Astellas Pharma has decided to end its collaboration with CytomX Therapeutics, the California-based biotech company, according to a March 17 announcement. The decision comes even as CytomX continues to report encouraging early results for its antibody platform targeting solid tumors.
The termination of the partnership is set to take effect in the second quarter of this year. CytomX disclosed in its 2025 earnings report that Astellas "chose not to advance the remaining preclinical research programs" under their agreement. Despite this development, CytomX said it still maintains partnerships with other major pharmaceutical companies including Regeneron, Amgen, Bristol Myers Squibb, and Moderna.
The collaboration between Astellas and CytomX began in March 2020. At that time, Astellas paid $80 million upfront to use CytomX’s Probody platform for several bispecific antibody programs aimed at undisclosed cancer types. Under the terms of the deal, CytomX was responsible for early research and discovery activities while Astellas was expected to handle preclinical and clinical development as well as regulatory and commercialization steps. The agreement could have brought more than $1.6 billion in milestone payments and tiered royalties on global sales if products reached the market.
CytomX’s Probody technology works by masking therapeutic antibodies so they remain inactive until reaching a tumor site. This approach is designed to exploit the tumor microenvironment—an area around cancer cells that can often hinder drugs or immune responses—by ensuring that active treatment targets only tumor tissue while minimizing harm to healthy cells.
Despite losing Astellas as a partner, CytomX reported positive results from its lead asset varsetatug masetecan, an antibody-drug conjugate. In a Phase 1 study involving patients with late-line metastatic colorectal cancer at a 10-mg/kg dose level, the drug achieved a confirmed response rate of 32 percent and a median progression-free survival of 7.1 months with an 84 percent disease control rate. The company plans to prioritize further development at both this dose and a lower one (8.6 mg/kg). Additional studies are ongoing or planned combining varsetatug masetecan with Roche’s Avastin and chemotherapy.
