The NLRP3 (NOD-like receptor family, pyrin domain–containing 3) inflammasome is a macromolecular structure involved in the inflammatory response to injury or infection.1 NLRP3 senses intracellular danger (eg, ischemia) or extracellular alarmin signaling during tissue injury.1 In myocardial ischemia reperfusion injury, NLRP3 activation leads to recruitment of ASC (apoptosis-associated speck-like protein containing carboxy-terminal caspase-recruitment domain) and caspase-1 leading to inflammasome formation and additional loss of viable myocardium.1–3
Several NLRP3 inhibitors have been tested in the animal model of ischemia reperfusion injury showing salvage of part of the myocardium at risk.1–8 To date, however, there are no NLRP3 inhibitors that are clinically available for human use. A recently developed small molecule NLRP3 inflammasome inhibitor, OLT1177, also known as dapansutrile (3-methanesulfonyl-propionitrile), has been shown to be effective in a variety of experimental models in vitro and in vivo9,10 and is the first NLRP3 inhibitor to complete phase I clinical testing and currently to be studied in phase IB-II clinical trials.11–13
In this study, we tested the effects of OLT1177 on infarct size and cardiac function in a murine model of myocardial ischemia reperfusion injury by transient left coronary artery ligation.
This study was conducted in 2 major parts: part 1, with experimental protocol 1, was developed to test potential effects of OLT1177 on myocardial contractility in healthy mice when administered intraperitoneally (IP) and to characterize the corresponding plasma exposure level; part 2, with experimental protocols 2–5, was designed to perform dose-ranging study to evaluate the effect of single IP administration of OLT1177 (6, 60, and 600 mg/kg) administered in mice subjected to acute myocardial infarction (AMI) (protocol 2), to define the effects of a single dose of OLT1177 on the cardiac function 7 days after AMI (protocol 3), to define the effects of OLT1177 following prolonged ischemia (protocol 4), and to evaluate the effect of delayed administration (60, 120, or 180 minutes after reperfusion) of OLT1177 (60 mg/kg IP) on infarct size and cardiac function (protocol 5). Figure 1 shows an outline of the experiments. OLT1177 was provided by Olatec Therapeutics LLC. Mice were randomly assigned to different treatments by an investigator not involved in the assessment of endpoints.
Determination of OLT1177 Plasma Concentration
The synthesis of OLT1177 is described in previous publications.9,10Figure 2 shows the chemical structure of OLT1177. Plasma OLT1177 exposure was measured in blood of healthy mice 5 minutes following IP injection (100 mg/kg). The plasma OLT11177 level was also measured 24 hours after induction of myocardial ischemia reperfusion injury following a single IP injection of 6, 60 or 600 mg/kg. Blood was collected in heparin tubes and centrifuged (2000 rpm for 10 minutes); plasma was harvested and snap frozen in liquid nitrogen.
The quantification of OLT1177 in biological matrix was performed as previously described9,10 using gas chromatography with detection by tandem mass spectrometry (MS/MS) at Syneos Health (Princeton, NJ). Dapansutrile and the internal standard, dapansutrile-d3, were extracted from 50 µL sodium heparin plasma of the mouse by a liquid–liquid extraction procedure. Extraction started with the addition of 20 µL of the internal standard working solution to all appropriate samples. The samples were vortexed gently, and then, 2 mL of ethyl acetate was added. Samples were vortexed, centrifuged, and placed in an acetone-dry ice-water bath. Supernatants were then transferred to clean tubes, evaporated to dryness, and reconstituted with N,O-bis trimethylsilyl trifluoroacetamide (1/25, vol/vol) with 1% trimethylchlorosilane/ethyl acetate. Samples were covered, vortexed, and transferred to clean amber vials. The extracts were chromatographed on a DB-17 gas chromatography column. The compounds were detected and quantified by tandem mass spectrometry in a positive ion mode on an Agilent 7890A. A method qualification run was performed, and the qualified quantitation range was 20.0–2000 ng/mL.
