The sponsor's development strategy is adjusted.

ABY-035-204 is a clinical study to assess the efficacy of IL-17 blocker ABY-035 in ankylosing spondylitis(AS). The primary objective is to estimate the relationship between different dose regimens of ABY-035 and clinical response as assessed by Assessment of Spondyloarthritis International Society 40 (ASAS40) response at Week 16 in subjects with active AS.

ABY-035-204 is a double-blind, randomized, parallel-group, placebo-controlled study.

The primary objective is to estimate the relationship between different dose regimens of ABY-035 and clinical response as assessed by Assessment of Spondyloarthritis International Society 40 (ASAS40) response at Week 16 in subjects with active AS.

The study will include the following 3 periods:

1. Screening Period: Up to 35 days prior to baseline randomization.

2. Treatment Period 1: Day 0-Week 16

   Cohort 1: Eligible subjects will be randomized 1:1:1:1 to receive 1 of 4 treatments (ABY-035 High Dose every 2 weeks (Q2W), ABY-035 Low Dose every 2 weeks (Q2W), ABY-035 High Dose every 4 weeks (Q4W), or placebo Q2W), and will remain on their allowable background medication.

   Cohort 2: Eligible subjects will be randomized 1:1:1 to receive 1 of 3 treatments (ABY-035 High Dose every week (QW), ABY-035 Low Dose every week (QW), or placebo QW), and will remain on their allowable background medication.

   Randomization will be stratified by region (North Eastern Asia and North America) and previous tumor necrosis factor alpha (TNFα) inhibitor exposure (TNFα inhibitor treated or TNFα inhibitor naïve). Maximum 30% of subjects will be TNFα inhibitor-treated subjects to ensure a representative population for the assessment of efficacy and safety.

   Treatment Period 1 ends at Week 16 after all trial assessments have been done and Treatment Period 2 starts at Week 16 with the IMP injection.

3. Treatment Period 2 (Extension Period): Week 16-Week 52 Cohort 1: Subjects will receive ABY-035 High Dose Q2W treatment in an open-label manner.

Cohort 2: Subjects will receive ABY-035 High Dose QW treatment in an open-label manner.

At Week24, subjects who could not achieve an ASAS20 response from baseline are defined as non-responders and will discontinue the study treatment.

Inclusion Criteria:

1. Male or female at least 18 years of age.

2. Subjects with active AS, determined by documented radiologic evidence (X-ray) fulfilling the Modified New York criteria for AS (1984).

   AND At least one SpA feature, according to ASAS criteria.

3. Subjects have moderate to severe active disease

4. Subjects must have inadequate response or intolerance to at least 2 NSAIDs, or contraindication to NSAID therapy.

5. Subjects may be TNFα inhibitor-naïve or may have received up to 2 prior TNFα inhibitor(s)..

Exclusion Criteria:

1. Subjects have active fibromyalgia or total spinal ankylosis ('bamboo spine'), or any other inflammatory arthritis.
2. Subjects have used medications in the manner as detailed by the exclusion criteria as detailed in the study protocol.
3. Subjects have received technetium-99 conjugated with methylene diphosphonate other than for diagnostic purpose within 5 years prior to baseline.
4. Have received any live (includes attenuated) vaccination within the 12 weeks prior to the baseline.
5. Subjects have received any non-biological therapy for AS not listed as detailed in the study protocol within or outside a clinical study in the 3 months or within 5 half-lives prior to the Baseline Visit (whichever is longer).
6. Subject has an active infection or history of infections
7. Have evidence of or test positive for hepatitis B virus (HBV)
8. Have evidence of or test positive for hepatitis C virus (HCV).
9. Have a historically positive human immunodeficiency virus (HIV) test or test positive at screening for HIV.
10. Subjects have known tuberculosis (TB) infection, at high risk of acquiring TB infection, or current or history of nontuberculous mycobacterium (NTMB) infection, or LTB.
11. Have a history of a lymphoproliferative disorder including lymphoma or current signs and symptoms suggestive of lymphoproliferative disease.
12. Subjects have active Crohn's disease (CD) or active ulcerative colitis (UC).
13. Subjects have active uveitis within 6 weeks prior to baseline.
14. Subjects have laboratory abnormalities at Screening.