What Is Pembrolizumab? A Plain-English Guide

Pembrolizumab - sold under the brand name Keytruda - has transformed cancer treatment over the past decade. It belongs to a class of drugs called checkpoint inhibitors, and for eligible patients, it can produce outcomes that were simply not achievable with older treatments. Here is what it is, how it works, and what it might mean for you.

How pembrolizumab works

The immune system has natural "checkpoints" - proteins that regulate how actively immune cells respond. One of these checkpoints is the PD-1 receptor, found on T cells (the immune cells responsible for targeting and destroying abnormal cells). When PD-1 binds to a protein called PD-L1 - which many tumours express on their surface - it sends an inhibitory signal that tells the T cell to stand down. The tumour, in effect, disguises itself as harmless tissue and escapes immune attack.

Pembrolizumab blocks this interaction. It is a monoclonal antibody that binds to PD-1 and prevents it from connecting with PD-L1. With the "off switch" blocked, T cells remain active and can recognise and attack the tumour. This is why pembrolizumab is described as releasing the immune system's brakes rather than directly targeting cancer cells.

What conditions does it treat?

Pembrolizumab has regulatory approval for a wide and growing range of cancers. These include:

• Non-small cell lung cancer (NSCLC)
• Melanoma (both advanced and adjuvant use after surgery)
• Head and neck squamous cell carcinoma (HNSCC)
• Urothelial carcinoma (bladder cancer)
• Cervical cancer
• Endometrial carcinoma
• Oesophageal cancer
• Colorectal cancer with microsatellite instability-high (MSI-H) status
• Biliary tract cancer

Eligibility typically depends on whether the tumour expresses PD-L1 above a threshold measured by laboratory testing on tumour tissue. In some cases - such as MSI-H tumours - eligibility does not depend on PD-L1 expression at all. This is why specialist oncologist assessment is essential before any treatment decision.

The subcutaneous formulation

Pembrolizumab was originally given as an intravenous (IV) infusion - a 30-minute drip administered in a hospital oncology unit, typically every three weeks. This meant regular hospital visits, IV access, and the associated time burden and clinical environment.

A subcutaneous (under the skin) formulation is now available. Clinical trials - including the pivotal KEYNOTE-D18 study - demonstrated bioequivalence: the subcutaneous and IV formulations produce the same drug exposure in the body, with no clinically meaningful difference in efficacy or safety. The subcutaneous injection takes 3-5 minutes and does not require hospital infrastructure, making home administration possible for eligible patients.

What the evidence shows

The KEYNOTE clinical trial programme - one of the largest in oncology history - has produced compelling long-term data. In non-small cell lung cancer with high PD-L1 expression, approximately 31% of patients were alive at five years, compared to 16% with chemotherapy. In advanced melanoma, five-year survival rates of around 34% have been reported - compared to historical rates of approximately 5% with older treatments.

These are not guarantees for any individual patient. Response rates vary substantially based on cancer type, PD-L1 expression, and other clinical factors. Some patients achieve durable remissions lasting years; others do not respond. The best predictor of likely benefit is a thorough clinical assessment by a qualified oncologist.

Is it right for you?

Pembrolizumab is not appropriate for everyone. Key eligibility factors include your cancer type, PD-L1 expression level, performance status, and whether you have conditions - such as active autoimmune disease - that increase the risk of immune side effects. The right starting point is a formal eligibility assessment, including review of your PD-L1 test results and medical history by an oncologist.