A 34-year-old man with Crohn's disease involving the terminal ileum and ileocolon (L3) has been on infliximab for 18 months. He develops loss of response (Harvey-Bradshaw index worsens, CRP rises to 28 mg/L). Infliximab trough level is 2.1 μg/mL with anti-drug antibodies (ADA) detected at high titre. The most appropriate next step per therapeutic drug monitoring (TDM) algorithms is:
- A Increase infliximab dose to 10 mg/kg
- B Switch to a different mechanism class (e.g., vedolizumab or ustekinumab) ✓
- C Switch to a different anti-TNF (adalimumab)
- D Add immunomodulator (azathioprine) to rescue infliximab
Explanation
In pharmacokinetic failure with low trough and high-titre anti-drug antibodies (ADA), switching within the same drug class (another anti-TNF) is less effective because ADA-mediated failure suggests broad class immunogenicity. The TAXIT and PRECISION studies support that high-titre ADA with sub-therapeutic troughs indicates immune-mediated clearance; switching to a non-TNF biologic (vedolizumab — gut-selective anti-integrin; or ustekinumab — anti-IL-12/23) is preferred for non-immunogenic mechanism-switching. Dose escalation in the setting of high ADA will not overcome antibody-mediated clearance. Adding AZA is a preventive measure for new biologic starts, not a rescue in ADA-positive loss of response.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.