Small molecule inhibitors of PSMA incorporating technetium-99m for imaging prostate cancer: Effects of chelate design on pharmacokinetics

Single amino acid chelate (SAAC) systems for the complexation of the M(CO)3 moiety (M = Tc/Re) have been successfully incorporated into novel synthetic strategies for radiopharmaceuticals and evaluated in a variety of biological applications. However, the lipophilicity of the first generation of 99mTc(CO)3 complexes has resulted in substantial hepatobiliary uptake when examined either as lysine derivatives or integrated into biologically active small molecules and peptides. Here, we designed, synthesized, and evaluated novel polar functionalized imidazole derived SAAC systems (SAAC II) which have been chemically modified to promote overall 99mTc(CO)3L3 complex hydrophilicity with the intent of reducing non-target effects and enhancing renal clearance of prostate specific membrane antigen (PSMA) targeting small molecules. The 99mTc-labeled compounds were prepared, purified, and evaluated for stability, lipophilicity, and tissue distribution in LNCaP xenograft mice. The Glu-urea-Lys-C11 analogs were prepared with a variety of chelators to form (19R,23S)-1-(X)-2-((Y)methyl)-13,21-dioxo-2,14,20,22-tetraazapentacosane-19,23,25-tricarboxylic acid where X = Y = (methyl)pyridin-2-yl (6), X = Y = (methyl)-1H-imidazol-2-yl (7), X = (methyl)pyridin-2-yl, Y = carboxymethyl (8), X = Y = 1-(carboxymethyl)-1H-imidazol-2-yl (9), X = 1-(carboxymethyl)-1H-imidazol-2-yl, Y = carboxymethyl (10), and X = Y = 1-(1-(2-(bis(carboxymethyl)amino)-2-oxoethyl)-1H-imidazol-2-yl)-2-((1-(2-(bis(carboxymethyl)amino)-2-oxoethyl)-1H-imidazol-2-yl) (11). 99mTc labeling was achieved at ligand concentrations as low as 10−6 M and the complexes were stable (>90%) for 24 h. These new SAAC II chelators were evaluated for their influence on binding of the Glu-urea-Lys-C11 analogs to PSMA-positive LNCaP cells and compared to pyridine- and N-methylimidazole-containing SAAC ligands. Tissue distribution of the 99mTc-complexes containing the more polar chelators, 9 and 11, demonstrated decreased liver (<2% ID/g) and increased LNCaP tumor (>11% ID/g) accumulation at 1 h post-injection.

Structures of the chelates employed to form the 99mTc-PSMA inhibitors derived from SAAC (6–8) and SAAC II (9–11) chelators.Figure optionsDownload as PowerPoint slideHighlights
► Designed polar imidazole derived SAAC II systems to promote hydrophilicity.
► SAAC II analogs (Glu-urea-Lys-C11) were evaluated for binding to PSMA+ cells.
► 99mTc-complexes were evaluated for tissue distribution in LNCaP xenograft mice.
► Results showed enhanced renal clearance and diminished hepatobiliary accumulation.
► Results demonstrated the dramatic influence the chelate on both affinity and pk.