Due to the importance of gene regulation in

Due to the importance of gene regulation in cancers, HDAC inhibitors have been studied extensively in cancer biology and are in current clinical use as anti-tumor therapies [17]. The HDAC inhibitors vorinostat, romidepsin, and belinostat have been approved for certain T-cell lymphomas, and panobinostat is approved for multiple myeloma; many more are in preclinical or phase II and III clinical trials (summarized in [18]). HDAC inhibitors have a range of effects on cancer cells including triggering apoptosis, autophagy, immune responses, DNA repair genes, signaling pathways, Entrectinib arrest, antiangiogenic effects, and more. This indicates that HDACs govern a program of responses instead of a specific, discrete cellular pathway [18]. Histone acetylation has also been studied in the heart due to its important role in cell survival. In mouse hearts subjected to I/R (ischemia 45 min and reperfusion 48 h), left ventricular HDAC activity nearly doubles [19]. No significant changes in HAT activity were observed [19]. Similarly, hypoxia induces HDAC activity in cultured neonatal mouse ventricular myocytes without affecting HAT activity [19]. Thus, it seems that HAT activity is not regulated during cardiac I/R. The most extensively studied HATs in muscle are p300 and the closely related coactivator, CREB-binding protein (CBP), enzymes that play critical roles in physiological and pathological growth of cardiac myocytes [20]. Recently, it was reported that rat hearts exposed to diabetic stress manifest increased HDAC activity at baseline and are more vulnerable to myocardial I/R injury compared with nondiabetic hearts. I/R injury further increased HDAC activity in diabetic rat hearts [21]. There are multiple HDAC isoforms in the heart responsible for reversing protein acetylation. Trichostatin A (TSA), an HDAC inhibitor specific to class I and class II HDACs, has been tested to determine whether it can reduce I/R injury. Du and colleagues have found that TSA treatment significantly reduces cardiomyocyte HDAC4 activity in the setting of I/R [22]. Further work by the same group revealed that cardiomyocyte-specific over-expression of a constitutively active HDAC4 (a His-976-Tyr mutation yielded an enzyme with a catalytic efficiency 1,000-fold higher than wild-type [23]) promotes larger infarct size [24]. Delivery of a chemical HDAC inhibitor attenuated the detrimental effects of active HDAC4 in I/R injury, revealing a pivotal role of active HDAC4 in response to myocardial I/R injury [24]. These results suggest that I/R injury derives at least in part from increased HDAC activity and subsequent relative de-acetylation of histones and proteins involved in a wide range of events.
HDAC inhibition reduces infarct size in preclinical studies I/R-associated increases in HDAC activity raise the prospect of HDAC inhibition as a potentially meaningful therapeutic target in I/R injury. Zhao and colleagues tested TSA in isolated mouse hearts exposed to I/R stress. Pretreatment of these hearts with TSA for 15 min (preconditioning) or 24 h (delayed pharmacologic preconditioning) markedly improved recovery of ventricular function and reduced infarct size [25]. Granger and colleagues tested multiple HDAC inhibitors, including Scriptaid and TSA in an in vivo model of I/R injury in mice. They demonstrated that chemical HDAC inhibitors reduced infarct size significantly, even when delivered one hour after the ischemic insult [19]. These data lend support to the concept of HDAC inhibition as a viable therapeutic agent, as it reduces infarct size even when structurally distinct inhibitors are administered at the time of reperfusion. In 2006, the HDAC inhibitor, vorinostat (Zolinza®, Merck) also known as suberanilohydroxamic acid (SAHA), was approved for human use in the treatment of cutaneous T cell lymphoma. Structurally, TSA and SAHA are very similar [26,27]. This opened the possibility of a clinical trial with a pharmaceutical grade compound. To pursue this, we first verified that SAHA reduces infarct size in mice when delivered at the time of reperfusion [26]. Next, we tested SAHA in a large animal (rabbit) I/R model [26]. Experiments were carried out in a blinded fashion with experimental rigor comparable to that used in a human clinical trial. Rabbits were randomized into three groups: vehicle control, SAHA pretreatment (one day prior and at surgery), and SAHA treatment only at the time of reperfusion. Each arm was subjected to I/R surgery. SAHA reduced infarct size robustly (around 40%) and partially rescued systolic function; importantly, the benefits observed were similar when drug was administered either before surgery (pretreatment) or exclusively at the time of reperfusion [26]. We also measured serum concentrations in rabbits to ensure that levels similar to those achieved in humans were seen. Of note, SAHA is the only FDA-approved HDAC inhibitor tested in a large animal model, an FDA pre-requisite to proceed to a first-in-human clinical trial. The protective effects of SAHA in a murine cardiac I/R model have also been verified in multiple, independent labs including the Menick lab [28] and the lab of one of the authors (M.X., unpublished data). These studies, then, lend strong support to the notion of pursuing pharmacological HDAC inhibition in I/R injury in patients (Fig. 1).