Abstract Body (Do not enter title and authors here): Background: While most current treatments can improve symptoms in patients with obstructive hypertrophic cardiomyopathy (HCM), they may be poorly tolerated and do not target the underlying pathophysiology. Mavacamten is the first and only cardiac myosin inhibitor approved for the treatment of adults with symptomatic New York Heart Association (NYHA) class II-III obstructive HCM but has not been evaluated in a pediatric age group. SCOUT-HCM (Study of MavaCamten in AdOlescents with Symptomatic ObstrUcTive HCM; NCT06253221) will evaluate the efficacy, safety, and pharmacokinetics of mavacamten in adolescents with symptomatic obstructive HCM.

Methods: SCOUT-HCM is a randomized, double-blind, placebo-controlled, international phase 3 clinical trial that will enroll 40 adolescents (age 12 to <18 years) with obstructive HCM (Figure). Patients will receive mavacamten or placebo during the first 28 weeks, followed by a 28-week double-blind active treatment period. Inclusion criteria include a diagnosis of HCM with presence of symptoms and left ventricular outflow tract (LVOT) obstruction. Patients with HCM phenocopies, LVEF <50% at rest in the past 6 months, planned escalation in HCM therapy or upcoming intervention (eg, major cardiac surgery) will be excluded. The primary endpoint is change in Valsalva LVOT gradient from baseline to week 28. Key secondary endpoints include other echocardiographic parameters, safety, pharmacokinetics, and exercise capacity.

Conclusions: SCOUT-HCM is the first randomized controlled trial evaluating a targeted myosin inhibitor in adolescents with symptomatic obstructive HCM. The results will inform the utility of mavacamten in adolescent patients with symptomatic obstructive HCM.