Benefit fades quickly when mavacamten stops
More detailed analysis of quality of life findings in the new report also showed that, while average KCCQ scores (both overall summary score and clinical summary score) steadily improved with mavacamten treatment, compared with control patients, through 18 weeks on treatment, the scores then roughly plateaued out to 30 weeks. This was followed by a sharp reversal back down to baseline levels and similar to control patients 8 weeks after stopping mavacamten, suggesting that the drug’s benefit quickly fades off treatment and hence must be taken chronically.
The responder analysis showed that 9% of patients on mavacamten had a worsening in their KCCQ overall summary scores by more than 5 points after 30 weeks, compared with 23% of the control patients. In contrast, a very large improvement in KCCQ score, defined as a rise of at least 20 points from baseline after 30 weeks, occurred in 36% of those who received mavacamten and in 15% of the controls. The between-group difference indicates a number needed to treat with mavacamten of roughly five to produce one additional patient with a very large improvement in KCCQ overall summary score, Dr. Spertus noted.
By design, all patients enrolled in EXPLORER-HCM had a left ventricular ejection fraction of at least 55%. During treatment, seven of the mavacamten-treated patients and two in the control arm had a transient decrease in their left ventricular ejection fraction to below 50%, although this later normalized in all affected patients. “An initial criticism” of the trial was that a significant percentage of mavacamten patients “developed left ventricular dysfunction” noted Dr. Martinez, but Dr. Spertus highlighted the poor apparent correlation between this phenomenon and quality of life self-assessment. Six of the seven patients on mavacamten who had a transient drop in their left ventricular ejection fraction had very large improvements in their KCCQ summary scores, Dr. Spertus reported.
Hypertrophic cardiomyopathy is a myocardial disorder characterized by primary left ventricular hypertrophy. Although a complex disease, HCM is broadly defined by pathologically enhanced cardiac actin-myosin interactions that result in hypercontractility, diastolic abnormalities, and dynamic left ventricular outflow tract obstruction. Mavacamten is a first-in-class, small-molecule, selective allosteric inhibitor of cardiac myosin ATPase developed to target the underlying pathophysiology of HCM by reducing actin-myosin cross-bridge formation, thereby reducing contractility and improving myocardial energetics.
EXPLORER-HCM was sponsored by MyoKardia, the company developing mavacamten and a subsidiary of Bristol-Myers Squibb. Dr. Spertus has been a consultant to MyoKardia, as well as to Abbott, Amgen, Bayer, Janssen, Merck, and Novartis. He has received research support from Abbott Vascular, and he holds the copyright for the KCCQ. Dr. Martinez has been a consultant to and received honoraria from Bristol-Myers Squibb.