Therapeutic Drug Monitoring — SCE Dermatology MCQ

Correct answer: B — TDM can distinguish between immunogenic failure (low levels + ADAs → switch class), pharmacokinetic failure (low levels, no ADAs → increase dose), and pharmacodynamic failure (adequate levels → switch class) — guiding rational treatment decisions

Therapeutic drug monitoring (TDM) in biologic-treated psoriasis measures drug trough levels and anti-drug antibodies to guide rational management of treatment failure. Three failure mechanisms can be distinguished: (1) Immunogenic failure (low drug level + positive ADAs — the immune system is neutralising the drug → switch to a different biologic class); (2) Pharmacokinetic failure (low drug level + negative ADAs — rapid clearance, often obesity-related → increase dose or shorten interval); (3) Pharmacodynamic failure (adequate drug level + negative ADAs — the drug reaches its target but the disease is driven by a different pathway → switch to a different mechanism class). TDM is increasingly used in clinical practice and recommended by BAD for managing biologic non-response.

Reference: BAD 2020 Biologic Guidelines; BJD TDM Review