Assessment of the Effects of OLT1177 on Cardiac Contractility In Vivo
To determine the effect of OLT1177 on cardiac contractility, serial changes in the left ventricular ejection fraction (LVEF) were measured in healthy mice before and after administration of OLT1177 (100 mg/kg), a matching volume of vehicle (NaCl 0.9%, in 0.2 mL), isoproterenol (Sigma Aldrich; 100 ng per mouse) that was used as positive inotropic agent, and Escherichia coli O111:B4 LPS (Sigma Aldrich, St. Louis, MO; 20 mg/kg) that was used as negative inotropic agent.14 Each group included 5–8 mice. Following determination of LVEF at baseline, ejection fraction was determined at 10–20 minutes, 80–110 minutes, and 180–240 minutes after administration of OLT1177, a vehicle, isoproterenol, or LPS. The effect of the treatment on contractility was expressed as the percentage of the LVEF interval change from baseline. Assessment of LVEF was performed off-line by an operator blinded to treatment allocation.
All mice underwent light anesthesia using pentobarbital (30–50 mg/kg; Diamondback Drugs, Scottsdale, AZ), and echocardiography was performed to measure left ventricular fractional shortening (LVFS) and to calculate the LVEF as previously described.3–5 Briefly, the sedated mice were shaved and placed supine on a VEVO770 platform, and ultrasound gel was applied. The left ventricle was visualized in bi-dimensional mode in transversal view at the mid-cavitary level, then M-mode sections were acquired and left ventricular end-diastolic (LVEDD) and left ventricular end-systolic diameters (LVESD) were recorded, and the LVFS = (LVEDD-LVESD)/LVESD × 100) averaged over several cycles. The LVEF was derived using the Teicholz formula, by which the LVEF is calculated based on the estimation of volumes as the third power of the diameter multiplied by a factor of 1.047 according to the ellipsoid shape in which the length is twice the diameter (LVEF = [LVEDD3 − LVESD3]/LVEDD3), as previously described.3,4 Assessment of LVEF was performed off-line by an operator blinded to treatment allocation. Figure 3 shows a representative contour analysis of the anterior/septal wall and posterior wall at M-mode echocardiography.
Measurement of Caspase-1 Activity in the Heart
The caspase-1 activity was determined on frozen heart tissue by cleavage of a fluorogenic substrate. The heart samples were powdered, and protein samples were prepared using extraction in RIPA buffer containing protease inhibitors using the Matrix Lysis D system (MP Biomedicals, Santa Ana, CA). After homogenation, the samples underwent 3 freeze-thaw cycles and were stored in at −80°C overnight and centrifuged at 10,000g for 20 minutes; the supernatant was collected for further analysis. Seventy micrograms of proteins were prepared in a 100 μL volume containing 32 μL of caspase buffer (32.5% sucrose, 0.325% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate and 325 mM HEPES pH 7.4), 10 mM di-thiotreitol, and 5-mM caspase-1 substrate Ac-YVAD-AMC (acetyl-Tyr-Val-Ala-Asp-7-amino-4-methylcoumarin; Enzo Life sciences, Farmingdale, NY) with or without the presence of 5-mM caspase-1 inhibitor (Ac-Tyr-Val-Ala-Asp-CHO; Enzo Life Sciences). The samples were incubated for 75 minutes at 30°C, and the net fluorescence between the sample with and without the inhibitor was used as a measure of caspase-1 activity and expressed as % of sham control.
Experimental AMI Model
All animal experiments were conducted under the guidelines of the “Guide for the care and use of laboratory animals” published by the National Institutes of Health (revised 2011). Adult male ICR (CD1) mice (8–12 weeks old) of 35–45 g of weight, supplied by Envigo (Indianapolis, IN) underwent experimental myocardial ischemia/reperfusion (I/R) by transient left coronary artery occlusion for 30 or 75 minutes, followed by reperfusion, as previously described.3,4 Briefly, mice were anesthetized using pentobarbital (50–70 mg/kg) followed by orotracheal intubation. After placing them in the right lateral decubitus position, the mice were subjected to left thoracotomy and pericardiectomy, and the proximal left coronary artery was ligated for 30 or 75 minutes and then released.
