Clinical Trial Protocol for VIOLET: A Single-Center, Phase I/II Trial Evaluation of Radioligand Treatment in Patients with Metastatic Castration-Resistant Prostate Cancer with [161Tb]Tb-PSMA-I&T.

[177Lu]Lu-PSMA is an effective class of therapy for patients with metastatic castration-resistant prostate cancer (mCRPC); however, progression is inevitable. The limited durability of response may be partially explained by the presence of micrometastatic deposits, which are energy-sheltered and receive low absorbed radiation with 177Lu due to the approximately 0.7-mm mean pathlength. 161Tb has abundant emission of Auger and conversion electrons that deposit a higher concentration of radiation over a shorter path, particularly to single tumor cells and micrometastases. 161Tb has shown in vitro and in vivo efficacy superior to that of 177Lu. We aim to demonstrate that [161Tb]Tb-PSMA-I&T will deliver effective radiation to sites of metastatic prostate cancer with an acceptable safety profile. Methods: This single-center, single-arm, phase I/II trial will recruit 30 patients with mCRPC. Key eligibility criteria include a diagnosis of mCRPC with progression after at least one line of taxane chemotherapy (unless medically unsuitable) and androgen receptor pathway inhibitor; prostate-specific membrane antigen-positive disease on [68Ga]Ga-PSMA-11 or [18F]DCFPyL PET/CT (SUVmax ≥ 20); no sites of discordance on [18F]FDG PET/CT; adequate bone marrow, hepatic, and renal function; an Eastern Cooperative Oncology Group performance status of no more than 2, and no prior treatment with another radioisotope. The dose escalation is a 3 + 3 design to establish the safety of 3 prespecified activities of [161Tb]Tb-PSMA-I&T (4.4, 5.5, and 7.4 GBq). The maximum tolerated dose will be defined as the highest activity level at which a dose-limiting toxicity occurs in fewer than 2 of 6 participants. The dose expansion will include 24 participants at the maximum tolerated dose. Up to 6 cycles of [161Tb]Tb-PSMA-I&T will be administered intravenously every 6 wk, with each subsequent activity reduced by 0.4 GBq. The coprimary objectives are to establish the maximum tolerated dose and safety profile (Common Terminology Criteria for Adverse Events version 5.0) of [161Tb]Tb-PSMA-I&T. Secondary objectives include measuring absorbed radiation dose (Gy), evaluating antitumor activity (prostate-specific antigen 50% response rate, radiographic and prostate-specific antigen progression-free survival, overall survival, objective response rate), and evaluating pain (Brief Pain Inventory-Short Form) and health-related quality of life (Functional Assessment of Cancer Therapy-Prostate and Functional Assessment of Cancer Therapy-Radionuclide Therapy). Conclusion: Enrollment was completed in February 2024. Patients are still receiving [161Tb]Tb-PSMA-I&T.

Combination Strategies and Targeted Radionuclide Therapies.

Combination models utilising treatments from two or more therapeutic classes are well established in cancer care. In the new era of theranostic (theragnostic) medicine there is an ongoing need to identify and refine novel combination strategies to optimise multidisciplinary care for conditions commonly encountered in nuclear medicine such as neuroendocrine neoplasms (NEN), prostate cancer (PCa), and thyroid cancer, along with seeking advancements in molecular imaging and therapy techniques for other tumour streams. This concise review explores the background of theranostic monotherapy, established approaches to combination strategies in theranostics, and emerging targeted radionuclide therapies in use or under active investigation, with a focus on Australian-led studies.

Quantitative calibration of Tb-161 SPECT/CT in view of personalised dosimetry assessment studies.

