Study of ALXN1920 in Adult Participants With Primary Membranous Nephropathy (PMN)

Inclusion Criteria:

• Participants who have a documented diagnosis of PMN, established by positiveantiPLA2R antibody level (> 20 RU/mL) at Screening, which must be confirmed by acentral laboratory

• Participants are willing to receive the background Standard of Care (SoC)

• Participants at high risk for disease progression, defined as:

1. Receiving ACE inhibitor or ARB for a minimum of 8 weeks prior to Screening,with the dose titrated to the maximally tolerated level. Participants with lessthan 8 weeks on ACE inhibitor or ARB before Screening or who have not yetreached maximally tolerated dose will enter the Run-in Period.

2. Participants who are on ACE inhibitor or ARB for a minimum of 8 weeks withSystolic Blood Pressure < 140 mmHg in ≥ 75% of the readings within last 8weeks.

3. Having two proteinuria measurements with each > 3.5 g/day, the secondmeasurement showing ≤50% decrease from the first measurement.

• eGFR60 mL/min/1.73 m2 during Screening calculated by CKD-EPI 2021 creatinine formula

• All participants must receive prophylactic treatment with appropriate antibioticswhile receiving Rituximab (RTX), and be willing to be vaccinated against Neisseriameningitidis

Exclusion Criteria:

• Documented rapid deterioration of kidney function

• History of life-threatening Nephrotic Syndrome within 1 year before Screening

• Diagnosis of anti- phospholipase A2 receptor (PLA2R) negative membranous nephropathy (MN) or anti-PLA2R positive MN but Screening serum anti-PLA2R < 20 RU/mL or kidneydisease other than PMN

• History of kidney transplant or planned kidney transplant or dialysis during theTreatment Period

• History of other solid organ (heart, lung, small bowel, pancreas, or liver) or bonemarrow transplant; or planned transplant during the Treatment Period

• History or presence of any clinically relevant co-morbidities

• History of intolerance or hypersensitivity to ACEi or ARB

• Initiation or dose adjustment of SGLT2i within 12 weeks prior to randomization orplanned adjustment of GSLT2i dose throughout the treatment period.

• Use of traditional Chinese medicines or Chinese proprietary medicines with systemicimmunosuppressive properties within 6 months prior to screening.

• Use of MRA, or ERA within 12 weeks prior to randomization and throughout the studyperiod

Note: Additional inclusion/exclusion criteria may apply, per protocol.