The MYOCIT study aims to evaluate the efficacy and safety of baricitinib in association with corticosteroids in new-onset patients with juvenile dermatomyositis (JDM) in a phase II trial with the objective to obtain a better efficacy than the conventional combination methotrexate (MTX) and corticosteroids over the 24 week study period. Thus, the investigators hypothesize that baricitinib could be used as a first line treatment in all forms of DMJ, including the most severe one, with a good safety profile.

Juvenile dermatomyositis (JDM) is a rare and severe paediatric-onset idiopathic inflammatory myopathy, associated with significant morbidity and mortality. The combination of corticosteroids and methotrexate (MTX) is recommended in new-onset JDM according to one randomized trial. However, in this trial, treatment failures were reported in 13/46 (28%) patients and severe JDM, (cutaneous or gastrointestinal ulceration, interstitial pulmonary disease, cardiomyopathy) were not taken into account. These data emphasize the need for a more efficient first-line treatment. Considering: 1) the strong implication of type IFN-I in the pathophysiology of JDM 2) the report of the efficacy and safety of JAK inhibitors (JAKis) (baricitinb, tofacitinib) in about 50 refractory DM patients, and 9 JDM, a trial which evaluates the efficacy and safety of baricitinib in combination with corticosteroids in new-onset JDM is warranted.

Inclusion Criteria:

• Patient aged 3-18 years with new-onset juvenile dermatomyositis, according to the ENMC 2018 dermatomyositis classification criteria
• Muscle weakness at MMT and/or CMAS (MMT < 74 and/or CMAS < 45)
• Seropositivity or vaccination for chickenpox
• For patients of childbearing age (following menarche) : Negative βHCG and effective method of contraception (sexual abstinence, hormonal contraception, intrauterine device or hormone-releasing system, cap, diaphragm or sponge with spermicide, condom) until the 7 days after administration of the last dose of Baricitinib
• Informed consent form signed by the patient or child' s parents Patient affiliated to a social security regime

Exclusion Criteria

• Amyopathic dermatomyositis (without muscle weakness)
• Inability to be treated by oral way or to take pills
• Previous treatment with JAK inhibitor
• Previous treatment of JDM with immunosuppressive drugs or biologics other than corticosteroids. Previous treatment with prednisone was allowed for no more than 1 month.
• Previous history of cancer
• Live vaccine within the 4 weeks before starting baricitinib therapy
• Current, or recent (< 4 weeks prior to baseline) of active infections according to investigator appreciation, but necessarily, including HBV, HCV, HIV, tuberculosis.
• Positive blood CMV PCR
• Creatinine clearance < 40 ml/min
• Lymphocytes < 0,5x109 cell/L and Neutrophils < 1x109 cell/L
• Hemoglobin < 8 g/dL
• Symptomatic herpes herpes simplex infection within 12 weeks prior to inclusion
• History of thrombosis or considered at high risk of venous thrombosis by the investigator
• Presence of severe JDM-related involvements: cardiovascular (requiring vasopressive drug and/or intensive care unit), respiratory (requiring oxygen and/or intensive care unit), gastrointestinal (requiring abdominal surgery).
• History of severe non-related JDM involvement: cardiovascular, respiratory, hepatic, gastrointestinal, endocrine, hematological, neurological or neuropsychiatric disorders or any other serious and/or instable illness that, in the opinion of the investigator, could constitute an unacceptable risk, when taking baricitinib.
• Actual or in project of pregrancy and breast-feeding until the 7 days after administration of the last dose of Baricitinib
• Patient on AME (state medical aid)
• Participation in another interventional study involving human participants or being in the exclusion period at the end of a previous study involving human participants