Synthesis and evaluation of a para-carboxylated benzyl-DOTA for labeling peptides and polypeptides

Radiolabeled peptides and low-molecular-weight (LMW) polypeptides show high and persistent radioactivity levels in the kidney. To develop a DOTA-based bifunctional chelating agent that provides a radiometabolite with a rapid elimination rate from the kidney, a para-carboxyl Bn-DOTA (p-COOH-Bn-DOTA) was designed, synthesized, and evaluated.

A precursor compound, p-COOH-Bn-DOTA(^tBu)₄, was synthesized in 9 steps using N-Boc-p-iodo-L-phenylalanine as the starting material. A synthetic somatostatin analog (TOC) was used as a representative peptide metabolized in the renal lysosomes. p-COOH-Bn-DOTA-conjugated TOC (DOTA-Bn-TOC) was synthesized by the conventional solid-phase peptide synthesis using p-COOH-Bn-DOTA(^tBu)₄. DOTA-tris(^tBu ester) was also conjugated with TOC to prepare DOTATOC. ¹¹¹In-labeling of the peptides was conducted under similar conditions. The radiochemical conversions, stability against apo-transferrin (apoTf), and in vivo behaviors were compared.

[¹¹¹In]In-DOTA-Bn-TOC was obtained with higher radiochemical conversions than [¹¹¹In]In-DOTATOC. Both ¹¹¹In-labeled TOC derivatives remained stable against apoTf. In biodistribution studies, [¹¹¹In]In-DOTA-Bn-TOC exhibited higher initial uptake in the kidney, followed by a faster elimination rate of radioactivity into the urine than [¹¹¹In]In-DOTATOC. The metabolic studies showed that the shorter residence time of the radiometabolite from [¹¹¹In]In-DOTA-Bn-TOC was responsible for the renal radioactivity decline.

p-COOH-Bn-DOTA provides stable ¹¹¹In-labeled peptides in high yields at low peptide concentrations. p-COOH-Bn-DOTA also provides a radiometabolite with a short residence time in the kidney. Such characteristics would render p-COOH-Bn-DOTA useful to the future application to radiolabeled LMW polypeptides with low renal radioactivity levels.