The immunogenic part of infliximab is the F(ab′)₂, but measuring antibodies to the intact infliximab molecule is more clinically useful

Objectives: To localise the immunogenic part of infliximab and evaluate the clinical usefulness of measuring antibodies against infliximab fragments. Design: Observational study. Settings: A specialised inflammatory bowel disease (IBD) centre in a tertiary hospital. Interventions Serum was collected from patients with IBD and controls. Antibodies against whole infliximab (ATI) and against the digested Fc, F(ab′)₂ and F(ab′) fragments were measured by a specifically developed ELISA and by western blotting. A separate ELISA was used to determine infliximab levels in serum. Results: 109 serum samples from 62 infliximab-treated patients were tested along with 64 control samples. Anti-F(ab′)₂ antibodies were found in 28/42 (67%) samples with positive ATI, all from infliximab-exposed patients. Anti-F(ab′)₂ antibodies were also present in 26 of the remaining 67 (39%) samples from exposed patients despite absent ATI. No specific anti-Fc antibodies were detected. Low trough infliximab level and high ATI level was found in 10/12 patients (83%) with complete loss of response to infliximab, but in only 5/14 patients (36%, p=0.02) who regained response to intensified infliximab regimen and in 2/24 patients (8%, p<0.001) in maintained remission while on 5 mg/kg/8 week infliximab treatment. Although Anti-F(ab′)₂ antibodies showed similar test characteristics to ATI in patients losing response to infliximab, they were also detected in 61% of patients in maintained remission, thereby limiting their clinical usefulness. No cross reactivity to adalimumab was noted. Conclusions: F(ab′) ₂ is the immunogenic fragment of infliximab. However, ATI level in serum - combined with measurement of trough infliximab level - is better correlated with the clinical response to infliximab or with its loss.