Production, quality control, biological evaluation and biodistribution modeling of Lutetium-177 maltolate as a viable bone pain palliative in skeletal metastasis

Hematological toxicity of [225Ac]Ac-PSMA-617 and [177Lu]Lu-PSMA-617 in RM1-PGLS syngeneic mouse model

BACKGROUND

Prostate cancer (PC) has a 34% 5-year survival rate after progressing to metastatic castration-resistant prostate cancer (mCRPC), which occurs in 20-30% of cases. Treatments like chemotherapy, immunotherapy, and PSMA-targeted radioligand therapy (RLT) show promise, but challenges remain with tumor resistance, side effects, and dose-limiting toxicity in kidneys and bone marrow. This study investigated the hematotoxicity, treatment efficacy, and recovery after [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 treatment in a syngeneic PC mouse model.

METHOD

Twenty-five male C57BL/6 mice were inoculated with RM1-PGLS cells and monitored using [68Ga]Ga-PSMA-11 PET/CT. The mice were divided into five groups as follows: (1) [225Ac]Ac-PSMA-617 treatment with tumors, (2) [177Lu]Lu-PSMA-617 treatment with tumors, (3) control group with tumors, (4) [225Ac]Ac-PSMA-617 treatment without tumors, and (5) [177Lu]Lu-PSMA-617 treatment without tumors. Tumor volume was measured weekly, and animals were sacrificed when tumors reached 1.5 cm³. Endpoint criteria included tumor size, survival, and body mass. Blood samples were collected at different time points to assess blood cell counts and liver and kidney function.

RESULTS

Both treatments significantly slowed tumor progression and extended survival. [225Ac]Ac-PSMA-617-treated mice had a median survival of 70 days, compared to 58 days for [177Lu]Lu-PSMA-617-treated mice and 30 days for the control group. Tumor volumes were significantly reduced in both treatment groups (P < 0.05). Hematological analysis showed that both treatments reduced WBCs, RBCs, and platelets, but values normalized within 35-42 days. Liver and kidney functions remained unaffected, and no significant renal or hepatic toxicity was observed.

CONCLUSION

Both [225Ac]Ac-PSMA-617 and [177Lu]Lu-PSMA-617 caused transient hematotoxicity without prolonged effects. The data do not explicitly support the necessity of immunocompetent models for studying therapeutic outcomes in this context. Future studies incorporating immune profiling are warranted to investigate immune system interactions in radioligand therapy further.

Hematological Toxicity in Mice after High Activity Injections of 177Lu-PSMA-617

Prostate cancer (PC) is one of the most common malignancies affecting men, with poor prognosis after progression to metastatic castration-resistant prostate cancer (mCRPC). Radioligand therapy (RLT) targeting the overexpressed PSMA on PC cells, with, e.g., ¹⁷⁷Lu-PSMA-617, has been effective in reducing tumor burden and prolonging survival in mCRPC. However, it is not a curative method with kidney and bone marrow toxicity limiting the activity given to patients. Previous preclinical models have reported transient hematotoxicity for up to 120 MBq. This activity may still be too low to investigate the effect on renal function since it corresponds to an absorbed dose below 10 Gy, whereas the kidneys in a clinical setting usually receive an absorbed dose more than double. Here we investigated the hematotoxicity and recovery after administered activities of 120, 160, and 200 MBq in a ¹⁷⁷Lu-PSMA-617 BALB/cAnNRj mouse model. The animals had an initial drop in white blood cells (WBC) starting 4 days post injection, which recovered after 21 days. The effect on red blood cells (RBC) and platelets was detected later; 17 days post-injection levels decreased compared to the control group. The reduction was restored again 32 days post injection. No correlation between injected activity and hematotoxicity was found. Our results suggest that activities up to 200 MBq of ¹⁷⁷Lu-PSMA-617 give transient hematotoxicity from which animals recover within a month and no radiation-related deaths. Injecting these high activities could allow animal studies with increased clinical relevance when studying renal toxicity in animal models.

Production and Clinical Applications of Radiopharmaceuticals and Medical Radioisotopes in Iran

During past 3 decades, nuclear medicine has flourished as vibrant and independent medical specialty in Iran. Since that time, more than 200 nuclear physicians have been trained and now practicing in nearly 158 centers throughout the country. In the same period, Tc-99m generators and variety of cold kits for conventional nuclear medicine were locally produced for the first time. Local production has continued to mature in robust manner while fulfilling international standards. To meet the ever-growing demand at the national level and with international achievements in mind, work for production of other Tc-99m-based peptides such as ubiquicidin, bombesin, octreotide, and more recently a kit formulation for Tc-99m TRODAT-1 for clinical use was introduced. Other than the Tehran Research Reactor, the oldest facility active in production of medical radioisotopes, there is one commercial and three hospital-based cyclotrons currently operational in the country. I-131 has been one of the oldest radioisotope produced in Iran and traditionally used for treatment of thyrotoxicosis and differentiated thyroid carcinoma. Since 2009, (131)I-meta-iodobenzylguanidine has been locally available for diagnostic applications. Gallium-67 citrate, thallium-201 thallous chloride, and Indium-111 in the form of DTPA and Oxine are among the early cyclotron-produced tracers available in Iran for about 2 decades. Rb-81/Kr-81m generator has been available for pulmonary ventilation studies since 1996. Experimental production of PET radiopharmaceuticals began in 1998. This work has culminated with development and optimization of the high-scale production line of (18)F-FDG shortly after installation of PET/CT scanner in 2012. In the field of therapy, other than the use of old timers such as I-131 and different forms of P-32, there has been quite a significant advancement in production and application of therapeutic radiopharmaceuticals in recent years. Application of (131)I-meta-iodobenzylguanidine for treatment of neuroblastoma, pheochromocytoma, and other neuroendocrine tumors has been steadily increasing in major academic university hospitals. Also (153)Sm-EDTMP, (177)Lu-EDTMP, (90)Y-citrate, (90)Y-hydroxyapatite colloid, (188/186)Re-sulfur colloid, and (188/186)Re-HEDP have been locally developed and now routinely available for bone pain palliation and radiosynovectomy. Cu-64 has been available to the nuclear medicine community for some time. With recent reports in diagnostic and therapeutic applications of this agent especially in the field of oncology, we anticipate an expansion in production and availability. The initiation of the production line for gallium-68 generator is one of the latest exciting developments. We are proud that Iran would be joining the club of few nations with production lines for this type of generator. There are also quite a number of SPECT and PET tracers at research and preclinical stage of development preliminarily introduced for possible future clinical applications. Availability of fluorine-18 tracers and gallium-68 generators would no doubt allow rapid dissemination of PET/CT practices in various parts of our large country even far from a cyclotron facility. Also, local production and availability of therapeutic radiopharmaceuticals are going to open exciting horizons in the field of nuclear medicine therapy. Given the available manpower, local infrastructure of SPECT imaging, and rapidly growing population, the production of Tc-99m generators and cold kit would continue to flourish in Iran.