The study will include a representative sample of 150 patients with active Psoriatic Arthritis (PsA) for whom the treating rheumatologist has decided to begin treatment with apremilast. This study is non-interventional, drug dosing and treatment duration will be at the sole discretion of the treating rheumatologist, in accordance with the local label and daily clinical practice. APOLO is a national (Belgium), multicentre, prospective, non-interventional, post-marketing study.

Patient care will follow routine clinical practice, involving regular follow-up visits, without any mandatory visit. In daily practice, patients are usually seen by their treating rheumatologist every 3 months. In this study, patients will be followed-up for a least 6 months and at most 18 months after apremilast treatment initiation. Patient data will be collected until 31st December 2018. Assuming a recruitment phase of 18-month duration starting as of December 2016, individual patient follow-up times will range from 6 to 18 months, depending on the inclusion date. During the first months of the study, the recruitment rate has been slower than expected. Therefore, it has been decided in October 2017 to increase the period of recruitment to 18 months in order to have enough evaluable patients to keep the accuracy required for the statistical analysis.

During the study, it is expected to collect data at inclusion and at 6 months after apremilast initiation in all patients. As per NIHDI recommendations, patients should consult their treating rheumatologists 6 months after apremilast initiation in order to evaluate treatment response and decide on treatment continuation for an additional period of 12 months. For patients recruited early in the study, data will be collected during the next follow-up visits up to 18 months after apremilast initiation. Only data pertaining to visits occurring at 9 (± 1), 12 (± 1), 15 (±1), and 18 (± 1) months after treatment initiation will be collected. If a study visit occurs approximately 3 (± 1) months after treatment initiation, the data will also be recorded.

All clinical data collected during this non-interventional study will be routinely documented in the patient's medical records, which are the main source of information. The study data will be collecting via an electronic Case Report Form (eCRF). Data from source documents including PRO-questionnaires will be entered in the eCRF by the investigator or other authorised appropriately designed and trained study site personnel. Data entered into the eCRF will be reviewed for consistency by the Data Manager using both automated logical checks (issuing in automatic queries generated by the system) and manual review (issuing in manual checks set by the Data Manager or the Monitor into the eCRF). All data collected within the eCRF will be approved and electronically signed and dated by the Investigator or designee. At the conclusion of the study, before the final statistical analysis, the eCRF and other study data will be locked to further additions or corrections.

During the study, the Clinical Research Associate (CRA) will contact each study site on a regular basis in order to check the progress and conduct of the study. If issues regarding study conduct arise, additional on-site visits may be performed. In particular, a quality analysis will also be performed and based on data quality, it will be decided whether on-site monitoring is necessary, for which site and on which percentage of patients. During monitoring visits, eCRFs, patient's source documents, and all other study documentation will be reviewed by the CRA. Accuracy will be checked by performing source data verification that is a direct comparison of the entries made onto the eCRF against the appropriate source documentation.

Adverse events AEs will be coded using the Medical Dictionary for Regulatory Activities (MedDRA) terminology.

The detailed methodology of the statistical analyses will be documented in a statistical analysis plan (SAP). The SAP will be written by the Clinical Research Organisation (CRO) in charge of the study and will be validated by sponsor prior to performing the analysis and obligatory before the database lock. A scientific committee has been selected for this study. This committee will provide advice on the SAP.

Inclusion Criteria:

- Patients who meet ALL the following criteria can be enrolled:

• Male or female aged at least 18 years,
• With a diagnosis of active Psoriatic Arthritis (PsA),
• For whom the treating physician has made the decision to commence apremilast treatment in accordance with the local label and reimbursement criteria (refer to the reimbursement conditions mentioned in Section 1.2),
• Able to follow the instructions of the study,
• Having signed an Informed Consent Form (ICF).

Exclusion Criteria:

- Patients who meet AT LEAST one of the following exclusion criteria will be excluded:

• Women who are pregnant, breastfeeding or planning on becoming pregnant,
• Non-menopausal women who are not using an adequate contraception method,
• Patients with hypersensitivity to apremilast or to one of its excipients.

Adult patients with active PsA visiting rheumatology practices in Belgium and eligible for treatment with apremilast according to the local label and reimbursement criteria. The decision to treat the patient with apremilast will be made prior to the decision to enter the patient into the study.