Panitumumab

• Class: Monoclonal antibody – anti-EGFR (epidermal growth factor receptor), fully human IgG2

Mechanism of Action (MOA)

• Binds EGFR on tumor cells → blocks ligand binding (EGF, TGF-α).
• Inhibits EGFR-mediated signaling pathways (RAS/RAF/MEK/ERK, PI3K/AKT).
• Results in:
  • Cell cycle arrest (G1 phase)
  • Apoptosis induction
  • Reduced angiogenesis and proliferation

Clinical Uses

• Metastatic colorectal cancer (mCRC):
  • RAS wild-type tumors only (KRAS/NRAS mutation excludes therapy).
  • Used as monotherapy in refractory mCRC or with FOLFOX/FOLFIRI in first/second-line therapy (RAS wild-type).

Dosing (Adults)

• 6 mg/kg IV infusion every 2 weeks or 9 mg/kg every 3 weeks.
• Infuse over 60–90 minutes initially; subsequent infusions may be shorter if tolerated.
• No premedication required (fully human → lower risk of hypersensitivity than cetuximab).

Toxicities

• Dermatologic toxicity (most common):
  • Acneiform rash, dry skin, pruritus → severity correlates with efficacy.
• Hypomagnesemia (monitor electrolytes).
• Diarrhea, fatigue, nausea.
• Rare: infusion reactions (lower risk than chimeric cetuximab), pulmonary toxicity.

Monitoring

• Electrolytes: magnesium, calcium, potassium (especially in combination with chemotherapy).
• Dermatologic: manage rash proactively (topical steroids, antibiotics).
• Renal and hepatic function for general chemotherapy regimens.
• Monitor for infusion reactions, though rare.

Summary

Panitumumab (Vectibix®) is a fully human anti-EGFR monoclonal antibody used in RAS wild-type metastatic colorectal cancer. Key concerns are skin toxicity, electrolyte disturbances, and diarrhea. Its fully human structure reduces infusion reactions compared to cetuximab.