Brush border enzyme-cleavable linkers: Evaluation for reducing renal uptake of radiolabeled prostate-specific membrane antigen inhibitors

Use of this brush border enzyme-cleavable linker reduced kidney uptake and resulted in improved tumor:kidney uptake ratios. Although further structural improvements are needed, this linker approach might be useful as a component in strategies for reducing renal uptake of radiolabeled PSMA inhibitors.

PSMA-769 (6), a derivative of the prototypical PSMA inhibitor (((S)‑1‑carboxy‑5‑(4‑iodobenzamido)pentyl)carbamoyl)glutamate (12) modified to contain a Gly-Tyr linker, and its protected tin precursor (11) were synthesized starting from the basic pharmacophore molecule Lys-urea-Glu. An analogue of 6 containing d‑tyrosine in lieu of l‑tyrosine (PSMA-769-d-tyrosine) and the corresponding tin precursor (d-11) also were synthesized. Both radioiodinated and 211At-labeled 6 were synthesized by radiohalogenation of 11 and deprotection in situ. Similarly, radioiodinated d-6 was synthesized from d-11. Paired label biodistribution of [125I]12 and [131I]6 was performed in normal mice and in SCID mice bearing both PC3 PIP (PSMA+) and PC3 flu (PSMA-) subcutaneous prostate carcinoma xenografts. The biodistribution of [131I]6 and [211At]6 was also evaluated in this tumor model. Biodistribution of the two radioiodinated diastereomers of 6 was evaluated in normal mice and urine samples were analyzed for the presence of 4‑iodohippuric acid.

Compounds [131I]6 and [211At]6 were synthesized from 11 in overall radiochemical yields of 32.5 ± 0.1% (n = 4) and 22% (n = 1), respectively; radiochemical purity was >95%. In normal mice, renal uptake of [131I]6 was 1.4-, 2.8- and 161-fold lower than that seen for co-injected [125I]12 at 1 h, 4 h and 21 h, respectively. In tumor-bearing mice, kidney uptake of [131I]6 was similar to that for [125I]12 (P > 0.05) at 1 h and 4 h but was 6- to 7-fold lower at 21 h; however, [131I]6 uptake in PC3 PIP tumors was also lower than that seen for [125I]12 at 21 h (12.6 ± 3.4%ID/g vs. 36.8 ± 12.4%ID/g). Uptake of [211At]PSMA-769 in PC3 PIP tumors was slightly higher than that seen for [131I]PSMA-769 at 4 h (9.6 ± 1.6%ID/g versus 7.8 ± 1.6%ID/g; P = 0.002); its uptake in a number of normal tissues also was higher. In normal mice, kidney uptake of [125I]PSMA-769 at 4 h was about 73% of that seen for [131I]PSMA-769-d-tyrosine. Activity in the urine of mice receiving [125I]PSMA-769 contained mainly 4‑[125I]iodohippuric acid while unmetabolized intact molecule was present in the case of [125I]PSMA-769-d-tyrosine.