A Study to Evaluate the Effect of Aficamten in Pediatric Patients With Symptomatic Obstructive Hypertrophic Cardiomyopathy (oHCM).

The overall objective of the trial is to determine the efficacy, safety, and tolerability of administration of aficamten in adolescents (12 to \< 18 years old) and children (6 to \< 12 years old) with symptomatic oHCM. Adolescents and children will be studied in a staged approach involving established favorable pharmacodynamic and safety profiles of aficamten in adolescents followed by further pharmacokinetic modeling to inform the dosing regimen in children. The 12 to \<18 years old cohort has enrolled all participants and is no longer recruiting, the 6 to \< 12 year old cohort is not yet recruiting. The trial will consist of 3 periods: 1. Period 1 is the randomized, double-blind, placebo-controlled treatment period that will assess the efficacy, safety and tolerability of aficamten in pediatric participants. 2. Period 2 is the open-label extension trial that will assess the long-term safety of aficamten in pediatric participants, and further assess efficacy and tolerability. 3. Period 3 is the long-term extension trial that will assess the long-term safety of aficamten in pediatric participants, and further assess efficacy and tolerability.