Daiichi Sankyo doses first patient with CD25-directed ADC DS1025

Daiichi Sankyo has dosed the first patient in a phase 1 trial of DS1025, an antibody-drug conjugate (ADC) directed at CD25, the company announced on August 27, 2026. The trial enrolls patients with advanced or metastatic solid tumors.

DS1025 takes aim at the immune system rather than directly at cancer cells. CD25 is a protein expressed at high levels on regulatory T cells (Tregs). Tregs normally keep immune responses from running out of control, but when they gather around a tumor they also hold back the immune cells that would attack it. DS1025 links an anti-CD25 antibody to a cytotoxic payload optimized for cancer immunotherapy, using Daiichi Sankyo's DXd technology. The idea is to reduce Tregs inside the tumor and let the suppressed antitumor response start up again. According to the company, no CD25-directed ADC has been approved for any type of cancer.

The trial runs in Asia, including Japan, and in Europe, and will enroll up to 45 patients whose disease progressed after at least one standard therapy. It will assess safety, tolerability and signs of efficacy. Forty-five is a typical size for a phase 1 study. In an ADC phase 1, though, the core of the work is watching what happens as the dose rises: how far the dose can go is settled before anyone asks whether the drug works.

The target raises a design question. If an ADC's payload comes off in the blood, it travels through the whole body, and CD25 is also found on T cells outside tumors. How selectively DS1025 can remove Tregs is therefore the safety question. Uniform conjugation (a controlled drug-to-antibody ratio) and site-specific attachment matter most for molecules like this one.

DXd is a platform Daiichi Sankyo has used across several products, changing the target each time. Payload and linker supply often limits how fast ADC programs can move, so owning the technology in-house counts for something. The release does not give the detailed start date, the dose levels or the trial sites in Japan.