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Pembrolizumab (Keytruda)

PD-1 Inhibitor Immunotherapy Monoclonal Antibody
Written by: Oncol.net editorial team | Reviewed by: Oncol.net editorial team | Last updated: January 24, 2026

⚠️ Important Safety Warning

Immune-Mediated Adverse Reactions: Pembrolizumab can cause severe and fatal immune-mediated adverse reactions including pneumonitis, colitis, hepatitis, endocrinopathies, nephritis, and skin reactions. Monitor closely and discontinue or withhold for severe reactions. This is a labeled warning. The current FDA label does not have a boxed warning.

Overview

Pembrolizumab (brand name: Keytruda) is a humanized monoclonal antibody that works as an immune checkpoint inhibitor. It belongs to a class of drugs called PD-1 (programmed death-1) inhibitors that help the immune system recognize and attack cancer cells.

Key Facts

  • Generic Name: Pembrolizumab
  • Brand Name: Keytruda
  • Manufacturer: Merck & Co., Inc.
  • FDA First Approval: September 2014
  • Drug Type: Humanized IgG4 monoclonal antibody
  • Route: Intravenous infusion; a separate product, Keytruda Qlex, is injected under the skin
  • Half-life: Approximately 22 days

Mechanism of Action

How Pembrolizumab Works

Step 1: Cancer cells express PD-L1/PD-L2 ligands

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Step 2: These ligands bind to PD-1 receptors on T cells

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Step 3: This binding "turns off" the T cell response

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Step 4: Pembrolizumab blocks PD-1 receptors

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Step 5: T cells remain active and can attack cancer

By blocking the PD-1 pathway, pembrolizumab prevents cancer cells from evading the immune system, allowing T cells to recognize and destroy tumor cells.

PD-1 Blocking in Plain Words

Your immune system has brakes. They stop it from attacking healthy tissue. These brakes are called checkpoints. One of them is a protein called PD-1, which sits on immune cells called T cells.

PD-1 has a partner protein called PD-L1. When the two join, the T cell gets an "off" signal and stops attacking. Some tumors make large amounts of PD-L1. This lets them switch off the T cells that would otherwise kill them.

Pembrolizumab sits on PD-1 so PD-L1 cannot reach it. The "off" signal never arrives, and the T cells can keep working. The medicine does not kill cancer cells directly. It lets your own immune cells do that.

What Are the "Blockers" Called, and How Do They Get Where They Are Needed?

The blocker is pembrolizumab itself. It is a monoclonal antibody, which is a lab-made immune protein. Doctors may call it a PD-1 blocker, a PD-1 inhibitor, or an immune checkpoint inhibitor. These names all mean the same type of drug. There is no separate ingredient inside the medicine that does the blocking.

Once given, the antibody moves through your bloodstream and attaches to PD-1 on T cells. It does not have to be aimed at the tumor. It works through T cells, wherever they are in the body. That body-wide action is also why side effects can show up in almost any organ.

Pembrolizumab stays in the body for weeks. Its half-life, the time for half of it to clear, is about 22 days. This is why doses can be spaced 3 or 6 weeks apart.

FDA-Approved Indications

Melanoma

FDA Approved
  • Unresectable or metastatic melanoma
  • Adjuvant treatment following resection

Non-Small Cell Lung Cancer

FDA Approved
  • First-line for PD-L1 ≥1%
  • With chemotherapy regardless of PD-L1

Head and Neck Cancer

FDA Approved
  • Recurrent or metastatic HNSCC
  • First-line with chemotherapy

Classical Hodgkin Lymphoma

FDA Approved
  • Relapsed or refractory cHL
  • Adult and pediatric patients

Urothelial Carcinoma

FDA Approved
  • Locally advanced or metastatic
  • BCG-unresponsive NMIBC

MSI-H/dMMR Tumors

FDA Approved
  • Any solid tumor with MSI-H/dMMR
  • Tissue agnostic indication

Triple-Negative Breast Cancer

FDA Approved
  • Advanced or metastatic: PD-L1 positive (CPS ≥10), with chemotherapy
  • High-risk early stage: with chemotherapy before surgery, then alone after surgery

