Study to Evaluate the Efficacy of Immunosuppression in Myocarditis or Inflammatory Cardiomyopathy.

Myocarditis can result in numerous complications, but there is paucity of data regarding optimal therapy, short- and long-term effects of possibly effective immunosuppressive therapy. The IMPROVE-MC study will provide high-quality scientific data about efficacy and safety of immunosuppressive therapy, non-invasive (MRI, biomarkers) and invasive diagnostics tests (endomyocardial biopsy), and prognosis in myocarditis. The objective of this multicenter, prospective, randomized, double-blind placebo-controlled trial is to assess the efficacy and safety of 12 - month treatment with prednisone and azathioprine comparing to placebo on top of guideline-recommended medical therapy in patients with biopsy-proven virus negative myocarditis or inflammatory cardiomyopathy and reduced ejection fraction (LVEF ≤ 45%). The study will also assess persistence of the treatment effects after 12 months.

Inclusion Criteria: To be eligible for inclusion in this study, patient must fulfill all of the following inclusion criteria: 1. Written informed consent to participate in the IMPROVE-MC study (including two EMBs and two cardiac CMRs) prior to any evaluation or procedure related to the study. 2. Patient with clinically suspected myocarditis or inflammatory cardiomyopathy (according to the criteria of the ESC Working Group on Myocardial \& Pericardial Diseases 2013 and ESC Heart Failure Guidelines 2021); OR/ AND, Patients with already diagnosed active myocarditis (lymphocytic or eosinophilic) or inflammatory cardiomyopathy who will undergo diagnostic right ventricular (or/and left ventricular) EMB during the screening; OR / AND, Patients with already diagnosed active myocarditis (lymphocytic or eosinophilic) or inflammatory cardiomyopathy confirmed by right ventricular (or/and left ventricular) EMB that was performed according to the IMPROVE-MC study protocol within 3 months from screening. 3. Men or women aged 18-65. Women of childbearing age must have a negative pregnancy test result. Female patients must be 1 year post-menopausal, surgically sterile, or using an acceptable method of contraception (with a failure rate of \< 1% per year) for the duration of the study (from the time they sign consent) and for 8 weeks after the last dose of study treatment to prevent pregnancy. Patients agreeing to total sexual abstinence can also be included, assuming it is their usual lifestyle. Women are considered postmenopausal and without the potential to have a child if they have 12 months of natural (spontaneous) amenorrhea with an appropriate clinical picture (e.g. appropriate age, history of vasomotor symptoms) or have undergone bilateral surgical ovariectomy (with or without hysterectomy) or tubal ligation at least six weeks ago. In the case of ovariectomy alone, only if the reproductive status of the woman has been confirmed by assessing hormone levels. 4. No significant improvement in clinical condition or worsening course of the disease despite the standard treatment in the investigator's opinion, in the last ≥ 3 months prior to the screening period. 5. LVEF 10 - 45% measured by echocardiogram taken during the screening period 1. No significant LVEF improvement in the last ≥3 months prior to the screening period in the investigator's opinion. 2. LVEF should be measured under stable conditions as assessed by the investigator. 3. LVEF should be verified in the CORE-LAB. 6. Histological and immunohistochemical evidence of active myocarditis (lymphocytic or eosinophilic) OR inflammatory cardiomyopathy during the screening period (EMB during the screening or within last 3 months). 7. Absence of cardiotropic viruses in cardiac tissue at PCR analysis during the screening period (EMB during the screening or within last 3 months). Exclusion Criteria: Patients fulfilling any of the following exclusion criteria are not eligible for inclusion in this study. No additional exclusions may be applied by the investigator, in order to ensure that the study population will be representative of all eligible patients. 1. Presence of contraindications to immunosuppressive therapy with steroids and/ or azathioprine (including hypersensitivity to azathioprine/ 6-mercaptopurine or prednisone, mainly untreated systemic infection, uncontrolled diabetes, poorly controlled endocrine diseases, osteoporosis, active gastric or duodenal ulcer, uncontrolled hypertension, leukocytopenia (leukocyte counts \