tocilizumab

MECHANISM: Binds both soluble and membrane-bound IL-6 receptors (sIL-6R and mIL-6R), preventing IL-6 signaling through the JAK/STAT pathway (gp130 signal transduction). IL-6 is a pro-inflammatory cytokine from macrophages, T cells, and fibroblasts. It drives the acute-phase response (CRP, fibrinogen, hepcidin, serum amyloid A, ESR) in the liver, B-cell differentiation and antibody production, and T-cell activation (Th17). Blocking it lowers inflammation and CRP/ESR.

INDICATIONS (high-yield): - Rheumatoid arthritis: moderate-to-severe disease with inadequate response to DMARDs or TNF inhibitors. Can be used as monotherapy, which is a notable advantage over TNF inhibitors, which are typically given with methotrexate. - Giant cell arteritis (GCA): FDA-approved. Often used with a steroid taper to allow steroid sparing. - Systemic JIA and polyarticular JIA. - Cytokine release syndrome (CRS) from CAR-T cell therapy or bispecific T-cell engagers. This is a first-line treatment for severe CRS. - COVID-19: used in hospitalized patients on corticosteroids who need oxygen or ventilatory support, because of the hyperinflammatory state. - Other uses: Castleman disease (IL-6 driven; siltuximab is an anti-IL-6 antibody used here), Takayasu arteritis, systemic sclerosis-associated interstitial lung disease, adult-onset Still disease.

ADVERSE EFFECTS: - Serious infections: bacterial, fungal, and TB reactivation. Screen for latent TB (PPD/IGRA) before starting. May blunt fever and CRP, which can mask infection. - Hepatotoxicity: elevated transaminases, with rare liver injury. Monitor LFTs. - Neutropenia and thrombocytopenia: monitor CBC. - Hyperlipidemia: raised total cholesterol, LDL, and triglycerides. - GI perforation: especially in patients with diverticulitis or on concurrent NSAIDs or steroids. - Infusion or injection-site reactions, and anaphylaxis (rare). - Demyelinating disorders (rare). - Avoid live vaccines.

LAB PITFALL: IL-6 blockade suppresses CRP and ESR, so these markers are unreliable for monitoring disease activity or infection. Serum IL-6 levels paradoxically rise because the receptor is blocked and the cytokine is not cleared.

CLASSIFICATION: Biologic DMARD. IL-6 receptor antagonist. Given IV or SC (the SC form is weekly or every other week). Sarilumab is another IL-6R antibody. Name stem: "-zumab" = humanized monoclonal antibody; "-li-" = immunomodulator (immune-system target).

BOARD COMPARISON OF BIOLOGICS: - TNF-α inhibitors (infliximab, adalimumab, etanercept, certolizumab, golimumab): reactivate TB, are contraindicated in advanced heart failure, and are linked to demyelination. - Anakinra: IL-1 receptor antagonist (used in gout, Still disease, and recurrent pericarditis). - Abatacept: CTLA-4 Ig, blocks T-cell costimulation (CD28/B7). - Rituximab: anti-CD20 (B cells). - Tofacitinib/baricitinib/upadacitinib: JAK inhibitors, which act on the same downstream pathway as IL-6 signaling. - Secukinumab: IL-17; ustekinumab: IL-12/23.

CLINICAL VIGNETTE: A 70-year-old woman with temporal headache, jaw claudication, and an elevated ESR has biopsy-proven giant cell arteritis. She relapses during a steroid taper. Tocilizumab is added as a steroid-sparing agent. In follow-up, a normal CRP does not reliably exclude active inflammation or infection.