A phase 2 study evaluating SSGJ-707 (PF-08634404), a PD-1/VEGF bispecific antibody, + chemotherapy (chemo) in patients (pts) with first-line (1L) advanced/recurrent endometrial cancer (EC).

5627 Background: SSGJ-707 is a fully human immunoglobulin G4 bispecific antibody targeting programmed death 1 (PD-1) and vascular endothelial growth factor (VEGF). SSGJ-707 has demonstrated promising efficacy and manageable safety alone and in combination with chemo in phase 2 studies in solid tumors. We report results from the phase 2 SSGJ-707-ST-II-02 study (NCT06522828) of 1L SSGJ-707 + chemo in advanced/recurrent EC. Methods: Pts with newly diagnosed stage lIl/IV or recurrent EC with low potential for cure by radiation therapy (tx) or surgery and systemic tx-naive were enrolled. Pts received SSGJ-707 5 mg/kg or 10 mg/kg Q3W + chemo (carboplatin AUC 5 + paclitaxel 175 mg/m 2 ) Q3W for 6 cycles, followed by SSGJ-707 maintenance tx until loss of clinical benefit or intolerable toxicity for up to 2 years. Primary endpoints were safety and ORR (RECIST 1.1). Secondary endpoints include duration of response (DOR), PFS, OS, PK, and biomarkers. Results: At data cutoff (Nov 28, 2025), 32 pts with EC (26 mismatch repair proficient [pMMR], 6 MMR deficient [dMMR]) received 5 mg/kg (n=16) or 10 mg/kg (n=16) SSGJ-707 + chemo. Median SSGJ-707 tx duration was 5.6 mo (range, 0.2-13.6); 8 pts (25.0%) discontinued tx. In the evaluable population, confirmed ORR was 85.7% (12/14 pts) for the 5 mg/kg dose and 80.0% (12/15) for the 10 mg/kg dose. Results for pMMR and dMMR groups are in the Table. Median PFS, OS, and DOR were not reached for either dose. Any-grade tx-related adverse events (TRAEs) were reported in 30/32 pts (93.8%) and grade ≥3 TRAEs in 22/32 pts (68.8%). The most common TRAEs (≥40%) included neutrophil count decreased (59.4%), white blood cell count decreased (59.4%), anemia (56.3%), and platelet count decreased (56.3%). No TRAEs led to death. TRAEs led to discontinuation of SSGJ-707 in 2 pts (6.3%). Immune-related AEs occurred in 6 pts (18.8%): hypothyroidism (n=4), hyperthyroidism (n=3), and rash (n=2). VEGF-related AEs occurred in 13 pts (40.6%): blood pressure elevation (n=6), proteinuria (n=6), hemorrhage (urinary occult blood positive; n=2), perforation and fistula (n=2), and venous thrombosis (n=1). Conclusions: SSGJ-707 + chemo demonstrated promising efficacy with a manageable safety profile in pts with tx-naive advanced/recurrent EC, supporting further investigation of SSGJ-707 in these pts. Clinical trial information: NCT06522828 . 5 mg/kg Q3W (n=14) 10 mg/kg Q3W (n=15) pMMR (n=12) dMMR (n=2) pMMR (n=11) dMMR (n=4) Confirmed ORR, n (%) [95% CI] 10 (83.3) [51.6, 97.9] 2 (100)[15.8, 100.0] 9 (81.8)[48.2, 97.7] 3 (75.0)[19.4, 99.4] Complete response 0 0 1 (9.1) 0 Partial response 10 (83.3) 2 (100) 8 (72.7) 3 (75.0) Stable disease 2 (16.7) 0 2 (18.2) 1 (25.0) Progressive disease 0 0 0 0 CR/PR pending – – 1 a 1 Unconfirmed ORR, n (%) [95% CI] 10 (83.3) [51.6, 97.9] 2 (100)[15.8, 100.0] 10 (90.9)[58.7, 99.8] 4 (100)[39.8, 100.0] a One additional pt in the 10 mg/kg group confirmed in Dec.