Daily Subcutaneous injections of Relaxin is a therapy for heart failure with preserved ejection fraction

Abstract Background and Significance: Relaxin (RLX), a hormone of pregnancy has engendered a great deal of excitement as a therapy for cardiovascular diseases. Clinical trials treated patients with acute decompensated Heart Failure (HF) with RLX (i.v. 30µg/kg/day-2 days) and reported a 37% decrease in mortality during a 6-months follow up. This led the FDA to declare RLX a ‘break-through’ therapy. However, a larger trial failed to meet the sought-after end points. Here, we propose that periodic subQ RLX pulses are more effective. We measured the pharmacokinetics of serum RLX following subQ injections of RLX in Sprague Dawley rats. Further, we treated rats with confirmed HFpEF with daily subQ RLX (2-weeks) to reverse the diastolic dysfunction (DD), arrhythmia and fibrotic profile. Methods Sprague-Dawley (250 g, n=20, either sex) received 2-subQ injections 12 hrs apart of RLX (from Relaxera) or the vehicle. Serial blood samples were drawn following each injection to measure [RLX]. ZSF1 rats (9-weeks old, either sex) were placed on a high fat diet (HFD) and serial echos tracked the DD. Once severe DD was established (25-35 weeks of HFD), rats received daily subQ injections of RLX (100µg/kg) or vehicle. After 2-weeks, hearts were perfused to optically map action potentials and Ca2+ transients, and analyze the arrhythmia profile. IF and Westerns were used to measure changes in fibrosis (collagen 1), Nav1.5, connexin 43, Wnt1 and β-catenin. Results After an injection, RLX reached a peak in ~60 min, fell to 50% in 5-6 hours. Injections of 0, 30, 100 or 500 µg/kg yielded peak levels of 0, 11.26±3.52, 58.33±16.10, and 209.42±29.04 ng/ml and residual levels after 24-hrs of 0, 4.9, 45.1 and 156 pg/ml, respectively. The 30µg/kg injections had no effect and 100 µg/kg increased Nav1.5 (25%), Cx43 (30%) and β-catenin (90%). The 500 µg/kg injections increased Nav1.5 and Cx43 but upregulated β-catenin, only slightly. In ZSF1 HFpEF rats that received the vehicle (n=12), a premature stimulus (at 30 ms) elicited arrhythmia in 67% of the hearts. RLX injections reversed DD, suppressed sustained atrial and ventricular arrhythmias (n=0/12) but not self-terminating arrhythmias, lasting < 10sec. RLX improved the conduction velocity (CV), particularly at fast rates (100ms) from 0.74 to 0.9 m/s (p<0.001, n=12). RLX significantly increased Cx43 expression, Nav1.5 and β-catenin at intercalated disks. RLX reduced collagen deposition in HFpEF rats, to normal levels and increased Wnt1 compared to control HFpEF rats. Conclusions The ZSF1 diabetic rat on a HFD recapitulates human HFpEF, high blood pressure, fibrosis, DD, AF and VF. SubQ RLX injections reversed DD i.e. enlarged Left Atrium, E/e’ ratio, fibrosis, and the arrhythmia vulnerability by increasing CV, Cx43, and Nav1.5. Daily subQ RLX injections were highly effective as a therapy for HFpEF.Echo parameterHFpEF + RLX