Certolizumab Structure — SCE Rheumatology MCQ

Correct answer: E — A PEGylated humanised anti-TNF Fab' fragment that lacks an Fc region

Certolizumab pegol is a humanised anti-TNF Fab' fragment conjugated to polyethylene glycol and, uniquely among licensed TNF inhibitors, lacks an Fc region. PEGylation delays elimination and gives a terminal half-life comparable with that of a whole antibody. The absent Fc region means certolizumab does not fix complement or cause antibody-dependent cell-mediated cytotoxicity in vitro and explains its low or negligible FcRn-mediated placental transfer. Option A describes etanercept, a soluble TNF-receptor–Fc fusion protein. Option B describes adalimumab. Option C describes an IL-6-receptor antibody rather than a TNF inhibitor, while currently licensed TNF inhibitors are biologic agents rather than oral small molecules. Pregnancy-related advantages are clinically relevant but are consequences of the defining structure, not the structural feature itself.

Reference: Cimzia 200 mg solution for injection in pre-filled pen, Summary of Product Characteristics, sections 2, 5.1 and 5.2, updated May 2026. https://www.medicines.org.uk/emc/product/7387/smpc