A Study of CC-99677 in Participants With Active Ankylosing Spondylitis

Percentage of participants who achieve an improvement in disease activity from baseline of ≥ 20% and ≥ 1 unit in at least 3 of the 4 SpondyloArthritis International Society (ASAS) domains on a scale of 0 to 10, and no worsening from baseline of ≥ 20% and ≥ 1 unit in the remaining domain on a scale of 0 to 10. Baseline is the last non-missing value on or before the date of the first dose of investigational product. The four ASAS Domains are: * Patient Global Assessment of Disease (0 to 10 unit Numerical Rating Scale \[NRS\]); * Total Back Pain NRS; * Function (the Bath Ankylosing Spondylitis Functional Index \[BASFI\] score NRS); * Inflammation (mean of Bath Ankylosing Spondylitis Disease Activity Index \[BASDAI\] NRS Questions #5 and #6 for morning stiffness).

Time frame: Week 12

Population: All treated participants who have non-missing response at week 12

ASDAS-CRP is a score of disease activity that combines patient reported assessments of back pain (Bath Ankylosing Spondylitis Disease Activity Index \[BASDAI\] question 2), duration of morning stiffness (BASDAI question 6), peripheral joint pain and/or swelling (BASDAI question 3), general wellbeing, and CRP in a weighted manner. The cut-off values for disease activity states and improvement scores are defined as follows: \<1.3 inactive disease, ≥1.3 and \<2.1 low disease activity, ≥2.1 and ≤3.5 high disease activity and 3.5 very high disease activity. The minimum clinically important difference (MCID) are defined as: change of at least 1.1 unit for 'clinically important improvement' and change of at least 2.0 units for 'major improvement'. Baseline is the last non-missing value on or before the date of the first dose of investigational product. ASDAS-CRP Formula: 0.12xBack Pain+0.06xDuration of Morning Stiffness+0.11xPatient Global+0.07xPeripheral Pain/Swelling+0.58xln(CRP+1)

Time frame: Baseline and Week 12

Population: All treated participants with baseline and week 12 measurements

Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) is a composite score based on a self-administered survey of six questions using a 0 to 10 unit numerical rating scale (NRS) that assesses five major symptoms of AS during the last week: 1) fatigue; 2) spinal pain; 3) peripheral joint pain/swelling; 4) areas of localized tenderness; 5a) morning stiffness severity upon wakening; 5b) morning stiffness duration upon wakening. To give each of the five symptoms equal weighting, the mean of the two scores relating to morning stiffness is taken. The resulting 0 to 50 score is divided by 5 to give a final 0 to 10 BASDAI score. A BASDAI score of 4 or greater is considered to be indicative of active AS disease. Baseline is the last non-missing value on or before the date of the first dose of investigational product.

Time frame: Baseline and Week 12

Population: All treated participants with baseline and week 12 measurements

Bath Ankylosing Spondylitis Functional Index (BASFI) is a composite score based on a self administered survey of ten questions using a 0 to 10 unit numerical rating scale (NRS) that assesses degree of mobility and functional ability during the last week. The questionnaire consists of eight questions regarding function in AS and the two last questions reflecting ability to cope with everyday life. The left-hand box of 0 represents easy, and the right-hand box represents impossible. The resulting 0 to 100 score is divided by 10 to give a final 0 to 10 BASFI score. A higher BASFI score correlates to reduced functional ability. Baseline is the last non-missing value on or before the date of the first dose of investigational product.

Time frame: Baseline and Week 12

Population: All treated participants with baseline and week 12 measurements

Change from Baseline in the Spondyloarthritis Research Consortium of Canada (SPARCC) scores of the sacroiliac joints. The SPARCC assesses 16 sites for enthesitis using a score of 0 for no activity or 1 for activity. Sites assessed include Medial epicondyle (left/right \[L/R\]), Lateral epicondyle (L/R), Supraspinatus insertion into greater tuberosity of humerus (L/R), Greater trochanter (L/R), Quadriceps insertion into superior border of patella (L/R), Patellar ligament insertion into inferior pole of patella or tibial tubercle (L/R), Achilles tendon insertion into calcaneum (L/R), and Plantar fascia insertion into calcaneum (L/R). The SPARCC is the sum of all site scores (range 0 to 16). Higher scores indicate more severe enthesitis. Baseline is the last non-missing value on or before the date of the first dose of investigational product.

Time frame: Baseline and Week 12

Population: All treated participants with baseline and week 12 measurements

Change from Baseline in the Spondyloarthritis Research Consortium of Canada (SPARCC) scores of the total spine. All 23 disco-vertebral units (DVU) of the spine (from C2 to S1) were scored for bone marrow edema. A single DVU has 18 scoring units, and each has score of 0 or 1, bringing the maximum total score to 414, the sum ranges from 0 to 414 with higher scores reflecting worse disease. Baseline is the last non-missing value on or before the date of the first dose of investigational product.

Time frame: Baseline and Week 12

Population: All treated participants with baseline and week 12 measurements

Percentage of participants who achieve an improvement in disease activity from baseline of ≥ 40% and ≥ 2 unit in at least 3 of the 4 SpondyloArthritis International Society (ASAS) domains on a scale of 0 to 10, and no worsening at all from baseline in the remaining domain. Baseline is the last non-missing value on or before the date of the first dose of investigational product. The four ASAS Domains are: * Patient Global Assessment of Disease (0 to 10 unit Numerical Rating Scale \[NRS\]); * Total Back Pain NRS; * Function (the Bath Ankylosing Spondylitis Functional Index \[BASFI\] score NRS); * Inflammation (mean of Bath Ankylosing Spondylitis Disease Activity Index \[BASDAI\] NRS Questions #5 and #6 for morning stiffness).

Time frame: Week 12

Population: All treated participants who have non-missing response at week 12

Percent change from baseline in high-sensitivity C-reactive protein (hsCRP). Baseline is the last non-missing value on or before the date of the first dose of investigational product.

Time frame: Baseline and Week 12

Population: All treated participants with baseline and week 12 measurements