Small Molecule Radiohalogenated (125I/211At), DOTA Containing PSMA Inhibitors: Metal Complexation and Competing Inhibitor Improve Biodistribution in Mice.

537 Objectives: Radiolabeled small-molecule PSMA inhibitors are currently under clinical investigation as imaging and radiotherapeutic agents for prostate cancer (PC). Astatine-211 is a 7.2 h radiohalogen emitting short range, high linear energy transfer alpha particles well suited for treatment of PC micrometastases. Our previous 211At agents suffered from high renal retention, resulting in radiation nephropathy, and deastatination in vivo. Here we report strategies for addressing those problems involving studies with a DOTA containing 211At- or 125I-labeled PSMA agent, its nonradioactive Lu complex, and co-injection of a well-characterized competing PSMA ligand, DCIBzL. Methods: Radiosyntheses were done using a one-pot multistep reaction sequence consisting of a) radiohalogenation of stannane 1 with 125I or 211At in methanol containing N-chlorosuccinimide and acetic acid, b) concentration, c) acid hydrolysis to give [125I/211At]2, and d) metal complexation with non-radioactive Lu to give [125I/211At]3. Final compounds were purified by HPLC. Biodistribution was performed after i.v. bolus of 37 kBq (1 µCi) into athymic mice bearing both PSMA+ PC3-PIP and PSMA- PC3-flu flank xenografts with and without co-injection of 0.5 nmol DCIBzL. Organs were harvested 1, 4, and 24 h post-injection (5 mice per time point). Results: Radiochemical yields (non-decay corrected) were 53-60% for [125I]3 and 12-26% for [211At]3. All agents had high uptake in PIP tumors (10-50%ID/g) with minimal uptake in flu tumors (0.1-1.0 %ID/g). Lutetium complexation ([125I]3 compared to [125I]2) reduced retention of radioactivity in potentially problematic tissues: a) kidneys (K) (4 h; 162±36 vs 18.2±4.9 %ID/g; 24 h, 30.6±14.0 vs 3.3±1.1%ID/g); b) salivary glands (SG) at 1 h (5.1±3.4 vs 1.0±0.3 %ID/g); and c) lacrimal glands (LG) at 1h (17.4±6.0 vs 4.1±1.3%ID/g). Co-injection of DCIBzL with [125I]3 or [211At]3 further reduced retention in K, SG, and LG: [125I]3 @1 h, K 4.2±1.3, SG 3.2±0.7, LG 0.5±0.1 %ID/g; [211At]3 @ 1 h, K 2.2±0.6, SG 0.4±0.15 %ID/g, LG below detection, @ 4h, K 0.7±0.1, SG 0.3±0.2 %ID/g, LG below detection, resulting in T/K ratios of 11, 27, and 44 @ 1, 4, and 24 h; and T/SG ratio of 64 and 62 @ 1 h and 4h for [211At]3. Stomach uptake of [125I]3 and [211At]3 was low and comparable ([125I]3, 1.8±0.2, 0.8±0.5, and 0.1±0.04%ID/g; [211At]3, 0.4±0.2, 0.4±0.05, 0.4±0.7 %ID/g @ 1, 4, and 24 h, respectively), suggesting negligible deastatination occurred in vivo. Conclusion: By combining a lutetium loaded radiohalogenated agent with an appropriate competitive PSMA ligand, ([211At]3 and DCIBzL), it was possible to obtain high (25.4±4.4% ID/g) and prolonged 211At uptake in PSMA+ PC3-PIP xenografts with low levels of activity in normal tissues including kidneys, salivary glands, and lacrimal glands, with no evidence of deastatination in vivo. These results represent a considerable improvement compared with previously described 211At-labeled PSMA inhibitors and further evaluation of the therapeutic potential of this promising combination strategy is warranted. Funding: NIH R01CA184228.