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Patients who meet all of the following criteria will be eligible to participate in the study: 1. Male or female patients at least 18 years of age; 2. Electrographically documented history of PSVT (e.g., electrocardiogram [ECG] obtained during an episode of PSVT, Holter monitoring, loop recorder, etc). If patient had a prior ablation for PSVT, patient must have documented ECG evidence of PSVT post-ablation; 3. History of sustained episodes of PSVT (i.e., typically lasting approximately 20 minutes or longer); 4. Females of childbearing potential who are sexually active with a male partner who is not surgically sterile (i.e., vasectomy) must agree to use a highly effective form of contraception from the time of signed informed consent until 30 days after the last administration of study drug. Females of childbearing potential should have a negative serum pregnancy test result at the Screening Visit and at the Final Study Visit, a negative urine pregnancy test at the Test Dose Randomization Visit and must use a highly effective form of contraception between the visits. The following categories define females who are NOT considered to be of childbearing potential: - Premenopausal females with 1 of the following: 1. Documented hysterectomy, 2. Documented bilateral salpingectomy or tubal ligation; or 3. Documented bilateral oophorectomy, or - Postmenopausal females, defined as having amenorrhea for at least 12 months without an alternative medical cause; 5. Male patients, except those who are surgically sterile, must use an approved highly effective form of contraception during the 3 days after any study drug administration; and 6. Signed written informed consent.

Patients who meet any of the following criteria will be excluded from participation in the study: 1. Systolic blood pressure <90 mmHg after a 5-minute rest in sitting position at the Screening Visit or before the test dose. In patients treated with a chronic prophylactic drug for PSVT (e.g., beta-blockers, verapamil, and diltiazem), the drug may be stopped for at least the equivalent of 5 half-lives, patients may be rescreened once, and chronic use of the drug cannot be restarted after randomization; 2. History of severe symptoms of hypotension, especially syncope, during episodes of PSVT; 3. History of atrial arrhythmia that does not involve the AV node as part of the tachycardia circuit (e.g., atrial fibrillation, atrial flutter, intra-atrial tachycardia); 4. History of allergic reaction to verapamil; 5. Current therapy with digoxin or any Class I or III antiarrhythmic drug, except if these drugs are stopped at least the equivalent of 5 half-lives before the Test Dose Randomization Visit; 6. Current chronic therapy with oral amiodarone, or have taken oral amiodarone within 30 days prior to the Test Dose Randomization Visit; 7. Evidence of ventricular pre-excitation (e.g., delta waves, short PR interval <100 msec, Wolff-Parkinson-White syndrome) on the ECG performed at the Screening Visit or before the test dose administration; 8. Evidence of a second- or third-degree AV block on the ECG performed at the Screening Visit or before the test dose administration; 9. History or evidence of severe ventricular arrhythmia (e.g., torsades de pointes, ventricular fibrillation, or ventricular tachycardia); 10. Current congestive heart failure defined by the New York Heart Association Class II to IV; 11. History of Acute Coronary Syndrome or stroke within 6 months of screening; 12. Evidence of hepatic dysfunction defined as alanine aminotransferase or aspartate aminotransferase >3 × the upper limit of normal (ULN) or total bilirubin >2 × ULN at the Screening Visit, unless due to Gilbert syndrome; 13. Evidence of End-Stage Renal Disease as determined by an estimated glomerular filtration rate assessed at the Screening Visit of <15 mL/min/1.73m2, or requiring hemodialysis; 14. Females who are pregnant or lactating; 15. Evidence or history of any significant physical or psychiatric condition including drug abuse, which, in the opinion of the Investigator, could jeopardize the safety of patients, or affect their participation in the study. Additionally, the Investigator has the ability to exclude a patient if for any reason the Investigator judges the patient is not a good candidate for the study or will not be able to follow study procedures; 16. Participation in any investigational drug or device study or the use of any investigational drug or device within 30 days of the Screening Visit; or 17. Previously enrolled in a clinical trial for etripamil and received study drug during a perceived episode of PSVT. Before randomization in the RAPID study, all patients will receive a test dose of an etripamil NS dosing regimen (an initial dose of etripamil NS 70 mg followed by a second dose of etripamil NS 70 mg not earlier than 10 minutes and not later than 15 minutes after the first dose) to evaluate tolerability and to train patients on the study procedures. Both doses of the etripamil dosing regimen must be administered for the test dose to be considered evaluable. A failure of the test dose is considered if patients meet any of the following criteria occurring after administration of the either the first or second dose of etripamil NS 70 mg: 1. Any symptoms consistent with clinically severe hypotension such as pre-syncope, medically significant lightheadedness, syncope, nausea, or vomiting; 2. For patients with a pre-test dose Systolic Blood Pressure above 100 mmHg: 1. Decrease in SBP ≥40 mmHg after test dose; or 2. Post-test dose SBP <80 mmHg; 3. For patients with a pre-test dose SBP between 90 mmHg and 100 mmHg (inclusive): a) Post-test dose SBP <75 mmHg; 4. Third-degree AV block, Mobitz II second-degree AV block, or Wenckebach with bradycardia ≤40 bpm; 5. New, significant sinus bradycardia Heart Rate ≤40 bpm or sinus pauses (≤3 seconds), if considered by the Investigator to put the patient's safety at risk if either were to occur while not under medical supervision; 6. Any new ventricular arrhythmia considered significant by the Investigator; or 7. Atrial fibrillation, atrial flutter or atrial tachycardia (event lasting longer than 30 seconds); 8. Refusal of second dose of etripamil test dose regimen. Patients who fail the test dose will proceed in the study as follows: - If the Investigator identifies a possible reversible cause of the initial test dose failure (e.g., concomitant medication such as beta-blocker), a re-challenge with a new test dose of etripamil NS 70 mg will be possible after elimination of the reversible cause (e.g., withdrawal of concomitant therapy with the appropriate washout period). Patients may be randomized if they pass the second test dose and the cause of the test dose failure is eliminated for the duration of the study; or - If the Investigator cannot identify a reversible cause of the initial test dose failure, or if the potential cause cannot be modified (e.g., necessary antihypertensive drug to control blood pressure), patients will not be randomized and will complete a Final Study Visit. Patients who fail the test dose will be part of the Test Dose Only Population.