The effects of OLT1177 in the ischemia/reperfusion model were tested using 4 different treatment protocols (protocols 2 to 5, Fig. 1).
Mice underwent ischemia for 30 minutes and were treated with OLT1177 (6, 60, or 600 mg/kg IP) or a matching volume of vehicle (NaCl 0.9%) at the time of reperfusion. Mice were killed at 24 hours of reperfusion for the assessment of infarct size and heart function (N = 5–9 per group).
Mice underwent ischemia for 30 minutes and were treated with a single dose of OLT1177 (60 mg/kg IP) or vehicle at reperfusion. Mice were killed 7 days after reperfusion to measure cardiac function.
Mice underwent ischemia for 75 minutes, were treated with OLT1177 (60 mg/kg IP) or vehicle at reperfusion, and killed at 24 hours after reperfusion to assess infarct size (N = 6–8 per group).
Mice underwent ischemia for 30 minutes, and a single dose of OLT1177 (60 mg/kg IP) was administered at 60-, 120-, or 180-minute postreperfusion. Cardiac function and infarct size were measured 24 hours following reperfusion.
Sham groups were used to measure the cardiac function of mice without AMI in part 2 (protocols 2–5) of the study.
Myocardial damage was determined by pathology assessment of viability using triphenyl tetrazolium chloride (Sigma–Aldrich) to stain the viable myocardium and phthalo blue staining to mark the nonrisk area, as previously described.3–5 Infarct size was expressed as a percentage of the whole left ventricular (LV) myocardium and as percentage of the area-at-risk (AAR). Assessment of infarct size was performed by an operator blinded to treatment allocation.
Data are presented as mean and standard error of the mean. To assess for differences between 3 or more groups, we used 1-way analysis of variance assessing for an absolute P value, for a P for linear trend (exploring for dose response), and when P < 0.05 it was followed by Dunnett 2-sided post hoc test comparing each intervention group to the vehicle control. Differences between 2 groups or interval changes in repeated measures within 1 group in echocardiographic data were analyzed comparing the individual percent changes in each group using paired t test. Data on OLT1177 plasma concentration were compared between groups after logarithmic transformation. Missing data (ie, mortality related to procedural mortality) were not imputed. P < 0.05 was considered significant throughout. SPSS (IBM) version 21.0 for Mac was used.
OLT1177 Plasma Level
The mean peak plasma concentration of OLT1177 at 5 minutes following 100 mg/kg IP injection was 134.3 ± 13.0 μg/mL (corresponding to a 1008 µM concentration), whereas all samples assayed from vehicle-treated animals (N = 6) were below the assay's limit of quantitation.
In a separate experiment, mice were injected with OLT1177 6, 60, or 600 mg/kg IP after myocardial ischemia, right after reperfusion was confirmed, and blood samples were obtained at 24 hours. OLT1177 was detectable in the plasma of mice treated with 60 and 600 mg/kg (291 ± 126 and 960 ± 441 ng/mL or 2.19 and 7.22 μM, respectively) and was below the limit detection in the mice treated with the 6 mg/kg dose (<20 ng/mL and <0.15 μM, P = 0.001 for linear trend) (Fig. 4).
Lack of Direct Effects of OLT1177 on Cardiac Contractility
No significant effect of OLT1177 on LVEF was measured in healthy control mice at the different time points (P > 0.05 vs. baseline for OLT1177 treated-group and P > 0.05 when compared with vehicle-treaded group) (Fig. 5A). There were no statistically significant differences in the maximal increase or maximal decrease in LVFS between OLT1177 and vehicle. Isoproterenol, used as a control for positive inotropism, led to a significant increase in LVFS (peak effect at 10–20 minutes) and LPS, used as a control for negative inotropism, led to a significant decrease in LVFS (peak effect at 180–240 minutes) (Fig. 5B).
