On May 19, 2025 CytomX Therapeutics, Inc. (Nasdaq: CTMX), a leader in the field of masked, conditionally activated biologics, reported that the first patient has been dosed with CX-801 in combination with KEYTRUDA (pembrolizumab), Merck’s anti-PD-1 therapy, in the ongoing Phase 1 dose escalation study (NCT06462794) evaluating safety and initial clinical activity in patients with metastatic melanoma (Press release, CytomX Therapeutics, MAY 19, 2025, View Source [SID1234653225]).

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CX-801 is a dually masked interferon (IFN) alpha-2b PROBODY cytokine. CX-801 has been intentionally designed by CytomX to harness the power of Interferon alpha-2b’s immune stimulating properties in combination with checkpoint inhibition. Interferon alpha-2b has well known single agent anti-cancer activity in multiple tumor types, including in melanoma. However, its use has been limited due to its poor tolerability arising from systemic toxicities. CX-801 is designed to localize the potency of IFN alpha-2b to tumors and reduce systemic toxicities, enabling combination strategies.

CytomX is conducting a focused Phase 1 dose escalation study of CX-801 in metastatic melanoma. The combination arm of the study evaluating CX-801 in combination with KEYTRUDA has been initiated following successful the clearance of the first three CX-801 monotherapy dose escalation cohorts.

"We are excited to begin evaluating this combination therapy that has the potential to provide significant clinical benefit to patients with PD-1 refractory melanoma, which remains an area of high unmet need," said Dr. Wayne Chu, M.D., chief medical officer of CytomX Therapeutics. "Utilizing CytomX’s proprietary conditional activation platform to maintain potency and widen interferon’s therapeutic index, CX-801 is well suited to combine with KEYTRUDA and could become an important component of combination immuno-oncology therapy. We look forward to initial Phase 1a translational and biomarker data in advanced melanoma in the second half of 2025."