REGN3918

An SAE is any untoward medical occurrence that at any dose: * Results in death * Is life-threatening * Requires in-patient hospitalization or prolongation of existing hospitalization. * Results in persistent or significant disability/incapacity * Is a congenital anomaly/birth defect. * Is an important medical event

An AESI (serious or non-serious) is one of scientific and medical concern specific to the sponsor's product or program, for which ongoing monitoring and rapid communication by the Investigator to the sponsor can be appropriate. Such an event might warrant further investigation in order to characterize and understand it

Any untoward medical occurrence in a patient administered a study drug which may or may not have a causal relationship with the study drug.

Any untoward medical occurrence in a patient administered a study drug which may or may not have a causal relationship with the study drug.

Cohort A

Cohort B Not requiring a red blood cell (RBC) transfusion per the protocol

Cohort B LDH ≤1.5 × ULN at each visit

The total MG-ADL score ranges from 0 to 24 points, with higher scores indicating greater functional impairment and disability

Open Label Treatment Period (OLTP)

Normalization of serum albumin was defined as serum albumin within the normal range at least 70 percent (%) of measurements between weeks 12 and 24, and no single albumin measurement of \<2.5 grams per deciliter (g/dL) between weeks 12 and 24, and no requirement for albumin infusion between weeks 12 and 24. Improvement in the following 4 prespecified clinical outcomes that were evaluable for improvement at baseline, without worsening of the others: Daily bowel movement frequency, the presence and severity of facial edema (physician-reported), the presence and severity of peripheral edema (physician-reported), and the participant/caregiver assessment of frequency of problematic abdominal pain. Percentage of participants with active disease at baseline who achieved normalization of serum albumin and improvement in prespecified clinical outcomes at Week 24 were reported.

Participants were considered to have had adequate control of intravascular hemolysis if all of their lactose dehydrogenase (LDH) readings from Week 4 through Week 26 inclusive had values less than or equal to ≤ 1.5 × upper limit of normal (ULN). Participants must have greater than or equal to (≥) 50 percent (%) of scheduled LDH measures in those weeks, must not have had more than (\>) 2 consecutive visits without LDH measures, must not have experienced breakthrough hemolysis, and must not have discontinued study treatment early. Participants were considered not to have had adequate control of intravascular hemolysis if they failed any of these criteria.

Transfusion avoidance was defined as not having received red blood cell (RBC) transfusion during the first 26 weeks. A transfusion was counted only if it was per-protocol, that is, it followed the predefined transfusion algorithm: RBC transfusion due to a post-baseline hemoglobin level \< 9 grams per deciliter (g/dL) (with anemia symptoms) or a post-baseline hemoglobin level \< 7 g/dL (without anemia symptoms).

Part A

Part B

Part A

The IBM-FRS is a 10-item interviewer-administered patient-reported outcomes (PRO) measure that assesses physical function in patients with IBM. The IBMFRS includes items assessing lower body physical function, general physical function, the ability to perform certain activities of daily living, fine motor skills, and swallowing. On a Likert scale ranging from 0 (unable to perform activity) to 4 (normal), patients are asked to rate their difficulty in performing tasks relative to each concept before disease onset. Scores on the IBMFRS range from 0 to 40, with higher scores indicating better functioning.

The sIFA is an 11-item self-administered PRO measure assessing physical function in patients with sIBM. This questionnaire will be completed by the patient on a numeric rating scale ranging from 0 (no difficulty) to 10 (unable to do), with a total score ranging from 0 to 100, where 100 indicates severe limitations. P atients will be asked to rate their difficulty with performing tasks involving lower and upper body physical function, general physical function, fine motor skills, and swallowing in the past 7 days.