Biologicals provide a tight disease control but not all patients respond favourably to treatment. Some patients do not respond at all (primary non-responders), while other patients respond initially but show loss of response over time (secondary non-responders). Drug concentrations in the serum of patients can be monitored and correlated with biological, clinical or endoscopic response. Therapeutic thresholds have been defined for infliximab and adalimumab. The European Medicines Agency has approved 3 biosimilars of infliximab and new biosimilars are waiting approval. Key Messages: Distinguishing primary non-responders from patients with insufficient drug exposure during induction through drug serum concentration determination will improve drug efficacy. Current algorithms to guide treatment of patients with secondary loss of response take into account that patients with high titers of anti-drug antibodies (ADA) do not respond to dose intensification and that patients with therapeutic drug concentrations cannot be switched to biologicals within class. For patients in clinical remission, the cost of biological treatment can be decreased by dose tapering patients with supra-therapeutic concentrations and/or by switching patients with adequate drug concentrations and no formation of ADA to biosimilar, whereas efficacy can be increased by dose-intensifying patients with low or transient ADA and by switching patients with persistent ADA to biologicals within or out-off class.
As an objective tool, therapeutic drug monitoring can identify patients who are eligible for dose tapering, intensification of treatment, cessation of treatment, switching within- or out-of-class and switching to biosimilar.