Actinium-225 Radiopharmaceutical RYZ401 Enters Human Testing for the First Time, Targeting SSTR-Positive Tumors

Radiopharmaceuticals must deliver enough energy into tumors to kill cancer cells while preventing normal organs from receiving excessive radiation. The key lies not only in “targeting,” but also in how quickly the drug leaves the body. The Huntsman Cancer Institute at the University of Utah in the United States said that the first three participants worldwide have received the experimental drug RYZ401, formally bringing this alpha-particle therapy for somatostatin receptor-positive tumors into human trials.

This Phase 1, first-in-human trial, identified as NCT07165132, is sponsored by Bristol Myers Squibb subsidiary RayzeBio and is enrolling patients with advanced neuroendocrine tumors, as well as certain other solid tumors that express somatostatin receptors. Treatment of the first three participants marks an important milestone in the study’s launch; however, the research team has not yet released human data on adverse reactions, tumor responses, or radiation doses within the body.

RYZ401 consists of a peptide that recognizes somatostatin receptor subtype 2 (SSTR2) linked to the radioactive isotope actinium-225. Many neuroendocrine tumor cells have a higher density of these receptors on their surfaces. The peptide acts as a navigation component, carrying the drug to the tumor; the alpha particles released by actinium-225 travel only a short distance and have a high energy density, with the goal of damaging cancer cells within a limited area while reducing radiation exposure to surrounding tissue.

This also extends the concept of “theranostics”: physicians first use receptor imaging to determine whether a tumor can be labeled, then deploy a therapeutic radiopharmaceutical directed at the same type of receptor. Imaging therefore does more than locate tumors; it can also help identify patients who may be more likely to be suitable for treatment. However, seeing the receptor on a scan does not mean the treatment has been proven effective.

The trial is divided into dose-escalation and dose-expansion stages. According to the study design, the initial stage is expected to enroll 6 to 24 participants to identify the recommended Phase 2 dose and dosing regimen. A subsequent expansion cohort of about 80 participants is planned, including about 40 with well-differentiated gastroenteropancreatic neuroendocrine tumors and about 40 with other SSTR-positive tumors. The study will evaluate dose-limiting toxicities, overall safety, pharmacokinetics, and preliminary antitumor signals.

Another design focus of RYZ401 is improving renal clearance and tumor retention. Some DOTATATE-based radiopharmaceuticals are reabsorbed by the kidneys, so amino acids must be infused concurrently during treatment to reduce renal radiation exposure, which may also cause nausea or electrolyte abnormalities. The RYZ401 trial currently does not plan this concurrent infusion. Mouse data presented by the sponsor showed a lower absorbed dose in the kidneys and inhibition of tumor growth, but these results cannot yet be used to infer that humans will receive the same convenience and safety advantages.

The most important questions ahead are not how powerful alpha particles are in theory, but what dose the human body can tolerate, which organs become dose-limiting, and whether the treatment can deliver durable tumor control. Treatment of the first three participants proves only that clinical testing has begun; until more participants complete follow-up, RYZ401 remains an early-stage therapy that has yet to be validated.