Ruxolitinib Treatment in Inclusion Body Myositis

Inclusion Criteria: * Age ≥ 45 years * Effective contraception for the duration of the clinical trial for fertile women of childbearing age * Defined diagnosis of IBM according to data-derived criteria (Llyod et al, 2014): Patient must fulfill the three following criteria for being diagnosed as IBM: (1) finger flexor or quadriceps weakness; and (2) muscle biopsy showing endomysial inflammation; and (3) muscle biopsy showing invasion of nonnecrotic muscle fibers or rimmed vacuoles * To be able to walk 6 min without assistance from another person (external assist devices permitted \[e.g., canes, walkers, or rollators\]) * Patient informed and having signed the consent for participation, possibly assisted by a trusted person Exclusion Criteria: * Pregnancy or breastfeeding * Patient under guardianship, curatorship, safeguard of justice or deprived of liberty * Patient with cognitive disorders or unable, according to the investigator, to understand the study and/or to give informed consent * Quadriceps weakness (manual muscle testing, MRC) below or equal 1 * Forced vital capacity (FVC) or forced expiratory volume (FEV) \< 50% of predicted value * Concomitant use of immunomodulatory drugs including previous treatment with JAK inhibitor, or medications acting on muscle anabolism or catabolism * Live vaccine within the 4 weeks before starting treatment * Comorbidity or active chronic disease which contraindicate ruxolitinib: * Lipid parameters abnormalities/elevations * Severe renal impairment (stage 4) and end-stage renal disease (stage 5) * Hepatic impairment * Cytopenia * Recent history (\<6 months) of cardiovascular or thromboembolic disease (documented coronaropathy or hospitalization for acute arterial thrombosis or stroke or deep venous thrombosis or pulmonary embolism) * Active smoking more than 20 pack-years or history of respiratory or skin cancer or recent history (\<6 months) of other neoplastic disease * Very high cardiovascular risk (red color) at SCORE 2 in case of recent history (\<6 months) of cardiovascular or thromboembolic disease and non-controlled cardiovascular risk factors * History of Stevens-Johnson's syndrome or Lyell's syndrome * Active SARS-CoV-2 infection (patient can be included once infection resolved) * Any medical condition which limits the ability of participant to participate in study * Necessity to use a drug incompatible with ruxolitinib * Hypersensitivity to the IMP's active substance (ruxolitinib) or to any of the excipients * Non-affiliation to a social security scheme or to another social protection scheme, patient on state medical aid * Foreseeable inability, according to the investigator, to participate in all the visits, treatments and measures provided for in the protocol * Concomitant participation in another clinical trial on medical product for human use, to a clinical investigation on a medical device, to interventional study involving human participants or in the exclusion period at the end of a previous clinical trial on medical product for human use, a clinical investigation on a medical device, or study involving human participants. Participation in non-interventional research is permitted.