Safety and Efficacy of REGN2477+REGN1033 in Patients With Sporadic Inclusion Body Myositis

The primary objective of the study is to evaluate the efficacy of REGN2477+REGN1033 in combination on total lean mass, as measured by Dual-energy X-ray absorptiometry (DXA) in patients with sporadic inclusion body myositis (sIBM). The secondary objectives of the study are: * To evaluate the efficacy of REGN2477+REGN1033 on the IBM-Functional Rating Scale (IBM-FRS) * To evaluate the efficacy of REGN2477+REGN1033 on the sIBM Physical Functioning Assessment (sIFA) * To evaluate the safety and tolerability of REGN2477+REGN1033 * To evaluate the effects of REGN2477+REGN1033 on body composition by DXA, including appendicular lean mass and total fat mass * To evaluate the efficacy of REGN2477+REGN1033 on measures of muscle performance and physical function * To evaluate the efficacy of REGN2477+REGN1033 on patient reported outcome measures including the fear of falling, falls and near falls, and health-related quality of life * To evaluate the pharmacokinetic(s) (PK) profile of REGN2477+REGN1033, including functional REGN2477 and functional REGN1033 concentrations in serum over time * To evaluate the immunogenicity of REGN2477+REGN1033