OLT117 Reduces Infarct Size
OLT1177 was administered intraperitoneally at reperfusion following 30 minutes of ischemia reduced infarct size in a dose-dependent manner (P < 0.001 for linear trend, −36%, −67%, and −62% for 6, 60, and 600 mg/kg, respectively; P = 0.010 vs. vehicle for 6 mg/kg; and P < 0.001 vs. vehicle for 60 and 600 mg/kg) (Fig. 6). Intraperitoneal administration of OLT1177 at reperfusion was shown to be effective in reducing infarct size also when prolonged ischemia (75 minutes) was performed (75 minutes; Fig. 7). No differences in the AAR were present between groups.
OLT1177 Reduces Caspase-1 Activity in the Heart
OLT1177 (600 mg/kg given immediately after reperfusion) significantly reduced caspase-1 activity in the heart tissue 24 hours after ischemia (30 minutes) by >50% (P = 0.028) as compared with vehicle-treated mice (Fig. 8).
OLT1177 Preserves Cardiac Function Following Myocardial Ischemia-Reperfusion Injury
Treatment with OLT1177 (6, 60, and 600 mg/kg IP) at reperfusion following 30 minutes of ischemia was shown to preserve LV systolic function at 24 hours when compared with the vehicle-treated group (Fig. 9A; all P < 0.05 vs. vehicle). Mice underwent the same experimental AMI procedure, and the group treated with OLT1177 (60 mg/kg) at reperfusion showed an increase in cardiac function at 7 days of reperfusion when compared with the control mice (Fig. 9B; all P < 0.05 vs. vehicle).
Window of Therapeutic Effects on Infarct Size and Cardiac Function
We next evaluated the effect of delayed administration of OLT1177 (60 mg/kg IP injection 60, 120, or 180 minutes after reperfusion) on infarct size and cardiac function at 24 hours of reperfusion. Treatment with OLT1177 60 minutes after reperfusion was shown to have a comparable efficacy on infarct size and LV function when compared to the administration at the time of reperfusion (Fig. 10; P < 0.001 vs. vehicle). Prolonged delay in administration of the NLRP3 inflammasome inhibitor (120 and 180 minutes after reperfusion) showed a decreased efficacy in the reduction of infarct size and preservation of LVEF compared with the no- and 1-hour–delayed strategies (Fig. 10). Treatment after 120 minutes of reperfusion showed a trend toward reduction (−27%) in infarct size compared with vehicle-treated mice (P = 0.07). Treatment with OLT1177 at 180-minute postreperfusion showed no difference on infarct size and LVEF when compared with the vehicle group (P > 0.05 vs. vehicle). These data demonstrate that a single dose of OLT1177 is sufficient to limit cardiac damage following ischemia/reperfusion injury when administrated within the first 1 hour.
Ischemia leads to anoxic cell death of the myocardium during AMI. Reperfusion following ischemia is able to salvage a significant proportion of the myocardium at risk. However, just as with ischemia, reperfusion is also associated with an intense inflammatory response that is itself responsible for loss of the myocardium.15 In the presence of tissue injury, the NLRP3 inflammasome is activated and becomes a key driver of the inflammatory response.1 Following formation, the NLRP3 inflammasome leads to caspase-1 activation, processing, and release of the proinflammatory mediators IL-1β and IL-18, and inflammatory cell death (pyroptosis).1 Genetic and pharmacologic inhibition of the NLRP3 inflammasome has been shown to reduce the extent of myocardial injury, with a similar therapeutic window.1–8 The results of NLRP3 inflammasome inhibitors in myocardial ischemia reperfusion differ substantially from those with inhibitors of the mitochondrial permeability transition pore that have shown to reduce infarct size in vivo in animals and in phase II clinical trials if given before reperfusion or immediately at reperfusion yet fail to reduce infarct size if given few minutes after reperfusion, thus showing a narrow therapeutic window.16
Of note, none of the pharmacological NLRP3 inhibitors tested hitherto, with the exception of OLT1177 (dapansutrile), are available for clinical use. In the current report, we show that OLT1177, which is already in phase II clinical development, significantly inhibits caspase-1 activity in the heart, reduces infarct size following moderate and extended ischemia (30–75 minutes), and preserves early and late systolic cardiac function following myocardial injury due to ischemia or reperfusion in the mouse. These effects were observed in absence of any reported inotropic activity in healthy mice.
