The infusion chair is the first thing many patients on Keytruda describe: a drip lasting about 30 minutes, repeated every three or six weeks. Pembrolizumab, the drug’s generic name, can also be given as an injection under the skin of the abdomen or thigh, which takes a nurse only one to two minutes. Doctors can switch a patient from one method to the other when it suits them.
Behind that routine is an unusual way of fighting cancer. Pembrolizumab binds to PD-1, a protein on the surface of immune cells called T cells. Tumors use that protein to hold the immune system back. With it blocked, the T cells are free to attack, which is why the drug is called an immune checkpoint inhibitor.
One drug, many cancers
The National Cancer Institute lists a long run of uses. They include biliary tract cancer, triple-negative breast cancer, cervical cancer, classic Hodgkin lymphoma, cutaneous squamous cell carcinoma, endometrial carcinoma, cancers of the esophagus and stomach, liver cancer and melanoma. Sometimes it is given alone. Often it comes with chemotherapy or another drug, and for some patients it is given before surgery, after it, or both.
Before treatment starts, patients may have blood tests for hepatitis B, hepatitis C and HIV, because the drug can weaken the immune system and wake a dormant virus. Doctors may also test the tumor for PD-L1 or certain gene changes. Blood tests continue during treatment to check blood cells, liver, kidneys and thyroid.
The cost of switching on the immune system
Side effects follow from the way the drug works. Cancer Research UK explains that because treatment affects the immune system, it can inflame tissue in several parts of the body. Some of these problems appear during treatment, others a few months after it ends.
Common ones, seen in more than ten percent of patients, can come in combination. Anaemia can leave a patient breathless and pale. A cough or breathlessness may signal infection or inflammation of the lungs (called pneumonitis), and the charity says to tell a doctor or nurse straight away. Tiredness is frequent, and sometimes traces to the thyroid or adrenal glands. Diarrhoea is another; the charity says to call if a patient has two or more extra loose stools in a day. Occasionally it points to inflammation of the bowel, which can be serious.
The same guidance adds a reassurance: it is very unlikely that anyone will have all of these. It also warns that fever or other signs of infection need a call to the advice line at once. Early treatment, it says, helps manage side effects better.
Keytruda is not a cure for every patient, and it asks for close watching. The encouraging part is how widely it can be used. Anyone starting it should talk to their doctor about which symptoms to report and when.
