Pembrolizumab (brand name Keytruda, manufactured by Merck) is a humanized monoclonal antibody and a cornerstone agent in modern oncology pharmacy.
Mechanism of Action
Pembrolizumab is a PD-1 (Programmed Death-1) inhibitor — a type of immune checkpoint inhibitor. It works by:
- Binding selectively to the PD-1 receptor on T-lymphocytes
- Blocking the interaction between PD-1 and its ligands PD-L1 and PD-L2, which tumor cells often overexpress to evade immune detection
- Restoring T-cell activity, allowing the immune system to recognize and destroy cancer cells
Classification
| Category | Detail |
|---|---|
| Drug class | Immune checkpoint inhibitor (ICI) |
| Subclass | Anti-PD-1 monoclonal antibody |
| IgG subtype | IgG4 kappa |
| Route | Intravenous (IV) infusion |
FDA-Approved Indications (select major ones)
Pembrolizumab has one of the broadest approval profiles of any oncology agent, including:
- Melanoma (adjuvant and metastatic)
- Non-small cell lung cancer (NSCLC) — first-line, with or without chemotherapy
- Head and neck squamous cell carcinoma (HNSCC)
- Classical Hodgkin lymphoma
- Urothelial carcinoma
- Colorectal cancer (MSI-H/dMMR)
- Gastric/gastroesophageal junction adenocarcinoma
- Triple-negative breast cancer (TNBC)
- Cervical cancer
- Hepatocellular carcinoma
- Tumor mutational burden-high (TMB-H) solid tumors — a tissue-agnostic approval
- MSI-H/dMMR solid tumors — another tissue-agnostic approval
Dosing & Administration
- Standard adult dosing: 200 mg IV every 3 weeks or 400 mg IV every 6 weeks (flat dosing, not weight-based)
- Infused over 30 minutes
- No pre-medication required routinely (unless managing prior infusion reactions)
Key Biomarkers Used in Practice
| Biomarker | Relevance |
|---|---|
| PD-L1 expression (TPS/CPS score) | Predicts response in many tumor types |
| MSI-H / dMMR | Strong predictor of response; tissue-agnostic indication |
| TMB-H (≥10 mut/Mb) | Tissue-agnostic predictor of benefit |
Notable Toxicities (Immune-Related Adverse Events — irAEs)
Because pembrolizumab broadly activates the immune system, it can cause immune-related adverse events (irAEs) affecting virtually any organ:
- Pneumonitis (potentially life-threatening)
- Colitis / diarrhea
- Hepatitis (transaminase elevation)
- Endocrinopathies — hypothyroidism, hypophysitis, adrenal insufficiency, type 1 diabetes
- Dermatitis / rash
- Nephritis
Management typically involves corticosteroids (e.g., prednisone or methylprednisolone) and, in severe cases, permanent discontinuation of therapy.
Pharmacy Practice Considerations
- Supplied as a concentrated solution requiring dilution in NS or D5W
- No dose adjustments required for renal or mild-to-moderate hepatic impairment
- Must be refrigerated (2–8°C); do not freeze or shake
- Diluted solution is stable for up to 96 hours refrigerated
- Requires monitoring of LFTs, TFTs, BMP, and CBC throughout therapy
Pembrolizumab represents a paradigm shift in oncology — moving from cytotoxic chemotherapy toward harnessing the patient’s own immune system to fight cancer, and its expanding indications make it one of the most clinically significant drugs in the oncology pharmacist’s toolkit.