Keytruda (pembrolizumab) is one of the most significant treatment advances for endometrial cancer in recent years, but how well it works depends heavily on a biological feature of your tumor called mismatch repair status. In tumors with deficient mismatch repair (dMMR), nearly half of patients see their tumors shrink. In tumors with proficient mismatch repair (pMMR), the more common type, Keytruda still improves outcomes but typically needs to be paired with other treatments to be effective.
Why Your Tumor’s Repair Status Matters
Every cell in your body has a system for catching and fixing errors when DNA copies itself. In some endometrial cancers, this repair system is broken. These tumors are called mismatch repair deficient (dMMR), sometimes also described as microsatellite instability-high (MSI-H). About 25 to 30 percent of endometrial cancers fall into this category.
Tumors with broken repair systems accumulate far more genetic mutations, which makes them look more “foreign” to the immune system. That’s exactly what Keytruda exploits. Cancer cells often hide from immune attack by displaying a protein called PD-L1, which acts like a “stand down” signal to T cells. Keytruda blocks the receptor that reads that signal, essentially removing the disguise and letting T cells recognize and kill tumor cells. Because dMMR tumors already have more features for the immune system to latch onto, Keytruda works especially well against them.
Your oncologist will test your tumor’s mismatch repair status before starting treatment. The FDA has cleared several companion diagnostic tests for this purpose, including tissue-based genetic sequencing and a panel that checks for four specific repair proteins (MLH1, PMS2, MSH2, and MSH6). This result shapes your entire treatment plan.
Response Rates in dMMR Tumors
In the KEYNOTE-158 study, which tested Keytruda as a single agent in patients with advanced dMMR endometrial cancer who had already tried other treatments, the objective response rate was 48 percent. That means nearly half of patients saw their tumors shrink meaningfully. The median time before the cancer progressed was 13.1 months, and many responses lasted years. The median duration of response was never formally reached during follow-up, with some patients still responding past four years.
These are striking numbers for patients whose cancer had already progressed on prior therapy. For context, chemotherapy alone in similar patients produces response rates well below this range.
First-Line Treatment With Chemotherapy
In June 2024, the FDA approved Keytruda in combination with standard chemotherapy (carboplatin and paclitaxel) as a first-line treatment for adults with primary advanced or recurrent endometrial cancer, regardless of mismatch repair status. This approval was based on the NRG-GY018 trial, which enrolled 810 patients split into dMMR and pMMR groups.
Adding Keytruda to chemotherapy reduced the risk of disease progression or death by 55 percent in dMMR patients and 36 percent in pMMR patients. In practical terms, the treatment course starts with Keytruda given alongside chemotherapy, then continues with Keytruda alone as maintenance for up to two years. Infusions are given either every three weeks (200 mg) or every six weeks (400 mg), delivered intravenously over about 30 minutes.
This approval was significant because it was the first time an immunotherapy was cleared for first-line use across all endometrial cancers, not just the dMMR subset.
Combination With Lenvatinib for Previously Treated Cancer
For patients whose endometrial cancer has progressed after prior treatment, Keytruda can also be combined with lenvatinib, a targeted pill that blocks blood vessel growth in tumors. The large KEYNOTE-775 trial compared this combination against standard chemotherapy, and five-year follow-up data show durable benefits.
Five-year overall survival rates for the combination versus chemotherapy alone tell the story clearly across tumor types:
- dMMR tumors: 36.5% alive at five years with the combination, versus 9.8% with chemotherapy
- pMMR tumors: 16.7% versus 7.3%
- All patients combined: 19.9% versus 7.7%
Five-year progression-free survival followed the same pattern: 26.4% versus 10.8% for dMMR patients, and 6.3% versus 2.1% for pMMR patients. These numbers confirm that even in the harder-to-treat pMMR group, the combination more than doubled the proportion of patients alive at five years compared to chemotherapy.
Side Effects and Tolerability
Keytruda on its own tends to be well tolerated compared to chemotherapy. The most common side effects include fatigue, skin rash, joint pain, and diarrhea. Because the drug activates the immune system broadly, it can sometimes trigger inflammation in healthy organs, including the thyroid, liver, lungs, or kidneys. These immune-related reactions are usually manageable with dose pauses or short courses of steroids, but they require monitoring.
The combination of Keytruda plus lenvatinib carries a heavier side-effect burden. In real-world data, 84 percent of patients experienced at least one serious (grade 3 or higher) adverse event. The most common were high blood pressure (49 percent of patients), anemia (26 percent), fatigue (14 percent), and low platelet counts (12 percent). More than half of patients needed at least one dose reduction of lenvatinib, and 40 percent had treatment temporarily paused. However, permanent discontinuation due to side effects was relatively uncommon: only 7 percent stopped lenvatinib and 2 percent stopped Keytruda entirely.
This means the combination works, but it requires active management. Most patients go through dose adjustments to find a tolerable level rather than stopping treatment altogether.
What These Numbers Mean in Practice
If you have dMMR/MSI-H endometrial cancer, Keytruda is among the most effective options available. Response rates are high, responses tend to last a long time, and the drug works both as a single agent and in combination. If you have pMMR endometrial cancer, Keytruda still offers meaningful improvement over chemotherapy alone, but it needs to be paired with either chemotherapy or lenvatinib to deliver its benefits. The gains are real but more modest than in the dMMR group.
Treatment duration is capped at about two years (35 cycles on a three-week schedule or 18 cycles on a six-week schedule). Some patients whose tumors respond well are able to stop treatment at that point and remain in remission. Others may eventually need additional lines of therapy. Your tumor’s molecular profile, how it responds in the first few months, and how well you tolerate the regimen all factor into the long-term picture.