The primary objective of this study was to analyze the cross-reactivity of antidrug antibodies to reference adalimumab (ADL) and SB5 (adalimumab biosimilar) in patients with inflammatory bowel disease (IBD) or rheumatoid arthritis (RA).
Sera from patients with IBD and RA with or without antibodies to adalimumab (ATA+ or ATA-, respectively) were tested for cross-reactivity with SB5 and ADL. Functional inhibition of tumor necrosis factor-α binding was measured. Sera from patients with antibodies to reference infliximab (ATI+) were examined for cross-reactivity to SB5. Sera were tested by enzyme-linked immunosorbent assay.
All 30 anti-ADL ATA+ sera from patients with IBD and all 4 anti-SB5 ATA+ sera from patients with RA were cross-reactive with ADL and SB5 (range of mean concentrations: IBD, 20.99-21.31 μg/ml; RA, 16.46-17.48 μg/ml). In general, there was no significant difference between mean ATA titers. A strong correlation was detected in all ATA+ samples (rho = 0.997 to >0.999; 
ADL and SB5 show cross-immunogenicity in sera from patients with IBD or RA, supporting shared immune-dominant epitopes. ATI+ sera did not cross-react with SB5, suggesting different immunogenic epitopes between infliximab and SB5.