Inclusion Criteria:
* Willing and able to provide informed consent
* Negative for anti-AAV1 neutralizing antibodies
* NYHA class II or III
* Left ventricular ejection fraction ≥ 50%
* Evidence of resting or exercise-induced left ventricle filling pressure
* On oral diuretic therapy
* Adequate birth control
Exclusion Criteria:
* NYHA class IV
* Heart failure requiring hospitalization in the past 3 months
* Manifested or provocable ischemic heart disease
* Atrial fibrillation
* History of congenital heart disease, restrictive or infiltrative cardiomyopathy, hypertrophic cardiomyopathy, acute myocarditis, pericardial disease, uncorrected thyroid disease or discrete left ventricular (LV) aneurysm
* History of amyloidosis
* Untreated left-sided valvular disease
* Severe COPD
* BMI \> 50 kg/m\^2
* Severe liver, kidney or hematologic dysfunction
* Cancer within the past 5 years
* Unstable concurrent conditions
View Inclusion and Exclusion Criteria at ClinicalTrials.gov