Merck's pembrolizumab cuts progression risk 43% in dMMR endometrial cancer

On 26 October 2026, Merck disclosed detailed results from KEYNOTE-C93, its Phase 3 trial of pembrolizumab as a single agent against platinum doublet chemotherapy in advanced or recurrent mismatch repair-deficient (dMMR) endometrial cancer ^[1]. Pembrolizumab cut the risk of disease progression or death by 43%, with median progression-free survival of 38.2 months versus 8.4 months for chemotherapy ^[1].

The figures came by press release. The trial's ClinicalTrials.gov entry (NCT05173987) still lists the study as active but no longer recruiting, with no results posted and a primary completion date of 27 May 2027 ^[3]. The efficacy numbers are Merck's disclosure ahead of that registry update.

Why it matters

The approved PD-1 option in this disease is GSK's dostarlimab (Jemperli), indicated in combination with carboplatin and paclitaxel followed by dostarlimab maintenance for primary advanced or recurrent endometrial cancer ^[5]. KEYNOTE-C93 tested pembrolizumab without chemotherapy. A median progression-free survival of 38.2 months from a chemo-free pembrolizumab regimen in dMMR patients compared to 8.4 months with chemotherapy ^[1][5].

The same day, Merck reported that sacituzumab tirumotecan, its TROP2 antibody-drug conjugate, cut the risk of death by 32% and extended median overall survival to 19.0 months from 12.1 months in patients who had already progressed on platinum chemotherapy and anti-PD-1/L1 therapy ^[2]. It also cut the risk of progression or death by 41% ^[2]. Merck describes it as the first global Phase 3 study of a TROP2 ADC to show both overall survival and progression-free survival benefit versus chemotherapy in this population ^[2].

Read together, the two readouts give Merck a position at both ends of the endometrial cancer treatment course: a chemo-free immunotherapy option in dMMR disease, and a new ADC for patients out of options after platinum and checkpoint therapy ^[1][2]. The two trials enroll different populations, so they complement each other rather than compete.

What to watch next

Merck framed both readouts as part of a set of first-time pivotal Phase 3 results it brought to ESMO 2026, previewed on 13 October 2026 ^[4]. The ClinicalTrials.gov record for KEYNOTE-C93 lists primary completion for 27 May 2027 and carries no posted results yet, so the registry and the press release are not yet reconciled; a registry update to completed status with posted data is the next concrete catalyst ^[3][4].

The now-what

In KEYNOTE-C93, pembrolizumab monotherapy showed median progression-free survival of 38.2 months versus 8.4 months for platinum doublet chemotherapy in dMMR advanced or recurrent endometrial cancer ^[1]. For GSK, whose dostarlimab is approved in combination with chemotherapy in this disease, the monotherapy data invites the question of whether the chemo backbone survives in dMMR, a question oncologists and payers will now ask ^[1][5]. Merck, for its part, has to convert a press release into a filing and a label, and none of that is yet public.

What Prognyx could not verify

The efficacy figures are Merck's own, disclosed by press release; no independent or analyst coverage in our source set cross-checks them, and no SEC filing in context carries this readout ^[1][2]. Overall survival for KEYNOTE-C93 is not in the disclosed figures. The sacituzumab tirumotecan trial's ClinicalTrials.gov record did not appear in our sources, so its registry status could not be confirmed. No FDA or EMA filing or label change tied to either result is public as of this briefing.

Verdict: a credible threat to the chemo-containing standard in dMMR endometrial cancer, with a wide window to act. No regulatory filing is yet public and overall survival for KEYNOTE-C93 is undisclosed, so competitors have until the full ESMO dataset and a Merck submission to recalibrate.