Serum trough infliximab and anti-infliximab antibodies in a cohort of gastroenterology and rheumatology patients' infliximab therapeutic drug monitoring

Background: Infliximab, a monoclonal antibody directed against tumour necrosis factor, is widely used in the treatment of chronic inflammatory conditions including Crohn's disease and rheumatoid arthritis. Its use is limited by development of anti-infliximab antibodies, which can lead to loss of therapeutic efficacy. Serum infliximab and anti-infliximab antibody measurements have recently become routinely available in the UK. The study aimed to assess the clinical utility of antibodies as an adjunct to trough infliximab. Methods: Serum trough infliximab was measured in 201 samples from 108 gastroenterology and rheumatology patients on maintenance infliximab therapy. Results were correlated with C-reactive protein concentrations. Total anti-infliximab antibodies were measured in 164 samples. Results: The median (25th-75th percentile) trough infliximab was 3.7 mu g/mL (1.2-5.2 mu g/mL) and 23% of samples had a concentration <= 1 mu g/mL. A notable proportion had positive anti-infliximab antibodies: 84/164 (51%), which subdivided to 85% and 28% with infliximab <= 1 and > 1 mu g/mL, respectively. Serum C-reactive protein was found to be significantly higher where infliximab was <= 1 compared to > 1 mu g/mL (10 mg/L [< 5-24 mg/L] vs. < 5 mg/L [< 5-8 mg/L], P< 0.01), although a strict correlation was not observed. The relationship between trough infliximab and C-reactive protein differed depending on antibody status and there was no association between C-reactive protein and the presence or absence of antibodies. Conclusion: Our findings support measurement of anti-infliximab antibodies only in the context of low infliximab concentrations < 1 mu g/mL. A higher therapeutic cut-off may be relevant in patients with negative antibodies. Further work is indicated to investigate the clinical significance of positive antibodies with therapeutic infliximab concentrations.