Checkpoint inhibitors are a type of cancer treatment that works by releasing the brakes on your immune system so it can recognize and attack tumor cells. Unlike chemotherapy, which directly poisons fast-growing cells, these drugs don’t target the cancer itself. Instead, they block the signals that cancer uses to hide from your body’s natural defenses. They’re now approved for more than a dozen cancer types, from melanoma and lung cancer to bladder cancer and Hodgkin lymphoma.
How They Work
Your immune system has built-in “checkpoints,” molecules on the surface of immune cells that act like off switches. These checkpoints exist for a good reason: they prevent your immune system from attacking your own healthy tissue. But cancer cells exploit this system. They display proteins on their surface that flip those switches, essentially telling your T-cells (the immune cells responsible for killing threats) to stand down.
Checkpoint inhibitors are antibodies designed to physically block that interaction. The three main targets are PD-1, PD-L1, and CTLA-4. PD-1 is a receptor on T-cells, and PD-L1 is the protein many tumors produce to engage that receptor. When PD-L1 locks onto PD-1, it tells the T-cell to ignore the cancer. Drugs that block either side of this handshake prevent the “off” signal from going through. CTLA-4 works at an earlier stage, dampening T-cell activation before the cells ever reach the tumor. Blocking CTLA-4 essentially lets the immune system mount a stronger response from the start.
The result, when these drugs work, is that your own immune system begins treating cancer cells as the foreign invaders they are.
Cancer Types They Treat
Checkpoint inhibitors are approved for a wide and growing list of cancers. According to the National Cancer Institute, current approvals cover breast cancer, bladder cancer, cervical cancer, colon cancer, head and neck cancer, Hodgkin lymphoma, liver cancer, lung cancer, kidney cancer, skin cancer (including melanoma), stomach cancer, and rectal cancer. There’s also a broader, tissue-agnostic approval: any solid tumor that can’t properly repair errors when its DNA copies itself (a feature doctors test for specifically) may qualify for treatment regardless of where the cancer originated.
Not every patient with these cancers will be offered a checkpoint inhibitor. Eligibility depends on the stage of cancer, previous treatments, and increasingly on biomarker testing.
How Doctors Decide Who Gets Them
Before starting a checkpoint inhibitor, your oncologist will often order tests to predict whether the drug is likely to work for your specific tumor. The most common test measures PD-L1 expression, which tells doctors how much of the “hide from the immune system” protein the tumor is producing. The thresholds vary depending on the cancer type: some approvals require PD-L1 levels of 50% or higher, others only 1%, and some don’t require PD-L1 testing at all.
A second key biomarker is called microsatellite instability-high (MSI-high) or mismatch repair deficiency. This identifies tumors with a high number of DNA repair errors, which tend to produce more abnormal proteins that the immune system can recognize. Tumors with this feature often respond well to checkpoint inhibitors regardless of where in the body they arose.
Tumor mutational burden (TMB), which measures the total number of mutations in a tumor, is another emerging predictor. Tumors with more mutations tend to look more “foreign” to the immune system, making them better targets. Other experimental biomarkers, including the presence of immune cells already infiltrating the tumor, are being studied but aren’t yet standard in clinical decision-making.
What Treatment Looks Like
Checkpoint inhibitors are given as intravenous infusions, typically in an outpatient cancer center. The dosing schedule varies by drug. Some are given every two weeks, others every three weeks, and some newer formulations stretch to every four or even six weeks. Dosing can be weight-based (calculated from your body weight) or a flat, fixed dose for all patients. The trend in recent years has been toward fixed dosing and longer intervals between infusions to simplify treatment and reduce the number of clinic visits.
How long you stay on treatment depends on how your cancer responds and how well you tolerate the side effects. Some patients remain on checkpoint inhibitors for a year or two; others continue longer if the drug is keeping the cancer in check.
Side Effects
Because checkpoint inhibitors activate the immune system broadly, the most characteristic side effects come from the immune system attacking healthy tissues. These are called immune-related adverse events, and they can affect nearly any organ system: skin rashes, thyroid problems, joint inflammation, gut issues like colitis, and less commonly, effects on the heart, lungs, liver, or nervous system.
The severity depends heavily on which drug (or drugs) you’re receiving. PD-1 and PD-L1 inhibitors cause serious (high-grade) side effects in roughly 10 to 20% of patients. CTLA-4 inhibitors carry higher risk, with about 27% experiencing high-grade toxicity. When both types are combined, that number jumps to around 55%.
Some immune-related side effects resolve after treatment is paused or stopped, but others can become chronic. Among patients who develop lasting non-hormonal side effects, the most commonly affected systems are joints and muscles (20%), the nervous system (19%), the gastrointestinal tract (16%), and the skin (14%). Endocrine side effects, particularly thyroid dysfunction, are especially common and often permanent, requiring lifelong hormone replacement. Your care team will monitor bloodwork and symptoms closely throughout treatment to catch these issues early.
Combining Two Checkpoint Inhibitors
Doctors sometimes pair a CTLA-4 inhibitor with a PD-1 or PD-L1 inhibitor, a strategy called dual checkpoint blockade. The logic is straightforward: blocking two different braking mechanisms at once can produce a stronger immune response than blocking just one.
This approach has shown clear benefits for specific patient groups. A large analysis combining data from six clinical trials in advanced non-small cell lung cancer found that patients whose tumors had no PD-L1 expression at all (the group least likely to respond to a single checkpoint inhibitor) nearly doubled their five-year survival rate with dual therapy: 16.6% were alive at five years compared to 9.3% with a single agent. Patients with a particular genetic mutation called STK11, which normally predicts poor response to immunotherapy, also did notably better on the combination.
For patients whose tumors already expressed PD-L1, however, adding the second drug didn’t provide a clear survival advantage. This is a case where biomarker testing directly shapes the treatment decision, since the combination carries significantly higher side effect risk and isn’t worth that tradeoff unless it’s likely to improve outcomes.
Newer Targets in Development
The current generation of checkpoint inhibitors targets three molecules: PD-1, PD-L1, and CTLA-4. A newer drug targeting LAG-3, another immune checkpoint, is already approved and used in combination with a PD-1 inhibitor. Early data shows this pairing produces modestly higher toxicity (high-grade side effects in about 19% of patients, compared to roughly 10% with a PD-1 inhibitor alone) but can improve responses in certain cancers like melanoma.
TIGIT is another checkpoint molecule that generated enormous industry interest. At least 21 companies pursued 30 different TIGIT-blocking drugs, enrolling nearly 49,000 patients in 220 clinical trials at an estimated cost of over $3 billion. Despite this investment, no TIGIT inhibitor has yet earned FDA or European approval. About a quarter of the programs have been terminated after failing to show meaningful benefit. Several candidates with different designs remain in late-stage trials, and results from those studies will determine whether TIGIT joins the list of viable targets.
The broader research landscape continues to explore additional immune checkpoints and combination strategies, but PD-1, PD-L1, and CTLA-4 inhibitors remain the backbone of cancer immunotherapy today.