Background Cardiac muscle hypercontractility is a key pathophysiological abnormality in hypertrophic cardiomyopathy, and a major determinant of dynamic left ventricular outflow tract (LVOT) obstruction. Available pharmacological options for hypertrophic cardiomyopathy are inadequate or poorly tolerated and are not disease-specific. We aimed to assess the efficacy and safety of mavacamten, a first-in-class cardiac myosin inhibitor, in symptomatic obstructive hypertrophic cardiomyopathy. Methods In this phase 3, randomised, double-blind, placebo-controlled trial (EXPLORER-HCM) in 68 clinical cardiovascular centres in 13 countries, patients with hypertrophic cardiomyopathy with an LVOT gradient of 50 mm Hg or greater and New York Heart Association (NYHA) class II-III symptoms were assigned (1:1) to receive mavacamten (starting at 5 mg) or placebo for 30 weeks. Visits for assessment of patient status occurred every 2-4 weeks. Serial evaluations included echocardiogram, electrocardiogram, and blood collection for laboratory tests and mavacamten plasma concentration. The primary endpoint was a 1•5 mL/kg per min or greater increase in peak oxygen consumption (pVO 2) and at least one NYHA class reduction or a 3•0 mL/kg per min or greater pVO 2 increase without NYHA class worsening. Secondary endpoints assessed changes in post-exercise LVOT gradient, pVO 2 , NYHA class, Kansas City Cardiomyopathy Questionnaire-Clinical Summary Score (KCCQ-CSS), and Hypertrophic Cardiomyopathy Symptom Questionnaire Shortness-of-Breath subscore (HCMSQ-SoB). This study is registered with ClinicalTrials.gov, NCT03470545.

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Key PointsQuestionIn patients with witnessed refractory out-of-hospital cardiac arrest, does early intra-arrest transport, extracorporeal cardiopulmonary resuscitation, and invasive assessment and treatment improve outcomes compared with standard resuscitation?FindingsIn this randomized clinical trial that included 256 patients, survival with neurologically favorable outcome (Cerebral Performance Category 1-2) at 180 days occurred in 31.5% in the invasive strategy group and 22.0% in the standard resuscitation group, a difference that was not statistically significant.MeaningAmong patients with refractory out-of-hospital cardiac arrest, the bundle of early intra-arrest transport, extracorporeal cardiopulmonary resuscitation, and invasive assessment and treatment did not significantly improve survival with neurologically favorable outcome at 180 days compared with standard resuscitation, although the trial was possibly underpowered to detect a clinically relevant difference.

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