Previous studies have shown that OLT1177 is a powerful and selective inhibitor of the NLRP3 inflammasome both in vivo and in vitro.9,10 Marchetti et al9 showed that the molecule prevents NLRP3-ASC and NLRP3–caspase-1 association by inhibition of NLRP3 ATPase activity, reducing release of mature IL-1β/IL-18, and pyroptosis. These effects were independent from other NLRs activation or off-target effects on other kinases.9 Pharmacokinetic studies in animals and humans showed a rapid peak in plasma OLT1177 concentrations (<5 minutes with parenteral injections to 1–2 hours with oral administration) and a terminal half-life of approximately 23 hours.9 In vitro efficacy of OLT1177 was evident across a wide range of molar concentrations (0.001–1000 µM).9 The peak plasma concentration observed in the mice 5 minutes after injection was within the therapeutic range. Plasma levels at 24 hours also showed a dose-dependent response with values within the therapeutic range for the higher doses (60 and 600 mg/kg).
In this study, we also focused in the determination of a therapeutic window of intervention of OLT1177 following myocardial ischemia reperfusion. These data collected show that NLRP3 inhibition with OLT1177 has a therapeutic value in the acute and subacute phase of myocardial infarction, extending the rescue of myocardium at risk by at least 1 hour. These data confirm the previous observation that NLRP3 activation is a secondary event that participates in the progression of myocardial damage due to the reperfusion injury, causing a wave-front progression of cardiomyocyte death that can be prevented and/or reduced by targeting NLRP3.1,3
The exact mechanisms by which OLT1177 protects the heart from injury are not explored in this study. We describe that parallel to a reduction in infarct size, OLT1177 reduces caspase-1 activity in the heart. Previous studies have shown that activation of the NLRP3 inflammasome induced inflammatory cell death (pyroptosis) in cardiomyocytes.2,5,17,18 Accordingly, NLRP3 inflammasome inhibitors rescued cardiomyocytes from dying following ischemia reperfusion and reduced the final infarct size.1–8
OLT1177 also lacks any direct effect on cardiac function, and therefore, the preservation of systolic function is to be attributed to myocardial salvage.
OLT1177 has already been tested in healthy volunteers in a phase I study,11 and a phase II clinical trial is ongoing in patients with acute gouty arthritis. A phase IB safety study is also ongoing in patients with chronic systolic heart failure.13
This study has been developed to define the efficacy of OLT1177 in different experimental settings to mimic various clinical scenarios, evaluating different times of ischemia and delayed treatment, with the long-term goal of guiding a thoughtful design of clinical studies. However, this study, as any preclinical study, presents several limitations. First, the use of a single animal breed (mouse) presents obvious differences with the human. Second, despite our attempt to simulate a clinical scenario (ie, treatment at reperfusion), the conditions seen in patients with AMI cannot be thoroughly reproduced in the experimental animal model. Third, the lack of cardiac biomarkers to validate the other techniques used to measure infarct size represents a limitation. Fourth, the use of echocardiogram to measure LV dimensions and functions has subjective variability. Fifth, although the selection of time points was informed by previous studies in preclinical models of ischemia/reperfusion, we cannot exclude the possibility that longer treatments or longer follow-up might reveal different effects. Sixth, the study is presented as a composition of different protocols, with different doses being used.
OLT1177 (dapansutrile) is rapidly detectable following intraperitoneal injection and produces no direct inotropic effect in healthy mice. In a preclinical model of myocardial ischemia/reperfusion, OLT1177 reduces infarct size and preserves cardiac systolic function across a wide range of therapeutic scenarios, confirming the potential clinical translational value of this novel cardioprotective strategy.
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