Clinical utility of random anti-tumour necrosis factor drug testing and measurement of anti-drug antibodies on long-term treatment response in rheumatoid arthritis.

BACKGROUND: Up to 40% of patients with rheumatoid arthritis treated with anti-tumour necrosis factor (TNF) drugs do not respond because of primary inefficacy or loss of response. Although one explanation is that immunogenicity leads to the development of anti-drug antibodies and low drug concentrations, the clinical usefulness of pharmacological monitoring is debated. Our aim was to assess whether the presence of anti-drug antibodies and non-trough drug concentrations could predict treatment response in patients with rheumatoid arthritis treated with anti-TNF drugs. METHODS: 331 patients were selected from a multicentre prospective cohort (160 treated with adalimumab, 171 etanercept). Serum samples were collected at 3, 6, and 12 months after treatment initiation. Anti-drug antibodies were measured with RIA, drug concentrations with ELISAs, and Disease Activity Score in 28 joints (DAS28) at each timepoint. Linear and logistic regression, generalised estimating equation (GEE), and receiver operating characteristic curves were used to test the association and predictive value of anti-drug antibodies and non-trough drug concentrations on treatment response (ΔDAS28). FINDINGS: 835 serial samples were tested (414 adalimumab, 421 etanercept). Anti-adalimumab antibodies were detected in 31 (24·8%) of 125 patients who had completed 12 month follow-up and none of the etanercept patients. The presence of anti-drug antibodies was associated with lower adalimumab concentrations (Spearman r=-0·66, p=0·0041). At 3 months, anti-drug antibody formation and low adalimumab concentrations were significant predictors of poor treatment response at 12 months (area under curve [AUC] 0·68, 95% CI 0·54-0·81, and 0·66, 0·55-0·77, respectively; and both combined 0·71, 0·57-0·85). Adalimumab concentration was the most significant independent predictor of ΔDAS28 after adjustment for confounders (regression coefficient 0·12, 95% CI 0·06-0·18; p=0·003). High etanercept concentrations were associated with better treatment response (p=0·01), but low concentrations at 3 months were not a significant predictor of poor treatment response at 12 months (AUC 0·58, 95% CI 0·46-0·70). In the combined GEE model including adalimumab and etanercept, a body-mass index of 30 kg/m(2) or more was associated with low drug concentrations (regression coefficient 0·78, 95% CI 0·37-1·18; p