Safety and Efficacy of Doxycycline in Patients With Non-Ischemic Cardiomyopathy

Inclusion Criteria: * Non-ischemic cardiomyopathy (LVEF\<40%) * Heart failure NYHA II-III Exclusion Criteria: * Age \<18 * Recent changes (previous 3 months) in HF maintenance medications (beta-blockers, angiotensin converting enzyme \[ACE\] inhibitors, aldosterone antagonists, vasodilators, cardiac glycosides, diuretics) * Hospitalization for worsening HF or acute decompensated HF within the previous 12 months * History of coronary or ischemic heart disease * Anticipated need for cardiac resynchronization therapy (CRT) or automated-implantable cardioverter defibrillator (AICD) or coronary revascularization or cardiac surgery * Angina or electrocardiograph (ECG) changes that limit maximum exertion during cardiopulmonary exercise testing * Active infection including chronic infection * Active cancer (or prior diagnosis of cancer within the past 10 years) * Recent (\<14 days) use of anti-inflammatory drugs (not including NSAIDs), Chronic inflammatory disorder (including but not limited to rheumatoid arthritis, systemic lupus erythematosus), malignancy, or any comorbidity limiting survival or ability to complete the study * Pregnancy * Inability to give informed consent * Other conditions limiting completion of cardiopulmonary exercise test or completion of the study