Increased Inotropism Following Parp Inhibition in the Setting of Myocardial Reperfusion Injury

A reduction in myocardial infarct size by poly(ADP-ribose) polymerase (PARP) inhibition was first described for the PARP inhibitor 3-aminobenzamide in 1997 (1, 2). Since then, numerous studies in rodents (3, 4) as well as in large animal models (5-7) reported the effects of different PARP inhibitors on myocardial ischemia and reperfusion injury, confirming a reduction in infarct size and an improvement in myocardial function. In a recent clinical phase 2 study, the potent PARP inhibitor INO-1001 was studied in 40 patients who had myocardial infarction diagnosed by ST-segment elevation and who were undergoing percutaneous coronary intervention within 24 h of insult. INO-1001 was well tolerated and caused a trend toward blunted plasma levels of IL-6 and C-reactive protein (8). In the current issue of Shock, Roesner and coworkers (9) provide some evidence for potential additional effects of INO-1001. In an established porcine model of left anterior descending artery occlusion for 1 h, followed by a 3-h reperfusion period, INO-1001 (1 mg/kg) was infused for 20 min beginning with the time of reperfusion. In addition to an untreated control group, swine subjected to ischemic preconditioning before the injury served as positive controls. In INO-1001-treated animals, myocardial function during reperfusion was improved compared with the other groups, as suggested by higher stroke volume and cardiac indexes as well as higher mixed venous oxygen saturation. However, the infarct size and plasma levels of troponin T, creatine kinase, and lactate dehydrogenase were only reduced in the group that underwent ischemic preconditioning, whereas INO-1001-treated pigs showed no improvement in these variables compared with untreated controls. Based on these findings, the authors concluded that the PARP inhibitor INO-1001 improves hemodynamic function during reperfusion after myocardial ischemia without reducing the infarct size. Two major questions arise from this study. First, why was there no reduction in infarct size as reported in numerous previous studies with the PARP inhibitor? Second, what is the mechanism underlying the INO-1001-associated improvement in myocardial function if it is not a reduction in cell death? An impaired reperfusion (e.g., no-reflow phenomenon) may explain the observed lack of effect of the PARP inhibitor. However, lack of refilling of the vessels is unlikely because the authors measured total coronary blood flow and confirmed coronary refill at the time of reperfusion. The low dose of 1 mg/kg INO-1001 represents another potential explanation. In this context, it may be noteworthy that it has been repeatedly reported that a total dose of 4 mg/kg might be necessary to achieve a complete inhibition of PARP in pigs (7, 10). In addition, we have been able to show an efficacy of INO-1001 using a 3-mg/kg bolus followed by a 0.3-mg·kg−1·h−1 continuous infusion in our ovine model of acute respiratory distress syndrome (11). Analysis of the immunohistochemical staining of active PARP in heart sections subjected to I/R (and treated with vehicle or PARP inhibitor) suggests that the dose chosen by the authors did indeed inhibit PARP activation. As the current study does contradict previous work, further evidence is required to support the notion of an adequate PARP inhibition. Unfortunately, no tissue levels of peroxynitrite, a potent activator of PARP, or poly(ADP ribose), the product of PARP, were quantitated in the present study using Western blot or enzyme-linked immunosorbent assay. In addition, plasma levels of PARP (e.g., in neutrophils) are not reported. Therefore, without an adequate dose response, the interpretation of the current data remains difficult. Against the background of an absence of a reduction in morphological injury, answering the second question about the mechanism of an increase in cardiac index and stroke volume is challenging. Positive inotropic effects of PARP inhibitors have been suggested in previous studies (7, 12), but definite evidence is still missing. In addition, INO-1001 had no effect on stroke volume in sham-operated animals as compared with the untreated sham group (especially when considering the baseline difference). In this context, however, it is important to consider that PARP activation results in the stimulation of various proinflammatory cytokines, such as TNF-α, and subsequent activation of polymorphonuclear cells (13). Both TNF-α and myocardial infiltration with neutrophils, lymphocytes, and monocytes have been reported to contribute to contractile dysfunction after I/R injury (14, 15). These inflammatory pathways might be attenuated by PARP inhibition. This theory would explain the improved myocardial function only in injured and not in sham animals. Another very interesting finding of the present study is the increased intrathoracic blood volume index in the INO-1001 group compared with control animals that may well explain an increase in cardiac index and stroke volume. As discussed by the authors, it has previously been shown that PARP is also activated outside the myocardium, specifically in leukocytes, after myocardial infarction (16). Notably, PARP induces nuclear factor-κB activation and thereby stimulates iNOS expression (11). The subsequent increase in NO production may cause a decrease in vascular tone of capacity vessels. As a consequence, PARP inhibition would prevent the increased NO production, which would explain the increase in intrathoracic blood volume, cardiac index, and stroke volume. However, as acknowledged by the authors, the present data are not adequate to support either of these theories. Taking the current literature as well as this interesting study by Roesner and coworkers (9) into account, it seems to us that PARP inhibition represents an interesting therapeutic strategy for reduction of the injury and/or dysfunction associated with acute myocardial infarction and possibly other conditions associated with I/R injury. Although the reasons for the lack of infarct size reduction in the current study remain unclear, the reported beneficial effects on stroke volume and cardiac index might be caused by anti-inflammatory properties of INO-1001. This hypothesis, however, needs to be verified in future studies.