Regeneron's ubamatamab hits 73% response rate in LGSOC trial

Regeneron Pharmaceuticals has reported early clinical data showing a 73% objective response rate for ubamatamab, its MUC16xCD3 bispecific antibody, in patients with recurrent low-grade serous ovarian cancer. The results, drawn from 11 patients treated at the highest dose tested in an ongoing Phase 1/2 trial, were presented in a late-breaking plenary session at the International Gynecologic Cancer Society 2026 Annual Global Meeting.

All 11 patients had received a median of four prior lines of therapy. At the 800 mg monotherapy dose, the data showed a 73% objective response rate (95% CI: 39.0 to 94.0), a nine-month median duration of response, and an 11-month median progression-free survival. All responses were partial responses. Regeneron noted that the study population was small and confidence intervals remain wide, so the results should be interpreted accordingly.

Clinical significance

LGSOC is biologically distinct from the more common high-grade serous ovarian cancer. It accounts for up to 10% of serous ovarian cancer cases, tends to occur in younger women, and is characterised by slower tumour growth but frequent recurrence and poor chemosensitivity. Historical data show objective response rates below 15% with conventional chemotherapy and endocrine therapy. The only currently approved therapy achieves a 44% response rate, but only in a biomarker-selected population representing roughly 30% of all LGSOC patients.

Ubamatamab's reported 73% response rate in an unselected population, if confirmed in larger studies, would represent a meaningful step forward in a disease with very limited options. Israel Lowy, clinical development unit head for oncology at Regeneron, said the results "highlight the potential of our bispecific approach to bring a differentiated and first-in-class treatment option to patients in need."

Safety data across all 19 patients in the LGSOC subset showed cytokine release syndrome in 90% of participants, though all cases were Grade 1 or 2. Serious treatment-emergent adverse events of Grade 3 or higher occurred in 21% of patients. Grade 3 or higher treatment-related adverse events included neutropenia, anaemia, and liver enzyme elevations. No deaths were attributed to the drug; one patient discontinued due to a stroke considered unrelated to ubamatamab.

Regulatory path and competitive landscape

Regeneron confirmed it is enrolling a dedicated, potentially registrational LGSOC cohort of up to 100 patients at the 800 mg monotherapy dose, and plans to discuss the data with the FDA in the coming months. The company also intends to initiate a confirmatory, randomised, controlled Phase 3 trial, signalling confidence in the signal despite the early stage of the evidence.

The LGSOC treatment landscape is sparse compared with HGSOC, where MEK inhibitors, PARP inhibitors, and bevacizumab-based regimens have established footholds. The lack of approved targeted therapy for unselected LGSOC patients has left clinicians largely reliant on endocrine agents, and Regeneron is not the only company directing bispecific antibody programmes toward gynaecological cancers. However, the MUC16 antigen, which is highly expressed in LGSOC, has attracted particular interest as a target given its near-uniform prevalence across the tumour type. Regulatory precedent in adjacent bispecific programmes suggests the FDA may be receptive to a registrational single-arm trial if the response rate and durability hold up in the larger cohort.