An adverse event (AE) is defined as any untoward medical occurrence in a patient or clinical investigation participant administered a pharmaceutical product and which does not necessarily have a causal relationship with this treatment. A serious adverse event (SAE) is an event that results in death, is life-threatening, requires or prolongs hospitalization, results in a congenital anomaly, persistent or significant disability/incapacity or is an important medical event that, based on medical judgment, may jeopardize the participant and may require medical or surgical intervention to prevent any of the outcomes listed above. Treatment-emergent events (TEAEs) are defined as an adverse event with an onset date that is on or after the first dose of study drug from Study M20-186, and no more than 30 days after the last dose of study drug from Study M20-186. For more details on adverse events please see the Adverse Event section.

Safety analysis set: all participants who received at least 1 dose of study drug in Study M20-186, grouped according to treatment sequence actually received

SLE Responder Index (SRI)-4 is defined as follows with all criteria compared to Baseline in Study M19-130:

• ≥4-point reduction in Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K) score
• No worsening of the overall condition (< 0.3 point increase in Physician's Global Assessment [PhGA])
• No new British Isles Lupus Assessment Group (BILAG) A or more than 1 new BILAG B disease activity scores (i.e., no organ system changes from baseline B/C/D/E to A and no more than 1 organ system changes from baseline C/D/E to B). A letter score is assigned to each organ system with following indications: A = severe, B = moderate, C = mild, D = inactive with prior history, and E = inactive with no history.

Full Analysis Set: all randomized participants who received at least 1 dose of study drug in Study M20-186; as observed (AO) analysis. When 50% of planned participants in Study M19-130 had completed Week 24 or withdrawn from the study, the ABBV-599 Low Dose and elsubrutinib 60 mg treatment groups were terminated as these groups did not meet projected efficacy. Per protocol, terminated groups were removed from the efficacy analyses.

BICLA is a composite responder index. Achievement of BICLA response is defined as improvement in all initial A and B BILAG scores, with no more than one new BILAG B score without worsening of the overall condition (no worsening in Physician's Global Assessment [PhGA], < 0.3 point increase) and no worsening of the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K) score.

Full Analysis Set: all randomized participants who received at least 1 dose of study drug in Study M20-186; as observed (AO) analysis. When 50% of planned participants in Study M19-130 had completed Week 24 or withdrawn from the study, the ABBV-599 Low Dose and elsubrutinib 60 mg treatment groups were terminated as these groups did not meet projected efficacy. Per protocol, terminated groups were removed from the efficacy analyses.

Participants'current use of steroid therapy was assessed at each study visit, and the amount of daily prednisone was documented.

Full Analysis Set: all randomized participants who received at least 1 dose of study drug in Study M20-186; as observed (AO) analysis. When 50% of planned participants in Study M19-130 had completed Week 24 or withdrawn from the study, the ABBV-599 Low Dose and elsubrutinib 60 mg treatment groups were terminated as these groups did not meet projected efficacy. Per protocol, terminated groups were removed from the efficacy analyses..

The SELENA SLEDAI flare index defines mild/moderate or severe SLE flares using the SLEDAI score, definitions of worsening signs and symptoms, treatment changes, and Physician's Global Assessment of Disease Activity.

Full Analysis Set: all randomized participants who received at least 1 dose of study drug in Study M20-186; as observed (AO) analysis. When 50% of planned participants in Study M19-130 had completed Week 24 or withdrawn from the study, the ABBV-599 Low Dose and elsubrutinib 60 mg treatment groups were terminated as these groups did not meet projected efficacy. Per protocol, terminated groups were removed from the efficacy analyses.