This is a multicenter, randomized, double-blind, placebo controlled Phase IIb study to explore the efficacy and safety of STL303 capsules in IgAN patients. About 15 patients dignosed with primary IgAN will be enrolled and randomized to three cohorts and take different dosage of STL303 or placebo capsules orally according to protocol.

This is a multicenter, randomized, double-blind, placebo-controlled study in approximately 15 patients with primary IgA nephropathy (IgAN).

Participants receiving background therapy will be randomized in a 1:1:1 ratio to receive STL303 capsules dose 1, dose 2, or placebo, administered orally once daily.

The study aims to evaluate the efficacy and safety of STL303 in patients with primary IgAN and to identify the optimal clinical dose.

Inclusion Criteria:

1. Male and female patients aged 18 years and older, with primary IgAN confirmed by renal biopsy:
2. eGFR (Chronic Kidney Disease Epidemiology Collaboration [CKD-EPI] formula) greater than or equal to 30 mL/min/1.73 m2 at screening and after completion of run-in.
3. UPCR greater than or equal to 0.75 g/g at screening and after completion of run-in.
4. Vaccinated against Neisseria meningitidis and Streptococcus pneumoniae before the first dose.
5. Have received stable treatment with RASis (ACEi or ARB) at the maximum recommended dose or MTD for at least 90 days prior to the first dose.
6. If the patient has been treated with SGLT2i, diuretics, other antihypertensive treatments, ERA, and/or hydroxychloroquine for IgAN prior to the first dose, the drug should also be used stably for at least 90 days.

Exclusion Criteria:

1. Secondary IgAN or unclear exclusion of secondary causes.
2. Rapidly progressive IgAN (eGFR decline greater than or equal to 50% in 3 months, or less than 50% but at high risk).
3. Other systemic diseases causing proteinuria/CKD or severe urinary obstruction.
4. Known or suspected immunodeficiency or hereditary complement deficiency.
5. Any organ transplant recipients except corneal.
6. Poorly controlled blood pressure (SBP greater than 150 or DBP great than 90).
7. Use of immunosuppressive drugs within 90 days or 5 half-lives.
8. Prior oral budesonide (Nefecon/Tarpeyo/Kinpeygo) within 6 months.
9. Prior complement inhibitors within 30 days, 5 half-lives, or residual effect period.
10. Major systemic diseases preventing participation (e.g., NYHA IV, severe pulmonary disease).
11. Significantly abnormal liver function (greater than 3× ULN enzymes or greater than 2× ULN bilirubin).
12. QTcF greater than 500 ms.
13. History of malignancy within 5 years (exceptions apply).
14. History of meningococcal, pneumococcal, or Hib infection.
15. Chronic/recurrent infections in past year (e.g., liver abscess, pyelonephritis).
16. Active systemic infections within 2 weeks or fever greater than 38°C within 7 days.

Individual participant data (IPD) will not be shared because participant consent and ethics approvals do not permit public data sharing. A clinical study report will be prepared at the end of the study and result will be shared with research sites and investigators will be able to discuss results with participants if needed.