An Extension Study of the Efficacy, Safety and Tolerability of BYM338 (Bimagrumab) in Patients With Sporadic Inclusion Body Myositis Who Previously Participated in the Core Study CBYM338B2203

Extension of the CBYM338B2203 Phase IIb/III Study to Evaluate the Long-term Efficacy, Safety and Tolerability of Intravenous BYM338 in Patients With Sporadic Inclusion Body Myositis

This extension study will provide data to further evaluate the efficacy, safety, and tolerability of three doses of BYM338 and to assess the long-term effects of BYM338 in patients with sporadic inclusion body myositis. The extension study was planned to consist of a Screening epoch (to assess patient eligibility), followed by a Treatment Period 1 epoch (double-blind and placebo-controlled), and a Treatment Period 2 epoch (open-label). A Post-treatment Follow-up (FUP) epoch was also planned for patients who discontinued prematurely. Patients who complete the core study and qualify for this extension study entered Treatment Period 1 and continued on the study drug to which they were randomized in the core study (either to one of the three bimagrumab doses (1 mg/kg, 3 mg/kg, and 10mg/kg) or placebo) during Treatment Period 1. Thus, Treatment Period 1 was double-blind and placebo-controlled. Participants were to continue in Treatment Period 1 until the dose with the best benefit-risk profile was determined from the core study data and selected (duration of Treatment Period 1 was estimated to be between 6 and 8 months). Once the dose with the best benefit-risk profile was selected, all participants (including those who were receiving placebo) were planned to enter Treatment Period 2 and switch to open-label treatment with bimagrumab at the selected dose. The core study has been completed but since the core study did not meet the primary end point (no bimagrumab dose was identified based on the core study efficacy results) the extension study was terminated as per protocol/sponsor's decision; therefore, no patients had entered Treatment Period 2. Instead, all patients were to return for the End of Treatment Period 1 (EOT1) visit at their next scheduled visit. As per protocol, all patients who discontinued study medication during Treatment Period 1 for any reason, including due to the study having been stopped as per protocol/sponsor's decision, were to have entered and complete the 6-month FUP after their EOT1 visit.

Due to the nature of the design of the core and extension studies and termination of study medication in the extension study, the treatment duration for individual patients varied considerably. Consequently, the number of patients contributing data to the efficacy analyses at Week 104 and later timepoints was decreased.

Inclusion Criteria:

• Patients who completed the core study
• Written informed consent must be obtained before any extension study assessment is performed.
• Able to communicate well with the investigator.
• Willing to participate for the entire duration of the extension study with commitment to follow study requirements and procedures.

Exclusion Criteria:

• Women who are pregnant
• Women of child-bearing potential unless they are using highly effective methods of contraception during dosing and for 6 months after the last BYM338 dose.
• Current use of prohibited treatments
• History of severe hypersensitivity reaction in the core study
• History of adverse event(s) (including those from the core study) prior to the start of study drug in the extension study that, in the judgment of the investigator, taking into account the subject's overall status, prevent the subject from entering the extension study
• Clinically significant abnormal liver function tests
• Any medical condition or laboratory finding which, in the opinion of the investigator may interfere with participation in the study, might confound the results of the study, or pose an additional safety risk in administering BYM338