Renal Cell Carcinoma

FDA Approved
  • First-line with axitinib
  • Adjuvant monotherapy

Additional Approved Indications: Gastric cancer, esophageal cancer, cervical cancer, hepatocellular carcinoma, biliary tract cancer, Merkel cell carcinoma, endometrial carcinoma, TMB-H tumors, cutaneous squamous cell carcinoma, malignant pleural mesothelioma, platinum-resistant ovarian cancer, primary mediastinal large B-cell lymphoma

Dosing & Administration

Standard Dosing Regimens

Indication Dose Frequency Infusion Time
Most adult indications 200 mg Every 3 weeks 30 minutes
Alternative adult dosing 400 mg Every 6 weeks 30 minutes
Pediatric (cHL, PMBCL, MSI-H/dMMR, Merkel cell, TMB-H; adjuvant melanoma at 12 years and older) 2 mg/kg (max 200 mg) Every 3 weeks 30 minutes
Melanoma adjuvant (adults) 200 mg or 400 mg Every 3 or 6 weeks, up to 12 months 30 minutes

Every 3 Weeks or Every 6 Weeks?

The FDA label allows either schedule for all adult uses. The 6-week dose is double the 3-week dose, so fewer visits does not mean less medicine. Your oncologist picks the schedule. If Keytruda is paired with chemotherapy, the plan may start on the same 3-week rhythm as the chemotherapy.

Keytruda Qlex: The Injection Under the Skin

Yes, a subcutaneous form exists. The FDA approved Keytruda Qlex on September 19, 2025. It contains pembrolizumab with an enzyme called berahyaluronidase alfa.

Pembrolizumab Dose Frequency Injection Time
395 mg (2.4 mL) Every 3 weeks 1 minute
790 mg (4.8 mL) Every 6 weeks 2 minutes
  • A health care provider gives the injection. It is not a shot you give yourself at home.
  • It goes into the thigh or abdomen, away from the 5 cm area around the navel.
  • It is approved for solid tumors in adults and patients 12 and older. The label lists no blood cancer use, so people with Hodgkin lymphoma should ask about the IV form.
  • The doses differ from the IV doses, and the two products cannot be substituted dose for dose. A switch is made only at the next scheduled dose.

Administration Guidelines

  • Administer as IV infusion over 30 minutes
  • Use 0.2-5 micron in-line filter
  • Do not co-administer other drugs through same line
  • Store in refrigerator; do not freeze
  • Dilute in NS or D5W to 1-10 mg/mL
  • Use within 6 hours at room temperature or 96 hours refrigerated

Dose Modifications

No dose reductions recommended. Management of adverse reactions may require withholding or discontinuation. The exact rule depends on the organ:

  • General rule: Withhold for Grade 3 reactions. Permanently discontinue for Grade 4 reactions
  • Lung: Withhold for Grade 2; permanently discontinue for Grade 3-4
  • Colon: Withhold for Grade 2-3; permanently discontinue for Grade 4
  • Heart: Permanently discontinue for Grade 2-4 myocarditis
  • Hormone glands: Withhold until stable for Grade 3-4, or discontinue depending on severity

How Long Treatment Lasts

There is no single answer. The length depends on the cancer, its stage, how well the drug works, and how you tolerate it. Treatment stops early if the cancer grows or side effects become too severe.

Situation Label Limit
Unresectable or metastatic melanoma No fixed limit; continues until progression or unacceptable toxicity
Most advanced cancers, such as lung, head and neck, bladder, and cervical Up to 24 months
After surgery for melanoma, non-small cell lung cancer, or kidney cancer Up to 12 months
High-risk early triple-negative breast cancer 24 weeks before surgery, then up to 27 weeks after

The drug and its effects can last after the final dose. Keep reporting new symptoms, because immune side effects can begin after treatment ends. Use birth control and avoid breastfeeding for 4 months after your last dose.

Side Effects

Common Side Effects (≥20%)

When pembrolizumab is used alone, the label lists these as reported in at least 20% of patients. Exact rates vary by cancer and study.

