A Multicenter Study to Evaluate the Dose Response Based on the Efficacy, Safety and Tolerability of Bimekizumab in Subjects With Active Psoriatic Arthritis Which is a Type of Inflammatory Arthritis.

Inclusion Criteria:

- Subject has a documented diagnosis of adult-onset PsA classified by Classification
Criteria for Psoriatic Arthritis (CASPAR) criteria with symptoms for at least 6 months
prior to Screening, with active psoriatic arthritis (PsA) at Baseline/Day 1, and must
have at Baseline tender joint count (TJC) >=3 out of 78 and swollen joint count (SJC)
>=3 out of 76

- Subject must be rheumatoid factor and anti-cyclic citrullinated peptide (CCP)
antibodies negative

- Subject must have active psoriatic lesion(s) and/or a documented history of psoriasis

- Subjects who are regularly taking nonsteroidal anti-inflammatory drug (NSAIDs)/COX-2
inhibitors as part of their PsA therapy are required to be on a stable dose/dose
regimen for at least 14 days before Baseline

- Subjects taking corticosteroids must be on an average daily dose of <=10mg/day
prednisone or equivalent for at least 14 days before Baseline and should remain on a
stable dose through the Week 16 visit

- Subjects taking methotrexate (MTX) (<=25mg /week) are allowed to continue their
medication if started at least 12 weeks prior to Baseline, with a stable dose for at
least 8 weeks before randomization

- Subjects taking leflunomide (LEF; <=20mg/day or an average of 20mg/day if not dosed
daily) are allowed to continue their medication if started at least 3 months prior to
Baseline, with a stable dose for at least 8 weeks before randomization. Dose and
dosing schedule should remain stable up to Week 16

- Subjects may be tumor necrosis factor (TNF) inhibitor naïve or may have received 1
prior TNF inhibitor. Subjects who have been on a TNF inhibitor previously must have:

1. experienced an inadequate response to previous treatment given for at least 3
months

2. been intolerant to administration (eg, had a side-effect/adverse event (AE) that
led to discontinuation)

3. lost access to TNF inhibitor for other reasons

Exclusion Criteria:

- Subjects with any current sign or symptom that may indicate an active infection (with
the exception of the common cold) or has had an infection requiring systemic
antibiotics within 2 weeks of Baseline/Day 1

- Subjects with a history of chronic or recurrent infections, or a serious or
life-threatening infection within the 6 months prior to the Baseline Visit

- Subjects with concurrent acute or chronic viral hepatitis B or C or human
immunodeficiency virus (HIV) infection

- Subjects with known history of or current clinically active infection with
Histoplasma, Coccidioides, Paracoccidioides, Pneumocystis, Blastomyces, or Aspergillus
or current active Candidiasis

- Subjects receiving any live (includes attenuated) 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 (LTBI), or current or history of nontuberculous
mycobacteria (NTMB) infection

- Subjects with a diagnosis of inflammatory conditions other than psoriasis or psoriatic
arthritis

- 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