Abstract Body (Do not enter title and authors here): Heart failure with preserved ejection fraction (HFpEF) is an increasingly prevalent and debilitating form of heart failure, associated with significant morbidity and mortality, yet it has few effective treatments. A hallmark of HFpEF pathophysiology is impaired intracellular calcium (Ca²⁺) handling, driven in part by reduced expression and function of the cardiac sarcoplasmic reticulum Ca²⁺ -ATPase (SERCA2a) pump. Direct SERCA2a activation through gene therapy holds promise to provide targeted myocardial relaxation for patients with HFpEF, with potential consequent clinical benefit. Delivery of an adeno-associated type 1 vector carrying SERCA2a (AAV1.SERCA2a) improved lusitropy in a dose-dependent manner in bioengineered, three-dimensional, human mini-heart tissue models of HFpEF created using human pluripotent stem cell-derived ventricular cardiomyocytes. The ongoing first-in-human phase 1b, open label trial (MUSIC-HFpEF), supported by the mini-heart potency assay, is designed to evaluate safety and explore efficacy in two escalating doses of AAV1.SERCA2a: 3E13 viral genome (vg)/patient (n=5) and 4.5 E13 vg/patient (n=5). The vector is delivered through a slow intracoronary infusion in patients (N=10) with HFpEF and a resting pulmonary capillary wedge pressure of at least 15 mmHg. The trial has received Fast Track Designation from the FDA. The primary endpoint is change in pulmonary capillary wedge pressure (PCWP) at baseline and different workloads. The secondary endpoints include changes in NYHA, 6-minute walk test (6MWT) distance, NYHA class, NT-proBNP, high sensitivity troponin, early diastolic mitral annular velocity (e′), and left ventricular tau. The trial has completed enrollment with five patients having received a dose of 3E13 vg/patient of AAV1.SERCA2a and five patients having received a dose of 4.5E13 vg/patient with follow-up between 1 to 20 months. There have been no gene therapy or procedure related serious adverse events. Four out of the five patients in the 3E13vg/patient group demonstrated improvements in NYHA heart failure classification at 6 & 12 months. Clinically meaningful improvements in 6MWT, and decrease/stabilization in NT-Pro-BNP and in troponin have also been observed in some of the patients. Preliminary data from this ongoing study indicate that AAV1.SERCA2a may represent a promising therapeutic avenue for patients with HFpEF, addressing a critical area of unmet clinical need.