Merck & Co. (Merck (MRK)) just dropped some promising news for patients with a hard-to-treat form of endometrial cancer. On Wednesday, the company announced that its blockbuster immunotherapy Keytruda (pembrolizumab) outperformed standard chemotherapy in a Phase 3 clinical trial called KEYNOTE-C93.

The trial focused on patients with mismatch repair-deficient (dMMR) advanced or recurrent endometrial cancer who either hadn't received systemic chemotherapy before or had a recurrence more than six months after finishing prior adjuvant therapy. These are patients with limited options, so the results matter.

The study hit its primary endpoint: progression-free survival (PFS). Patients on Keytruda lived longer without their cancer getting worse compared to those on platinum doublet chemotherapy. According to Merck, Keytruda is now the first and only PD-1 inhibitor to show a statistically significant and clinically meaningful improvement in PFS as a monotherapy in this setting in a Phase 3 trial. That's a big deal.

At a pre-specified interim analysis, an independent Data Monitoring Committee also spotted a trend toward improvement in overall survival (OS), the trial's other primary endpoint. But the OS data aren't mature yet—meaning not enough patients have had events to draw firm conclusions. The trial is ongoing, and Merck will evaluate OS for the full study population at a future analysis.

The analysis also showed a clinically meaningful overall response rate (ORR), complete response rate (CRR), and duration of response (DOR) for Keytruda. And the safety profile looked consistent with what's been seen in earlier studies—no new safety signals popped up.

This is just the latest win for Keytruda in endometrial cancer. In the U.S., it's already the only anti-PD-1 therapy with three approved indications for certain types of the disease. And last week, the FDA approved Keytruda and Keytruda Qlex (pembrolizumab with berahyaluronidase alfa-pmph) in combination with Padcev (enfortumab vedotin-ejfv) as neoadjuvant treatment for muscle-invasive bladder cancer (MIBC), followed by continued use after cystectomy as adjuvant treatment. Those approvals marked the first PD-1 inhibitor plus antibody-drug conjugate regimens approved for adults with MIBC regardless of cisplatin eligibility.

Investors seemed pleased. Merck shares were up 2.43% at $123.71 on Wednesday afternoon.