The normal zone, area at risk and infarct zones from both sides of the top five slices and the proximal side of the apical slice were traced onto acetate sheets. Planimetric determination of infarct size and area at risk was made using a Numonics Digitizer Tablet (1000 points/inch, Numonics Corp., Montgomeryville, PA) and Sigma-Scan software (Version 3.92; Jandel Scientific, Corte Madera, CA).
Exclusion criteria. A total of 69 pigs underwent the 40-min coronary artery occlusion. Seventeen pigs were excluded from the final statistics (see the accompanying table) for the following reasons. Nine pigs fibrillated and were not able to be converted. Three pigs needed more than six DC conversion attempts (15-25 J) for successful resuscitation. Five pig hearts had areas at risk that were deemed atypically small (<14% of total LV) and consequently were not included.
| Reason for Exclusion | Treatment Group |
| Death after ventricular | 4 animals before drug or vehicle |
| fibrillation | 2 animals preconditioning group |
| 1 animal low-dose AMP 579 group | |
| 2 animals high-dose AMP 579 group | |
| >6 DC conversion attempts | 1 animal control group |
| 1 animal preconditioning group | |
| 1 animal high-dose AMP 579 group | |
| Area at risk judged atypically | 2 animals control group |
| small (<14% LV) | 1 animal low-dose AMP 579 group |
| 2 animals high-dose AMP 579 group |
Statistical analysis. Hemodynamic data were analyzed with a repeated measures ANOVA followed by Newman-Keuls' post-hoc test. Analysis of area at risk and infarct size was also performed with ANOVA followed by Newman-Keuls' post-hoc test. Differences were considered statistically significant if P < 0.05.
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Results |
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Introduction
Materials & Methods
Results
Discussion
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Preconditioning and the effects of AMP 579 when administered before ischemia. Effects on hemodynamics: MAP and HR data are shown in figure 2, where Isch-x is time of ischemia in minutes, and Reperf-x is time of reperfusion in minutes. Administration of AMP 579 (3 µg/kg and 0.3 µg/kg/min i.v.) before ischemia did not cause significant alterations in MAP or HR. Similarly, there were no significant differences in CBF through the LAD in any of the pretreatment groups (fig. 3).
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Effects of AMP 579 when administered before myocardial reperfusion. Effects on hemodynamics: Administration of the low dose (3 µg/kg and 0.3 µg/kg/min i.v.) of AMP 579 led to no significant change in MAP or HR (fig. 6). In contrast, a higher dose of AMP 579 (30 µg/kg and 3.0 µg/kg/min i.v.) administered before reperfusion significantly reduced MAP and HR during drug infusion, but values rapidly returned to normal after the infusion was terminated (fig. 6). Similarly, infusion of the A1 agonist GR79236 (3 µg/kg and 0.3 µg/kg/min i.v.) also resulted in a significant reduction in HR (fig. 6) compared with control pigs, although this agonist did not cause any significant reduction in BP. The adenosine-regulating agent acadesine (5 mg/kg bolus and 0.5 mg/kg/min i.v.) did not change HR or BP (fig. 6). Bolus administration of the adenosine receptor antagonist CGS15943 (1 mg/kg i.v.) tended to cause a short-lasting increase in BP when administered before AMP 579 or its vehicle (fig. 6).
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Discussion |
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Introduction
Materials & Methods
Results
Discussion
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Coronary artery disease is one of the major causes of premature
death in developed nations. Thus a large unmet medical need exists for
drugs that can be used safely during the acute phase of myocardial
infarction to increase the degree of tissue salvage that is achieved
when blood flow is restored to the ischemic myocardium after
thrombolysis or angioplasty. Evidence suggests that drugs that can
reduce infarct size will confer significant benefits in terms of short-
and long-term morbidity and mortality (Chareonthaitawee et
al., 1995; Miller et al., 1995; Schaer et
al., 1996). Given that both adenosine A1 and
A2 receptor agonists have been shown to be cardioprotective
in several species (Downey, 1992; Norton et al., 1992;
Schlack et al., 1993), we postulated that a potent agonist
at both of these receptor subtypes should possess an effective cardioprotective profile. Thus the aim of this study was to examine the
cardioprotective profile and pharmacology of AMP 579, a novel adenosine
agonist with high affinity for the A1
(Ki = 5 nM) and A2a
(Ki = 56 nM) receptor subtypes.
