After several rounds of standard chemotherapy, when few options seem to remain, the word 'immunotherapy' can sound like renewed hope. Hearing that someone 'received Keytruda (generic name pembrolizumab) and had almost no side effects' can make anyone want to grasp at any possibility. Still, understanding how this drug works, when it tends to help, and whether 'no side effects' is accurate can make a conversation with your care team much calmer.
Immune checkpoint inhibitors such as pembrolizumab do not attack cancer cells directly the way traditional chemotherapy does. Immune cells (T cells) carry 'brakes' that prevent them from over-attacking, and cancer cells press these brakes to hide from immune surveillance. Drugs like pembrolizumab release those brakes so the body's existing immune cells can recognize and fight the cancer again. When it works, responses can last a long time — but only when conditions allow the immune system to 'see' the tumor.
In colorectal cancer, the key markers that predict this are 'MSI-H (microsatellite instability-high)' or 'dMMR (mismatch repair deficient).' Such tumors accumulate many genetic errors, so they display many abnormal signals on their surface, and immune cells recognize them as foreign. By contrast, most colorectal cancers are 'MSS/pMMR,' and unfortunately these usually respond poorly to immunotherapy alone. This is why testing a tissue sample or running genetic testing (NGS) to confirm these markers first is an important step before considering immunotherapy.
The phrase 'almost no side effects' also deserves a closer look. Immunotherapy tends to cause fewer of the classic chemotherapy effects such as hair loss or severe vomiting, so many people do feel relatively comfortable. But when the released immune system attacks healthy organs, 'immune-related adverse events' can occur — thyroid dysfunction, lung inflammation (pneumonitis), diarrhea from colitis, hepatitis, skin rash, and hormone-gland problems, for example. Most are manageable, but some can be serious enough to need prompt attention. It is safer to think of these effects as 'different in kind' rather than 'absent.'
Cost and insurance come up often as well. In Korea, national health insurance coverage for immunotherapy generally applies only when set conditions — such as the marker criteria above — are met. If the markers do not match, coverage is difficult; and even where a non-covered prescription is possible, the cost is high and the expected benefit may be limited. Actual coverage and cost depend on your test results, the hospital, and timing, so confirming with your medical team and pharmacy is the accurate path.
Finally, when liver function has declined or ascites is significant, whether the body can tolerate any systemic therapy must be weighed carefully. Whether to try a new treatment or focus on easing symptoms is a decision made by considering the patient's overall condition and wishes together with the test numbers, not the numbers alone. Ask your care team your questions, and where helpful, seeking a second opinion from another hospital can also be valuable.
This article is for general information only and does not replace individual medical care or judgment. Please discuss any treatment choices, medication use, and side-effect management with your own medical team.