This is a phase 2, randomized, double-blind, placebo-controlled, multicenter study to evaluate the efficacy and safety of AK111 in subjects with active ankylosing spondylitis.

The purpose of this study is to evaluate the efficacy and safety of AK111 in the treatment of subjects with active ankylosing spondylitis. Subjects will be randomized to receive AK111 or placebo following subcutaneous injection. The study period for each subject will be 25 weeks, including a screening period of up to 35 days, followed by a treatment period of 12 weeks and a follow up period of 8 weeks.

Inclusion Criteria:

• Male or female subjects aged 18-75 years old.
• Ankylosing spondylitis has been diagnosed for at least 6 months before screening, with radiological evidence that meets the Modified New York Criteria for Ankylosing Spondylitis.
• Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) score ≥ 4 and Spinal pain score≥ 4 (based on BASDAI question 2).
• Subjects received at least 1 kind of non-steroidal anti-inflammatory drugs (NSAIDs), prior to randomization with an inadequate response or failure to respond.
• Subjects who are regularly taking NSAIDs, weak opioids or oral glucocorticoids as part of their AS therapy are required to be on a stable dose for at least 14 days before randomization.
• Subjects taking methotrexate (MTX) (≤25mg/week) or Sulfasalazine (≤3g/day) are allowed to continue their medication if started at least 12 weeks prior to Baseline, with a stable dose for at least 4 weeks before randomization.

Exclusion Criteria:

• Subjects with total ankylosis of the spine.
• Subjects with progressive or uncontrolled diseases of respiratory, circulatory, digestive, urogenital, endocrine, nervous or mental systems, or with other chronic diseases that are not suitable to participate to the study.
• Subjects with other inflammatory diseases or autoimmune diseases except Ankylosing spondylitis (AS).
• Subjects with any severe systemic or local infection within 2 months before screening.
• Subjects who are using strong opioid analgesics.
• Combined use of Disease Modifying Anti-Rheumatic Drugs (DMARDs) other than methotrexate and sulfasalazine, including but not limited to thalidomide, iguratimod, etc. within 4 weeks before randomization.
• Received any live vaccine within 2 months before screening or planned to receive any live vaccine during the study period.
• Previous exposure to secukinumab, ixekizumab or any other biologic drug directly targeting IL-17 or IL-17 receptor.
• Received Natalizumab or other drugs that regulate B cells or T cells within 6 months before screening.
• Received more than 2 kinds of Tumor necrosis factor alpha(TNF-α) inhibitors before screening.