Anti-PD-1 is a type of cancer immunotherapy that works by removing a molecular “brake” on your immune system, freeing your T cells to recognize and attack tumor cells. These drugs are now a standard treatment for more than a dozen cancer types, including melanoma, lung cancer, kidney cancer, and head and neck cancers. They’re given as intravenous infusions, typically every few weeks, and have transformed survival rates for cancers that were previously very difficult to treat.
How the PD-1 Pathway Works
Your immune system has built-in safeguards called checkpoints that prevent T cells from attacking healthy tissue. One of the most important involves a protein on the surface of T cells called PD-1 (programmed death-1). When PD-1 binds to a matching protein called PD-L1 on another cell, it sends an “off” signal that tells the T cell to stand down. Under normal circumstances, this is a good thing. It keeps your immune response from becoming so aggressive that it damages your own organs.
Cancer cells exploit this system. Many tumors produce large amounts of PD-L1 on their surface, essentially disguising themselves as normal tissue. When a T cell approaches the tumor and its PD-1 receptor locks onto the tumor’s PD-L1, the T cell receives a shutdown signal and backs off. The cancer grows unchecked, hidden behind a molecular shield.
Anti-PD-1 drugs are lab-made antibodies that physically block the PD-1 receptor on T cells. With PD-1 covered, the tumor’s PD-L1 can no longer deliver that “off” signal. The T cells stay active, recognize the cancer cells as threats, and mount an immune attack against them.
Approved Anti-PD-1 Drugs
Three anti-PD-1 drugs have received FDA approval and are widely used:
- Nivolumab (Opdivo), first approved in 2014 for melanoma, then expanded to lung cancer, kidney cancer, and several other tumor types.
- Pembrolizumab (Keytruda), approved in 2016 for advanced lung cancer and now one of the most broadly prescribed cancer drugs in the world, covering dozens of indications.
- Cemiplimab (Libtayo), approved in 2018 initially for advanced cutaneous squamous cell carcinoma, a form of skin cancer, in patients who aren’t candidates for surgery or radiation.
These drugs continue to receive new approvals as clinical trials demonstrate their effectiveness in additional cancer types. Pembrolizumab alone has been approved for use in ovarian cancer, bladder cancer, certain colorectal cancers, and many others beyond its original indication.
Cancers Treated With Anti-PD-1
Anti-PD-1 therapy is now part of standard care for a wide range of malignancies. Melanoma was the first cancer where these drugs proved transformative, and it remains one of the best-studied applications. Advanced non-small-cell lung cancer (NSCLC) is another major use, along with kidney cancer, head and neck squamous cell carcinoma, Merkel cell carcinoma, and malignant pleural mesothelioma.
In many of these cancers, anti-PD-1 therapy is used for advanced or metastatic disease, meaning the cancer has spread or can’t be surgically removed. However, approvals have increasingly moved into earlier-stage settings. Some patients now receive these drugs after surgery to reduce the risk of their cancer coming back.
How Doctors Decide Who Gets Treatment
Not every patient with a qualifying cancer type will benefit from anti-PD-1 therapy. A key factor is how much PD-L1 the tumor produces, which is measured through a biopsy and lab test. Pathologists score the percentage of tumor cells that stain positive for PD-L1, called the Tumor Proportion Score (TPS).
For lung cancer, a TPS of 1% or higher means the tumor has PD-L1 expression and the patient may be eligible for treatment. A TPS of 50% or higher is considered high expression, and these patients often qualify for anti-PD-1 as a first-line therapy, meaning it can be the initial treatment they receive. Patients with lower expression (1% to 49%) may be eligible after other treatments have been tried first. Tumors with less than 1% PD-L1 expression generally don’t qualify for anti-PD-1 monotherapy, though they may still benefit from combination approaches.
Other scoring systems exist for different cancer types. Some use a Combined Positive Score (CPS), which counts immune cells around the tumor in addition to tumor cells themselves. The specific threshold varies depending on the cancer and the drug being considered.
What Treatment Looks Like
Anti-PD-1 drugs are given as intravenous infusions at a cancer center or infusion clinic. The typical schedule for pembrolizumab is 200 mg every three weeks or 400 mg every six weeks. Nivolumab can be given at 240 mg every two weeks or 480 mg every four weeks, with clinical trials showing that the less frequent, higher-dose schedules maintain the same effectiveness and safety profile as more frequent dosing.
Treatment continues as long as the cancer responds and side effects remain manageable. For some patients, that means months; for others, it can extend to two years or longer. The optimal duration is still being studied, with ongoing trials testing whether less frequent dosing or shorter treatment courses can deliver similar results.
How Effective Anti-PD-1 Therapy Is
In advanced melanoma, anti-PD-1 monotherapy produces five-year overall survival rates of roughly 39%, a dramatic improvement over earlier treatments for a cancer that was once nearly always fatal at the metastatic stage. Patients who achieve a complete response (meaning no detectable cancer remains) have even better outcomes. In the landmark KEYNOTE-001 trial, pembrolizumab-treated patients who reached complete response had a two-year disease-free survival rate of about 91%.
Combining anti-PD-1 with another checkpoint inhibitor targeting a different protein (CTLA-4) can push response rates higher. In the CheckMate 067 trial for melanoma, the combination of nivolumab and ipilimumab achieved an objective response rate of 58%, compared to 44% for nivolumab alone and 19% for ipilimumab alone. Complete responses, where tumors disappeared entirely on scans, occurred in 19% of combination patients versus 16% for nivolumab alone. For lung cancer patients whose tumors had high PD-L1 expression (50% or more), the combination therapy response rate reached 92%.
Side Effects and Immune Reactions
Because anti-PD-1 drugs work by releasing the brakes on the immune system, the most significant side effects come from that same immune system attacking healthy tissue. These are called immune-related adverse events, and they affect roughly 40% of patients to some degree. The most common involve the gut, skin, and hormone-producing glands. Patients may experience diarrhea, skin rashes, fatigue, or thyroid problems.
Most of these reactions are mild to moderate. However, about 20% of patients develop severe (grade 3 or 4) immune-related side effects that require treatment interruption or additional medication, typically steroids to calm the overactive immune response. The pattern of side effects varies by cancer type. Skin and gut reactions tend to be more common in melanoma patients, while joint pain and skin inflammation appear more frequently in lung cancer patients.
Combination therapy with both anti-PD-1 and anti-CTLA-4 drugs carries a higher risk of severe immune reactions than anti-PD-1 alone, which is an important tradeoff patients and oncologists weigh against the improved response rates.
Why Some Patients Don’t Respond
Despite its effectiveness, anti-PD-1 therapy doesn’t work for everyone. Some patients never respond at all (primary resistance), while others respond initially but then see their cancer return (secondary resistance). Several biological factors explain this.
Tumors with few genetic mutations tend to look less “foreign” to the immune system, giving T cells fewer targets to latch onto even after PD-1 is blocked. Some cancers lose the ability to display proteins on their surface that T cells need to identify them as abnormal. Others create a hostile local environment filled with immune-suppressing cells that neutralize T cells before they can do their work.
In patients who initially respond but later relapse, T-cell exhaustion is a common culprit. Over time, T cells that are constantly activated can become worn out, turning on multiple additional “off” switches beyond PD-1. The tumor may also develop new mutations that help it evade detection or resist the immune signals that would normally trigger its destruction. These overlapping resistance pathways are a major reason why researchers are testing anti-PD-1 in combination with other drugs rather than relying on it alone.