BACKGROUND: Terbium-161 (161Tb)-based radionuclide therapy poses an alternative to current Lutetium-177 (177Lu) approaches with the additional benefit of secondary Auger and conversion electron emissions capable of delivering high doses of localised damage to micro-metastases including single cells. Quantitative single-photon emission computed tomography, paired with computed tomography (SPECT/CT), enables quantitative measurement from post-therapy imaging. In view of dosimetry extrapolations, a Tb-161 sensitivity SPECT/CT camera calibration was performed using a method previously validated for 177Lu. METHODS: Serial imaging of a NEMA/IEC body phantom with Tb-161 was performed on SPECT/CT with low-energy high-resolution collimators employing a photopeak of 75 keV with a 20% width. Quantitative stability and recovery coefficients were investigated over a sequence of 19 scans with buffered 161Tb solution at total phantom activity ranging from 70 to 4990 MBq. RESULTS: Sphere recovery coefficients were 0.60 ± 0.05, 0.52 ± 0.07, 0.45 ± 0.07, 0.39 ± 0.07, 0.28 ± 0.08, and 0.20 ± 0.08 for spheres 37, 28, 22, 17, 13, and 10mm, respectively, when considered across all activity and scan durations with dual-energy window scatter correction. Whole-field reconstructed sensitivity was calculated as 1.42E-5 counts per decay. Qualitatively, images exhibited no visual artefacts and were comparable to 177Lu SPECT/CT. CONCLUSIONS: Quantitative SPECT/CT of 161Tb is feasible over a range of activities enabling dosimetry analogous to 177Lu whilst also producing suitable imaging for clinical review. This has been incorporated into a prospective trial of 161Tb-PSMA for men with metastatic prostate cancer.

AlphaBet: Combination of Radium-223 and [17 7Lu]Lu-PSMA-I&T in men with metastatic castration-resistant prostate cancer (clinical trial protocol).

Background: [177Lu]Lu-PSMA is a radioligand therapy used in metastatic castration-resistant prostate cancer (mCRPC). Despite a survival benefit, the responses for many patients receiving [177Lu]Lu-PSMA are not durable, and all patients eventually develop progressive disease. The bone marrow is the most common site of progression. Micrometastases in this area likely receive an inadequate dose of radiation, as the emitted beta-particles from 177Lu travel an average range of 0.7 mm in soft tissue, well beyond the diameter of micrometastases. Radium-223 (223Ra) is a calcium-mimetic and alpha-emitting radionuclide approved for use in men with mCRPC with bone metastases. The range of emitted alpha particles in soft tissue is much shorter (≤100 µm) with high linear energy transfer, likely more lethal for osseous micrometastases. We anticipate that combining a bone-specific alpha-emitter with [177Lu]Lu-PSMA will improve eradication of micrometastatic osseous disease, and thereby lead to higher and longer responses. Methods: This is a single-center, single-arm phase I/II trial evaluating the combination of 223Ra and [177Lu]Lu-PSMA-I&T in men with mCRPC. Thirty-six patients will receive 7.4 GBq of [177Lu]Lu-PSMA-I&T, concurrently with 223Ra in escalating doses (28 kBq/kg - 55kBq/kg), both given intravenously every six weeks for up to six cycles. Eligible patients will have at least two untreated bone metastases visible on bone scintigraphy, and PSMA-positive disease on PSMA PET scan. Patients must have adequate bone marrow and organ function and be willing to undergo tumor biopsies. Patients with discordant disease visible on FDG PET scan (defined as FDG positive disease with minimal or no PSMA expression and no uptake on bone scan) will be excluded. Other key exclusion criteria include the presence of diffuse marrow disease, prior treatment with 223Ra or [177Lu]Lu-PSMA, or more than one prior line of chemotherapy for prostate cancer. The co-primary objectives of this study are to determine the maximum tolerated dose of 223Ra when combined with [177Lu]Lu-PSMA-I&T and the 50% PSA response rate. Conclusion: The AlphaBet trial is a phase I/II study combining 223Ra with [177Lu]Lu-PSMA-I&T in patients with mCRPC. We aim to enroll the first patient in Q3 2022, and recruitment is anticipated to continue for 24 months. Study registration: NCT05383079.