On October 1, 2026 Regeneron Pharmaceuticals, Inc. (NASDAQ: REGN) reported the first results for the investigational ubamatamab in patients with low-grade serous ovarian cancer (LGSOC). The data are from an ongoing Phase 1/2 trial in recurrent platinum-resistant high- or low-grade serous ovarian cancer that were presented in a late-breaking plenary oral session at the International Gynecologic Cancer Society (IGCS) 2026 Annual Global Meeting. Ubamatamab is a MUC16xCD3 bispecific antibody designed to bridge MUC16 on cancer cells with CD3-expressing T cells to facilitate local T-cell activation. Based on these preliminary trial results, Regeneron has been enrolling a single-arm, potentially registrational LGSOC trial and will be discussing the data with the U.S. Food and Drug Administration (FDA) in the coming months. Regeneron is also planning to initiate a confirmatory, randomized, controlled Phase 3 trial to support the use of ubamatamab in LGSOC.

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LGSOC is a biologically distinct variant from the more common high-grade serous ovarian cancer (HGSOC), comprising up to 10% of serous ovarian cancer, with typically uniformly high MUC16 expression and limited treatment options. The disease often affects younger women and is characterized by slower-growing tumors than those seen in HGSOC, but frequently recurs and is less responsive to conventional chemotherapy with few treatment options. LGSOC has a median overall survival of approximately seven years from initial diagnosis, compared to approximately four years for HGSOC. In recurrent advanced LGSOC, historical trial data have shown that objective response rates (ORRs) generally remain below 15% with chemotherapy and endocrine therapy, while the only approved therapy has an ORR of 44% in biomarker-selected patient populations (representing about 30% of the overall LGSOC population).

"The high rate of durable responses observed thus far with ubamatamab in low-grade serous ovarian cancer, without requiring biomarker selection, are highly encouraging and reinforce our excitement about MUC16 as a new therapeutic target in this difficult-to-treat disease," said Israel Lowy, M.D., Ph.D., Clinical Development Unit Head, Oncology, at Regeneron. "This initial clinical evidence highlights the potential of our bispecific approach to bring a differentiated and first-in-class treatment option to patients in need. We are pleased with the safety profile of the drug as well, and are eager to build on these findings through our ongoing trial and planned confirmatory Phase 3 trial."

Data presented at IGCS focused on the LGSOC subset of 19 patients who had received a median of four prior lines of therapy and were dosed with intravenous ubamatamab ranging from 3 to 800 mg weekly or every three weeks. Ubamatamab was given either as monotherapy or in combination with Libtayo (cemiplimab).

Included in the presentation were 11 patients treated with 800 mg of ubamatamab monotherapy, eight treated weekly and three treated every three weeks. Results at the 800 mg doses showed a:

73% ORR (95% Confidence Interval [CI]: 39.0 to 94.0), with all responses being partial responses
9-month median duration of response (95% CI: 6.9 to not estimable [NE]), per Kaplan-Meier estimate
11-month median progression-free survival (95% CI: 3.0 to NE), per Kaplan-Meier estimate
Safety was assessed across all 19 patients in the LGSOC subset. Treatment-emergent adverse events (TEAEs) occurred in 100% of participants (n=19). Cytokine release syndrome occurred in 90% of participants (n=17), all of which were Grade 1 or 2. Serious TEAEs grade 3 or higher occurred in 21% (n=4) of participants and treatment-related adverse events (TRAEs) occurred in 74% of participants (n=14). Grade 3 or higher TRAEs included neutropenia (21%; n=4), anemia (11%; n=2), increased alanine aminotransferase (11%; n=2), increased aspartate aminotransferase (11%; n=2), abdominal pain (5%; n=1) and ileus (5%; n=1). Most adverse events occurred during the step-up dosing period. One participant discontinued treatment due to a stroke considered unrelated to ubamatamab treatment; no TEAEs resulted in death.

The potential uses of ubamatamab described above are investigational, and its safety and efficacy have not been evaluated by any regulatory authority.

About the Trial
The ongoing, open-label, Phase 1/2 trial is evaluating ubamatamab alone and in combination with Libtayo in adult patients with recurrent platinum-resistant ovarian cancer and other recurrent MUC16-expressing cancers.

The Phase 1 portion of the trial is primarily assessing safety, tolerability, dose-limiting toxicities and pharmacokinetics of ubamatamab, with efficacy evaluated as a secondary endpoint through measures such as ORR, duration of response and progression-free survival. The Phase 2 portion is evaluating the anti-tumor activity of ubamatamab, primarily through ORR, while continuing to assess safety and tolerability.

A prospective, dedicated, potentially registrational cohort in the Phase 2 portion of this trial is evaluating ubamatamab 800 mg monotherapy every three weeks in up to 100 patients with recurrent LGSOC and is currently enrolling. If you are interested in learning more about this trial, please contact us ([email protected], +1 844 734 6643), or visit our clinical trial website.

About Ovarian Cancer
Ovarian cancer is one of the most common gynecologic cancers and remains a leading cause of cancer-related mortality among women. It arises from the uncontrolled growth of abnormal cells in the ovary and is frequently diagnosed at an advanced stage, when disease has spread beyond the ovary. In 2026, an estimated 21,010 new cases of ovarian cancer were expected to be diagnosed in the U.S., and approximately 12,450 women were expected to die from the disease. Despite advances in treatment, ovarian cancer remains a significant unmet medical need, with many patients ultimately experiencing disease recurrence and progression.

LGSOC is a distinct form of ovarian cancer that occurs in younger women and comprises up to 10% of new epithelial ovarian cancer cases. While LGSOC tumor cells tend to grow and divide more slowly than those seen in high-grade serous ovarian cancer, the disease frequently recurs and is less responsive to conventional chemotherapy. LGSOC has a median overall survival of approximately seven years from initial diagnosis, compared to approximately four years for HGSOC. Although endocrine therapy and targeted therapies are used in selected patients, treatment options remain limited for many people with recurrent disease.

(Press release, Regeneron, OCT 1, 2026, View Source [SID1234671235])