Because pigs and humans are very similar in cardiac anatomy, including
a poor collateral circulation, experiments were carried out in a
porcine model of myocardial infarction. In the first part of this
study, we determined whether AMP 579 could reduce infarct size when
given before myocardial ischemia. In addition, we compared its efficacy
to that of ischemic preconditioning, generally accepted as one of the
most effective strategies for reducing infarct size (Downey, 1992;
Baxter and Yellon, 1994). However, there are a limited number of
clinical situations where cardiologists have the ability to administer
compounds before myocardial ischemia. Thus a major focus of this study
was to determine whether AMP 579 was able to reduce infarct size when
administered after the onset of ischemia, just before reperfusion. In
this way, we sought to ensure that the results would have greater
relevance to the potential of the compound as an adjunctive therapy in
acute myocardial infarction. Similarly, the duration of the ischemic insult was chosen in an effort to parallel (as closely as possible) the
degree of injury that has occurred in humans at the point (generally
between 2 and 5 hr) where they undergo therapy to open an occluded
coronary artery. Recent clinical estimates in patients who have
suffered an acute myocardial infarction and received reperfusion
therapy estimate infarct size to be in the range of 50 to 70% of the
area of left ventricle subjected to ischemia (Miller et al.,
1995; Schaer et al., 1996). For comparison, in our porcine
experimental model, the LAD coronary artery was occluded for 40 min,
which produced an infarct size of 55% of the ischemic area in control
animals.
Administration of AMP 579 before myocardial ischemia. The results demonstrate that when administered 30 min before myocardial ischemia and through the first hour of reperfusion, AMP 579 produced marked cardioprotection, reducing infarct size by 98% and completely abolishing the incidence of ischemia-induced VF. The dose of AMP 579 employed did not evoke any change in BP or HR, which indicates that the protective effects are not a consequence of decreased afterload or oxygen demand. In addition, this dose of AMP 579 did not significantly change CBF.
Preconditioning (two cycles of 5 min of ischemia followed by 10 min of reperfusion) also resulted in a marked cardioprotective effect, inducing a 70% reduction in infarct size. These data compare well with previous reports (Schott et al., 1990; Van Winkle et al., 1994; Ovize et al., 1995) demonstrating an infarct
size reduction in the range of 78 to 91% after ischemic
preconditioning in anesthetized swine. However, as suggested by Kloner
(1995), it seems that, in contrast to the anti-infarct effect of
preconditioning, the effect on arrhythmias is highly dependent on the
species and experimental conditions used. In agreement with Ovize
et al. (1995), we did not see a significant protective
effect of preconditioning against ischemia-induced fibrillation in the
pig; however, in contrast to these workers, we did not find evidence to
suggest that preconditioning accelerates the onset of fibrillation.
This difference may simply be due to slightly different experimental
conditions, including the use of a different anesthetic and
preconditioning protocol in the two studies.
