A Phase 2, Open-label Study Evaluating Dosimetry, Randomized Dose Optimization, Dose Escalation and Efficacy of Ac-225 Rosopatamab Tetraxetan in Participants With PSMA PET-Positive Castration-Resistant Prostate Cancer
This is a four-part study evaluating the safety and efficacy of a PSMA-directed radioantibody (rosopatamab tetraxetan, conjugated to either In-111 or Ac-225). Part 1 will consist of one administration of In-111-rosopatamab tetraxetan to characterize the biodistribution of the radioantibody to target organs and prostate cancer lesions. Participants then will be enrolled into either Part 2 (Dose Optimization) or Part 3 (Dose Escalation and Expansion) depending on their prior treatment history. Part 4 will be an extended regimen in dose escalation and expansion. Participants qualifying for Part 2 will be randomized to receive Ac-225 rosopatamab tetraxetan in a single fractionated cycle (dose administration on Day 1 and Day 15) at either 45 or 60 kBq/Kg. Participants qualifying for Part 3 must have received prior Lu-177-PSMA-radioligand therapy and will receive Ac-225 rosopatamab tetraxetan in a single fractionated cycle at 45, 55, or 60 kBq/Kg. Dose limiting toxicities (DLTs) will be monitored in Part 3 to determine the recommended phase 2 dose (RP2D), and the study may enroll additional participants to be treated with the RP2D dose level. Participants qualifying for Part 4 will initially receive two doses (dose administration on Day 1 and Day 15) at the dose level selected in Part 2, followed by a single third dose of Ac-225 rosopatamab tetraxetan administered approximately 12 weeks after completion of the Part 2 fractionated dosing regimen. The starting dose level for Part 4 will be 22 kBq/kg (or fixed activity equivalent). Dose limiting toxicities (DLTs) will be monitored following administration of the third dose in Part 4 to determine the recommended phase 2 dose (RP2D) of a third dose of Ac-225 rosopatamab tetraxetan. Participants enrolled into any part will attend study visits which will include blood samples, electrocardiogram (ECG), radiographic imaging, and physical examinations along with other assessments.
• Progressive CRPC defined as castrate levels of testosterone and progressing by at least one of the following criteria:
‣ Serum PSA progression as defined by PCWG3 (rising PSA consisting of two consecutive increases measured at least 3 weeks apart, of a rise of \>25%, and with an absolute rise of 2.0 ng/mL.
⁃ Soft tissue progression defined as a ≥20% increase in the sum of the diameter (short axis for nodal lesions and long axis for non-nodal lesions) of all target lesions based on the smallest sum of the diameter since the previous treatment was started or the appearance of one or more new lesions by CT/magnetic resonance imaging (MRI)
⁃ Progression of bone disease defined by PCWG3 as evaluable disease or new bone lesions by bone scan
⁃ Identification of new soft tissue or bone lesions on PSMA PET imaging
• Metastatic disease defined as either or both of the following:
‣ Parts 1, 2, 3, and 4: Documented M1 disease on conventional imaging (CT/MRI of the chest/abdomen/pelvis and/or Technetium 99m \[99mTc\] whole-body bone scan)
⁃ Parts 1, 2, and 4 only: Identification of bone lesion(s), extra-pelvic soft tissue lesion(s), or visceral metastases on PSMA PET imaging with an FDA-approved imaging agent
• PSMA PET-positive disease, defined as at least one PSMA-positive metastatic lesion and no PSMA-negative lesions
• Progression following treatment with ADT and at least one ARSI (e.g., enzalutamide, apalutamide, darolutamide, and/or abiraterone acetate)
• The standard of care use (in the setting of metastatic CRPC with significant burden of active bone metastases) of antiresorptive bone-targeted agents (e.g., zoledronic acid, denosumab) is required for all participants without a contraindication, for at least 4 weeks prior to administration of Ac-225 rosopatamab tetraxetan.
• Participants with HIV are eligible if they are well-controlled (i.e, an undetectable HIV viral load (\<50 copies/mL) within 6 months of enrollment and a stable ART regimen for at least 6 months prior to enrollment) and at low risk for HIV-related illness
∙ Part 3 Only:
• Prior treatment with Lu-177-PSMA-radioligand therapy
• Prior treatment with up to only one taxane-based chemotherapy regimen is allowed