A Study to Evaluate the Efficacy and Safety of ABC008 for Inclusion Body Myositis

Inclusion Criteria: * Adult males and females age \>40 years at the time of the first dose of study medication; * Weight \>40 and \<150 kg; * Diagnosis of either clinico-pathologically defined IBM, clinically defined IBM, or probable IBM according to the European Neuromuscular Centre (ENMC) IBM 2011 research diagnostic criteria (Rose et al., 2013). Documented histopathology results must be available prior to Baseline (Day 1) to confirm eligibility; * Able to arise from a chair (with armrests), with use of their arms but without support from another person or device (e.g., cane, walking stick), at Screening and Baseline (Day 1); * Able to walk 3 meters, turn around, walk back to the chair, and sit down, with or without assistive device. Once arisen from the chair, subject may use any walking device but cannot be supported by another person, furniture, or a wall; Exclusion Criteria: * Any other form of myositis or myopathy other than IBM, e.g., metabolic or drug-induced myopathy, drug-induced myositis, anti-synthetase syndrome, polymyositis or dermatomyositis, cancer-associated myositis (myositis diagnosed within 3 years, either before or after), myositis in overlap with another autoimmune disease (e.g., systemic lupus, systemic sclerosis, rheumatoid arthritis), or muscular dystrophy; * Any condition, e.g., severe degenerative arthritis with limited range of motion, which precludes the ability to quantitate muscle strength or perform functional assessments (e.g., mTUG), in the Investigator's opinion;. * Presence of another autoimmune or autoinflammatory disease other than indication under study, e.g., rheumatoid arthritis, psoriatic arthritis, axial spondyloarthropathy, inflammatory bowel disease, systemic lupus erythematosus. Subjects with Sjogren's syndrome, T-cell large granular lymphocyte leukemia (T-LGLL), or well-controlled thyroid disease are permitted;