The precise mechanism of the anti-infarct effect of preconditioning
remains unknown. However, in experiments using rabbits, dogs and pigs
(Liu et al., 1991; Auchampach and Gross, 1993; Schulz et al., 1995), there is evidence to suggest that the
mechanism involves the generation of endogenous adenosine in the
ischemic tissue and that subsequent stimulation of adenosine
A1 receptors before ischemia renders the myocardium more
resistant to ischemia. Moreover, there is evidence in all species so
far examined that adenosine A1 receptor agonists can mimic
the anti-infarct effect of preconditioning (Liu et al.,
1991; Auchampach and Gross, 1993; Van Winkle et al., 1994;
Lee et al., 1995). Thus we were not surprised that the
high-affinity adenosine A1/A2 receptor agonist
AMP 579 reduced infarct size when administered before ischemia in this model. It is worth noting that stimulation of adenosine A3
receptors is unlikely to contribute to the ability of AMP 579 to reduce infarct size, because this compound possesses low affinity
(Ki ~ 1 µM; unpublished observations,
manuscript submitted) for this receptor subtype. In addition to infarct
size reduction, AMP 579 demonstrated a marked anti-arrhythmic effect, a
result consistent with previous reports indicating that adenosine
A1 receptor stimulation in vivo attenuates
ischemia-induced VF (Boachie-Ansah et al., 1993; Wainwright
and Parratt, 1993). However, it is interesting that AMP 579 appeared to
produce a more marked cardioprotective effect than preconditioning in
terms of both infarct size reduction and anti-arrhythmic effects. A
possible explanation for the latter observation is that the
preconditioning protocol used in the current study was not optimal for
infarct size reduction. Alternatively, the constant exposure of the
myocardium to a stable (t1/2 ~ 30-60 min after
i.v. administration to pigs; unpublished observations) exogenous
adenosine agonist during both the period of ischemia and the
reperfusion period may simply provide a more powerful cardioprotective
stimulus. In any event, these data suggest potential clinical utility
for AMP 579 in situations where therapeutic agents can be given before
an ischemic insult
e.g., before coronary artery bypass
grafting.
Administration of AMP 579 before reperfusion. AMP 579 was still able to reduce infarct size significantly when administered 30 min after the onset of myocardial ischemia, just before reperfusion. Post-mortem visual examination of the tetrazolium-stained left ventricular slices suggests that AMP 579-induced salvage of myocardium was generalized with viable tissue present in endo-, mid- and epicardial regions, interspersed with variably sized islands of necrosis. In contrast, infarcts in vehicle-treated left ventricles were large, solid and generally transmural.
The low dose of AMP 579 caused a 53% reduction in infarct size, and this effect was observed in the absence of any change in HR or BP. This suggests that (as when the compound was given before ischemia) the cardioprotective effect of AMP 579 was not dependent on changes in afterload or myocardial oxygen demand. Moreover, the ability of AMP 579 to reduce infarct size was completely dependent on adenosine receptor stimulation, because this effect was blocked in animals pretreated with the adenosine A1/A2 receptor antagonist CGS15943 (Williams et al., 1987). Although previous reports
in the literature (see review by Forman et al., 1993) have
indicated that adenosine itself can reduce infarct size when administered before reperfusion, there have been some conflicting findings (Goto et al., 1991; Vander Heide & Reimer, 1996),
and concerns have been raised about whether lidocaine pretreatment contributed to any apparent cardioprotective effect of adenosine (Homeister et al., 1990). However, the present observations,
made in a collateral deficient species in the absence of
lidocaine or any other antiarrhythmic, demonstrate that
adenosine receptor stimulation before reperfusion can result in reduced
infarct size. Therefore, these observations tend to support the view
that lethal reperfusion injury (Hearse and Bolli, 1991; Kloner, 1993)
does occur and can be pharmacologically modulated.
A 10-fold higher dose of AMP 579 was also cardioprotective but did not
reduce infarct size to a greater extent than the lower dose. The higher
dose of AMP 579 also caused a significant reduction in BP and HR. This
was not surprising, because it is well documented (Pelleg and Porter,
1990) that adenosine A1 and A2 receptor
stimulation can evoke bradycardia and vasodilation, respectively.
However, the observations with the lower dose of agonist indicate that the myocardial adenosine receptors responsible for the
cardioprotective properties of AMP 579 can be stimulated at
concentrations of AMP 579 below those that are necessary to stimulate
adenosine A1 or A2 receptors mediating
bradycardia or peripheral vasodilation. Similar observations have
previously been made for other adenosine A1 and
A2 receptor agonists (Norton et al., 1992). This
may well reflect a difference in the efficiency of stimulus-response
coupling for adenosine receptors in different cardiac and vascular
tissues. Regardless of mechanism, this is a positive observation with
regard to the potential utility of adenosine agonists such as AMP 579 in the treatment of acute myocardial infarction, where substantial vasodepressor activity is undesirable.
