A Study of IBI311 in Subjects With Inactive or Active Thyroid Eye Disease

Key Inclusion Criteria:

1. Written informed consent.

2. Male or female subject between the ages of 18 and 80 years at screening.

3. Weight between 50 kg and 100 kg.

4. Moderate-to-severe active TED:

• CAS ≥ 3 in the study eye at screening and baseline;

• Usually associated with at least two of the following: lid retraction ≥ 2 mm,moderate or severe soft tissue involvement, exophthalmos ≥ 3 mm above normal,and/or inconstant or constant diplopia;

• ≤ 12 months since the onset of active TED symptoms according to subjects' chiefcomplaint or medical record at screening; Inactive TED:

• According to subjects' chief complaint or medical record at screening, initialdiagnosis of TED > 12 months but < 10 years prior to screening.

• CAS ≤ 2 in both eyes at screening and baseline and CAS ≤ 2 in both eyes for atleast 6 months prior to screening or all of the following at least 6 monthsprior to screening: a. no progression in proptosis; b. no progression indiplopia; c. no new inflammatory TED symptoms.

• Exophthalmos ≥ 3 mm above normal.

5. Exophthalmos ≥ 20 mm in the study eye at baseline.

6. Female subjects should be fertile women who are sterile or have a negative bloodpregnancy test during the screening period and who agree to take contraceptivemeasures within 120 days from the screening period to the last medication; Malesubjects should agree to use contraception from the screening period to 120 daysafter the last dose.

Key Exclusion Criteria:

Subjects will be ineligible for study participation if they meet any of the following criteria:

1. Baseline CAS decreased by ≥ 2 points, or baseline proptosis decreased by ≥ 2 mm ascompared with screening.

2. Visual function impairment due to optic neuropathy, defined as ≥ 2 lines of visionloss, new visual field defect, or color vision impairment secondary to optic nerveinvolvement within the past 180 days;

3. Corneal ulcers with no relief after treatment as determined by the investigator;

4. TED patients who need immediate corticosteroid therapy, orbital radiotherapy, ororbital decompression;

5. At any time prior to baseline or during the study period planned to receive orbitalradiation therapy or TED surgery, including orbital decompression, strabismussurgery, and eyelid retraction correction;

6. Poorly controlled thyroid function, which was defined as free triiodothyronine (FT3)or free tetraiodothyronine (FT4) deviated more than 50% from the normal referencerange of the local research center laboratory at screening.

7. Either ear had a history of tinnitus or other hearing impairment; or abnormal puretone audiometry (defined as mean bone conduction threshold [0.5, 1, 2, 4 kHz] ≥25 dBor any bone conduction threshold ≥ 40 dB);

8. Poorly controlled diabetes at screening, defined as HbA1C ≥ 9.0% at screening, orany new medication for diabetes within 60 days prior to screening, or any doseadjustment for diabetes drugs > 10%);

9. Systemic use of glucocorticoids ≤ 30 days prior to screening;

10. Periorbital use of glucocorticoids ≤ 90 days prior to screening;

11. Systemic use of immunosuppressants ≤ 90 days prior to screening;

12. Use glucocorticoid eye drops or immunosuppressive eye drops ≤ 30 days prior toscreening

13. Use IBI311 or TEPEZZA at any time prior to screening; 13 Use CD20 antibody ≤ 1 year prior to screening, or IL-6R antibody ≤ 180 days prior to screening; 14. Subjects had participated in other interventional clinical trials ≤ 90 days prior to screening, or attempting to participate in other clinical trials during the study period.