Randomized Trial to Examine a Differential Therapeutic Response in Symptomatic Patients With Non-obstructive Coronary Artery Disease

Inclusion Criteria: * Age 18 - 85 years * Recurrent angina symptoms provoked by exercise and/or repeated attacks of angina at rest (both at least for 4 weeks) * Absence of flow-limiting coronary artery stenosis (as defined by any coronary artery diameter reduction \>50% or fractional flow reserve ≤0.80) * Left ventricular ejection fraction (LVEF) \>50% * Written informed consent Exclusion Criteria: * Pregnancy, planned pregnancy, or breast-feeding * Female patients of childbearing potential who are unwilling to use a highly effective contraception method during trial participation according to CTFG. In addition, a negative serum or urine pregnancy test must be available prior to randomization. * Expected life expectancy \<1 year * Contraindications to withholding nitrates, calcium channel blockers, and beta blockers for 48 hours before invasive coronary reactivity testing (e.g. clinical need for rate control in case of permanent atrial fibrillation, recurrent angina symptoms without any possibility to wihthold ongoing medication) * Known hypersensitivity or contraindication to bisoprolol or diltiazem or any of its excipients. * Concomitant therapy with systemic drugs that are strong inhibitors of both CYP3A4 and P-gp (azole antimycotics such as ketoconazole and itraconazole or HIV protease inhibitors such as ritonavir) * Concomitant therapy with drugs that are strong CYP3A4 inducers (e.g. carbamazepine, phenytoin, rifampicin, St. John's wort) * Bradycardia (\<50/min) at ti…