Abstract
OBJECTIVES: Up to 40% of psoriatic arthritis (PsA) patients experience first-line Tumour Necrosis Factor inhibitors (TNF-i) failure. Lower serum drug levels (SDL) have been associated with lower response in autoimmune conditions. This study aimed to: (i) establish the relationship between adalimumab (ADL) and etanercept (ETN) SDL and 3-month response; and (ii) identify optimal non-trough SDL thresholds in PsA. METHODS: PsA patients commencing ADL or ETN were recruited to the UK observational study OUTPASS. Patients were seen pre-TNF-i and at 3 months when response was measured, and non-trough serum samples collected. Response was defined according to the PsARC or EULAR criteria. Descriptive statistics and concentration-effect curves established differences in SDL based on response. Receiver operating characteristics and regression identified optimal SDL thresholds. RESULTS: PsA ETN (n = 97) PsARC and EULAR good responders had significantly higher 3-month SDL compared with non-responders (p= 0.006 and p= 0.020 respectively). Non-trough 3-month ETN SDL discriminated PsARC responders from non-responders (AUC = 0.70), with a threshold of 1.8 µg/ml being 63% specific and 69% sensitive. EULAR good and non-/moderate responders were discriminated with an AUC of 0.65 with a threshold of 2.0 µg/ml being 57% specific and 69% sensitive. ADL prescribed (n = 104) EULAR good responders had significantly higher 3-month SDL (p= 0.049). Non-trough 3-month ADL SDL discriminated EULAR good and non-/moderate responders (AUC = 0.63) with a threshold of 3.6 µg/ml being 48% specific and 81% sensitive. CONCLUSION: Higher 3-month SDL were detected in responders. Interventions to optimise SDL may improve treatment response earlier. This study suggests 3-month SDL thresholds which may be useful in clinical practice to optimise treatment response.