Conference Coverage

EXPLORER trial hints at potential new drug option in obstructive hypertrophic cardiomyopathy


An investigational drug that targets part of the molecular machinery underlying obstructive hypertrophic cardiomyopathy (HCM) can improve both symptoms and functional status in patients with the genetic disorder, suggests a placebo-controlled phase 3 trial.

Iacopo Olivotto, MD, Careggi University Hospital, Florence, Italy

Dr. Iacopo Olivotto

Treatment with mavacamten (MyoKardia) worked partly by alleviating high-pressure gradients in the left ventricular outflow tract (LVOT), a key characteristic of obstructive HCM. Its effects appeared consistent across a wide range of objective and patient-assessed endpoints.

Mavacamten is “the first potential medical therapy addressing the underlying biology of symptoms in hypertrophic obstructive cardiomyopathy,” observed Iacopo Olivotto, MD, Careggi University Hospital, Florence, Italy.

Patients in the EXPLORER-HCM trial who took the new drug showed improvements in “every aspect of objective performance and subjective well-being,” Dr. Olivotto said at a preview for journalists before his formal online presentation of the results during the virtual European Society of Cardiology Congress 2020, staged in lieu of the traditional annual meeting because of the COVID-19 pandemic.

Dr. Olivotto, also lead author on the study’s same-day publication in The Lancet, was exuberant about the findings. “It is really hard to convey what this actually means for a scientific and clinical community that has spent over 60 years trying to understand and cure hypertrophic cardiomyopathy.”

MyoKardia released abbreviated top-line results of EXPLORER-HCM in May, which were reported by | Medscape Cardiology at the time.

“I think it’s pretty exciting. We certainly need more and better drugs for this patient population,” Arnon Adler, MD, who is not associated with the trial but follows HCM at the Peter Munk Cardiac Centre, Toronto General Hospital, said in an interview.

The trial compared the new drug to placebo rather than full contemporary drug therapy for obstructive HCM, Adler cautioned, and had a fairly short follow-up time. But he was impressed that mavacamten’s apparent benefits seemed consistent not only for endpoints like change in New York Heart Association (NYHA) functional class and quality of life but also for more objective measures like peak VO2 and LVOT gradients.

“I think the results were promising across the board,” he told.

Unique mechanism of action

Mavacamten is described as a first-in-class, small-molecule, selective allosteric inhibitor of cardiac myosin adenosine triphosphatase that addresses the underlying pathophysiology of HCM by reducing actin–myosin cross-bridge formation. It thereby inhibits the excessive myocyte contractility that is a key mechanism of the disorder’s tell-tale hypertrophy, something the available HCM drug therapies don’t do.

Almost three-fourths of patients in the trial were initially in NYHA class 2. Such patients in practice tend to be treated pharmacologically, with more invasive but generally effective surgical myectomy and alcohol septal ablation performed more often for patients in NYHA class 3.

“In the EXPLORER-HCM trial, patients enrolled did not have any immediate indication for surgery,” although many of them in NYHA class 2 would likely progress to NYHA 3, Dr. Olivotto said in an interview.

Based on the trial, he said, it’s possible that mavacamten could lead to “earlier and broader treatment of obstruction symptoms in patients who would never have qualified for surgery in the first place because their symptoms may not be severe enough, but they are still limited.”

Notably, the published report notes, 27% of patients taking mavacamten achieved what was defined as a complete response – that is, a reduction of all LVOT gradients to less than 30 mm Hg in the total absence of symptoms.

Only 1% of patients in the placebo-treated control group met that goal, “showing that mavacamten might be capable of achieving marked relief of symptoms and LVOT obstruction,” the report states.

In the trial, “treatment with mavacamten led to clinically meaningful improvements in hemodynamic status, functional capacity, and subjective well-being in patients with obstructive hypertrophic cardiomyopathy,” agrees an editorial accompanying the EXPLORER-HCM publication.

Mavacamten might even compare favorably to surgery and ablative therapy, speculated the editorialists, Michael Papadakis, MBBS, MD, and colleagues of St. George’s University Hospitals NHS Foundation Trust, London. The drug appeared to reduce the peak LVOT gradient “to less than the guideline-based threshold for septal reduction therapy, 50 mm Hg, in 74% of patients, compared with 21% in the placebo group, indicating that mavacamten could represent a valid alternative to highly specialized invasive therapy,” they wrote.


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