Milestone Pharmaceuticals MIST announced that the FDA issued a Refusal to File (RTF) letter for the new drug application (NDA) seeking approval for self-administered etripamil nasal spray for the treatment of paroxysmal supraventricular tachycardia (PSVT).

Per the FDA, the NDA for etripamil to treat PSVT, submitted in October 2023, was not sufficiently complete to permit substantive review. The regulatory body has requested clarification about the time of data recorded for adverse events related to treatment with etripamil in phase III studies supporting the NDA.

However, Milestone Pharma stated that the FDA has not shown any concern about the nature or severity of the adverse events. The company is currently preparing to meet with the FDA to seek clarification for the RTF issued to the NDA seeking approval for etripamil to treat PSVT and figure out a way forward.

Milestone Pharma’s stock plunged 30.9% in the last trading session as investors were disappointed by the unfavorable regulatory update on the NDA for etripamil to treat PSVT. Year to date, MIST shares have plummeted 49.7% compared with the industry’s 16.1% fall.

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Etripamil, a novel calcium channel blocker nasal spray, is the company’s lead and only investigational candidate. The FDA has approved a conditional brand name for etripamil, Cardamyst.

The NDA submission was based on positive results from the completed phase III RAPID study of etripamil, evaluating the safety and efficacy of the candidate for treating PSVT compared with placebo. The late-stage study met its primary endpoint observing 64% of patients who self-administered etripamil converted from supraventricular tachycardia to sinus rhythm within 30 minutes compared with 31% on placebo.

At the one-hour mark, the benefit was observed in 73% of the patients who received etripamil. Additionally, patients who took etripamil achieved significant reductions in time to conversion, which were evident early and durable. The median time to conversion for patients treated with etripamil was 17 minutes compared with 54 minutes for patients treated with placebo.

The company further stated that the data from the RAPID study demonstrated statistically significant improvement in multiple defined symptoms of PSVT in patients receiving etripamil compared with placebo, determined using a patient-reported outcome questionnaire.

PSVT is a type of arrhythmia or abnormal heart rhythm and is characterized by episodes of rapid heartbeats often exceeding 150 to 200 beats per minute. Per Milestone Pharma, approximately two million people suffer from PSVT in the United States. This represents a significant unmet medical need.