Complete renal response is defined as: (1) urinary protein:creatinine ratio (uPCR) < 0.5 mg/mg with ≥ 50% reduction of uPCR from Day 1 (Baseline; from a 24 hour urine collection); and (2) estimated glomerular filtration rate (eGFR) within normal range. Partial renal response is defined as: (1) ≥ 50% reduction in uPCR from Day 1 (Baseline; from a 24-hour urine collection) and, (2) with one of the following: (a) uPCR of < 1.0 mg/mg if the Day 1 (Baseline) was ≤ 3.0 mg/mg, or, (b) uPCR < 3.0 mg/mg if the Day 1 (Baseline) ratio was > 3.0 mg/mg; and stabilization of renal function (eGFR + or - 25% of Day 1 [Baseline] or serum creatinine within normal range).

Participants in the modified intent-to-treat (mITT) population (participants in ITT population except for those who withdrew from study due to study early termination. Includes participants who completed Week 44 infusion and Visits at Week 52/Early Withdrawal and Week 56/End of Study but withdrew due to study early termination.)

Complete renal response is defined as uPCR < 0.5 mg/mg with ≥ 50% reduction of uPCR from Baseline (from a 24-hour urine collection) and eGFR within normal range.

Participants in the mITT population (participants in ITT population except for those who withdrew from study due to study early termination. Includes participants who completed Week 44 infusion and Visits at Week 52/early withdrawal and Week 56/End of Study but withdrew due to study early termination.)

Duration of response was calculated as the days in between the date of Week 52 visit and the date when the participant last became complete renal responder on or before Week 52 visit. Complete renal response: (1) uPCR <0.5 mg/mg with ≥ 50% reduction of uPCR from Day 1 (Baseline) (from a 24 hour urine collection); and (2) eGFR within normal range.

Participants in the mITT population (participants in ITT population except for those who withdrew from study due to study early termination. (Includes participants who completed Week 44 infusion and Visits at Week 52/Early Withdrawal and Week 56/End of Study but withdrew due to study early termination.)

Onset of renal response was calculated as weeks elapsed from baseline date to first visit where renal response was achieved. Complete renal response is defined as: (1) uPCR <0.5 mg/mg with ≥ 50% reduction of uPCR from Day 1 (Baseline) (from a 24 hour urine collection); and (2) eGFR within normal range. Partial renal response is defined as: (1) ≥ 50% reduction in uPCR from Day 1 (Baseline; from a 24-hour urine collection) and, (2) with one of the following: (a) uPCR of < 1.0 mg/mg if the Day 1 (Baseline) was ≤ 3.0 mg/mg, or, (b) uPCR < 3.0 mg/mg if the Day 1 (Baseline) ratio was > 3.0 mg/mg; and stabilization of renal function (eGFR + or - 25% of Day 1 [Baseline] or serum creatinine within normal range). Estimated from the Kaplan-Meier Curve.

Participants in the ITT population who achieved a renal response at Week 52. The ITT population included all participants who were randomized and received at least 1 dose of study treatment (BIIB023 or placebo).

Participants with a uPCR > 3.0 mg/mg at Baseline in the mITT population (participants in ITT population except for those who withdrew from study due to study early termination. Includes those who completed Week 44 infusion and Visits at Week 52/Early Withdrawal and Week 56/End of Study but withdrew due to study early termination.)

Active urinary sediment is defined by 1 of the following (in the absence of a urinary tract infection or menses): > 5 red blood cell/high power field (RBC/HPF) or above the reference range for the laboratory, and > 5 white blood cell/high power field (WBC/HPF) or above the reference range for the laboratory, and presence of cellular casts (RBC or WBC). Inactive urinary sediment is defined as: < 5 RBC/HPF and < 5 WBC/HPF, or within the laboratory reference range, and no cellular casts (no RBC or WBC casts).

Participants with active urinary sediment at Day 1 in the mITT population (participants in ITT population except for those who withdrew from study due to study early termination. (Includes participants who completed Week 44 infusion and Visits at Week 52/Early Withdrawal and Week 56/End of Study but withdrew due to study early termination.)

AEs that had an onset on or after dosing of MMF on run-in Day 1 up to the first double-blind dose, or any pre-existing condition that worsened. AE: any untoward medical occurrence that does not necessarily have a causal relationship with this treatment. SAE: any untoward medical occurrence that at any dose: results in death; in the view of the Investigator, places the participant at immediate risk of death (a life-threatening event); requires inpatient hospitalization or prolongation of existing hospitalization; results in persistent or significant disability/incapacity; or results in a congenital anomaly/birth defect. An SAE may also be any other medically important event that, in the opinion of the Investigator, may jeopardize the participant or may require intervention to prevent one of the other outcomes listed above.

All enrolled participants

AEs that had an onset on or after dosing of BIIB023 or placebo, or any pre-existing condition that worsened. AE: any untoward medical occurrence that does not necessarily have a causal relationship with this treatment. SAE: any untoward medical occurrence that at any dose: results in death; in the view of the Investigator, places the participant at immediate risk of death (a life-threatening event); requires inpatient hospitalization or prolongation of existing hospitalization; results in persistent or significant disability/incapacity; or results in a congenital anomaly/birth defect. An SAE may also be any other medically important event that, in the opinion of the Investigator, may jeopardize the participant or may require intervention to prevent one of the other outcomes listed above.

The safety population was defined as all subjects who received at least 1 dose of study treatment (including placebo or BIIB023).

Number of days between first visit with response to last consecutive visit with partial or complete response. Complete renal response is defined as: (1) uPCR <0.5 mg/mg with ≥ 50% reduction of uPCR from Day 1 (Baseline) (from a 24 hour urine collection); and (2) eGFR within normal range. Partial renal response is defined as: (1) ≥ 50% reduction in uPCR from Day 1 (Baseline; from a 24-hour urine collection) and, (2) with one of the following: (a) uPCR of < 1.0 mg/mg if the Day 1 (Baseline) was ≤ 3.0 mg/mg, or, (b) uPCR < 3.0 mg/mg if the Day 1 (Baseline) ratio was > 3.0 mg/mg; and stabilization of renal function (eGFR + or - 25% of Day 1 [Baseline] or serum creatinine within normal range). Estimated from the Kaplan-Meier Curve.

The ITT population included all participants who were randomized and received at least 1 dose of study treatment (BIIB023 or placebo).