Bone formation in ankylosing spondylitis during anti-tumour necrosis factor therapy imaged by (18)F-fluoride positron emission tomography

OBJECTIVES: Excessive bone formation is an important hallmark of AS. Recently it has been demonstrated that axial bony lesions in AS patients can be visualized using (18)F-fluoride PET-CT. The aim of this study was to assess whether (18)F-fluoride uptake in clinically active AS patients is related to focal bone formation in spine biopsies and is sensitive to change during anti-TNF treatment. METHODS: Twelve anti-TNF-naïve AS patients [female 7/12; age 39 years (SD 11); BASDAI 5.5 ± 1.1] were included. (18 )F-fluoride PET-CT scans were performed at baseline and in two patients, biopsies were obtained from PET-positive and PET-negative spine lesions. The remaining 10 patients underwent a second (18)F-fluoride PET-CT scan after 12 weeks of anti-TNF treatment. PET scans were scored visually by two blinded expert readers. In addition, (18)F-fluoride uptake was quantified using the standardized uptake value corrected for individual integrated whole blood activity concentration (SUV(AUC)). Clinical response to anti-TNF was defined according to a ⩾ 20% improvement in Assessment of SpondyloArthritis international Society criteria at 24 weeks. RESULTS: At baseline, all patients showed at least one axial PET-positive lesion. Histological analysis of PET-positive lesions in the spine confirmed local osteoid formation. PET-positive lesions were found in the costovertebral joints (43%), facet joints (23%), bridging syndesmophytes (20%) and non-bridging vertebral lesions (14%) and in SI joints (75%). After 12 weeks of anti-TNF treatment, (18)F-fluoride uptake in clinical responders decreased significantly in the costovertebral (mean SUV(AUC) −1.0; P < 0.001) and SI joints (mean SUV(AUC) −1.2; P = 0.03) in contrast to non-responders. CONCLUSIONS: (18)F-fluoride PET-CT identified bone formation, confirmed by histology, in the spine and SI joints of AS patients and demonstrated alterations in bone formation during anti-TNF treatment.