A Phase 1, Open-label, Dose-Ascending Study to Evaluate the Safety and Tolerability of the Therapeutic Radiopharmaceutical ¹³¹I-MIP-1095 for the Treatment of Metastatic Castration-Resistant Prostate Cancer

PURPOSE ¹³¹I-MIP-1095 is a targeted radiotherapeutic that contains ¹³¹I, a β-particle emitter, and MIP-1095, a urea-based ligand for prostate-specific membrane antigen. We report the first phase 1, dose-escalation study of ¹³¹I-MIP-1095 in patients with metastatic castration-resistant prostate cancer (mCRPC). Methods This study enrolled men with mCRPC refractory to second-generation antiandrogen(s) and taxane chemotherapy. Dosimetry/biodistribution assessments were performed. Safety and tolerability were determined in subjects who qualified for therapeutic administration of ¹³¹I-MIP-1095 with maximum tolerated activity examined in a dose-ascending manner (3 + 3 design methodology). Disease outcomes including prostate-specific antigen (PSA) change, tumor response, survival, and circulating tumor cell concentration were assessed. Results A total of 9 subjects with mCRPC were included in this study. On the basis of dosimetry results, 5 of 9 patients were treated: 3 in cohort 1 (50 mCi) and 2 in cohort 2 (75 mCi). Accrual stopped at the cohort 2 activity level in response to the US Food and Drug Administration mandate for ¹³¹I-MIP-1095 manufacturing concerns. Parotid/salivary glands (3.5 Gy/Bq), liver (2.2 Gy/Bq), kidneys (1.3 Gy/Bq), and spleen (0.7 Gy/Bq) demonstrated the greatest extent of ¹³¹I-MIP-1095 exposure. There were no deaths, serious adverse events, or drug discontinuations due to treatment-emergent adverse events. Grade 1-2 thrombocytopenia, anemia, leukopenia, and dry mouth most commonly occurred. One subject (33.3%) exhibited maximum decline for the PSA response of 50% or greater. Conclusion ¹³¹I-MIP-1095 demonstrated favorable dosimetry, biodistribution, and safety, as well as a modest PSA response supporting further investigation for treatment of men with mCRPC.