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

Abstract

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.

Keywords: Biologic; FDG; Longitudinal; Positron emission tomography; Psoriatic; Spondylitis.

Figure 1

Pre-treatment relationships between ¹⁸F-FDG uptake in clinically affected joints/entheses and disease activity. (A) Example of an AS patient displaying elevated ¹⁸F-FDG uptake at the knees. (B) Example of a PsA patient displaying ¹⁸F-FDG uptake at the right knee and left trochanter areas. (C) Comparison of ¹⁸F-FDG uptake in clinically involved vs clinically uninvolved joints/entheses. Reported p-values are from sign tests comparing the median ¹⁸F-FDG uptake between the involved/uninvolved regions. (D,E) represent the associations between ¹⁸F-FDG uptake, as measured using the composite score gSUV_max, and disease activity in AS (D) and PsA (E). Correlation coefficients (r), together with respective p-values, are reported in the boxes.

Figure 2

(A) Comparison of ¹⁸F-FDG uptake in clinically involved sacroiliac joints vs clinically uninvolved joints/entheses. Reported p-values are from sign tests comparing the median ¹⁸F-FDG uptake between the involved/uninvolved regions. (B) Associations between ¹⁸F-FDG uptake in the sacroiliac joints and disease activity in AS (BASDAI). Correlation coefficients (r), together with respective p-values, are reported in the box.

Figure 3

Longitudinal relationships between ¹⁸F-FDG uptake in clinically affected joints/entheses and disease activity. (A,B) Example of pre-treatment vs 3-month post-treatment ¹⁸F-FDG PET scans of an AS (A) and PsA (B) patient. For the AS patient (A), the pre-treatment scan shows elevated ¹⁸F-FDG uptake at the clinically involved areas (shoulders, knees, and hips). After 3 months of biologic treatment, the follow-up ¹⁸F-FDG PET scan shows clear reductions in ¹⁸F-FDG uptake in the aforementioned regions. For the PsA patient (B), the pre-treatment scan showed elevated ¹⁸F-FDG uptake at the right knee and left trochanter areas. After 3 months of treatment with biologics, the follow-up scan revealed significantly lower ¹⁸F-FDG uptake in the knee and slightly lower uptake in the trochanter area. (C) Comparison of ¹⁸F-FDG uptake, as measured using gSUV_max, before and 3 months after treatment. Reported p-values are from sign tests comparing the median gSUV_max between the pre- and post-treatment scans. (D,E) Results from linear mixed effects models showing the adjusted mean change from baseline in BASDAI for AS (D) and in DAPSA for PsA (E) at 3 and 6 months after biological treatment initiation. Results are shown for the first (Q1) and third quartile (Q3) of the 3-month change in gSUV_max. Stronger reductions in gSUV_max (Q3) were associated with greater clinical improvements at follow-up (blue lines).

Figure 4

(A) Example of pre-treatment (left) vs 3-month post-treatment (right) ¹⁸F-FDG PET scans of an AS patient with clinical involvement at the sacroiliac level. The follow-up scan reveals significantly lower ¹⁸F-FDG PET signal at the sacroiliac joints. (B) Comparison of ¹⁸F-FDG uptake at the sacroiliac joints before and 3 months after treatment. Reported p-values are from sign tests comparing the median SUV_max between the pre- and post-treatment scans. (C) Results from linear mixed effects models showing the adjusted mean change from baseline in BASDAI for AS at 3 and 6 months after biological treatment initiation. Results are shown for the first (Q1) and third quartile (Q3) of the 3-month change in SUV_max at the sacroiliac joints.

Figure 5

Results from linear mixed effects models showing the adjusted mean change from baseline in BASDAI for AS (A) and in DAPSA for PsA (B) at 3 and 6 months after biological treatment initiation. Results are shown for the first (Q1) and third quartile (Q3) of the pre-treatment gSUV_max. No statistically significant associations between pre-treatment gSUV_max and clinical improvements at follow-up were found.