This is a multicenter, randomized, double-blind, placebo controlled Phase IIb study to explore the efficacy and safety of FXS6837 capsules in IgAN patients. About 60 patients dignosed with primary IgAN will be enrolled and randomized to three cohorts and take different dosage of FXS6837 or placebo capsules orally according to protocol.

This is a multicenter, randomized, double-blind, placebo-controlled study in approximately 60 patients with primary IgA nephropathy (IgAN).

Participants receiving background therapy will be randomized in a 1:1:1 ratio to receive FXS6837 capsules dose 1,dose 2, or placebo, administered orally once daily.

The study aims to evaluate the efficacy and safety of FXS6837 in patients with primary IgAN and to identify the optimal clinical dose.

Inclusion criteria:

1. Adult male or female patients aged ≥18 years with biopsy-confirmed primary IgA nephropathy (IgAN), meeting all of the following:

   1. A qualifying renal biopsy performed within the past 8 years;
   2. ≤50% tubulointerstitial fibrosis;
   3. Crescent formation present in ≤50% of glomeruli;
   4. If a historical biopsy is not available, a biopsy may be performed during screening.

2. Estimated glomerular filtration rate (eGFR) ≥30 mL/min/1.73 m² at screening and at the end of the run-in period.

3. Urine protein-to-creatinine ratio (UPCR) ≥0.75 g/g at screening and at the end of the run-in period.

4. Vaccination against Neisseria meningitidis and Streptococcus pneumoniae is required prior to initiation of study treatment. If not previously vaccinated or if a booster is required, 5. vaccination should be administered according to local regulations at least 2 weeks prior to first dose. If treatment must begin earlier, prophylactic antibiotic therapy should be initiated.

5. Patients must have received a stable dose of angiotensin-converting enzyme inhibitors (ACEi) or angiotensin receptor blockers (ARB), at the locally approved maximum daily dose or maximally tolerated dose (per investigator judgment), for at least 90 days prior to first dose. If receiving sodium-glucose cotransporter-2 inhibitors (SGLT2i), endothelin receptor antagonists (ERA), or hydroxychloroquine, doses must also be stable for at least 90 days prior to first dose (per investigator judgment).

Exclusion criteria:

1. Secondary IgA nephropathy (IgAN), as defined by the investigator.
2. Rapidly progressive IgAN, defined as ≥50% decline in eGFR (CKD-EPI) within 3 months, or <50% decline but considered by the investigator to be at risk of rapid renal function deterioration.
3. Other systemic diseases associated with proteinuria or chronic kidney disease (e.g., diabetic nephropathy, lupus nephritis, ANCA-associated vasculitis), or severe urinary tract obstruction or dysuria.
4. Prior treatment with immunosuppressive agents, including but not limited to cyclophosphamide, rituximab, infliximab, eculizumab, canakinumab, mycophenolate mofetil (MMF), mycophenolate sodium (MPS), cyclosporine, tacrolimus, sirolimus, everolimus, or systemic corticosteroids within 90 days (or 5 half-lives, whichever is longer) prior to first dose.
5. Prior treatment with oral budesonide (Nefecon®) within 6 months prior to first dose.
6. Prior treatment with other complement inhibitors within 30 days (or 5 half-lives, whichever is longer) prior to first dose.
7. Positive test results for HIV; active syphilis infection; chronic hepatitis B infection (HBsAg positive with HBV DNA > lower limit of quantification [LOQ]); or hepatitis C infection (positive HCV antibody with detectable HCV RNA).
8. Active tuberculosis at screening.
9. Clinically significant abnormal liver function at screening, defined as any of the following: ALT, AST, GGT, or ALP >3 × upper limit of normal (ULN), or total bilirubin >2 × ULN.
10. History of meningococcal infection.
11. Active systemic bacterial, viral (including COVID-19), or fungal infection within 14 days prior to first dose, or body temperature >38°C within 7 days prior to first dose.