This study will evaluate the effectiveness of apremilast in AS as measured by improvement in patients' signs and symptoms of the disease and changes in imaging. Additionally the safety and tolerability of apremilast in AS will be assessed. The patients will be recruited from hospitals by Consultant referral. The patients will have had AS for at least 2 years and their symptoms will have been uncontrolled on conventional non−steroidal anti−inflammatory drugs such as ibuprofen. Patients will be randomised to either receive apremilast or a placebo and treated over a period of 12 weeks. They will then be followed up for 28 days after the treatment period.