Targeted alpha-particle therapy (TAT) aims to selectively deliver radionuclides emitting α-particles (cytotoxic payload) to tumors by chelation to monoclonal antibodies, peptides or small molecules that recognize tumor-associated antigens or cell-surface receptors. Because of the high linear energy transfer (LET) and short range of alpha (α) particles in tissue, cancer cells can be significantly damaged while causing minimal toxicity to surrounding healthy cells. Recent clinical studies have demonstrated the remarkable efficacy of TAT in the treatment of metastatic, castration-resistant prostate cancer. In this comprehensive review, we discuss the current consensus regarding the properties of the α-particle-emitting radionuclides that are potentially relevant for use in the clinic; the TAT-mediated mechanisms responsible for cell death; the different classes of targeting moieties and radiometal chelators available for TAT development; current approaches to calculating radiation dosimetry for TATs; and lead optimization via medicinal chemistry to improve the TAT radiopharmaceutical properties. We have also summarized the use of TATs in pre-clinical and clinical studies to date.

New effective therapies are greatly needed for metastatic uveal melanoma, which has a very poor prognosis with a median survival of less than 1 y. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanoma metastases, and a MC1R-specific ligand (MC1RL) with high affinity and selectivity for MC1R was previously developed. Methods: The 225 Ac-DOTA-MC1RL conjugate was synthesized in high radiochemical yield and purity and was tested in vitro for biostability and for MC1R-specific cytotoxicity in uveal melanoma cells, and the lanthanum-DOTA-MC1RL analog was tested for binding affinity. Non-tumor-bearing BALB/c mice were tested for maximum tolerated dose and biodistribution. Severe combined immunodeficient mice bearing uveal melanoma tumors or engineered MC1R-positive and-negative tumors were studied for biodistribution and efficacy. Radiation dosimetry was calculated using mouse biodistribution data and blood clearance kinetics from Sprague-Dawley rat data. Results: High biostability, MC1R-specific cytotoxicity, and high binding affinity were observed. Limiting toxicities were not observed at even the highest administered activities. Pharmacokinetics and biodistribution studies revealed rapid blood clearance (,15 min), renal and hepatobillary excretion, MC1R-specific tumor uptake, and minimal retention in other normal tissues. Radiation dosimetry calculations determined pharmacokinetics parameters and absorbed α-emission dosages from 225 Ac and its daughters. Efficacy studies demonstrated significantly prolonged survival and decreased metastasis burden after a single administration of 225 Ac-DOTA-MC1RL in treated mice relative to controls. Conclusion: These results suggest significant potential for the clinical translation of 225 Ac-DOTA-MC1RL as a novel therapy for metastatic uveal melanoma.

Lipophilicity is explored in the biodistribution (BD), pharmacokinetics (PK), radiation dosimetry (RD), and toxicity of an internally administered targeted alpha-particle therapy (TAT) under development for the treatment of metastatic melanoma. The TAT conjugate is comprised of the chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), conjugated to melanocortin receptor 1 specific peptidic ligand (MC1RL) using a linker moiety and chelation of the 225Ac radiometal. A set of conjugates were prepared with a range of lipophilicities (log D 7.4 values) by varying the chemical properties of the linker. Reported are the observations that higher log D 7.4 values are associated with decreased kidney uptake, decreased absorbed radiation dose, and decreased kidney toxicity of the TAT, and the inverse is observed for lower log D 7.4 values. Animals administered TATs with lower lipophilicities exhibited acute nephropathy and death, whereas animals administered the highest activity TATs with higher lipophilicities lived for the duration of the 7 month study and exhibited chronic progressive nephropathy. Changes in TAT lipophilicity were not associated with changes in liver uptake, dose, or toxicity. Significant observations include that lipophilicity correlates with kidney BD, the kidney-to-liver BD ratio, and weight loss and that blood urea nitrogen (BUN) levels correlated with kidney uptake. Furthermore, BUN was identified as having higher sensitivity and specificity of detection of kidney pathology, and the liver enzyme alkaline phosphatase (ALKP) had high sensitivity and specificity for detection of liver damage associated with the TAT. These findings suggest that tuning radiopharmaceutical lipophilicity can effectively modulate the level of kidney uptake to reduce morbidity and improve both safety and efficacy.

