On April 9, 2024 Fusion Pharmaceuticals Inc. (the "Company") reported a poster at the American Association for Cancer Research (AACR) (Free AACR Whitepaper) Annual Meeting 2024 which showed interim efficacy and safety data from the Phase 2 TATCIST open-label clinical trial evaluating FPI-2265, an actinium-225 based PSMA-I&T targeting radioconjugate ("RC") for the treatment of metastatic castration-resistant prostate cancer ("mCRPC") (Press release, Fusion Pharma, APR 9, 2024, View Source [SID1234641932]). Results demonstrate that FPI-2265 is active in heavily pretreated patients with progressive mCRPC, including patients who received prior lutetium-based RCs. Safety, tolerability and clinical activity data were generally consistent with other published studies of small molecule-based 225Ac-PSMA RCs.

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As of the March 1, 2024 data cutoff, 35 patients received at least one dose of FPI-2265, with 25 patients having at least 12 weeks of follow-up. The analysis included 25 patients for safety evaluation and 20 patients for assessing prostate-specific antigen ("PSA") response. Four participants were identified as superscan patients and were excluded from the efficacy analyses and reported separately in the safety analysis. One participant was not included in the efficacy analysis due to uninterpretable PSA response. Patients in the study were pretreated with a median of four prior lines of anticancer therapy, with 20 out of 25 (80%) receiving prior chemotherapy, including 10 patients who received at least two prior lines of taxanes. Nine out of 25 patients received a prior 177Lu-based PSMA RC.

From the efficacy-evaluable patient population, PSA50 (≥50% decline in PSA by 12 weeks after first treatment) response was achieved in 10 out of 20 patients (50%) regardless of prior lutetium treatment. PSA50 was achieved in 61% of lutetium-naïve participants and 42% of lutetium-treated participants. In an exploratory subset analysis of 13 patients, including six patients who received prior 177Lu-based PSMA RC treatment, with baseline PSMA Mean Standardized Uptake Value (SUVmean) >6, PSA50 response was observed in nine patients (69%).

FPI-2265 demonstrated meaningful improvement in secondary endpoints which include maximum % PSA decline, and independent reviewer-assessed response rates based on RECIST v1.1 criteria, and the rate of disease progression in bone per Prostate Cancer Working Group 3 (PCWG3) criteria.

FPI-2265 was generally well tolerated and in line with prior published data, with predominantly Grade 1-2 treatment related adverse events ("TRAEs") observed, including xerostomia (dry mouth), thrombocytopenia, anemia, fatigue and dry eye. Xerostomia, the most common TRAE, was primarily Grade 1 with all incidences being Grade 1-2 (62% Grade 1 and 24% Grade 2). One treatment-related death due to cerebral hemorrhage was reported in a superscan patient. Three out of 25 participants discontinued treatment due to TRAEs, including two participants in the superscan group, however there were no discontinuations due to xerostomia.