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Checkpoint Inhibitor: Cancer Immunotherapy Explained

Checkpoint inhibitors are cancer drugs that reactivate the immune system to attack tumor cells. They are a key part of modern immunotherapy.

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Things worth knowing about "Checkpoint inhibitor"

Checkpoint inhibitors are cancer drugs that reactivate the immune system to attack tumor cells. They are a key part of modern immunotherapy.

What is a Checkpoint Inhibitor?

A checkpoint inhibitor is a type of drug used in modern cancer therapy that belongs to the class of immune checkpoint blockers. It works by releasing natural brakes on the immune system, enabling the body's own defense cells – particularly T cells – to recognize and destroy cancer cells. Checkpoint inhibitors are a central pillar of immuno-oncology and represent one of the most significant advances in cancer medicine in recent decades.

Mechanism of Action

The immune system has built-in control points known as checkpoints that prevent it from overreacting and damaging healthy tissue. Cancer cells exploit these mechanisms by displaying inhibitory proteins on their surface that silence T cells, allowing tumors to evade immune detection.

Checkpoint inhibitors block these inhibitory signals. The most important target structures include:

PD-1 / PD-L1: PD-1 is a protein found on T cells; PD-L1 is expressed by tumor cells. When PD-L1 binds to PD-1, the T cell is inactivated. Drugs such as pembrolizumab, nivolumab (anti-PD-1) and atezolizumab (anti-PD-L1) prevent this binding.
CTLA-4: This protein on T cells dampens immune activation at an earlier stage. Ipilimumab blocks CTLA-4 and thereby amplifies the immune response against the tumor.

By blocking these checkpoints, T cells are reactivated and can mount an effective attack against the tumor.

Indications

Checkpoint inhibitors are approved for a wide range of cancers or are being investigated in clinical trials, including:

Malignant melanoma (skin cancer)
Non-small cell lung cancer (NSCLC)
Renal cell carcinoma
Bladder cancer (urothelial carcinoma)
Hodgkin lymphoma
Colorectal cancer with mismatch repair deficiency
Head and neck cancers

Checkpoint inhibitors may be used as a monotherapy or in combination with chemotherapy, targeted therapies, or other immunotherapies.

Dosage and Administration

Checkpoint inhibitors are typically administered as an intravenous infusion, usually every two to four weeks in a hospital or oncology clinic. The exact dosage and duration of treatment depend on the specific drug, the type of cancer, and the response to therapy. Treatment is continued as long as it remains effective and side effects are manageable.

Side Effects

Because checkpoint inhibitors activate the entire immune system, they can sometimes cause the immune system to attack healthy tissue. These are called immune-related adverse events (irAEs) and can affect almost any organ:

Skin: Rash, itching, and rarely severe skin reactions
Gastrointestinal tract: Diarrhea, colitis (inflammation of the colon)
Liver: Elevated liver enzymes, hepatitis
Lungs: Pneumonitis (non-infectious lung inflammation)
Endocrine glands: Thyroid dysfunction, adrenal insufficiency, hypophysitis
Joints and muscles: Arthritis, myositis

Severe side effects are generally treated with corticosteroids or other immunosuppressive medications. For mild symptoms, supportive care is often sufficient. Patients should always report any new or worsening symptoms to their treatment team promptly.

Who Benefits from Checkpoint Inhibitor Therapy?

Not all patients respond equally well to checkpoint inhibitors. Certain biomarkers can help predict the likelihood of treatment success:

PD-L1 expression in tumor tissue
Tumor Mutational Burden (TMB): Tumors with a high number of mutations often respond better
Microsatellite instability-high (MSI-H): A marker for mismatch repair deficiency

Biomarker testing is therefore often performed before starting therapy to identify patients most likely to benefit.

References

Topalian SL, Taube JM, Anders RA, Pardoll DM. Mechanism-driven biomarkers to guide immune checkpoint blockade in cancer therapy. Nature Reviews Cancer, 2016; 16(5): 275–287.
Haanen JBAG et al. Management of toxicities from immunotherapy: ESMO Clinical Practice Guidelines. Annals of Oncology, 2018; 29 (Suppl 4): iv264–iv266.
National Cancer Institute (NCI). Immune Checkpoint Inhibitors. Available at: https://www.cancer.gov/about-cancer/treatment/types/immunotherapy/checkpoint-inhibitors (accessed 2024).

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