CytomX Therapeutics to Present CX-801 Phase 1 Monotherapy Biomarker Data at the Society for Immunotherapy of Cancer (SITC) Annual Meeting

Rhea-AI Summary

CytomX Therapeutics (Nasdaq: CTMX) will present initial Phase 1 translational biomarker data for CX-801 (PROBODY interferon alpha-2b) at SITC 2025 (Nov 7–9).

Data demonstrate tumor-selective activation of interferon signaling and induction of immune cell populations and interferon-stimulated genes (including PD-1 and PD-L1) in paired tumor biopsies, and are reported as generally well tolerated in patients with metastatic melanoma refractory to prior checkpoint inhibitors. CytomX says these findings support the ongoing Phase 1 combination study of CX-801 with KEYTRUDA, with combination clinical data expected in 2026.

Poster: “Pharmacodynamic Activity of CX-801…” (Abstract 606) on Nov 8, 2025, 5:10–6:35 PM ET. Additional preclinical poster for CX-908 (Abstract 961) on Nov 7, 2025.

Positive

• Phase 1 biomarker data show tumor-selective interferon signaling activation
• Paired biopsies show induction of immune cell populations and interferon-stimulated genes
• Generally well tolerated in patients with metastatic melanoma refractory to checkpoint inhibitors
• Data support ongoing CX-801 + KEYTRUDA Phase 1 combination study

Negative

• Initial translational data from Phase 1—efficacy outcomes not reported
• Combination clinical results are pending in 2026, no readout yet
• Findings described from paired biopsies; sample size and clinical impact unclear

Insights

Translational Oncology Analyst

Early Phase 1 biomarker signals show tumor‑localized interferon activation and tolerability; promising but preliminary for clinical impact.

CytomX reports that CX-801 induced tumor‑localized interferon signaling and immune cell activation in paired biopsies from patients with advanced melanoma, and that initial safety appears acceptable. These findings align with the stated mechanism of a masked PROBODY® interferon and provide biological proof‑of‑mechanism at the tissue level rather than clinical efficacy data.

Key dependencies and risks include small Phase 1 sample sizes, lack of disclosed quantitative biomarker metrics, and absence of clinical response or durability data in the release. The translational signal supports the rationale for the ongoing combination study with KEYTRUDA®, but the clinical benefit of that combo remains unproven until combination safety and efficacy readouts appear.

Watch for the full poster on November 8, 2025 and the announced combination clinical data expected in 2026; those items will determine whether tissue biomarker activity translates into measurable patient benefit over a meaningful time horizon.

- CX-801 (PROBODY^® interferon alpha-2b) Phase 1 data demonstrate activation of tumor-selective interferon signaling in patients with advanced melanoma -

- Data supportive of CX-801’s mechanism of action and the ongoing Phase 1 combination study with KEYTRUDA^® (pembrolizumab) -

SOUTH SAN FRANCISCO, Calif., Nov. 04, 2025 (GLOBE NEWSWIRE) -- CytomX Therapeutics, Inc. (Nasdaq: CTMX), a leader in the field of masked, conditionally activated biologics, today announced that initial translational data from the ongoing Phase 1 study of CX-801 in patients with advanced melanoma will be presented at the Society for Immunotherapy of Cancer (SITC) 40^th Anniversary Annual Meeting, being held in National Harbor, MD on November 7-9, 2025.

“CX-801 was intentionally designed to unlock the powerful immune-modulating effects of interferon alpha-2b by leveraging our PROBODY^® therapeutic platform. We are excited to share initial Phase 1 biomarker data for CX-801 which suggest the molecule 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,” said Marcia Belvin, Ph.D. SVP, chief scientific officer of CytomX Therapeutics.

“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. These initial data support the rationale for the ongoing Phase 1 combination study of CX-801 combined with KEYTRUDA^® in melanoma, an area of significant unmet need. We look forward to providing Phase 1 clinical data of CX-801 combined with KEYTRUDA^® in 2026,” said Dr. Wayne Chu, M.D., chief medical officer of CytomX Therapeutics.

KEYTRUDA^® is a registered trademark of Merck Sharp & Dohme LLC, a subsidiary of Merck & Co., Inc., Rahway, NJ, USA

Details for CytomX Therapeutics Presentations at SITC 2025

CX-801 poster presentation:
Presentation Title: Pharmacodynamic Activity of CX-801, a Masked IFNα2b PROBODY^® Cytokine, in Patients with Advanced Melanoma
Abstract Number: 606
Session Date and Time: Saturday, November 8, 2025, 5:10 pm – 6:35 pm ET

Preclinical Masked T-cell Engager Targeting CDH3:
Presentation Title: CX-908, a PROBODY^® T Cell Engager Targeting CDH3 and CD3, Induces Tumor Regressions and Improves the Therapeutic Window in Preclinical Studies
Abstract Number: 961
Session Date and Time: Friday, November 7, 2025, 5:35 pm – 7:00 pm ET

