JTV-519

The ryanodine receptor 2 (RyR2) is present in both the heart and kidneys, and plays a crucial role in maintaining intracellular Ca2+ homeostasis in cells in these organs. This study aimed to investigate the impact of M201‐A on RyR2, as well as studying its effects on cardiac and renal functions in preclinical and clinical studies.

Following the administration of M201‐A (1,4‐benzothiazepine‐1‐oxide derivative), we monitored diastolic Ca2+ leak via RyR2 and intracellular Ca2+ concentration in isolated rat cardiomyocytes and in cardiac and renal function in animals. In a clinical study, M201‐A was administered intravenously at doses of 0.2 and 0.4 mg·kg−1 once daily for 20 min for four consecutive days in healthy males, with the assessment of haemodynamic responses.

In rat heart cells, M201‐A effectively inhibited spontaneous diastolic Ca2+ leakage through RyR2 and exhibited positive lusi‐inotropic effects on the rat heart. Additionally, it enhanced natriuresis and improved renal function in dogs. In human clinical studies, when administered intravenously, M201‐A demonstrated an increase in natriuresis, glomerular filtration rate and creatinine clearance, while maintaining acceptable levels of drug safety and tolerability.

The novel drug M201‐A inhibited diastolic Ca2+ leak via RyR2, improved cardiac lusi‐inotropic effects in rats, and enhanced natriuresis and renal function in humans. These findings suggest that this drug may offer a potential new treatment option for chronic kidney disease and heart failure.

FKBP12.6, a binding protein to the immunosuppressant FK506, which also binds the ryanodine receptor (RyR2) in the heart, has been proposed to regulate RyR2 function and to have antiarrhythmic properties. However, the level of FKBP12.6 expression in normal hearts remains elusive and some controversies still persist regarding its effects, both in basal conditions and during β-adrenergic stimulation. We quantified FKBP12.6 in the left ventricles (LV) of WT (wild-type) mice and in two novel transgenic models expressing distinct levels of FKBP12.6, using a custom-made specific anti-FKBP12.6 antibody and a recombinant protein. FKBP12.6 level in WT LV was very low (0.16 ± 0.02 nmol/g of LV), indicating that <15% RyR2 monomers are bound to the protein. Mice with 14.1 ± 0.2 nmol of FKBP12.6 per g of LV (TG1) had mild cardiac hypertrophy and normal function and were protected against epinephrine/caffeine-evoked arrhythmias. The ventricular myocytes showed higher [Ca²⁺]_i transient amplitudes than WT myocytes and normal SR-Ca²⁺ load, while fewer myocytes showed Ca²⁺ sparks. TG1 cardiomyocytes responded to 50 nM Isoproterenol increasing these [Ca²⁺]_i parameters and producing RyR2-Ser2808 phosphorylation. Mice with more than twice the TG1 FKBP12.6 value (TG2) showed marked cardiac hypertrophy with calcineurin activation and more arrhythmias than WT mice during β-adrenergic stimulation, challenging the protective potential of high FKBP12.6. RyR2^R420Q CPVT mice overexpressing FKBP12.6 showed fewer proarrhythmic events and decreased incidence and duration of stress-induced bidirectional ventricular tachycardia. Our study, therefore, quantifies for the first time endogenous FKBP12.6 in the mouse heart, questioning its physiological relevance, at least at rest due its low level. By contrast, our work demonstrates that with caution FKBP12.6 remains an interesting target for the development of new antiarrhythmic therapies.