A Phase III, Randomized, Double-blind, Parallel-group, Placebo controlled Study to Evaluate the Pharmacokinetics, Pharmacodynamics, Efficacy, and Safety of IV Anifrolumab in Pediatric Participants 5 to < 18 Years of Age with Moderate to Severe Active Systemic Lupus Erythematosus While on Background Standard of Care Therapy

Trial status: Authorised SRI(4) responders at Wk. 52 (Y/N): meet all the following: Minimum 4-point reduction from baseline (BL) SLEDAI2K score No new BILAG-2004 (BILAG) scores from BL (≥ 1 new A or ≥ 2 new BILAG B scores) No worsening in BL lupus disease activity (increase ≥ 0.30 points on a PGA 3-point VAS), Time to first flare through Week 52, where flare is defined as either ≥ 1 new BILAG-2004 A, or ≥ 2 new BILAG-2004 B items compared with the previous visit., • PK: Anifrolumab serum concentrations • Immunogenicity: ADA • PD: Change from baseline through Week 52 in: o Anti-dsDNA antibodies o C3, C4, and CH50 complement levels o Type I IFN 21-gene signature, Participants who are PRINTO/ACR cSLE responders (Y/N) at W52, defined as at least 50% improvement from baseline in any 2 of 5 core set outcome measures and no more than one of the remaining worsening > 30%, Reduction of OCS background dose from baseline through Week 52 After single dose or after dose adjustment: • Serum PK concentrations PK parameters: Cmax, AUC, Cmin after the first dose or after dose adjustment, BICLA responders at W52 (Y/N), defined as follow: Reduction of all baseline BILAG A to B/C/D & B to C/D & no BILAG worsening in other organ systems (≥ 1 new BILAG A or ≥ 2 new BILAG B) No worsening from baseline S-2K (increase of > 0 points) No worsening from baseline in SLE activity (increase ≥ 0.30 points on a PGA 3-point VAS)

SRI(4) responders at Wk. 52 (Y/N): meet all the following: Minimum 4-point reduction from baseline (BL) SLEDAI2K score No new BILAG-2004 (BILAG) scores from BL (≥ 1 new A or ≥ 2 new BILAG B scores) No worsening in BL lupus disease activity (increase ≥ 0.30 points on a PGA 3-point VAS), Time to first flare through Week 52, where flare is defined as either ≥ 1 new BILAG-2004 A, or ≥ 2 new BILAG-2004 B items compared with the previous visit., • PK: Anifrolumab serum concentrations • Immunogenicity: ADA • PD: Change from baseline through Week 52 in: o Anti-dsDNA antibodies o C3, C4, and CH50 complement levels o Type I IFN 21-gene signature, Participants who are PRINTO/ACR cSLE responders (Y/N) at W52, defined as at least 50% improvement from baseline in any 2 of 5 core set outcome measures and no more than one of the remaining worsening > 30%, Reduction of OCS background dose from baseline through Week 52

Age group: 0-17 years Gender: Female, Male Target enrollment: 20 participants