ASAS 20 was defined as achieving an improvement from baseline of ≥ 20% and ≥ 1 unit in at least 3 of 4 ASAS domains on a scale of 0 to 10 units and no worsening from baseline of ≥ 20% and ≥ 1 unit in the remaining ASAS domain on a scale of 0 to 10 units. The 4 ASAS domains are:

1. Patient Global Assessment of Disease (0 - 10 unit Numerical Rating Scale [NRS]); participant marks a box with an X on a 0 - 10 unit NRS; the left-hand box of 0 = not active and the right-hand box = very active
2. Total Back Pain (0 to 10 unit NRS); participant marks a box with an X on a 0 - 10 unit NRS; the left-hand box of 0 = "no pain" and the right-hand box = "most severe pain"
3. Function (Bath AS Functional Index [BASFI] NRS 0 - 10 unit); participant provides a self-administered survey of 10 questions assessing for degree of mobility and functional ability
4. Inflammation domain is determined by the mean of 2 Bath AS Disease Activity Index NRS Questions #5 and #6 for morning stiffness) (0 - 10 unit)

The mITT population included participants who were randomized and received at least one dose of investigation product (IP). Participants who discontinued early due to lack of efficacy were counted as nonresponders, and last observation carried forward (LOCF) imputation was used for participants who discontinued for other reasons.

The BASFI is a composite score based on a self-administered survey of 10 questions using a 0 to 10 unit numerical rating scale (NRS) that assesses the degree of mobility and functional ability. The survey consists of 8 questions regarding function in AS and the last 2 reflect the ability to manage everyday life. The patient marks a box with an X on a 0 to 10 unit NRS for 10 questions; the left-hand box of 0 = easy; the right-hand box = impossible. The resulting 0 to 100 score is divided by 10 to give a final 0 to 10 BASFI score. The overall score is the mean of the 10 items and ranges from 0 to 10. A higher score correlates to reduced functional ability.

The mITT population included those who were randomized and received at least one dose of IP. Missing data were imputed as baseline carried forward for participants who escaped early or discontinued early (prior to Week 24) due to lack of efficacy and LOCF for other early discontinuations

The BASDAI is a composite score based on a participant self-administered survey of six questions measured using a 0 to 10 unit numerical rating scale (NRS) that assessed the participants' five major symptoms of AS: 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. The participant was asked to mark the box with an X on a 0 to 10 unit NRS for each of the 6 questions. To give each of the five symptoms equal weighting, the mean of the two scores relating to morning stiffness was taken. The resulting 0 to 50 score was divided by 5 to give a final 0 to 10 BASDAI score. A BASDAI score of 4 or greater was considered to be indicative of active AS disease.

The mITT population included those who were randomized and received at least one dose of IP. Missing data were imputed as baseline carried forward for participants who escaped early or discontinued early (prior to Week 24) due to lack of efficacy and LOCF for other early discontinuations.

ASAS 20 was defined as achieving an improvement from baseline of ≥ 20% and ≥ 1 unit in at least 3 of 4 ASAS domains on a scale of 0 to 10 units and no worsening from baseline of ≥ 20% and ≥ 1 unit in the remaining ASAS domain on a scale of 0 to 10 units. The 4 ASAS domains are:

1. Patient Global Assessment of Disease (0 - 10 unit Numerical Rating Scale [NRS]); participant marks a box with an X on a 0 - 10 unit NRS; the left-hand box of 0 = not active and the right-hand box = very active
2. Total Back Pain (0 to 10 unit NRS); participant marks a box with an X on a 0 - 10 unit NRS; the left-hand box of 0 = "no pain" and the right-hand box = "most severe pain"
3. Function (Bath AS Functional Index [BASFI] NRS 0 - 10 unit); participant provides a self-administered survey of 10 questions assessing for degree of mobility and functional ability
4. Inflammation domain is determined by the mean of 2 Bath AS Disease Activity Index NRS Questions #5 and #6 for morning stiffness) (0 - 10 unit)

The mITT population included participants who were randomized and received at least one dose of investigation product (IP). Participants who discontinued early due to lack of efficacy were counted as nonresponders, and last observation carried forward (LOCF) imputation was used for participants who discontinued for other reasons.

