Safety, Tolerability, and Exploratory Efficacy of AGP100 in Patients With Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT)

Inclusion Criteria: 1. Signed informed consent prior to any study-related procedures 2. Male or female, aged between 18 and 75 years (inclusive) 3. Clinical diagnosis of CPVT based on proven RYR2 mutation AND reproducible premature ventricular contraction with exercise or polymorphic or bidirectional ventricular tachycardia with exercise 4. Able and willing to undergo exercise testing (bicycle test) AND exhibits exercise-induced ventricular ectopic beats at Screening (at least 1 point on the VA scale) 5. On stable, maximum tolerated, dose of non-selective β-blocker for at least 4 weeks before Visit 1. The dosage and choice of β-blocker are to be determined by the patients' physician(s) before entry into the study and must remain unchanged throughout the conduct of the study. Participants taking a stable dose of flecainide for at least 4 weeks, in addition to β-blocker, are also eligible. 6. Clinical laboratory evaluations including clinical chemistry, haematology, urinalysis, thyroid function (including thyroid stimulating hormone, triiodothyronine, thyroxine, and free T4) and coagulation testing (activated partial thromboplastin time, and international normalized ratio) within the reference range, unless deemed not clinically significant by the Investigator 7. Willing to refrain from strenuous or new exercise for 24 hours before each study visit 8. Women of childbearing potential (WOCBP) agree to implement accepted and highly effective means of contraception from study entry until at least 33 days after study drug discontinuation (as per the Clinical Trials Facilitation and Coordination Group guidelines). The main exclusion criteria are: 1. Diagnosis of structural heart disease, including coronary artery disease or heart failure with reduced ejection fraction (left ventricular ejection fraction \<45%) 2. Participants who have had arrhythmias causing hemodynamic instability at previous exercise tests (performed while on the current standard of care treatment) 3. Participants having a sustained VT (VA score of 5) during the exercise tests performed as part of the screening activities 4. Participation in another clinical study with an investigational product or device within 60 days of 5 half-lives prior to Baseline (whichever is longer) 5. Medical history of severe anaphylactic reactions to any component(s) of the IMP 6. Sensitivity to any of the study treatments, or components thereof, or any drug or other allergy that, in the opinion of the Investigator precludes participation in the study 7. Hypersensitivity or contraindication to PDE2 inhibitor drugs 8. Use of PDE3, PDE4, or PDE5 inhibitor drugs. 9. Participants taking any antiarrhythmic drug(s) except flecainide and non-selective β-blockers 10. Significant hypertension (defined as systolic blood pressure of \>160 mmHg and/or diastolic blood pressure of \>95 mmHg). If the blood pressure results are out of range at Screening, the measurements can be repeated on the same day more than once, or at another convenient visit 11. Prolonged PR and/or QTc interval at Screening, defined as PR \>240 ms or QTc \>480 ms