To help further define the pharmacology of AMP 579, we next determined
the profile of a selective adenosine A1 receptor agonist, GR79236 (Gurden et al., 1993), when administered prior to
reperfusion. The dose of GR79236 used was identical to the low dose of
AMP 579, because in vitro data in rat adipocytes indicate
that the two compounds have almost identical affinity and efficacy as
agonists at the adenosine A1 receptor (Merkel et
al., 1995; unpublished observations, manuscript submitted).
Administration of GR79236 resulted in a bradycardia but no significant
fall in BP, a profile consistent with stimulation of adenosine
A1 receptors. However, in contrast to the effect of AMP
579, no reduction in infarct size was observed. Because the
cardioprotective effect of AMP 579 was blocked by the specific
adenosine A1/A2 receptor blocker CGS15943 but
was not mimicked by the selective A1 receptor agonist GR79236, adenosine A2 receptors seem to play an important
role in the ability of AMP 579 to reduce infarct size when given before reperfusion. This is consistent with previous reports indicating that
A2 receptor stimulation before reperfusion in pig (Schlack et al., 1993), rabbit (Norton et al., 1992) and
dog (Jordan et al., 1997) models of myocardial ischemia
results in reduced infarct size. However, it should be noted that in
other species, it has also been suggested that A1 receptor
stimulation before reperfusion can exert a cardioprotective effect
(Norton et al., 1992; Lee et al., 1995).
If A2 receptors are important in the cardioprotective
effect of AMP 579, what is the underlying mechanism? Although the
concept of lethal reperfusion injury remains controversial (Hearse and Bolli, 1991; Kloner, 1993), evidence suggests that neutrophil adhesion,
neutrophil activation and the generation of free radicals contribute to
lethal cell injury in the early period after reperfusion (Granger and
Korthuis, 1995). In addition, it has been established that stimulation
of A2 receptors on neutrophils results in the inhibition of
adhesion, degranulation and injury to cardiomyocytes (Cronstein
et al., 1990; Bullough et al., 1995). Indeed,
Jordan et al. (1997) recently reported that the selective
A2a receptor agonist CGS21680 reduced infarct size when
given just before reperfusion in a canine model of myocardial
infarction and that this protection was associated with decreased
neutrophil infiltration. Thus it is likely that stimulation of
A2 receptors on neutrophils contributes to the ability of
AMP 579 to reduce infarct size.
Because there exists considerable clinical interest in the therapeutic
application of adenosine-based cardioprotective treatments, the effects
of AMP 579 were also contrasted with those of the adenosine-regulating
agent acadesine (AICA riboside). Our hope was that a comparative study
with acadesine would help clarify whether adenosine receptor agonists
(such as AMP 579) or adenosine-regulating agents are likely to offer
the most effective approach to inhibition of lethal reperfusion injury.
Acadesine is an adenosineregulating agent (Mullane, 1993) that has
undergone both preclinical and clinical evaluation for its ability to
attenuate the effects of myocardial ischemia/reperfusion. The compound
is thought to act by increasing the generation of endogenous adenosine
in ischemic tissue (Mullane, 1993). The results indicate that acadesine
does not reduce infarct size when given before myocardial reperfusion in vivo, and this is one of the first studies to examine the
efficacy of the compound in that paradigm. The results are perhaps not surprising, because it has previously been reported (Galinanes et
al., 1992) that in a rat isolated heart model of
ischemia/reperfusion-induced dysfunction, acadesine was protective only
if administered before the ischemic period. Similarly, all other
reports we are aware of related to cardioprotective effects of
acadesine involve administration of the compound before myocardial
ischemia. It is possible that when it is given after the onset of
ischemia, acadesine has insufficient access to the sites of endogenous
adenosine production. These data suggest that adenosine agonists such
as AMP 579 may offer a more effective cardioprotective approach than
adenosineregulating agents in the context of reducing lethal
myocardial injury after ischemia/reperfusion.
Conclusions. These data indicate that the novel high-affinity adenosine A1/A2 receptor agonist AMP 579 is a highly effective cardioprotective agent in a porcine model of myocardial infarction. Given before ischemia, AMP 579 almost completely prevents myocardial necrosis and abolishes the incidence of VF. Given before reperfusion, AMP 579 still effectively reduces infarct size by approximately 50%. Finally, the cardioprotective effects of AMP 579 are a consequence of adenosine receptor stimulation and occur at doses below those at which undesirable reductions in BP and HR are observed. Thus the pharmacological profile of AMP 579 is consistent with potential utility in the treatment of acute myocardial infarction.
