First Patient Is Dosed in SANTANA-225 Trial of ES-SCLC and MCC Therapy

The first patient has been dosed in the phase 1/2 SANTANA-225 study (NCT06939036) with ²²⁵Ac-SSO110 for the treatment of extensive-stage small cell lung cancer (ES-SCLC) and Merkel cell carcinoma (MCC).¹

The SANTANA-225 clinical trial is a global, open-label study designed to assess the safety, tolerability, preliminary efficacy, and recommended phase 2 dose of 225Ac-SSO110 in patients with ES-SCLC who are treated with checkpoint inhibitors in first-line maintenance therapy, or in patients with MCC who are treated with checkpoint inhibitors in first-line therapy. The trial is expected to have a total enrollment of 20 patients in the dose escalation phase, followed by expansion cohorts.

²²⁵Ac-SSO110 is a potentially first- and best-in-class actinium 225–labeled antagonist of the somatostatin type 2 receptor (SSTR2). SSTR2 is highly overexpressed in neuroendocrine tumors (NETs), making it an ideal target for radioligand therapies (RLTs).

In February 2025, the FDA granted orphan drug designation to ²²⁵Ac-SSO110 for the treatment of SCLC.

“RLTs are redefining precision oncology by enabling targeted delivery of radiation directly to tumor cells while minimizing exposure to healthy tissue,” said Germo Gericke, MD, chief medical officer of Ariceum Therapeutics, study sponsor, in a news release. “²²⁵Ac-SSO110 is the first SSTR2 antagonist RLT in clinical development, designed to deliver higher doses of alpha radiation directly to patients’ tumors while maintaining a favorable safety profile for individuals with neuroendocrine cancers, including ES-SCLC and MCC. Dosing the first patient in the SANTANA-225 trial is a significant step for our lead program and an important milestone toward addressing urgent patient needs in these aggressive cancers. We expect to report initial safety data from the SANTANA-225 trial in 2026, which may support expansion into additional [NET] indications and further validate the differentiated mechanism of action of ²²⁵Ac-SSO110.”

The SANTANA-225 trial aims to address a high unmet need faced by patients with ES-SCLC and MCC. ²²⁵Ac-SSO110 is the first SSTR2-targeting antagonist radiolabeled with actinium 225 to undergo human trials in combination with checkpoint inhibitors for ES-SCLC and MCC NETs.

Patient inclusion criteria include, but are not limited to, having ES-SCLC, or recurrent locally advanced or metastatic MCC, being eligible to receive immune checkpoint inhibitor monotherapy at the start of treatment, and having an ECOG status of 0 or 1.²

Patient exclusion criteria include, but are not limited to, having unstable spinal cord compression, having any previous systemic radioligand therapy or extensive radiotherapy, receiving or planned to receive consolidative chest radiation, and having a history of primary immunodeficiency, transplantation, or chimeric antigen receptor T-cell therapy.

REFERENCES

1. Ariceum Therapeutics doses first patient in SANTANA-225 phase 1/2 clinical trial of 225Ac-SSO110 in patients with extensive-stage small cell lung cancer or Merkel cell carcinoma. News release. Ariceum Therapeutics. November 11, 2025. Accessed November 11, 2025. https://tinyurl.com/bde7haxk

2.Study of 225Ac-SSO110 in subjects with ES-SCLC or MCC (SANTANA-225) (SANTANA-225). ClinicalTrials.gov. Updated October 28, 2025. Accessed November 11, 2025. https://clinicaltrials.gov/study/NCT06939036