Background: Psoriatic arthritis (PsA) is a common chronic inflammatory disease with a prevalence up to 670 every 100,000 subjects. Patients with PsA has an increased risk of cardiovascular disease (CVD) which is one of the major causes of death. The investigators hypothesize that metformin in combination of a treat-to-target (T2T) strategy aiming at tight disease control is more effective in preventing progression of subclinical arthrosclerosis than T2T strategy alone in non-diabetic PsA patients.

Objective: To investigate the vascular effects of metformin in PsA patients without diabetes mellitus. The metabolic and anti-inflammatory roles of metformin will also be explored. Study design: This is a 1-year, single-centered, pilot, open-labelled, randomized controlled trial. A total of 24 enrolled patients with PsA being followed at the Prince of Wales Hospital rheumatology clinics will be recruited and randomized to either metformin group or control group in a 1:1 ratio. Participants randomized to the metformin group will be instructed to take 500 mg metformin daily for 1 week before titrating up to twice a day (one with the morning meal, one with the evening meal) to reduce gastro-intestinal adverse events. Expected outcomes: The data from this study will support that there will be significant difference in the proportion of subjects with carotid plaque progression between the metformin group and control group over a period of 1 year.