Mechanism of action
Pembrolizumab is a highly selective, humanized IgG4 monoclonal antibody that binds to the programmed cell death protein 1 (PD-1) receptor on T lymphocytes. Under normal conditions, tumor cells exploit the PD-1/PD-L1 pathway to evade immune surveillance. When PD-L1 (or PD-L2) on tumor cells binds to PD-1 on activated T cells, it delivers an inhibitory signal that suppresses T-cell proliferation, cytokine production, and cytotoxic activity.
By blocking PD-1, pembrolizumab prevents this inhibitory signal, effectively releasing the brakes on T-cell-mediated antitumor immunity. The result is restoration of the immune system's ability to recognize and destroy cancer cells. Unlike traditional cytotoxic chemotherapy, pembrolizumab does not directly kill tumor cells; instead, it enhances the host immune response.
The efficacy of pembrolizumab is highest in tumors with high PD-L1 expression or high tumor mutational burden, which generate more neoantigens and thus more potential immune targets. This mechanism explains both the therapeutic benefit and the spectrum of immune-related adverse effects.
- Pembrolizumab is a PD-1 checkpoint inhibitor that restores T-cell antitumor immunity by blocking the PD-1/PD-L1 interaction.
- Approved for multiple cancers including melanoma, NSCLC, and MSI-high/dMMR tumors (tissue-agnostic indication).
- Most testable adverse effects: immune-related pneumonitis, colitis, hepatitis, and endocrinopathies (especially thyroiditis).
- Management of irAEs: withhold pembrolizumab and give high-dose corticosteroids; taper steroids slowly to prevent rebound.
- Pembrolizumab is contraindicated in pregnancy; it may disrupt immune tolerance to the fetus.
- PD-L1 expression and tumor mutational burden are predictive biomarkers for response.
- Pseudoprogression (initial apparent worsening due to immune infiltrate) can occur; re-image before stopping therapy.
- Vitiligo in melanoma patients is a favorable prognostic sign indicating robust immune response.
Indications
Pembrolizumab is FDA-approved for a wide range of malignancies, making it one of the most broadly indicated cancer immunotherapies. Key indications include:
- Melanoma: unresectable or metastatic melanoma, and adjuvant treatment after complete resection
- Non-small cell lung cancer (NSCLC): first-line for metastatic NSCLC with high PD-L1 expression (≥50% tumor proportion score) and no EGFR/ALK mutations, or in combination with chemotherapy for metastatic non-squamous NSCLC
- Head and neck squamous cell carcinoma (HNSCC): recurrent or metastatic disease, first-line in combination or as monotherapy
- Classical Hodgkin lymphoma: refractory or relapsed disease
- Primary mediastinal large B-cell lymphoma (PMBCL): refractory disease
- Urothelial carcinoma: locally advanced or metastatic disease, especially after platinum-based chemotherapy
- MSI-high or dMMR tumors: tissue-agnostic indication for unresectable or metastatic solid tumors with mismatch repair deficiency or microsatellite instability-high status
- Gastric and gastroesophageal junction adenocarcinoma
- Cervical cancer: recurrent or metastatic with PD-L1 expression
- Hepatocellular carcinoma
- Merkel cell carcinoma
- Renal cell carcinoma: in combination with axitinib
- Endometrial carcinoma: advanced disease with dMMR
- Tumor mutational burden-high (TMB-H) tumors: another tissue-agnostic indication
The tissue-agnostic approvals for MSI-high/dMMR and TMB-high tumors represent a paradigm shift in oncology, where treatment is guided by molecular characteristics rather than tumor histology alone.
Pharmacokinetics
Pembrolizumab is administered as an intravenous infusion. Dosing follows the product label. As a large monoclonal antibody (approximately 149 kDa), pembrolizumab does not undergo hepatic metabolism via cytochrome P450 enzymes. Instead, it is catabolized into small peptides and amino acids by proteolytic enzymes throughout the body.
The steady-state volume of distribution is approximately 6 liters, and the terminal half-life is about 22 days. Steady-state concentrations are reached by 16 weeks with repeated dosing. Pembrolizumab exhibits dose-proportional pharmacokinetics. No dose adjustments are required for renal or hepatic impairment based on available data, though severe impairment has not been extensively studied.
Unlike small molecules, pembrolizumab is not renally excreted, and hemodialysis does not significantly alter its clearance. The pharmacokinetics are not significantly affected by age, sex, race, tumor type, or baseline PD-L1 expression.
Adverse effects
The adverse effect profile of pembrolizumab is dominated by immune-related adverse effects (irAEs), which result from the drug's mechanism of enhancing immune activity. These can affect virtually any organ system and range from mild to life-threatening.
