The small-molecule troponin activator CK-136 increases myocyte contractility in human heart failure

Direct sarcomere activating drugs (myotropes) have emerged as one alternative to inotropes in heart failure. The only myotropes that have been evaluated clinically involve myosin modulation, which does not increase maximum calcium activated tension (Tmax), observed in heart failure with reduced (HFrEF) and obese preserved (HFpEF) ejection fraction. Here, we tested if troponin modulation with the sarcomere activator CK-136 (1 uM) can augment Tmax. We isolated permeabilized myocytes from HFrEF (n = 5), HFpEF (n = 9, 5 w/severe obesity and reduced Tmax and 4 w/o), and non-failing human (n = 6) and porcine controls and obtained tension-Ca relationships with CK-136. Consistent with its calcium sensitizing effect, CK-136 reduced Ca for 50% Tmax (EC50) by 25% in non-failing controls, 19% in HFrEF (p = 0.01), 16% in HFpEF w/o severe obesity (p = 0.05), and 18% in HFpEF w/ severe obesity (p = 0.01). Moreover, treatment with CK-136 increased Tmax by 16% in non-failing controls, 28% in HFrEF (p = 0.02, p-interaction = 0.98), 20% in HFpEF w/o severe obesity (p = 0.04), and 50% in HFpEF with severe obesity (p = 0.01). We performed x-ray diffraction on permeabilized cardiac muscle strips to understand potential mechanisms for the latter. CK-136 increased equatorial intensity ratio in relaxing conditions (pCa 8) by 36%, 60%, and 49% in porcine control, non-failing controls, and human HFrEF, respectively, suggesting that despite being a troponin activator, CK-136 increases the proportion of myosin heads for force generation. Subsequent studies in skinned cells from porcine myocardium found that CK-136 increased the stiffness by 18% at pCa 8, suggesting it increases the proportion of weakly bound crossbridges. These preliminary results suggest that CK-136 may have a role for treating depressed myocyte contractility in cases where Tmax is reduced and diastolic stiffness is not increased, including human HFrEF and HFpEF w/ severe obesity.