Synthesis and preliminary evaluation of 211 At-labeled inhibitors of prostate-specific membrane antigen for targeted alpha particle therapy of prostate cancer.

Introduction: The high potency and short tissue range of α-particles are attractive features for targeted radionuclide therapy, particularly for cancers with micro-metastases. In the current study, we describe the synthesis of a series of ²¹¹ At-labeled prostate-specific membrane antigen (PSMA) inhibitors and their preliminary evaluation as potential agents for metastatic prostate cancer treatment.
Methods: Four novel Glu-urea based PSMA ligands containing a trialkyl stannyl group were synthesized and labeled with ²¹¹ At, and for comparative purposes, ¹³¹ I, via halodestannylation reactions with N-chlorosuccinimide as the oxidant. A PSMA inhibitory assay was performed to evaluate PSMA binding of the unlabeled, iodinated compounds. A series of paired-label biodistribution experiments were performed to compare each ²¹¹ At-labeled PSMA ligand to its ¹³¹ I-labeled counterpart in mice bearing subcutaneous PC3 PSMA+ PIP xenografts.
Results: Radiochemical yields ranged from 32% to 65% for the ²¹¹ At-labeled PSMA inhibitors and were consistently lower than those obtained with the corresponding ¹³¹ I-labeled analogue. Good localization in PC3 PSMA+ PIP but not control xenografts was observed for all labeled molecules studied, which exhibited a variable degree of in vivo dehalogenation as reflected by thyroid and stomach activity levels. Normal tissue uptake and in vivo stability for several of the compounds was markedly improved compared with the previously evaluated compounds, [ ²¹¹ At]DCABzL and [*I]DCIBzL.
Conclusions and Implications for Patient Care: Compared with the first generation compound [ ²¹¹ At]DCABzL, several of the novel ²¹¹ At-labeled PSMA ligands exhibited markedly improved stability in vivo and higher tumor-to-normal tissue ratios. [ ²¹¹ At]GV-620 has the most promising characteristics and warrants further evaluation as a targeted radiotherapeutic for prostate cancer.
Competing Interests: Declaration of competing interest G.V., R.M., I.L., M.P. and M.R.Z. are co-inventors on filed patent applications that cover (211)At- and (131)I-labeled PSMA inhibitors including the molecules comprising this study. We declare no other competing interests related to this work.
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