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Footnotes |
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Accepted for publication April 28, 1998.
Received for publication December 19, 1997.
Send reprint requests to: Dr. Ken Clark, Department of Cardiovascular Discovery (NW4), Rhone-Poulenc Rorer, 500 Arcola Road, Collegeville, PA 19426-0107.
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Abbreviations |
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MAP, mean arterial blood pressure; LVP, left ventricular pressure; CBF, coronary blood flow; IS, infarct size; VF, ventricular fibrillation; AMP 579, ([1S-[1a,2b,3b,4a(S*)]]-4-[7-[[2-(3-chloro-2-thienyl)-1-methylpropyl]amino]-3H-imidazo[4,5-b] pyridyl-3-yl] cyclopentane carboxamide) ; BP, blood pressure; LAD, left anterior descending coronary artery.
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Introduction
Materials & Methods
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Discussion
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generation, respectively.
J Clin Invest
85:
1150-1157.This article has been cited by other articles:
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  R. D. Lasley, G. Kristo, B. J. Keith, and R. M. Mentzer Jr. The A2a/A2b receptor antagonist ZM-241385 blocks the cardioprotective effect of adenosine agonist pretreatment in in vivo rat myocardium Am J Physiol Heart Circ Physiol, January 1, 2007; 292(1): H426 - H431. [Abstract] [Full Text] [PDF]
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E. A. Reid, G. Kristo, Y. Yoshimura, C. Ballard-Croft, B. J. Keith, R. M. Mentzer Jr, and R. D. Lasley In vivo adenosine receptor preconditioning reduces myocardial infarct size via subcellular ERK signaling Am J Physiol Heart Circ Physiol, May 1, 2005; 288(5): H2253 - H2259. [Abstract] [Full Text] [PDF]
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D. K. Glover, L. M. Riou, M. Ruiz, G. W. Sullivan, J. Linden, J. M. Rieger, T. L. Macdonald, D. D. Watson, and G. A. Beller Reduction of infarct size and postischemic inflammation from ATL-146e, a highly selective adenosine A2A receptor agonist, in reperfused canine myocardium Am J Physiol Heart Circ Physiol, April 1, 2005; 288(4): H1851 - H1858. [Abstract] [Full Text] [PDF]
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G. Kristo, Y. Yoshimura, B. J. Keith, R. M. Stevens, S. A. Jahania, R. M. Mentzer Jr., and R. D. Lasley Adenosine A1/A2a receptor agonist AMP-579 induces acute and delayed preconditioning against in vivo myocardial stunning Am J Physiol Heart Circ Physiol, December 1, 2004; 287(6): H2746 - H2753. [Abstract] [Full Text] [PDF]
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J. A. Auchampach, Z.-D. Ge, T. C. Wan, J. Moore, and G. J. Gross A3 adenosine receptor agonist IB-MECA reduces myocardial ischemia-reperfusion injury in dogs Am J Physiol Heart Circ Physiol, July 11, 2003; 285(2): H607 - H613. [Abstract] [Full Text] [PDF]
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J. Pye, F. Ardeshirpour, A. McCain, D. A. Bellinger, E. Merricks, J. Adams, P. J. Elliott, C. Pien, T. H. Fischer, A. S. Baldwin Jr., et al. Proteasome inhibition ablates activation of NF-kappa B in myocardial reperfusion and reduces reperfusion injury Am J Physiol Heart Circ Physiol, March 1, 2003; 284(3): H919 - H926. [Abstract] [Full Text] [PDF]
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Z. Xu, M. V. Cohen, J. M. Downey, T. L. Vanden Hoek, and Z. Yao Attenuation of oxidant stress during reoxygenation by AMP 579 in cardiomyocytes Am J Physiol Heart Circ Physiol, December 1, 2001; 281(6): H2585 - H2589. [Abstract] [Full Text] [PDF]
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