AlphaBet: A phase I/II trial evaluating the combination of radium-223 and [¹⁷⁷Lu]Lu-PSMA-I&T in patients (pts) with metastatic castration-resistant prostate cancer (mCRPC).

TPS280 Background: [ 177 Lu]Lu-PSMA-I&T (LuPSMA) is a small molecule radioligand that delivers radiation via beta-particulate emission to cells expressing prostate-specific membrane antigen (PSMA). Despite a survival benefit in mCRPC, responses for many are not durable, with the majority of pts developing progressive osseous disease. Due to the lack of crossfire radiation in micrometastatic disease, small cancer cell clusters in the bone marrow may not receive an adequate dose of radiation from 177 Lu to induce cytotoxic double-stranded DNA breaks. Alpha-emitters have a shorter path length (≤100 µm) and higher linear energy transfer, making them better suited for treating micrometastatic disease. As a calcium-mimetic, radium-223 ( 223 Ra) is a bone-specific alpha-emitter which targets osteoblastic bone metastases. We hypothesise that the combination of LuPSMA with 223 Ra will be well tolerated and lead to improved response durability. Methods: This phase I/II, single-arm, single-centre study will enrol 36 patients with mCRPC who have progressed on a prior second-generation antiandrogen. Up to 6 cycles of LuPSMA (7.4 GBq) and 223 Ra (28 kBq/kg - 55 kBq/kg) will be given intravenously every 6 weeks, along with background androgen deprivation and bone protective therapy. Key eligibility criteria include a diagnosis of mCRPC with at least two untreated bone metastases visible on bone scintigraphy and PSMA-positive disease on PSMA PET/CT (SUVmax ≥20). Sites of FDG-positive disease must be either PSMA-positive or have increased uptake on bone scan. Other eligibility criteria include an ECOG status of 0-2, and adequate bone marrow and organ function. Patients treated with more than one line of chemotherapy for metastatic prostate cancer are not eligible. Blood samples and biopsies will be taken at baseline, on treatment, and at progression to develop tumor- and immune-based biomarkers that predict treatment response and associated survival benefit. A traditional 3+3 dose-escalation model will be utilized initially, escalating the dose of 223 Ra. The dose of LuPSMA will remain fixed. The co-primary objectives of this study are to determine the maximum tolerated dose of 223 Ra when combined with LuPSMA, and the 50% PSA response rate. Secondary objectives include measuring the frequency and severity of adverse events, assessment of efficacy through overall, radiographic and PSA progression-free survival, overall survival, objective tumor response rate, and evaluation of pain and health-related quality of life. Enrolment commenced in September 2022 and will continue for 24 months. Clinical trial information: NCT05383079 .