Study to Test the Efficacy and Safety of Bimekizumab and Certolizumab Pegol in Patients With Active Ankylosing Spondylitis.

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. 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; <=25mg/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 3grams/day) or hydroxychloroquine (up to 400mg
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-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-tumor necrosis
factor alpha (TNFα) agent. Subjects may not have been on more than 1 anti-TNFα agent.

- Subject has high-sensitive C-Reactive Protein (hsCRP) levels >=3mg/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, 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