  • Fatigue
  • Musculoskeletal pain and other pain
  • Rash and pruritus (itching)
  • Diarrhea, constipation, nausea, and abdominal pain
  • Fever
  • Cough and shortness of breath
  • Decreased appetite
  • Hypothyroidism (underactive thyroid)

Immune-Related Adverse Events (irAEs)

Serious irAEs Requiring Immediate Attention

  • Pneumonitis (3.4%): Cough, dyspnea, chest pain
  • Colitis (1.7%): Diarrhea, abdominal pain, blood in stool
  • Hepatitis (0.7%): Elevated liver enzymes, jaundice
  • Nephritis (0.3%): Elevated creatinine, decreased urine
  • Endocrinopathies: Hypothyroidism (8%), hyperthyroidism (3.4%), adrenal insufficiency (0.8%), hypophysitis (0.6%), thyroiditis (0.6%), type 1 diabetes (0.2%)
  • Skin reactions (1.4%): SJS/TEN, severe rash
  • Myocarditis (<1%): Chest pain, heart failure

Percentages are from 2,799 patients who took pembrolizumab alone in the label's safety data.

Immune Side Effects by Organ: What to Report

With the brake released, the immune system can attack healthy tissue. This can happen at any time, including after treatment stops. Early reporting lets problems be treated before they worsen.

Organ What Can Happen Symptoms to Report Right Away
Lungs Pneumonitis New or worse cough, shortness of breath, chest pain
Bowel Colitis Diarrhea or more bowel movements than usual; black, tarry, bloody, or mucus-filled stools; severe belly pain
Liver Hepatitis Yellow skin or eyes, severe nausea or vomiting, pain in the right upper belly, tea-colored urine, easy bleeding or bruising
Hormone glands Thyroid, pituitary, or adrenal problems; type 1 diabetes Lasting or unusual headaches, extreme tiredness, fast heartbeat, sweating, weight change, feeling cold, dizziness or fainting, mood changes, more thirst, hunger, or urination
Kidneys Nephritis Less urine, blood in urine, swollen ankles, loss of appetite
Skin Rash, severe skin reactions Rash or itching; blistering or peeling skin; painful sores in the mouth, nose, throat, or genital area; fever or flu-like symptoms; swollen lymph nodes
Heart Myocarditis Chest pain, irregular heartbeat, shortness of breath, swollen ankles
Brain and nerves Encephalitis, meningitis, nerve inflammation Confusion, sleepiness, memory problems, stiff neck, balance problems, tingling or numbness in the arms or legs
Eyes Eye inflammation Blurry or double vision, eye pain, light sensitivity
Muscles and joints Myositis, arthritis Lasting or severe muscle pain or weakness, cramps, joint pain

Infusion Reactions

Reactions can happen during an infusion and can be severe. Tell your nurse right away about chills or shaking, itching or rash, flushing, shortness of breath or wheezing, dizziness, feeling faint, fever, or back pain.

Management of irAEs

Severity Management
Grade 1 Continue pembrolizumab, monitor closely
Grade 2 Hold pembrolizumab for some organs, corticosteroids if needed
Grade 3 Withhold or discontinue depending on the organ, high-dose corticosteroids
Grade 4 Usually discontinue, high-dose corticosteroids, may need additional immunosuppression

Monitoring Requirements

Before Each Dose

  • Symptoms assessment
  • Liver function (AST, ALT, bilirubin)
  • Renal function (creatinine)
  • Thyroid function (TSH)

Every 3-6 Cycles

  • Complete metabolic panel
  • Free T4 if TSH abnormal
  • Cortisol if indicated
  • Blood glucose

As Clinically Indicated

  • Chest imaging for pneumonitis
  • Colonoscopy for severe colitis
  • Cardiac workup if symptoms
  • Endocrine consultation

Drug Interactions

⚠️ Corticosteroids

Tell your care team if you take steroids. Steroids are a main treatment for serious immune side effects, and some chemotherapy plans use them to prevent nausea and reactions. The Keytruda IV dosing instructions do not call for steroid premedication. Do not stop or start steroids on your own.