About CytomX Therapeutics

CytomX is a clinical-stage, oncology-focused biopharmaceutical company focused on developing novel conditionally activated, masked biologics designed to be localized to the tumor microenvironment. By pioneering a novel pipeline of localized biologics, powered by its PROBODY^® therapeutic platform, CytomX’s vision is to create safer, more effective therapies for the treatment of cancer. CytomX’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. CytomX’s current clinical-stage pipeline includes CX-2051 and CX-801. CX-2051 is a masked, conditionally activated ADC directed toward epithelial cell adhesion molecule (EpCAM), armed with a topoisomerase-1 inhibitor payload. CX-2051 has potential applicability across multiple EpCAM-expressing epithelial cancers, including CRC, and was discovered in collaboration with ImmunoGen. CX-801 is a masked interferon alpha-2b PROBODY^® cytokine with broad potential applicability in traditionally immuno-oncology sensitive as well as insensitive (cold) tumors. CytomX has established strategic collaborations with multiple leaders in oncology, including Amgen, Astellas, Bristol Myers Squibb, Regeneron and Moderna. For more information about CytomX and how it is working to make conditionally activated treatments the new standard-of-care in the fight against cancer, visit www.cytomx.com and follow us on LinkedIn and X (formerly Twitter).

CytomX Therapeutics Forward-Looking Statements

This press release includes forward-looking statements. Such forward-looking statements involve known and unknown risks, uncertainties and other important factors that are difficult to predict, may be beyond our control, and may cause the actual results, performance, or achievements to be materially different from any future results, performance or achievements expressed or implied in such statements, including those related to CX-801. Accordingly, you should not rely on any of these forward-looking statements, including those relating to the potential benefits, safety and efficacy or progress of CytomX’s or any of its collaborative partners’ product candidates, including CX-801, the potential benefits or applications of CytomX’s PROBODY^® therapeutic platform, CytomX’s or its collaborative partners’ ability to develop and advance product candidates into and successfully complete clinical trials, including the ongoing and planned clinical trials of CX-801 and the timing of initial and ongoing data availability for our clinical trials, including CX-801, and other development milestones. Risks and uncertainties that contribute to the uncertain nature of the forward-looking statements include: the unproven nature of CytomX’s novel PROBODY^® therapeutic technology; uncertainties around the Company’s ability to raise sufficient funds to carry out its planned research and development; CytomX’s clinical trial product candidates are in the initial stages of clinical development and its other product candidates are currently in preclinical development, and the process by which preclinical and clinical development could potentially lead to an approved product is long and subject to significant risks and uncertainties, including the possibility that the results of preclinical research and early clinical trials, may not be predictive of future results; the possibility that CytomX’s clinical trials will not be successful; the possibility that current preclinical research may not result in additional product candidates; CytomX’s dependence on the success of CX-2051 and CX-801; CytomX’s reliance on third parties for the manufacture of the Company’s product candidates; possible regulatory developments in the United States and foreign countries, including China and the European Union; and the risk that we may incur higher costs than expected for research and development or unexpected costs and expenses. Additional applicable risks and uncertainties include those relating to our preclinical research and development, clinical development, and other risks identified under the heading “Risk Factors” included in CytomX’s Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. The forward-looking statements contained in this press release are based on information currently available to CytomX and speak only as of the date on which they are made. CytomX does not undertake and specifically disclaims any obligation to update any forward-looking statements, whether as a result of any new information, future events, changed circumstances or otherwise.

PROBODY is a U.S. registered trademark of CytomX Therapeutics, Inc. All other trademarks are the properties of their respective owners.

Company Contact:
Chris Ogden
SVP, Chief Financial Officer
cogden@cytomx.com

Investor Contact:
Precision AQ (formerly Stern Investor Relations)
Stephanie Ascher
stephanie.ascher@precisionaq.com

Media Contact:
Redhouse Communications
Teri Dahlman
teri@redhousecomms.com

FAQ

What did CytomX (CTMX) announce about CX-801 at SITC 2025?

CytomX will present Phase 1 biomarker data showing tumor-selective interferon signaling activation and immune modulation in advanced melanoma.

When and where will CTMX present CX-801 data at SITC 2025?

CX-801 poster (Abstract 606) is scheduled for Nov 8, 2025, 5:10–6:35 PM ET at the SITC annual meeting in National Harbor, MD.

What do the CX-801 Phase 1 biomarker results show for CTMX?

Initial results indicate tumor-localized activation of immune cells and interferon-stimulated genes, including PD-1 and PD-L1, in paired biopsies.

Does CytomX report CX-801 is safe in the Phase 1 study?

CytomX reports CX-801 is generally well tolerated to date in the studied metastatic melanoma patients.

Will CytomX present combination data of CX-801 with KEYTRUDA and when?

CytomX said combination clinical data for CX-801 + KEYTRUDA are expected to be provided in 2026.

What other CytomX program is being presented at SITC 2025?

A preclinical poster on CX-908, a masked T-cell engager targeting CDH3 and CD3 (Abstract 961), is scheduled for Nov 7, 2025.