The European Medicines Agency has required pre-treatment genotyping of CYP2C19 to determine the initial and maximum dosage, in order to avoid overexposure to mavacamten associated with a decrease in ventricular ejection fraction below 50% in slow metabolisers of CYP2C19 (AUC multiplied by 3.4). The study is a requalification for the search for DNA samples obtained during treatment in order to genotype the genes of interest.The primary objective of the study is to estimate, for each of the CYP2C19 phenotypes of interest determined by genotyping (ultra-rapid, rapid, normal/extensive and intermediate metabolisers), the proportion of patients who are non-responders to mavacamten at each time point (D0, Week 4, Week 8, Week 12 and Week 24 in the treatment of HCM). This is a multicentre (3 centres which are hospitals of APHP) study aiming to include 300 patients with obstructive hypertrophic cardiomyopathy treated with Mavacamten who underwent or are undergoing CYP2C19 genotyping at the start of treatment. The inclusion period is 36 months and the follow-up period is 6 months. The total duration of the study is 42 months.

Before the arrival of mavacamten, preventive and symptomatic management of patients with obstructive hypertrophic cardiomyopathy relied on the use of non-vasodilating beta-blockers or non-dihydropyridine calcium channel blockers, or the addition of disopyramide in combination therapy with a beta-blocker. When dynamic obstruction of the left ventricular outflow tract (LVOT) persisted, invasive septal reduction treatments were offered, which were associated with significant morbidity and mortality.

The introduction of mavacamten, the first reversible and selective inhibitor of cardiac myosin ATPase activity, made it possible to relieve LVOT obstruction and reduce myocardial hypercontractility after a period of titration, which varied in length depending on the patient, guided by echocardiography. The relationship between plasma concentrations of mavacamten and echocardiographic parameters has not been clearly established. However, this new drug is characterised by significant inter-individual pharmacokinetic variability, linked in part to genetic polymorphisms of CYP2C19, the hepatic enzyme responsible for its metabolism. Therefore, to ensure its safe use, the European Medicines Agency has required pre-treatment genotyping of CYP2C19 to determine the initial and maximum dosage, in order to avoid overexposure to mavacamten associated with a decrease in ventricular ejection fraction below 50% in slow metabolisers of CYP2C19 (AUC multiplied by 3.4). The study is a requalification for the search for DNA samples obtained during treatment in order to genotype the genes of interest. The primary objective of the study is to estimate, for each of the CYP2C19 phenotypes of interest determined by genotyping (ultra-rapid, rapid, normal/extensive and intermediate metabolisers), the proportion of patients who are non-responders to mavacamten at each time point (D0, Week 4, Week 8, Week 12 and Week 24 in the treatment of HCM). This is a multicentre (3 centres : La pitie Salpetriere hospital, Bicetre hospital and Ambroise Paré hospital). The study involves 2 non recruiting centres : the Molecular Genetics - Pharmacogenetics Service (which realize the genetic analyses) and the Biological ressources center (which preserves the bioological collection). The study aiming to include 300 patients with obstructive hypertrophic cardiomyopathy treated with Mavacamten who underwent or are undergoing CYP2C19 genotyping at the start of treatment. The inclusion period is 36 months. The recruitment of the patient is retrospective and prospective. The follow-up period is 6 months. The total duration of the study is 42 months.

Inclusion Criteria:

• - Patients aged ≥18 years
• Diagnosis of obstructive hypertrophic cardiomyopathy (OHCM), based on the guidelines of the European Society of Cardiology (Arbelo et al. 2023), with unexplained left ventricular hypertrophy and a maximum LVOT gradient ≥ 50 mmHg at rest, or after Valsalva manoeuvre or exercise at the time of diagnosis, and an LVOT gradient with Valsalva manoeuvre ≥ 30 mmHg at selection
• With cardiac symptoms defined as NYHA class II/III, persistent despite background treatment (beta-blockers or calcium channel blockers)
• LVEF ≥55% at the start of treatment with mavacamten
• Initiation of treatment with mavacamten or patient already receiving treatment
• Prescription of pre-treatment CYP2C19 genotyping performed
• Consent to participate signed by the patient
• Beneficiary of health insurance

Exclusion Criteria:

• Minors
• Adults under guardianship (legal guardianship and curatorship) or judicial protection
• Pregnant or breastfeeding women

300 patients with obstructive hypertrophic cardiomyopathy treated with Mavacamten who underwent or are undergoing CYP2C19 genotyping at the start of treatment.