P001 What is the role of antidrug antibody and drug level testing in patients treated with infliximab and adalimumab?
Authors: Cox Miriam , Smith Ruth , Wild Graeme , Dunkley Lisa 
Published: April 2021

Abstract
Background/Aims
Approximately 30-40% of rheumatology patients fail to respond to first-line biologics. Secondary inefficacy is mediated by immune complex formation between biologic agents and anti-drug antibodies. Anti-drug antibody testing has been undertaken at Sheffield Teaching Hospitals since October 2015. However, there are currently no national guidelines or consensus on what levels of anti-drug antibodies are clinically significant or what changes to therapy are suggested as a consequence of these tests. We aimed to review the reasons for and outcomes of anti-drug antibody levels tested at STH in patients on Adalimumab or Infliximab.
Methods
Retrospective review of records of all Rheumatology patients having antidrug antibody levels tested October 2015 - April 2019.
Results
237 patients were included in this analysis. The mean age of patients was 48 years. 43% were male. The most common reasons for testing antibody levels were clinical evidence of a flare in disease (n = 92) and patient reported worsening of symptoms (n = 88). 66% (n = 157) of antibody levels tested were negative, 21% (n = 49) of tests were strongly positive (antibody titre &gt;50). Serum drug concentrations were subtherapeutic in 20 % (n = 47), therapeutic in 22% (n = 51) and supratherapeutic in 38% (n = 91). In 51% of patients (n = 119) the current treatment regime was continued. However, 38% (n = 90) changed biologics, and dosing schedule was changed in 2% (n = 6). Antibody titres were more likely to be strongly positive in patients who had clinically active disease compared to those who had symptoms but no clinical evidence of disease (30% vs 10% p = 0.009). Those with strongly positive antibodies were more likely to switch biologics than those with normal antibodies (84% vs 28%, p = 0.01). Patients with clinically active disease but normal antibodies and drug levels were more likely to switch biologics than patients with no evidence of active disease but positive antibodies (p = 0.03). Underlying diagnosis (p = 0.23) or concomitant DMARD use (p = 0.92) were not associated with positive autoantibodies. Of the 47 patients with subtherapeutic drug levels, 61% (n = 29) had strongly positive anti-drug antibodies and 73% (n = 34) subsequently switched biologics. 49% (n = 111) of patients had both therapeutic drug levels and normal antibodies. Of these, 22% (n = 25) switched biologics. Of the 25 patients that switched biologics 24% (n = 6) did not have evidence of active disease and 76% (n = 19) had active disease.
Conclusion
33% of patients had positive autoantibodies. 39% of patients switched biologics following testing. There was no protective effect of DMARDs identified. Patients with active disease were more likely to have positive antibodies and to switch biologics than those with no clinical evidence of disease. 25% of patients had subtherapeutic drug levels. However, only 2% of patients had a dose schedule adjustment. Therefore, dosing schedule alterations could be considered in these patients prior to escalating to a more expensive biologic.
Disclosure
M. Cox: None. R. Smith: None. G. Wild: None. L. Dunkley: None.