Merck & Co. (NYSE:MRK) dropped two late-stage clinical readouts on Monday at the European Society for Medical Oncology Congress 2026, and both involve endometrial cancer. One is for the company's investigational antibody-drug conjugate, sacituzumab tirumotecan. The other is for Keytruda, the immunotherapy that has been Merck's financial engine for years. Both showed statistically significant survival improvements. The stock, for whatever it's worth, went the other direction.

TroFuse-005: The Antibody-Drug Conjugate Delivers

Start with the newer asset. In the Phase 3 TroFuse-005 study, developed alongside Kelun-Biotech, sacituzumab tirumotecan met its dual primary endpoints in patients who had already been treated with platinum-based chemotherapy and immunotherapy. That is a sick population, and the bar for success is high.

The trial marks the first global Phase 3 study for an antibody-drug conjugate to demonstrate statistically significant survival gains over chemotherapy in this population. That is a meaningful first.

The numbers: sacituzumab tirumotecan reduced the risk of death by 32% compared to physician's choice chemotherapy, extending median overall survival to 19.0 months versus 12.1 months. It also cut the risk of disease progression or death by 41%, delivering a median progression-free survival of 7.2 months compared to 3.9 months. And the objective response rate came in at 35.1%, roughly double the chemotherapy arm's 17.3%.

The safety profile remained consistent with earlier evaluations, with no new safety signals. Following the TroFuse-005 findings, the U.S. Food and Drug Administration accepted a Biologics License Application under priority review, with a decision date of June 15, 2027.

KEYNOTE-C93: Keytruda Does Its Thing

Then there is the incumbent. Merck also highlighted results from the Phase 3 KEYNOTE-C93 trial assessing Keytruda monotherapy in patients with mismatch repair deficient endometrial cancer. Keytruda lowered the risk of disease progression or death by 43% compared to platinum doublet chemotherapy.

Patients receiving Keytruda reached a median progression-free survival of 38.2 months versus 8.4 months for chemotherapy recipients. Read that again. Thirty-eight months versus eight. Keytruda also demonstrated a favorable overall survival trend, recording a 36-month survival rate of 70.7% compared to 62.0% for chemotherapy. Complete response rates reached 33.8% for Keytruda versus 26.2% for chemotherapy, and researchers identified no new safety concerns.

The Stock Did Not Celebrate

You might expect a pair of wins like this to move shares higher. Instead, Merck shares were down 2.37% at $141.82 at the time of publication on Monday, according to market data. Markets are funny that way. Sometimes good data is already priced in, sometimes investors fixate on something else entirely, and sometimes a Monday is just a Monday.

What Merck has, at minimum, is a clearer path in endometrial cancer: a priority-reviewed BLA for its antibody-drug conjugate with a decision expected by mid-2027, and fresh evidence that Keytruda remains a potent option in the right genetic subgroup. Whether the stock agrees is a separate question.