The purpose of the study is to evaluate the efficacy and safety of bimekizumab compared to certolizumab pegol in the treatment of subjects with active ankylosing spondylitis (AS).

Inclusion Criteria:

• Documented diagnosis of active adult-onset ankylosing spondylitis (AS) as defined by documented radiologic evidence (X-ray) fulfilling the Modified New York criteria for AS (1984) of at least 3 months' symptom duration and age of onset <45 years

• Subject has moderate to severe active disease at the Screening Visit as defined by each of the following:

  1. Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) score >=4
  2. Spinal pain >=4 on a 0 to 10 numeric rating scale (NRS) (from BASDAI Item 2)

• Subjects must have had an inadequate response to, have a contraindication to, or have been intolerant to at least 2 nonsteroidal anti-inflammatory drugs (NSAIDs)

• Subjects taking corticosteroids must be on a maximum daily dose of <=10mg/day oral prednisolone or equivalent

• Subjects taking methotrexate (MTX; <=25 mg/week) are allowed to continue their medication if they received a stable dose for at least 12 weeks before randomization

• Subjects taking sulfasalazine (up to 3 grams/day) or hydroxychloroquine (up to 400 mg per day total) are allowed to continue their medication if started at least 12 weeks prior to randomization

• Subject who has been on an anti-tumor necrosis factor alpha (TNFα) agent must have experienced an inadequate response to previous or current treatment given at an approved dose for at least 3 months or have been intolerant to at least 1 administration of an anti-TNFα agent. Subjects may not have been on more than 1 anti-TNFα agent

• Subject has high-sensitive C-Reactive Protein (hsCRP) levels >=3 mg/L at the Screening Visit

• Female subjects must be postmenopausal, permanently sterilized or, if of childbearing potential, must be willing to use a highly effective method of contraception up till 20 weeks after last administration of investigational medicinal product (IMP)

• Male subjects with a partner of childbearing potential must be willing to use a condom when sexually active, up till 20 weeks after the last administration of IMP

Exclusion Criteria:

• Subject has received previous or current biological treatment other than TNFα inhibitor treatment

• Subjects with a total ankylosis of the spine, or a diagnosis of any other inflammatory arthritis eg, rheumatoid arthritis (RA), sarcoidosis, systemic lupus erythematosus, or reactive arthritis

• Subjects with any current sign or symptom that may indicate an active infection (except for the common cold)

• Subject has received previous or current biological treatment other than TNFα inhibitor treatment

• Subject has chronic, recurrent, recent serious / life-threatening or current infection, as defined in the protocol

• Subject has history of certain atypical infections, viral hepatitides, human immunodeficiency virus (HIV) infection, tuberculosis, as defined in the protocol

• Subjects receiving any live vaccination within the 8 weeks prior to Baseline

• Subjects with known tuberculosis (TB) infection, at high risk of acquiring TB infection, with latent TB infection or current or history of nontuberculous mycobacteria (NTMB) infection

• Subject has immunosuppressive condition or treatment, recent history of malignancy (some exceptions) or demyelinating disease

• Subjects with concurrent malignancy or a history of malignancy during the past 5 years will be excluded, with following exceptions that may be included:

  1. <= 3 excised or ablated basal cell carcinomas of the skin
  2. One squamous cell carcinoma of the skin (stage T1 maximum) successfully excised, or ablated only (other treatments, ie, chemotherapy, do not apply), with no signs of recurrence or metastases for more than 2 years prior to Screening
  3. Actinic keratosis (-es)
  4. Squamous cell carcinoma-in-situ of the skin successfully excised, or ablated, more than 6 months prior to Screening

• Subject has history of psychiatric disorder, including suicidality (as defined in the protocol

• Subject has major abnormalities on laboratory testing, as defined in the protocol

Data from this study may be requested by qualified researchers six months after product approval in the US and/or Europe, or global development is discontinued, and 18 months after trial completion. Investigators may request access to anonymized IPD and redacted study documents which may include: raw datasets, analysis-ready datasets, study protocol, blank case report form, annotated case report form, statistical analysis plan, dataset specifications, and clinical study report. Prior to use of the data, proposals need to be approved by an independent review panel at www.clinicalstudydatarequest.com and a signed data sharing agreement will need to be executed. All documents are available in English only, for a pre-specified time, typically 12 months, on a password protected portal. This plan may change if a determination is made that the data cannot be adequately anonymized.

Data from this study may be requested by qualified researchers six months after product approval in the US and/or Europe or global development is discontinued, and 18 months after trial completion.

Qualified researchers may request access to anonymized IPD and redacted study documents which may include: raw datasets, analysis-ready datasets, study protocol, blank case report form, annotated case report form, statistical analysis plan, dataset specifications, and clinical study report. Prior to use of the data, proposals need to be approved by an independent review panel at www.clinicalstudydatarequest.com and a signed data sharing agreement will need to be executed. All documents are available in English only, for a pre-specified time, typically 12 months, on a password protected portal.

This is an investigator-blind and subject-blind study.