Anti-TNF Therapy

## TNF-Alpha Biology

Tumor necrosis factor-alpha (TNF-alpha) is a pro-inflammatory cytokine produced primarily by activated macrophages, T cells, and NK cells. It signals through two receptors: TNFR1 (ubiquitous, mediates inflammation and apoptosis) and TNFR2 (restricted to immune and endothelial cells, mediates proliferation). TNF-alpha drives synovial inflammation, cartilage destruction, osteoclast activation, and systemic inflammatory responses.

In autoimmune diseases, dysregulated TNF production sustains chronic inflammation. TNF levels are markedly elevated in synovial fluid of rheumatoid arthritis patients and intestinal mucosa of Crohn's disease patients.

## Approved Anti-TNF Agents

Five anti-TNF biologics are currently approved, differing in structure and pharmacokinetics:

- **Infliximab**: Chimeric (mouse/human) IgG1 monoclonal antibody. IV infusion every 6-8 weeks
- **Adalimumab**: Fully human IgG1 monoclonal antibody. Subcutaneous every 2 weeks
- **Etanercept**: TNFR2-Fc fusion protein (soluble decoy receptor). Subcutaneous weekly
- **Certolizumab pegol**: PEGylated Fab' fragment (no Fc region). Subcutaneous every 2-4 weeks
- **Golimumab**: Fully human IgG1 monoclonal antibody. Subcutaneous monthly or IV every 8 weeks

Structural differences matter clinically. Etanercept binds both TNF-alpha and lymphotoxin-alpha but is less effective in granulomatous diseases (Crohn's, sarcoidosis). Certolizumab lacks Fc-mediated effects (complement activation, ADCC) and does not cross the placenta, making it preferred in pregnancy.

## Clinical Indications

Anti-TNF therapy has transformed treatment of multiple autoimmune conditions:

- **Rheumatoid arthritis**: First-line biologic, often combined with methotrexate
- **Inflammatory bowel disease**: Infliximab, adalimumab, certolizumab, golimumab for moderate-to-severe Crohn's and ulcerative colitis
- **Psoriasis and psoriatic arthritis**: All five agents approved
- **Ankylosing spondylitis**: All five agents approved
- **Juvenile idiopathic arthritis**: Adalimumab, etanercept

## Safety Considerations

**Infections** are the primary concern. TNF is essential for granuloma formation and maintenance, so anti-TNF therapy increases risk of tuberculosis reactivation, histoplasmosis, and other granulomatous infections. Screening for latent TB (tuberculin skin test or interferon-gamma release assay) is mandatory before initiation.

Other risks include hepatitis B reactivation, demyelinating disorders (rare), heart failure exacerbation (contraindicated in NYHA Class III/IV), and a modest increase in lymphoma risk, though confounded by underlying disease activity.

**Immunogenicity**: Anti-drug antibodies (ADAs) reduce efficacy and cause infusion reactions. Co-administration with methotrexate reduces ADA formation.

## Key Takeaways

- Five anti-TNF agents offer distinct structural and pharmacokinetic profiles
- Mandatory TB screening before initiation due to granuloma-dependent defense disruption
- Structural differences (Fc presence, PEGylation) affect placental transfer and disease specificity
- Anti-drug antibodies drive secondary treatment failure; methotrexate co-therapy mitigates this