Development of a C1q-immobilized (Cim) assay to measure total antibodies to infliximab and its clinical relevance in patients with inflammatory bowel disease.

OBJECTIVE: Determination of antibodies to infliximab (ATI) is desirable for the management of patients with inflammatory bowel disease (IBD) who receive infliximab. Conventional ligand-binding ATI-assays detect only free-form of ATI, potentially increasing the proportion of patients with undetectable ATI, but with adequate trough infliximab (TRI) level who experience loss of response (LOR) to infliximab. We investigated this assertion using a novel ATI-Cim assay.

METHODS: An ATI-Cim assay was developed by utilizing a C1q-immobilized plate, detecting free-form and ATI-infliximab complexes. Plasma ATI in 137 consecutive IBD patients, 56 with sustained clinical response (SCR), 76 with LOR and 5 with infusion reactions was measured.

RESULTS: ATI levels reached a plateau following addition of up to 25 μg/mL infliximab to different concentrations of free-form ATI. ATI concentration did not significantly change during infliximab infusion (P = 0.4316). ATI concentration > 0.153 μg/mL was associated with LOR (odds ratio 3.0: 95%, confidence interval 1.5 to 6.1, P = 0.0029). The number of patients with undetectable ATI was higher in SCR than in LOR, 53.6% vs 22.4% (P = 0.0004). Patients with SCR and LOR were divided into 4 subgroups by combined cut-off ATI and TRI values. (A) ATI > 0.153 μg/mL and TRI ≤ 2 μg/mL; (B) ATI > 0.153 μg/mL and TRI > 2 μg/mL; (C) ATI ≤ 0.153 μg/mL and TRI ≤ 2 μg/mL; (D) ATI ≤ 0.153 μg/mL and TRI > 2 μg/mL. The frequency of LOR showed a decreasing trend from subgroup A to D, 80.8%, 64.1%, 55.2% and 36.8%, respectively (P = 0.0003).

CONCLUSIONS: The measured ATI level appeared to define the patients' response to infliximab. Combining ATI and trough infliximab levels should help to understand the mechanism of LOR and make therapeutic algorithms.