Fusion Pharmaceuticals has dosed the first subject in the Phase II segment of the Phase II/III AlphaBreak clinical trial of FPI-2265 (²²⁵Ac-PSMA I&T) to treat metastatic castration-resistant prostate cancer (mCRPC).

The open-label, randomised, multicentre AlphaBreak trial aims to determine the safety and efficacy of FPI-2265 in patients who have previously undergone ¹¹⁷Lu-PSMA radiotherapy.

The Phase II dose optimisation part will compare additional safety or efficacy benefits of two alternative dosing regimens against the previously researched dosage of 100kBq/kg administered every eight weeks.

This segment of the trial is anticipated to conclude the enrolment of around 60 patients by the end of this year.

Upon completion and analysis of the Phase II data, Fusion Pharmaceuticals plans to hold an end-of-Phase II meeting with the US Food and Drug Administration to finalise the recommended dosing regimen for Phase III.

The subsequent global registration segment of the AlphaBreak trial is set to begin in 2025, with an estimated 550 patients participating.

FPI-2265 is an actinium-225-based prostate-specific membrane antigen (PSMA) targeting radioconjugate (RC).

Actinium-225, an alpha particle emitter, offers the advantage of delivering a higher radiation dose over a shorter distance. This could result in effective cancer cell destruction with targeted delivery, potentially reducing damage to surrounding healthy tissue.

Fusion Pharmaceuticals chief medical officer Dmitri Bobilev said: “Actinium-based PSMA targeting RCs have demonstrated compelling safety and clinical activity, which is supported by Fusion's encouraging data from the Phase II TATCIST trial reported recently at the AACR Annual Meeting.

“We believe FPI-2265 represents an important potential new treatment option for patients with mCRPC, and with the initiation of the AlphaBreak trial, we are pleased to move forward.”