Synthesis and radiopharmacological evaluation of ⁶⁴Cu-labeled bombesin analogs featuring a bis(2-pyridylmethyl) 1,4,7-triazacyclononane chelator

The bifunctional chelating agent 2-[4,7-bis(2-pyridylmethyl)-1,4,7-triazacyclononan-1-yl]acetic acid, DMPTACN-COOH, has been found to bind strongly to copper(II), resulting in a radiocopper(II)-ligand complex that exhibits high in vivo stability. The pendant carboxylic acid group enables this derivative to be conjugated to the N-terminal amino acid residues of peptides. Exploiting this, two stabilized bombesin (BBN) derivatives, ßAla-ßAla-[Cha¹³,Nle¹⁴]BBN(7-14) and ßhomo-Glu-ßAla-ßAla-[Cha¹³,Nle¹⁴]BBN(7-14) have been coupled to DMPTACN-COOH and radiolabeled with the positron emitter copper-64 (⁶⁴Cu-1 and ⁶⁴Cu-3). The in vitro binding characteristics of the [⁶⁴Cu]Cu-labeled bombesin conjugates in gastrin-releasing peptide receptor (GRPR) over-expressing prostate cancer (PC-3) cells have been evaluated. Biodistribution studies performed in Wistar rats indicate a specific uptake in the GRPR-rich pancreas and rapid renal elimination for both ⁶⁴Cu-1 and ⁶⁴Cu-3. Small animal PET imaging studies performed in NMRI nu/nu mice bearing the human prostate tumor PC-3 demonstrated a very high degree of tumor accumulation for ⁶⁴Cu-1 and ⁶⁴Cu-3. Incorporation of a single additional glutamic acid residue within the spacer between bombesin and the radiolabeled complex (⁶⁴Cu-3) leads to a higher tumor-to-muscle uptake ratio (amounting to >30 at 100 min post injection) compared to ⁶⁴Cu-1.