Other Considerations

  • Other immunosuppressants: Tell your care team about any medicines that suppress the immune system
  • Vaccines: Ask your care team before getting any vaccine
  • No known CYP interactions: Antibody not metabolized by CYP450

Patient Information

Before Starting Treatment

  • Review all medications including OTC and supplements
  • Discuss vaccination status
  • Report any autoimmune conditions
  • Pregnancy/breastfeeding counseling
  • Understand immune side effects

During Treatment

What to Report Immediately

  • New or worsening cough, shortness of breath
  • Severe diarrhea or abdominal pain
  • Yellowing of skin or eyes
  • Unusual fatigue or weakness
  • Chest pain or irregular heartbeat
  • Severe headaches or vision changes
  • Skin rash or blistering

Lifestyle Considerations

  • Carry medical alert card about immunotherapy
  • Inform all healthcare providers about treatment
  • Use effective contraception during and 4 months after
  • Do not breastfeed during treatment or for 4 months after your last dose
  • Ask before getting any vaccine
  • Stay hydrated
  • Report any new symptoms promptly

Cost & Access

Financial Considerations

Average wholesale price: Approximately $10,000-12,000 per infusion

Annual cost: $150,000-200,000 (varies by dosing schedule)

Financial Assistance Programs

  • Merck Access Program
  • Patient assistance programs
  • Co-pay assistance for eligible patients
  • Foundation support programs

Insurance Coverage: Generally covered for FDA-approved indications. Prior authorization typically required.

Clinical Pearls

  • irAEs can occur at any time, even after discontinuation
  • Endocrinopathies may be permanent, requiring hormone replacement
  • PD-L1 testing may be required for certain indications
  • Can be combined with chemotherapy or other targeted agents
  • Response may take longer than with chemotherapy (pseudo-progression possible)
  • Some patients achieve durable responses lasting years

Frequently Asked Questions

What are the blockers in Keytruda called?

The blocker is pembrolizumab itself. It is a lab-made antibody that attaches to a protein called PD-1 on T cells, so doctors call it a PD-1 blocker, PD-1 inhibitor, or immune checkpoint inhibitor. There is no separate blocking ingredient inside the medicine.

How does Keytruda reach the cancer once it is given?

Keytruda goes into a vein, or under the skin with Keytruda Qlex, and then travels through the bloodstream. It does not attack the tumor itself. It attaches to PD-1 on T cells, and those T cells do the work of finding and killing cancer cells.

How often is Keytruda given?

For adults, the IV form is 200 mg every 3 weeks or 400 mg every 6 weeks, given over 30 minutes. Children usually receive it every 3 weeks. Keytruda Qlex under the skin is also given every 3 or 6 weeks.

Is there a Keytruda shot instead of an IV?

Yes. The FDA approved Keytruda Qlex on September 19, 2025. A health care provider injects it under the skin of the thigh or abdomen. The dose takes about 1 minute every 3 weeks or about 2 minutes every 6 weeks. It is approved for solid tumors in adults and in patients 12 and older, so it is not an option for every Keytruda use.

How long will I stay on Keytruda?

It depends on the cancer and the reason for treatment. For advanced cancers, the label often allows treatment for up to 24 months, or until the cancer grows or side effects become too severe. After surgery, treatment often lasts up to 12 months. Metastatic melanoma has no fixed limit in the label.

Does Keytruda have a black box warning?

No. The current FDA label does not have a boxed warning. It does list serious warnings, including severe and sometimes fatal immune-mediated reactions, infusion reactions, and harm to an unborn baby.

Important Safety Information

This is not a complete list of side effects or drug information. Always consult the full prescribing information and discuss with your oncology team. Report side effects to FDA at 1-800-FDA-1088.

References

  1. Keytruda (pembrolizumab) [prescribing information]. Merck & Co., Inc.; 2026.
  2. NCCN Guidelines. Management of Immunotherapy-Related Toxicities. Version 1.2026.
  3. FDA Oncology Drug Approvals. Pembrolizumab indication updates.

Sources