Camzyos Could Become the First Drug Approved for Teenagers With Obstructive HCM on September 30th

The first sign is usually a kid who cannot keep up. A 14-year-old who used to be the fastest on the team starts stopping to catch her breath halfway through a drill. A boy gets dizzy climbing the stairs to third period and tells nobody. A pediatrician hears a murmur at a sports physical, or does not, because in obstructive hypertrophic cardiomyopathy the murmur can come and go with position and effort. When an echocardiogram finally shows a heart wall far thicker than it should be, the family learns 2 things in the same appointment: that this is a genetic disease that will not go away, and that every drug the cardiologist can offer was approved for adults and has never been tested properly in someone their child's age.

By September 30, 2026 the FDA is due to decide whether that second part changes.

What Obstructive Hypertrophic Cardiomyopathy Does to a Teenager's Heart

Hypertrophic cardiomyopathy is one of the most common inherited heart diseases, affecting about 1 in 500 people worldwide, and its symptoms most often first appear in adolescence or young adulthood. The problem is not a weak heart. It is a heart that squeezes too hard. In most patients the wall between the 2 lower chambers, the septum, grows thick, and in the obstructive form that thickened wall bulges into the channel that blood has to pass through on its way out to the body. The harder the heart contracts, the more the channel narrows, so exactly the moments when a teenager needs the most blood flow, sprinting or climbing or playing, are the moments the heart delivers the least.

Cardiologists measure that narrowing as a pressure gradient in millimeters of mercury, the same units as blood pressure. A normal heart has essentially no gradient across its outflow tract. The adolescents who entered the SCOUT-HCM trial had gradients around 80 mm Hg when they held their breath and bore down, the Valsalva maneuver that mimics what exertion does to the heart. That number, the Valsalva LVOT gradient, is the one the whole trial was built to move.

The disease is inherited in most families, in an autosomal dominant pattern. Variants in MYH7 and MYBPC3, 2 genes that build the sarcomere, the basic contracting unit of heart muscle, account for most of the identified cases, and a parent with HCM has a 50% chance of passing the variant to each child. The damage of a hyperactive sarcomere accumulates over years: fainting, an irregular rhythm that can be life-threatening, and in a small number of people heart failure. Most people with HCM live a normal lifespan, but the disease carries a risk of sudden death even in people with no symptoms, which is the reason a 16-year-old with this diagnosis is often told to stop competitive sport.

The medicines a cardiologist reaches for first in obstructive HCM are old and were never designed for the disease. Beta-blockers such as metoprolol slow the heart and blunt the force of each beat, which reduces obstruction as a side effect of reducing everything. Calcium channel blockers do a version of the same thing. Neither carries an FDA indication for HCM in children. Pediatric cardiologists prescribe them off-label because there has been nothing else, and their side effects land hard on teenagers: fatigue, low mood, cold hands, and a heart rate that will not climb when they try to exercise.

When drugs fail, the next step is a surgeon. Septal myectomy opens the chest and shaves the thickened wall down; alcohol septal ablation threads a catheter into a small artery and deliberately scars part of the septum so it shrinks. Both work well in expert hands, and both are open-heart or catheter procedures on a 15-year-old. The NEJM paper that reported the adolescent trial opened with a sentence that sums up the situation families have been in for decades.

“Approved pharmacologic therapies for pediatric hypertrophic cardiomyopathy are lacking.”

Rossano et al., The New England Journal of Medicine, March 29, 2026

Adults got something new on April 28, 2022, when the FDA approved Camzyos as the first cardiac myosin inhibitor. Mavacamten was discovered by MyoKardia, a South San Francisco company built on the idea that if a hyperactive sarcomere causes the disease, the right drug should reach into the sarcomere and calm it directly. Bristol Myers Squibb bought MyoKardia for $13.1 billion in November 2020. In the 251-adult EXPLORER-HCM trial, 37% of patients on the drug improved on a combined measure of exercise capacity and symptoms, against 17% on placebo. By May 2026, BMS said, it had been prescribed to almost 25,000 patients in the United States. The label covered only adults.

Inside the SCOUT-HCM Trial of Mavacamten in Adolescents

SCOUT-HCM (NCT06253221) is the first randomized, placebo-controlled trial of any drug in adolescents with obstructive HCM. It began enrolling on April 17, 2024 and randomized 44 patients aged 12 to 17 across 47 sites in 9 countries, at children's hospitals from Philadelphia and Toronto to Munich, Madrid and Barcelona. 23 were assigned to mavacamten and 21 to placebo, with an average age of about 14 and a half in both groups. All had symptomatic disease, NYHA class II or III, and a provoked gradient high enough to count as obstructive.

The design has 3 parts, and understanding them matters because only the first has been reported. For 28 weeks, patients took mavacamten or a matching placebo without anyone knowing which. At week 28, everyone on placebo crossed over to the real drug for a second 28 weeks, so that by the end of the first year every adolescent in the study is on treatment. After that, an open-label extension follows them for up to 144 more weeks, close to 3 years, to see what happens to a growing heart on a myosin inhibitor over time. The FDA application rests on the first 28 weeks. The trial itself runs to 200.

SCOUT-HCM Results at Week 28

The primary endpoint was the change in the Valsalva LVOT gradient from the start of the trial to week 28. On mavacamten it fell by 48.5 mm Hg. On placebo it fell by 0.5. The difference of 48.0 mm Hg had a 95% confidence interval of 67.7 to 28.3 mm Hg and a p-value below 0.001. From a starting point around 80 mm Hg, the treated group ended the 28 weeks with, on average, well under half the obstruction they came in with, and the placebo group ended where they started.