IDO1 is an enzyme catalyzing the initial and rate-limiting step in the catabolism of tryptophan along the kynurenine pathway. IDO1 expression could suppress immune responses by blocking T-lymphocyte proliferation locally, suggesting a role of IDO in the regulation of immune responses. The goal of this study was to evaluate the potential of radiofluorinated carboximidamides as selective PET radioligands for IDO1. Specific binding correlated with IDO1 expression as measured through in vitro, microPET experiments. Specific accumulation of the new radiotracer [18F]IDO49 was observed in IDO1-expressing tumors and confirmed by Western blot and IHC analyses. These results suggest that [18F]IDO49 has substantial potential as an imaging agent that targets IDO1 in tumors, and therefore may be utilized as a companion diagnostic for IDO1 targeted therapies.

New effective therapies are greatly needed for uveal melanoma, the most common ocular malignancy, which has a very poor prognosis with a median survival of less than one year when metastatic disease develops. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanomas but is not expressed in normal tissues of concern for toxicity. Previously, we developed an MC1R specific ligand (MC1RL) with high affinity and selectivity for MC1R. We used the MC1RL as a targeting scaffold for development of a radiopharmaceutical by conjugation of 225Ac chelate to form 225Ac -MC1RL-DOTA. 225Ac is a therapeutic alpha-emitting radionuclide. 225Ac -MC1RL-DOTA was synthesized with high radiochemical yield and purity (>95 and >99 % respectively) with an excellent in vitro stability, i.e. 90% after 10 days in human serum at 37ºC. An MTT cytotoxicity study demonstrated MC1R specific antiproliferative effect in cutaneous, ocular and uveal melanoma cell lines. A maximum tolerated dose (MTD) study was completed, where a range of 0-148 kBq (0-4 µCi) of 225Ac -MC1RL-DOTA was administered to groups of BALB/c mice. The radiopharmaceutical was well tolerated at even the highest doses and animals did not reach any clinical endpoints, such as weight loss, loss of kidney function or abnormal pathology. Biodistribution studies on MC1R expressing tumor bearing mice revealed tumor selectivity and a combination of renal and hepatic clearance with minimal retention in other normal tissues. A blood PK study in rats showed rapid clearance of the agent from the blood in <60 min. To determine radiodosimetry, gamma spectra were acquired for tumors, blood and other tissues and organs over a 21-day time period and activity curves generated for each tissue for 225Ac and other gamma-emitting daughter products in the decay chain. An exponential line-fitting of these curves allowed the estimation of clearance kinetics, tissue biological half-life and total absorbed dose (Gy) for each tissue. Doses were highest in MC1R positive tumors and clearance organs (liver, kidney and spleen) and low in all other tissues. In vivo efficacy studies were performed in human A375/MC1R cutaneous and MEL20 uveal melanoma tumors in SCID mice, or syngeneic mouse B16 melanoma tumors in C57BL/6 mice (n=10/group) injected with either 225Ac -MC1RL-DOTA, 225Ac-scrambled-MC1RL-DOTA, La-MC1RL-DOTA (non-radioactive control agent) or saline. In every case a tumor growth delay was observed in treated mice relative to controls, and a percentage of mice bearing the human tumors had complete remission. In conclusion, we evaluated the in vitro and in vivo properties of 225Ac-MC1RL-DOTA in this study. This agent demonstrated a significant MC1R-specific in vivo therapeutic efficacy in a mouse model of uveal melanoma with low systemic toxicity. Citation Format: Narges K. Tafreshi, Nella C. Delva, Christopher J. Tichacek, Michael L. Doligalski, Darpan N. Pandya, Nikunj B. Bhatt, HyunJoo Kil, Mikalai M. Budzevich, Epifanio Ruiz, Thaddeus J. Wadas, Mark L. McLaughlin, David L. Morse. Targeted alpha particle therapy for uveal melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5198. doi:10.1158/1538-7445.AM2017-5198