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Infection and Clozapine Levels — MRCPsych Paper B MCQ

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HardSchizophrenia & PsychosisInfection and Clozapine LevelsMRCPsych Paper B

A 34-year-old man with schizophrenia has taken clozapine 400 mg daily at a stable dose for 6 months. Before becoming unwell, a 12-hour trough sample showed clozapine 0.48 mg/L and norclozapine 0.32 mg/L. During febrile pneumonia, with a C-reactive protein of 180 mg/L, a repeat 12-hour trough shows clozapine 1.10 mg/L and norclozapine 0.28 mg/L. His adherence, smoking and caffeine consumption are unchanged, he has received no interacting medicines, and his renal and hepatic function remain normal. Which mechanism best explains the change?

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Correct answer: E — Cytokine-mediated downregulation of hepatic CYP1A2 activity

The correct answer is E. Febrile pneumonia with marked inflammation, unchanged dose and exposures, and comparable trough sampling indicates reduced clozapine metabolism. The increased clozapine-to-norclozapine ratio further supports impaired metabolic conversion. Pro-inflammatory cytokines can downregulate CYP1A2, producing functional phenoconversion to a poor-metaboliser state; clozapine clearance falls and its plasma concentration may rise into the toxic range. Reduced renal clearance is unlikely because clozapine is extensively hepatically metabolised and only trace unchanged drug is excreted. Haemoconcentration would not adequately explain the altered parent-to-metabolite ratio. Sampling error is excluded by matched 12-hour trough samples. Increased absorption is not the recognised explanation for infection-associated clozapine elevation. UK product information advises plasma-level monitoring during pneumonia or other serious infection.

Reference: de Leon J, Ruan C-J, Verdoux H, Wang C. Reflections on the Complex History of the Concept of Clozapine-Induced Inflammation during Titration — section on inflammation, cytokines and CYP1A2 inhibition of clozapine metabolism, 2022. https://pubmed.ncbi.nlm.nih.gov/36256975/ (compatible UK product information: Clozapine SmPC, section 4.2/4.4 blood clozapine level monitoring in pneumonia or other serious infection, https://www.medicines.org.uk/emc/product/15497/smpc)