Safety is where families should read most carefully, because it is where a myosin inhibitor differs from a beta-blocker. Mavacamten works by reducing how hard the heart contracts, and the danger is that it reduces contraction too far. In the adult EXPLORER-HCM trial, 7 of 123 patients on the drug (6%) had their ejection fraction fall below 50% at some point, and all 7 recovered when the drug was paused. In SCOUT-HCM, no adolescent's ejection fraction fell below 50% in 28 weeks. Adverse events were similar in the 2 groups. 2 patients on mavacamten had serious adverse events, 1 who fainted twice and 1 who received an inappropriate shock from an implanted defibrillator; 2 on placebo did as well, 1 with chest pain and 1 with depression. Nobody died.

What the 28-week data cannot tell anyone is what the drug does over a decade to a heart that is still growing, whether the gradient reduction holds up at year 3, or how often a teenager's ejection fraction will dip once thousands rather than 23 are taking it. Those are the questions the extension phase and post-approval registries exist to answer. The FDA's decision is about whether 28 weeks of data in 44 patients, on top of 4 years of adult use, is enough to let physicians prescribe it now with the same monitoring rules that adults follow.

The Boxed Warning and REMS Program, Explained for a 15-Year-Old's Parents

Camzyos carries the FDA's most serious label warning, a boxed warning for heart failure, and it can only be dispensed through a Risk Evaluation and Mitigation Strategy called the CAMZYOS REMS Program. Neither of those is expected to change with an adolescent indication, so it helps to know what they mean in practice before the decision arrives.

Echo before starting

The label says not to start the drug if LVEF is below 55%. A baseline echocardiogram is required, and the Valsalva gradient from it sets the starting point for dosing.

Echo at weeks 4, 8 and 12

During the initiation phase the dose is adjusted at each visit based on 2 numbers: LVEF must stay at or above 50%, and the dose goes up only if the Valsalva gradient is still 30 mm Hg or higher.

Echo every 6 months once stable

After the first 12 weeks, patients whose gradient is under 30 mm Hg and whose LVEF is 55% or higher move to a maintenance schedule with an echo roughly every 6 months, plus one 4 weeks after any dose increase.

Pause if LVEF falls below 50%

The drug is stopped, the heart is rechecked in 4 weeks, and it is restarted at a lower dose if the number recovers. In the adult trial it always did.

Drug interactions

Strong CYP2C19 inhibitors are contraindicated, and the label specifically names over-the-counter omeprazole, esomeprazole and cimetidine as drugs that raise mavacamten levels. Every new medication a teenager takes, including acne drugs and antidepressants, needs to be checked.

REMS enrollment

The prescriber, the pharmacy and the patient must all be enrolled. The drug ships only from certified pharmacies. For a minor this means a parent handles enrollment and the family commits to the echo schedule.

Pregnancy

The drug may cause fetal harm. The label requires ruling out pregnancy before starting in anyone who could become pregnant, effective contraception during treatment and for 4 months after, and it notes that mavacamten can weaken some hormonal contraceptives. This will be a real conversation for the families of teenage girls.

Read together, those rules describe a drug that asks a great deal of a family: a certified pharmacy, a cardiologist willing to do 4 echoes in the first 3 months, and a teenager who takes a capsule every day and tells someone when a new prescription shows up. The trade is that it treats the mechanism of the disease instead of slowing the whole heart down, and in the trial it did that with adverse events no more frequent than on placebo.

Three Clocks Behind the September 30 Decision

The dates tell their own story about how pediatric drug development works. Adults have had this drug since April 2022. The adolescent trial did not enroll its first patient until April 2024, 2 years later, and did not publish until March 2026. Once the application was filed, the FDA moved fast: priority review compresses the standard 10-month clock to 6, and the agency accepted the application on May 29, 2026 with a September 30 target.

That 4-year gap is typical of how pediatric indications follow adult ones rather than unusual. SCOUT-HCM's 47-site footprint, large for a 44-patient study, is what a sponsor has to build to find enough symptomatic teenagers with obstructive disease who are willing to risk 28 weeks on placebo.

What Changes for Families if the FDA Approves Camzyos for Adolescents

The immediate change is insurance. Today a pediatric cardiologist who believes a 16-year-old should be on mavacamten has to prescribe it off-label and argue with a payer that will point to the line in the label saying safety and effectiveness have not been established in pediatric patients. An approved indication removes that line for ages 12 to 17 and gives the prescription a code the insurer recognizes. Prior authorization will still be required, and the REMS and echo requirements will still apply, but the conversation moves from whether the drug is appropriate to whether the paperwork is complete.

The second change is the order of options. For a symptomatic teenager who is not doing well on a beta-blocker, the next step has been to think about surgery. In the adult VALOR-HCM trial, 112 patients who had already been referred for septal reduction took mavacamten or placebo for 16 weeks; at the end only 18% of the mavacamten group still met criteria for surgery or chose it, against 77% on placebo. Nobody has run that trial in adolescents, but it is the reason cardiologists expect an approved myosin inhibitor to sit between beta-blockers and the operating room rather than after it.

The third change is what does not change. The drug does not thin the heart wall back to normal, it does not remove the risk of dangerous rhythms, and it does not on its own reverse a decision to keep a teenager out of competitive sport. Those decisions rest on other measures, including the wall thickness and any history of fainting or arrhythmia, and a lower gradient is 1 input among several.

The decision could also go the other way, or the FDA could ask for more. If a complete response letter came, the efficacy result is unambiguous enough that the question would more plausibly concern the size of the safety database. In that case the crossover and extension data now accumulating in SCOUT-HCM would be the answer, and the timeline would stretch by a year or more.