This study is a randomized, parallel, double-blind, placebo-controlled, subcutaneous administration Phase II dose-exploration clinical trial aimed at evaluating the efficacy, safety, PK, PD, and immunogenicity characteristics of SLN12140 at different doses in IgA nephropathy subjects who have previously received standard treatment (the standard treatment drugs allowed in this study include: angiotensin-converting enzyme inhibitors [ACEi], angiotensin II receptor blockers [ARB], and sodium-glucose co-transporter 2 inhibitors [SGLT2i]) but have poor control.

The study is divided into four stages, including a screening period of up to 8 weeks, an introduction period of up to 12 weeks, a 40-week double-blind period (including a 36-week treatment period and a 4-week safety follow-up period; all subjects in the three dose groups who are willing to continue treatment and are judged by the investigator to potentially benefit from subsequent treatment will enter the open-label extension period for continued treatment after completing the double-blind period), and a 56-week open-label extension period (all subjects in the three dose groups who are willing to continue treatment and are judged by the investigator to potentially benefit from subsequent treatment will continue SLN12140 at the same dose group [the optimal dose], including a 52-week open treatment period and a 4-week safety follow-up period).

Inclusion Criteria:

• The subject has fully understood the purpose, nature, methods and potential adverse reactions of this trial, volunteers to participate as a subject, is able to communicate effectively with the investigators, understands and complies with all requirements of this study, and voluntarily signs the Informed Consent Form (ICF) prior to initiation of any study procedures.
• Male and female subjects aged 18 to 80 years (inclusive) at the time of signing the ICF, with body weight ≥ 50 kg for males and ≥ 45 kg for females; body mass index (BMI) 18.0-35.0 kg/m² (inclusive).
• Patients with IgA nephropathy confirmed by biopsy within the past 3 years before screening, with renal tubulointerstitial fibrosis < 50%.
• Estimated glomerular filtration rate (eGFR) ≥ 30 mL/min/1.73 m² during the screening period, calculated using the 2021 creatinine-based Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation.
• During the screening period, 24-hour urine protein-to-creatinine ratio (24h UPCR) ≥ 1.0 g/g; after completion of the run-in period, UPCR ≥ 0.75 g/g or urine protein excretion (UPE) ≥ 0.75 g/day.
• Must have received standard of care therapy for at least 12 weeks (and at a stable dose for at least 4 weeks) prior to the first dose of study medication, and agree to maintain stable dosing throughout the study period. (Note: Permitted standard of care medications in this study include ACEi/ARB and SGLT2i.)
• Males and women of childbearing potential (WOCBP; as defined by the Clinical Trials Facilitation and Coordination Group [CTFG] 2020) must agree to follow the contraception guidelines specified in the protocol from the screening period until 6 months after the last dose of study medication.
• The subject is willing to receive quadrivalent meningococcal vaccine and pneumococcal vaccine at least 14 days before dosing (for subjects vaccinated within 14 days before dosing, antibiotic prophylaxis for at least 14 days is required after the first vaccination). Re-vaccination is not required for those who have received quadrivalent meningococcal vaccine within 3 years before the first dose, or pneumococcal vaccine within 5 years before the first dose.

Exclusion Criteria:

• A known history of hypersensitivity, allergy, or anaphylactic reaction to any component of the investigational product, including hypersensitivity to human, humanized, or murine monoclonal antibodies, or known hypersensitivity to any ingredient of the product.
• Secondary IgA nephropathy as determined by the investigator, such as that caused by Henoch-Schönlein purpura, systemic lupus erythematosus, hepatitis, ankylosing spondylitis, infection, or other etiologies.
• Diagnosis of IgA vasculitis.
• Current presence or history of nephrotic syndrome.
• Clinical suspicion of IgA nephropathy with rapidly progressive glomerulonephritis in accordance with the Kidney Disease: Improving Global Outcomes (KDIGO) 2025 guidelines (≥50% decline in eGFR within 3 months prior to ICF signature, or a <50% decline but at risk of rapid renal function deterioration as assessed by the investigator).
• Clinical suspicion or biopsy-confirmed chronic kidney disease caused by any disease other than IgA nephropathy, including other glomerulopathies or podocytopathies.

Other protocol-defined exclusion criteria may apply.

It is not decided yet at this moment if we will share IPD with other researchers, or when will IPD will be shared, or with whom will IPD be shared, or by what mechanism will IPD be shared.