Pembrolizumab

Immune checkpoint inhibitors
Immune checkpoint inhibitors
High alert
An error with this medicine can cause serious harm: double checks and protocols are used.

An anti-PD-1 antibody that reactivates T cells against many cancers, including melanoma, lung cancer and any tumour with mismatch-repair deficiency.

Updated Nov 1, 2026

Also called: anti-PD-1 antibody

Quick facts

Group: Immune checkpoint inhibitors
Target: Programmed cell death protein 1 (PD-1) on T cells
Used for: Melanoma · Non-small cell lung cancer · Urothelial
Key effects: Fatigue · Rash and itching · Diarrhoea
Monitor: Thyroid function · Glucose · Stool frequency
ATC: L01FF02

No doses: the clinician sets them by the current guideline.

Mechanism of action

Programmed cell death protein 1 (PD-1) on T cellsPembrolizumab

The crystal shows the extracellular domain of human PD-1 bound by the pembrolizumab Fab, which covers the surface PD-L1 would use.

Target in 3D: Programmed cell death protein 1 (PD-1) on T cells. PDB structure 5GGS (RCSB PDB, public domain). View the structure at RCSB PDB

Pembrolizumab is a humanised IgG4 antibody that binds PD-1 on activated T cells and prevents it from engaging PD-L1 and PD-L2. Tumours that display PD-L1 use this pathway to exhaust the T cells that infiltrate them; blocking it restores T-cell activity, cytokine release and killing of tumour cells. Tumours with many mutations (for example mismatch-repair deficient or MSI-H) make more abnormal proteins for T cells to recognise and respond more often.

Tumour PD-L1 switches off PD-1-positive T cells
Pembrolizumab binds PD-1 and blocks PD-L1/PD-L2
T cells regain activity inside the tumour
Tumour cells are killed; autoimmunity is possible

Indications

Melanoma, advanced or after resection of high-risk disease. Melanoma →
Non-small cell lung cancer, alone (high PD-L1) or with chemotherapy. Lung cancer →
Head and neck squamous cell carcinoma. Head and neck squamous cell carcinoma (oral cavity and oropharynx, HPV) →
Urothelial (bladder) cancer. Bladder cancer →
Classical Hodgkin lymphoma. Hodgkin lymphoma →
Merkel cell carcinoma. Merkel cell carcinoma →
Triple-negative breast cancer, with chemotherapy. Breast cancer →
Any solid tumour with mismatch-repair deficiency or high microsatellite instability (tissue-agnostic), including colorectal cancer. Colorectal cancer →

Adverse effects

Common

Fatigue
Rash and itching
Diarrhoea, nausea, decreased appetite
Cough and breathlessness
Muscle and joint pain
Thyroid dysfunction

Contraindications

Previous severe immune-mediated reaction to a checkpoint inhibitor (generally not re-exposed)
Caution: active autoimmune disease, organ or allogeneic stem-cell transplant

Key interactions

Systemic corticosteroids and other immunosuppressants: Used to treat moderate to severe immune-mediated reactions; they damp the immune response.
Allogeneic stem-cell transplant before or after treatment: A recognised caution: the stronger immune response can cause serious transplant complications, so the team weighs it before use.

What is monitored

Thyroid function (TSH) before starting and regularly
Liver tests and creatinine before each dose
Glucose; cortisol if fatigue or low blood pressure
Stool frequency, cough and breathlessness at each visit
Signs of myocarditis (chest pain, palpitations) and muscle weakness

Administration and safety

Intravenous infusion only, never as a push or bolus, through a line as the product instructions specify.
Look-alike names: the -mab checkpoint inhibitors (nivolumab, pembrolizumab, atezolizumab, cemiplimab, ipilimumab) are checked by full name; products combined with hyaluronidase for subcutaneous use are not interchangeable with the intravenous ones.
Watch for infusion reactions (chills, fever, rash, breathlessness) during the infusion.

Pregnancy and breastfeeding

Blocking PD-1/PD-L1 or CTLA-4 can harm the fetus (IgG antibodies cross the placenta); effective contraception is used during treatment and for months after, and breastfeeding is avoided.

Nursing care

Before each dose, ask about diarrhoea, cough, rash, headache, fatigue, dizziness, thirst or passing more urine, and review thyroid, liver, kidney and glucose results.
Teach the patient that side effects can start weeks or months after a dose, even after treatment ends, and must be reported early.
Teaching: report diarrhoea or blood in the stool, new cough or breathlessness, yellow skin, severe headache, extreme tiredness, chest pain or muscle weakness at once.
Make sure the patient carries information that they receive immunotherapy, for any emergency visit.

Pharmacokinetics

Absorption: Intravenous infusion.
Elimination: Broken down like natural IgG.
Half-life: About 26 days.

Test yourself

Think of the answer, then tap the question to see it.

What does pembrolizumab bind, and what does it block?

PD-1 on T cells; it blocks its interaction with PD-L1 and PD-L2.

Which tumour feature makes pembrolizumab usable regardless of the organ of origin?

Mismatch-repair deficiency or high microsatellite instability (MSI-H).

A patient on pembrolizumab has fatigue, low blood pressure and low sodium. Which immune-related problem should be considered?

Adrenal insufficiency, primary or from hypophysitis.

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Connections

← Back to the class: Immune checkpoint inhibitors

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Pembrolizumab Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).

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Representative model, for educational purposes only.