The ASQoL is a validated disease specific patient reported outcomes instrument to assess the impact of ankylosing spondylitis on the quality of life of individuals with emphasis on the ability of the person to fulfill his or her needs. It consisted of 18 items requesting a yes (score=1) or no (score=0) response to questions related to the impact of pain on sleep, mood, motivation, ability to cope, activities of daily living, independence, relationships, and social life. The summary score ranges 0-18 with higher scores indicating worse quality of life.

The mITT included those who were randomized and received at least one dose of IP. Missing data were imputed as baseline carried forward for participants who escaped early or discontinued early (prior to Week 24) due to lack of efficacy and LOCF for other early discontinuations.

The Medical Outcome Study Short Form 36-Item Health Survey, Version 2 (SF-36) was a self-administered instrument that measures the impact of disease on overall quality of life and consists of 36 questions in eight domains (physical function, pain, general and mental health, vitality, social function, physical and emotional health). Norm-based scores (based on US general population with mean of 50 and standard deviation of 10) were used in analyses. Higher scores indicate a higher level of functioning. The PCS encompasses physical functioning, role-physical, and bodily pain, as well as general health and vitality. A positive change from baseline score indicates an improvement

The mITT included those who were randomized and received at least one dose of IP. Missing data were imputed as baseline carried forward for participants who escaped early or discontinued early (prior to Week 24) due to lack of efficacy and LOCF for other early discontinuations.

The BASMI-Linear was designed to assess axial status (ie, cervical, dorsal and lumbar spine, hips, and pelvic soft tissue) and to define clinically significant changes in spinal movement. Five dimensions of movement (lateral lumbar flexion, tragus to wall, forward lumbar flexion, maximal intermalleolar distance, and cervical rotation) were measured and normalized on 0 to 10 unit NRS. The average of these scores was the total BASMI score, ranging from 0-10 with higher values indicating more severe limitation in spinal mobility.

The mITT population included those who were randomized and received at least one dose of IP. Missing data were imputed as baseline carried forward for participants who escaped early or discontinued early (prior to Week 24) due to lack of efficacy and LOCF for other early discontinuations

The Modified Stoke Ankylosing Spondylitis Spine Score is a scoring method used to determine the amount or degree of ankylosing spondylitis disease that is in the spine based on x-ray radiographs of the spine. The m-SASSS scores 0-3.

0 = No abnormality, 1 = Erosion, Sclerosis or Squaring, 2 = Syndesmophyte, 3 = Total bony Bridging at each Site. An increase in the m-SASSS indicated a worsening of AS disease.

The radiograph subset analysis population included randomized participants who received at least one dose of IP and had at least a baseline radiograph available. Includes apremilast participants as treated who had a baseline and at least one post-baseline score. Missing scores were imputed using the LOCF.

A TEAE was an adverse event (AE) with a start date on or after the date of the first dose of IP and no later than 28 days after the last dose of IP for participants who discontinued early. A serious AE = results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect; or constitutes an important medical event. The severity of AEs was assessed based on the following scale: Mild = asymptomatic or mild symptoms, clinical or diagnostic observations only; Moderate = symptoms cause moderate discomfort; Severe = symptoms causing severe pain discomfort.

Safety population includes all participants who were randomized and received at least one dose of IP. Includes data through Week 16 for placebo-treated and apremilast 20 mg BID treated participants who escaped early and data up to Week 24 for all other participants.

A TEAE was an adverse event (AE) with a start date on or after the date of the first dose of IP and no later than 28 days after the last dose of IP for participants who discontinued early. A serious AE = results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect; or constitutes an important medical event. The severity of AEs was assessed based on the following scale: Mild = asymptomatic or mild symptoms, clinical or diagnostic observations only; Moderate = symptoms cause moderate discomfort; Severe = symptoms causing severe pain discomfort.

Apremilast Subjects as Treated (AAT) were those who received at least 1 dose of apremilast at any time during the study. Participants were included in the treatment group corresponding to the apremilast dosing regimen they actually received, irrespective of the treatment group to which they were randomized or re-randomized.