Astellas Pharma and CytomX Therapeutics first partnered up in March 2020. For $80 million upfront, the Japanese pharma gained access to the biotech’s Probody platform to generate masked antibody therapies for cancer.
Astellas Pharma has pulled the plug on its years-long cancer partner CytomX Therapeutics looking to advance masked bispecific antibody therapies for solid tumors—even as the California biotech continues to report positive early data for its platform.
Astellas “chose not to advance the remaining preclinical research programs” under the partnership, CytomX revealed in its 2025 earnings report on Monday. As a result, the collaboration will be terminated, effective in the second quarter of this year. The biotech retains other high-profile partners, including Regeneron, Amgen, Bristol Myers Squibb and Moderna, CytomX said on Monday.
CytomX and Astellas linked up in March 2020, with the Japanese pharma paying $80 million upfront to leverage the biotech’s Probody platform for multiple bispecific programs for undisclosed cancer indications. Under the agreement, CytomX led early research and discovery activities and Astellas was supposed to assume responsibility of preclinical and clinical development, as well as subsequent regulatory and commercialization activities.
CytomX would have received more than $1.6 billion in preclinical, clinical and commercial milestones, plus tiered royalties on net global sales of products that reach the market.
CytomX’s Probody technology, the centerpiece of the Astellas agreement, masks therapeutic antibodies, rendering them inert until they are in the vicinity of a tumor. This approach—which the biotech on its website claims is designed to “outsmart cancer”—exploits a tumor’s microenvironment, the ecosystem surrounding cancer cells that otherwise can help disable drugs and the immune system.
Probody limits the activity of antibodies by blocking their active binding sites with masks that are cleaved off after entering the tumor microenvironment. This helps minimize toxic effects on healthy tissues, according to the biotech, while also making sure that the drug’s therapeutic window is concentrated on its target tumor.
So far, CytomX’s masking approach has been paying off. On Monday, alongside the announcement of Astellas’ abandonment, CytomX revealed that its lead asset varsetatug masetecan, an antibody-drug conjugate, achieved a 32% confirmed response rate in a Phase 1 study of late-line metastatic colorectal cancer (CRC) at a 10-mg/kg dose level. Median progression-free survival was 7.1 months, with an 84% disease control rate.
CytomX is prioritizing this 10-mg/kg dose, as well as a weaker 8.6-mg/kg strength, for further development, according to the Monday release. The Phase 1 study is ongoing. The biotech has initiated another early-stage CRC study combining varsetatug masetecan with Roche’s Avastin, and a Phase 1b/2 trial testing a combo regimen with Avastin and chemotherapy is set to start by the end of the year.
