Persistence of Infliximab in Circulation for 7 Years?

A 55-year-old man with ankylosing spondylitis presented with increasing symptoms despite treatment with adalimumab. The decreased therapeutic response raised concern that he had produced antibodies against the drug or that he had cross-reacting antibodies made against a similar drug, infliximab, with which he was treated 7 years earlier. Tests for antibodies against the drugs were negative, but the serum adalimumab and infliximab concentrations, reported automatically with results for antidrug antibodies, were 4.4 and 4.6 g/mL.