Common immune-related adverse effects
- Pneumonitis: immune-mediated lung inflammation, potentially fatal; presents with dyspnea, cough, chest pain
- Colitis and diarrhea: immune-mediated inflammation of the GI tract
- Hepatitis: elevated transaminases, bilirubin; can progress to hepatic failure
- Endocrinopathies: hypothyroidism (most common), hyperthyroidism (often preceding hypothyroidism), hypophysitis (anterior pituitary inflammation), adrenal insufficiency, type 1 diabetes mellitus with DKA
- Dermatologic: rash, pruritus, vitiligo (especially in melanoma patients, correlates with response)
- Nephritis: immune-mediated kidney injury with elevated creatinine
Less common but serious irAEs
- Myocarditis: rare but potentially fatal; presents with troponin elevation, arrhythmias, heart failure
- Neurologic: encephalitis, myasthenia gravis, Guillain-Barré syndrome, aseptic meningitis
- Hematologic: immune thrombocytopenia, hemolytic anemia
- Ocular: uveitis, episcleritis
- Rheumatologic: arthritis, myositis
Other adverse effects
- Fatigue (very common)
- Infusion-related reactions (usually mild)
- Decreased appetite, nausea
- Musculoskeletal pain
Management principle: Most irAEs are managed with temporary or permanent discontinuation of pembrolizumab and administration of high-dose corticosteroids. Endocrinopathies often require lifelong hormone replacement. Early recognition and prompt treatment of irAEs is critical to prevent serious outcomes.
Contraindications and warnings
There are no absolute contraindications to pembrolizumab, but several warnings and precautions merit attention:
Immune-mediated adverse reactions: Pembrolizumab can cause severe and fatal immune-mediated adverse effects including pneumonitis, colitis, hepatitis, endocrinopathies, nephritis, dermatologic reactions, and other organ toxicities. These reactions can occur at any time during or after treatment and may affect multiple organ systems simultaneously. Patients should be monitored closely for signs and symptoms of irAEs. Baseline and periodic assessment of thyroid function, liver enzymes, creatinine, and blood glucose is recommended. Management involves withholding treatment and administering corticosteroids for moderate to severe reactions. Permanent discontinuation is required for life-threatening reactions, recurrent severe reactions, or inability to taper corticosteroids.
Infusion reactions: Mild to moderate infusion-related reactions can occur; severe reactions are rare. Interrupt or slow the infusion rate if reactions occur.
Complications in hematopoietic stem cell transplant (HSCT): Increased risk of severe graft-versus-host disease (GVHD) and other transplant-related complications in patients who receive allogeneic HSCT after pembrolizumab. Consider benefit versus risk carefully.
Pregnancy and lactation: Pembrolizumab may cause fetal harm based on its mechanism of action. PD-1/PD-L1 pathway blockade can disrupt immune tolerance to the fetus. Verify pregnancy status before initiating treatment. Effective contraception should be used during treatment and for 4 months after the last dose. Breastfeeding is not recommended during treatment and for 4 months after the final dose, as it is unknown whether pembrolizumab is excreted in human milk.
Autoimmune disease: Use with caution in patients with preexisting autoimmune conditions, as pembrolizumab may exacerbate underlying disease. The risk-benefit ratio must be carefully considered.
Drug interactions
Pembrolizumab has minimal traditional pharmacokinetic drug interactions because it is a monoclonal antibody that does not undergo metabolism by cytochrome P450 enzymes and does not affect CYP enzyme activity.
Immunosuppressants: Concurrent use of systemic corticosteroids or other immunosuppressants at baseline may diminish the efficacy of pembrolizumab by blunting the immune response that the drug aims to restore. However, corticosteroids used to manage irAEs after they develop do not appear to compromise antitumor efficacy. Baseline immunosuppression for organ transplants or autoimmune conditions is a relative contraindication.
Live vaccines: Avoid administration of live vaccines during treatment with pembrolizumab, as the safety and efficacy have not been established. The enhanced immune activation could theoretically increase the risk of vaccine-related adverse events.
Other checkpoint inhibitors: Pembrolizumab is sometimes combined with other checkpoint inhibitors (e.g., CTLA-4 inhibitors like ipilimumab) in clinical trials, but combination therapy significantly increases the risk and severity of immune-related adverse effects compared to monotherapy.
Chemotherapy and targeted therapy: Pembrolizumab is frequently combined with chemotherapy or targeted agents (e.g., axitinib, lenvatinib) in approved regimens. These combinations are studied in clinical trials to establish safety and efficacy; they do not have direct pharmacokinetic interactions but may have additive toxicities.
No dose adjustments are needed based on concomitant medications, renal function, or hepatic function.
Clinical pearls
- Hyperprogression: A small subset of patients may experience rapid tumor growth (hyperprogression) after starting pembrolizumab. The mechanism is not fully understood, but it underscores the importance of early imaging assessment.
- Pseudoprogression: Some patients show initial radiographic progression due to immune cell infiltration into tumors, followed by subsequent response. This phenomenon, called pseudoprogression, can mimic true progression. Re-imaging and clinical correlation are necessary before discontinuing therapy.
