Efficacy of Endothelin Receptor Antagonism in Coronary Artery Spasm

Inclusion Criteria: 1. Definitive diagnosis of epicardial vasospasm on maximal acetylcholine dose of 100µg (at iCFT) 2. At least 18 years of age 3. On optimal regular care ( current or previous treatment with at least 2 daily anti-anginal medicines i.e. nitrate and calcium channel blocker 4. Continuing episodes of angina(-like) complaints at least once weekly despite 3. 5. Signed online informed consent for participation in NL-CFT registry (the Netherlands Registry of invasive coronary vasomotor function testing), or willing to co-sign for registry at time of inclusion in EDIT-CAS 6. Written informed consent for EDIT-CAS Exclusion Criteria: 1. Systolic blood pressure (SBP) \<85 mmHg measured at Visit 1 2. Significant hepatic impairment at time of iCFT lab (ASAT/ALAT \>3x upper limit of normal (ULN)) or history of liver cirrhosis (Child-Pugh 7-15) 3. Severe anemia (Hb\<6.0mmol/L) without identified cause at time of inclusion 4. Patients with limited life expectancy (\<1 year) 5. Participation in another randomized clinical study with an use of an Investigational Medicinal Product (IMP) up to one month prior to enrolment. 6. Pregnancy, active desire to become pregnant or unwilling to take adequate\* contraceptive measures when of child bearing potential for the duration of 6 months (active medication period + 3 months safety). 7. Known heart failure with reduced ejection fraction\<35% 8. Known pulmonary hypertension of any type 9. Potentially dangerous interaction due to the use of another CYP3A4 or CYP2C9 substrate (e.g. ciclosporin A, glibenclamide, fluconazole, rifampicin, tacrolimus/sirolimus, lopinavir/ritonavir) 10. Repeat spasm provocation test deemed unsafe (e.g. allergic reaction at iCFT) * Adequate in this case meaning if on hormonal contraceptives, additional measures to be taken (e.g. condom).