Preparation and preclinical evaluation of 68Ga-iPSMA-BN as a potential heterodimeric radiotracer for PET-imaging of prostate cancer

Bispecific radioligands targeting prostate‐specific membrane antigen and gastrin‐releasing peptide receptors on the surface of prostate cancer cells

“…After cartridge purification, the final product ([ 68 Ga] GaiPSMA-BN) was obtained in an RY of 62 ± 4% and radiochemical purity (RCP) greater than 99%. 46 In a later study by the same group, tracer iPSMA-BN 6 (18.0 nmol, 0.05 mg, MW = 2763 g/mol) was labeled with 177 Lu (740 MBq), resulting in [ 177 Lu]LuiPSMA-BN (molar activity = 41.1 MBq/nmol) with high RC 99 ± 1% (n = 12). 47 Another example of a PSMA/GRPr heterodimer, 7, refers to a 64 Cu-labeled ligand [DUPA (2-[3-(1,3dicarboxypropyl)ureido]pentanedioic acid)-6-Ahx-(NODAGA)-5-Ava-BN(7-14)NH 2 ].…”

“…28,44 In a recent study, a similar approach was followed by coupling the iPSMA (PSMA inhibitor) (2-Nal-Lys-CO-Glu-OH) 55 with DOTA (1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid) (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and combining it with the Lys 3 -(1-14), agonist, (iPSMA-BN) 6 (Scheme 2). 46,47,56 The Lys 3 -BN peptide was conjugated to the N-γmaleimidobutyl-oxysuccinimidyl ester moiety at the Lys 3 and then coupled to Glu-CO-Lys-2-Nal-Cys-DOTA through the formation of a thioether with the Cys residue. Compound 6 was labeled with 68 Ga and then assessed in vitro for its PSMA/GRPr binding affinity.…”

“…The affinity of the ligand 6 was estimated using a saturation assay. 46 The manuscript reports the K d values of the heterodimer in comparison with the monomers. SCHEME 2 Heterodimers based on DOTA, NODAGA (chelator group: black; gastrin-releasing peptide receptor [GRPr] pharmacophore: green; spacer group: red; and prostate-specific membrane antigen [PSMA] pharmacophore: blue) Heterodimer 6, [ 68 Ga]GaiPSMA-BN, presented comparable binding affinity with the monomers for both receptors GRPr and the PSMA (Table 1).…”

“…Moreover, it was found to have a greater cell uptake and internalization for both cell lines in comparison with the monomers [ 68 Ga] GaiPSMA and [ 68 Ga]GaBN. 46 In a recent study, ligand 6, labeled with 177 Lu, was tested against the nonradioactive monomers iPSMA and BN in order to determine inhibitory concentration IC 50 in LNCaP and PC-3 cells (IC 50 GRPr 3.49nM, PSMA 5.62nM). The same procedure was performed to determine IC 50 46,47 The orientation of the heterodimer's minimized energy conformer reduced the steric hindrance for the two active sites GRPr and PSMA resulting in increased affinity for both receptors.…”

“…However, the higher cell binding/internalization reported for [ 68 Ga]Ga/ [ 177 Lu] Lu6 was attributed to the position of the chelator and not to the combinatorial effect of the two pharmacophores. 46 Moreover, in all cases, the spacers used were rather short and aimed at minimizing the intramolecular interactions with the other parts of the heterodimer (chelator and second binding motif) as well as changing the pharmacokinetics of the ligands. Consequently, for these molecules, improved cell uptake occurred due to the improved binding kinetics, and increased local ligand concentration, and not because of simultaneous binding to both receptors.…”