- Duration of treatment: In many cancers, pembrolizumab is continued until disease progression, unacceptable toxicity, or a maximum of 2 years. Some patients achieve durable remission and may remain in remission after stopping treatment.
- Biomarkers: PD-L1 expression (measured by tumor proportion score or combined positive score) and tumor mutational burden are predictive biomarkers. However, responses can occur even in PD-L1-negative tumors in some cancer types.
- MSI-high/dMMR: Pembrolizumab's approval for MSI-high or mismatch repair-deficient tumors was a landmark tissue-agnostic indication. These tumors have high mutational loads and are highly responsive to PD-1 blockade regardless of primary site.
- IrAE timing: Most immune-related adverse effects occur within the first few months, but they can develop at any time, even after discontinuation. Late-onset irAEs have been reported more than a year after stopping therapy.
- Steroid dosing: High-dose corticosteroids (e.g., prednisone 1-2 mg/kg/day or equivalent) are the cornerstone of irAE management. Taper slowly over at least 4-6 weeks to prevent rebound inflammation.
- Vitiligo in melanoma: Development of vitiligo in melanoma patients treated with pembrolizumab is associated with improved clinical outcomes and is considered a favorable immune response.
- Exam tip: USMLE and similar exams frequently test recognition of immune-related adverse effects, especially thyroiditis, pneumonitis, and colitis, and the management approach (hold drug, give steroids).
Practice questions
1. A 62-year-old man with metastatic melanoma is started on pembrolizumab. Three months into therapy, he develops progressive dyspnea and dry cough. Chest CT shows diffuse ground-glass opacities. Oxygen saturation is 89% on room air. Laboratory studies show normal white blood cell count and no fever. Which of the following is the most appropriate next step in management?
2. A 58-year-old woman with advanced non-small cell lung cancer is receiving pembrolizumab. She presents with fatigue, weight gain, cold intolerance, and constipation. Laboratory studies show TSH 42 mIU/L (normal 0.5-5), free T4 0.4 ng/dL (normal 0.8-1.8). Which of the following is the most appropriate management?
3. A 55-year-old man with metastatic colorectal cancer undergoes tumor molecular testing, which reveals microsatellite instability-high (MSI-H) status and mismatch repair deficiency. He has failed two lines of chemotherapy. Which of the following is the most appropriate next treatment?
Frequently asked questions
How does pembrolizumab work?
Pembrolizumab works by blocking the PD-1 receptor on T cells, preventing tumor cells from inactivating the immune response via the PD-L1/PD-1 pathway. By removing this inhibitory signal, pembrolizumab restores T-cell-mediated killing of cancer cells. It does not directly destroy tumors but enhances the body's own immune system to recognize and attack malignant cells. This mechanism explains both its therapeutic efficacy and its propensity to cause immune-related adverse effects.
What are the side effects of pembrolizumab?
Pembrolizumab causes immune-related adverse effects (irAEs) by enhancing immune activity. Common irAEs include pneumonitis, colitis, hepatitis, hypothyroidism, and skin rash. Less common but serious toxicities include myocarditis, hypophysitis, adrenal insufficiency, type 1 diabetes, nephritis, and neurologic disorders like encephalitis. Fatigue and infusion reactions also occur. Most irAEs are managed by holding the drug and administering high-dose corticosteroids; endocrinopathies often require lifelong hormone replacement.
What cancers is pembrolizumab used for?
Pembrolizumab is approved for melanoma, non-small cell lung cancer, head and neck cancer, Hodgkin lymphoma, urothelial carcinoma, gastric cancer, cervical cancer, hepatocellular carcinoma, Merkel cell carcinoma, renal cell carcinoma, and endometrial carcinoma. It also has tissue-agnostic approvals for any solid tumor with MSI-high/dMMR or high tumor mutational burden. It is used as monotherapy or in combination with chemotherapy or targeted therapy depending on the cancer type and stage.
What is the difference between pembrolizumab and nivolumab?
Both pembrolizumab and nivolumab are PD-1 checkpoint inhibitors with similar mechanisms of action and overlapping indications. Pembrolizumab is an IgG4 antibody, while nivolumab is also an IgG4 antibody; their clinical efficacy and safety profiles are comparable. The choice between them often depends on approval status for specific cancer types, institutional preference, and availability. Cross-trial comparisons are difficult, but head-to-head studies show no major differences in efficacy or toxicity for shared indications.
Is pembrolizumab a chemotherapy drug?
No, pembrolizumab is not chemotherapy; it is an immunotherapy agent. Unlike chemotherapy, which directly kills rapidly dividing cells, pembrolizumab enhances the immune system's ability to recognize and destroy cancer cells by blocking the PD-1 immune checkpoint. This fundamental difference in mechanism results in a distinct adverse effect profile: immunotherapy causes immune-related toxicities rather than the myelosuppression, mucositis, and alopecia typical of traditional cytotoxic chemotherapy.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: L01XC18
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.