Baricitinib in the Treatment of New-onset Juvenile Dermatomyositis (MYOCIT)

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 i…