¹⁸F-FDG positron emission tomography as a marker of disease activity and treatment response in ankylosing spondylitis and psoriatic arthritis

The ability of ¹⁸F-FDG positron emission tomography (PET) to track disease activity and treatment response in patients with Ankylosing Spondylitis (AS) or Psoriatic Arthritis (PsA) remains unclear. Here, we assessed whether ¹⁸F-FDG uptake is a marker of disease activity and treatment response in AS or PsA, and explored the ability of ¹⁸F-FDG to predict treatment response. Patients with AS (n = 16) or PsA (n = 8) who were scheduled to initiate treatment with biologics were recruited. Participants underwent a clinical evaluation and an ¹⁸F-FDG scan prior to therapy initiation. Eleven participants underwent a follow-up ¹⁸F-FDG scan 3 months post-treatment. Images were quantified using a composite measure that describes the inflammatory status of the patient. Clinically involved joints/entheses had higher ¹⁸F-FDG uptake compared to unaffected areas (median difference > 0.6, p < 0.01). Among patients with AS, pre-treatment ¹⁸F-FDG uptake was strongly associated with disease activity (r = 0.65, p = 0.006). Longitudinal ¹⁸F-FDG scans demonstrated that decreases in uptake at 3 months were associated to clinical response (β_ΔgSUVmax > 8.5, p < 0.001). We found no significant association between pre-treatment ¹⁸F-FDG uptake and subsequent clinical response. ¹⁸F-FDG PET shows potential as a marker of disease activity in AS and PsA, allowing for monitorization of biological treatment efficacy in these patients.