CytomX Therapeutics has announced promising initial translational data from its ongoing Phase 1 study of CX-801 in patients with advanced melanoma, with findings to be presented at the Society for Immunotherapy of Cancer (SITC) 40th Anniversary Annual Meeting in National Harbor, MD on November 7-9, 2025.
Mechanism of Action Validated Through Biomarker Analysis
The Phase 1 biomarker data demonstrate that CX-801, a masked interferon alpha-2b PROBODY cytokine, is working as designed by inducing tumor-localized activation of immune cell populations and interferon-stimulated genes in paired tumor biopsies, including PD-1 and PD-L1. According to Dr. Marcia Belvin, SVP and chief scientific officer of CytomX Therapeutics, "CX-801 was intentionally designed to unlock the powerful immune-modulating effects of interferon alpha-2b by leveraging our PROBODY therapeutic platform."
The data suggest the molecule is successfully activating tumor-selective interferon signaling in patients with advanced melanoma, supporting the underlying mechanism of action for this conditionally activated biologic.
Clinical Progress in Treatment-Resistant Population
The Phase 1 study has shown initial evidence that CX-801 is generally well tolerated and can modulate the immune tumor microenvironment in patients with metastatic melanoma refractory to prior immune checkpoint inhibitor therapy. This represents a significant development for a patient population with limited treatment options.
Dr. Wayne Chu, chief medical officer of CytomX Therapeutics, noted, "We are pleased with the Phase 1 progress for CX-801 to-date, including initial evidence that CX-801 is generally well tolerated and can modulate the immune tumor microenvironment in patients with metastatic melanoma refractory to prior immune checkpoint inhibitor therapy."
Combination Strategy and Future Development
The initial data support the rationale for the ongoing Phase 1 combination study of CX-801 combined with KEYTRUDA (pembrolizumab) in melanoma, which the company describes as "an area of significant unmet need." CytomX expects to provide Phase 1 clinical data of CX-801 combined with KEYTRUDA in 2026.
PROBODY Platform Technology
CX-801 represents CytomX's PROBODY therapeutic platform approach to creating masked, conditionally activated biologics designed to be localized to the tumor microenvironment. The company's multi-modality technology platform has produced therapeutic candidates across multiple treatment modalities including antibody-drug conjugates (ADCs), T-cell engagers, and immune modulators such as cytokines.
CX-801 is described as having broad potential applicability in traditionally immuno-oncology sensitive as well as insensitive (cold) tumors, expanding its potential therapeutic reach beyond melanoma.
Conference Presentation Details
The CX-801 data will be presented in a poster session titled "Pharmacodynamic Activity of CX-801, a Masked IFNα2b PROBODY Cytokine, in Patients with Advanced Melanoma" (Abstract Number: 606) on Saturday, November 8, 2025, from 5:10 pm to 6:35 pm ET.
CytomX will also present preclinical data on CX-908, a PROBODY T Cell Engager targeting CDH3 and CD3, demonstrating the breadth of the company's pipeline development efforts.
