Phase 3 maintenance trial targets the relapse gap in MMRp endometrial cancer
A new phase 3 study is asking a direct clinical question in endometrial cancer: can mocertatug rezetecan, given with physician’s choice of immunotherapy, prolong disease control after initial treatment in patients with mismatch repair proficient disease? The trial, BEHOLD-Endometrial02, is recruiting participants with advanced or recurrent endometrial carcinoma and compares the combination against standard maintenance immunotherapy alone. It is an open-label, randomized, multicenter study sponsored by GlaxoSmithKline and is designed to evaluate efficacy, safety, tolerability, and health-related quality of life.
The setup reflects a common problem in gynecologic oncology. Patients can achieve an initial response to platinum-based chemotherapy paired with a checkpoint inhibitor, yet microscopic residual disease may continue to drive later progression. In mismatch repair proficient tumors, which are biologically less immunogenic than mismatch repair deficient disease, maintenance strategies often face a tougher target. The trial is built around that reality: keep the immune pressure in place, then add a new agent that may deepen and prolong tumor control.
BEHOLD-Endometrial02 is enrolling an estimated 610 participants and carries the ClinicalTrials.gov identifier NCT07684599. Study start is listed as 2026-09-24, with primary completion estimated for 2029-07-23 and final completion estimated for 2031-09-18. The study remains recruiting, and it spans 12 locations across Canada, Japan, and South Korea.
A randomized test of added maintenance pressure
Participants in the investigational arm receive mocertatug rezetecan in combination with physician’s choice of immunotherapy, either dostarlimab or pembrolizumab. The control arm receives the same physician’s choice of checkpoint blockade alone, representing current standard maintenance care. This design is clinically important because it isolates the incremental effect of the added drug on top of a regimen already accepted in practice.
The maintenance window begins soon after induction therapy ends. Eligible participants must be able to start study treatment within 3 to 9 weeks of the final dose of first-line therapy. They also must have no clinical or radiographic progression after induction, which means the trial is focusing on patients who have already demonstrated at least some disease control and are being tested at the point where prolonging remission matters most.
The biology of the enrollment criteria suggests a trial aimed at a high-risk but still treatment-sensitive population. Participants must have received six cycles of first-line therapy consisting of platinum-based doublet chemotherapy, such as carboplatin and paclitaxel, together with immune checkpoint inhibition as part of standard care or via trial run-in with dostarlimab. In a narrower circumstance, four cycles of chemotherapy are allowed if treatment stopped because of toxicity, provided the participant still received at least 12 weeks of checkpoint inhibitor therapy during induction. This preserves the trial’s focus on patients who have already had substantial exposure to standard frontline therapy.
One sentence captures the strategic logic of the study: residual disease after induction is exactly where maintenance therapy either earns its place or fails.
Why the trial focuses on mismatch repair proficient disease
The protocol specifically targets mismatch repair proficient endometrial cancer. That subgroup matters because it has not historically matched the dramatic immunotherapy responsiveness seen in mismatch repair deficient disease. A maintenance strategy in this setting has to overcome an immune environment that is often less naturally inflamed, less neoantigen-rich, and more resistant to checkpoint blockade alone.
By pairing mocertatug rezetecan with a PD-1 inhibitor, investigators are testing whether a second mechanism can intensify control beyond what immunotherapy provides by itself. The source record does not describe the drug’s mechanism, so the practical clinical interpretation stays grounded in trial design: the combination is being positioned as a way to deepen durable suppression of residual endometrial carcinoma during the maintenance phase.
The study also makes room for a broad pathologic spectrum. Histologically confirmed endometrial carcinoma is required, including endometrioid, serous, clear cell, and endometrial carcinosarcoma histologies, and mixed epithelial carcinomas are permitted. That breadth should help the study reflect the heterogeneity encountered in daily gynecologic oncology practice, where high-risk histologies often complicate prognosis and response assessment.
Endpoints extend beyond progression control
The primary purpose is to see how well the combination works as maintenance therapy compared with standard immunotherapy alone. Safety and tolerability are explicit goals, as is health-related quality of life. Those secondary dimensions are not cosmetic. A maintenance regimen only matters if patients can actually stay on it, preserve function, and avoid trading a delayed progression for chronic toxicity that erodes the gains.
This trial also carries a practical bedside implication for oncologists treating patients after induction. Maintenance is no longer being treated as a passive holding pattern. Instead, it is being tested as an active therapeutic phase, with randomized evidence planned to determine whether continued checkpoint blockade is enough or whether a combination approach offers a real advantage.
Another sentence stands on its own: a maintenance strategy that cannot be tolerated long enough to matter will fail before it ever reaches the endpoint.
Global enrollment and broad investigator network
The study has sites in Toronto and Montreal, several cities across Japan including Fukuoka, Fukushima, Hokkaido, Ibaraki, Miyagi, Numakunai, Osaka, Saitama, and Tokyo, and Daegu in South Korea. Principal investigators listed on the record include Stephanie Lheureux, Lucy Gilbert, Hideaki Yahata, Shu Soeda, Hidemichi Watari, Toyomi Satoh, Shogo Shigeta, Tsukasa Baba, Satoe Fujiwara, Kosei Hasegawa, Mayu Yunokawa, and So-Jin Shin.
The network is broad enough to support a large phase 3 effort in a disease that often requires specialized gynecologic oncology infrastructure. It also suggests an intention to enroll a diverse population across multiple health systems, which may help the results translate more cleanly into routine care if the combination succeeds.
The trial record does not yet provide outcomes, and no efficacy or safety data are posted. The most immediate clinical question remains whether adding mocertatug rezetecan to checkpoint inhibitor maintenance can extend control without compromising tolerability in a population already treated intensively with platinum-based induction therapy. The answer will depend on whether the combination can improve on standard maintenance in a disease setting where recurrence after frontline therapy remains a central threat.