This is a non-confirmatory, multicenter, open-label, non-randomized trial which will extend active treatment to those patients that participated in the preceding proof-of-concept study (CBYM338X2205) in order to collect long-term safety and tolerability data. Up to 14 patients with sIBM will be invited to enroll into the study to receive BYM338 open-label for a period of approximately 3 years or until BYM338 is commercially available, whichever comes first. The study consists of a maximum 28-day screening period, a 5-day baseline period, and a treatment period consisting of the site visits at 4-week intervals for treatment administration, safety and pharmacokinetic follow-up. All patients will be administered a medium level i.v. BYM338 dose regardless of their treatment allocation in the prior study.