Merck (NYSE: MRK) reported that pembrolizumab (Keytruda) monotherapy met its primary progression-free survival endpoint in the Phase III KEYNOTE-C93 trial, making it the first PD-1 inhibitor to demonstrate statistically significant PFS improvement over platinum doublet chemotherapy in the frontline dMMR endometrial cancer setting. The result opens the door to a chemotherapy-free option for a biomarker-selected population that has historically shown high sensitivity to immune checkpoint blockade.
Pembrolizumab blocks the PD-1 receptor on T cells, preventing tumor-mediated immune evasion and restoring cytotoxic T-cell activity against cancer cells.
KEYNOTE-C93 is a randomized, open-label Phase III trial enrolling 299 patients with dMMR advanced or recurrent endometrial cancer who had received no prior systemic chemotherapy, or who relapsed more than six months after completing adjuvant therapy. Patients received either pembrolizumab 400 mg intravenously every six weeks for up to 18 cycles, or carboplatin plus paclitaxel every three weeks for six cycles. At a pre-specified interim analysis, the trial met its primary PFS endpoint; a trend toward improved overall survival — the trial's co-primary endpoint — was observed but OS data were not mature. Specific hazard ratios and response rates were not disclosed and will be presented at a forthcoming medical meeting. No new safety signals were identified.
The commercial and regulatory implications are material. Pembrolizumab is already approved in the US for three endometrial cancer indications, including as a single agent for previously treated dMMR or MSI-H endometrial carcinoma and in combination with carboplatin and paclitaxel for primary advanced or recurrent endometrial carcinoma regardless of MMR status. A successful KEYNOTE-C93 submission could expand the single-agent label to include the frontline dMMR population — a chemotherapy-free positioning that none of its competitors currently hold in approved form.