Symbiotic-GI-03: A randomized, double-blind, phase 3 study of first-line PF-08634404, a PD-1/VEGF bispecific antibody, in combination with mFOLFOX6 in patients with metastatic colorectal cancer (mCRC).

TPS3675 Background: Multi-agent chemotherapy regimens (eg, mFOLFOX6) ± anti-vascular endothelial growth factor (VEGF) or anti-epidermal growth factor regimens are currently the standard of care for patients (pts) with mCRC without targetable mutations. However, the 5-year survival rate remains low at approximately 16% (SEER, 2025). The addition of biological therapies (eg, bevacizumab [bev], an anti-VEGF monoclonal antibody) to immune checkpoint inhibitors (ICIs) has demonstrated a synergistic antitumor effect in solid tumors. However, ICI monotherapy has limited activity in mismatch repair proficient (pMMR)/microsatellite stable (MSS) mCRC. Simultaneous bispecific targeting of programmed death 1 (PD-1) and VEGF in conjunction with cytotoxic chemotherapy has the potential to enhance antitumor activity. PF-08634404 is a fully human immunoglobulin G4 (IgG4) bispecific antibody targeting PD-1 and VEGF. A phase 2 study of PF-08634404 with XELOX or mFOLFOX6 demonstrated promising antitumor activity with a manageable safety profile in first-line (1L) mCRC (Xu X, ESMO 2025; Abstract 796P). Methods: Symbiotic-GI-03 (NCT07222800) is an ongoing, double-blind, randomized, phase 3 study evaluating the efficacy and safety of 1L PF-08634404 + mFOLFOX6 vs bev + mFOLFOX6. Inclusion criteria are age ≥18 y; mCRC with measurable disease per RECIST 1.1; no prior systemic therapy for metastatic disease; ECOG PS 0 or 1; and adequate hematologic, hepatic, and renal function. Pts with BRAF V600E-mutant or mismatch repair deficient (dMMR)/microsatellite instability-high (MSI-H) CRC, active central nervous system metastases, or clinically significant risk of hemorrhage/fistula will be excluded. Approximately 800 pts will be randomized 1:1 to receive PF-08634404 + mFOLFOX6 or bev + mFOLFOX6 until disease progression, unacceptable toxicity, withdrawal of consent, or death. Randomization will be stratified by region (North America, Europe, or rest of world), RAS status (mutant or wildtype), and presence of liver metastasis (yes or no). The dual primary endpoints are progression-free survival (PFS) by blinded independent central review and overall survival; for each, a stratified log-rank test will be used to compare arms. Hazard ratios with corresponding 2-sided 95% CIs will be estimated using a stratified Cox proportional hazards model, and medians will be estimated using the Kaplan-Meier method with corresponding 2-sided 95% CIs calculated using the log(-log) method. Secondary endpoints include PFS by investigator, objective response rate, duration of response, PFS after next-line therapy, safety/tolerability, pharmacokinetics, immunogenicity, and patient-reported outcomes. Enrollment began in December 2025. Clinical trial information: NCT07222800 .