Radiometal Complexes as Pharmacokinetic Modifiers: A Potent 68Ga-Labeled Gastrin-Releasing Peptide Receptor Antagonist Based on the Macrocyclic Metal Chelator NODIA-Me.

The gastrin-releasing peptide receptor (GRPr) is overexpressed in various cancer types including prostate and breast carcinomas, making it an attractive target for molecular imaging and therapy. In this work, we designed a novel GRPr antagonistic probe comprising metal chelator NODIA-Me. This 1,4,7-triazacyclononane-based chelator forms positively charged metal complexes due to its neutral methylimidazole arms. Because a positive charge at the N-terminus of GRPr conjugates is responsible for high receptor affinity as exemplified by the current gold standard DOTA-RM2, we investigated if a positively charged radiometal complex can be used as a pharmacokinetic modifier to also produce high-affinity GRPr conjugates. In this respect, the bioconjugate NODIA-Me-Ahx-JMV594 was prepared by a combination of solid-phase peptide synthesis and solution-based reactions in a 94% yield. Radiolabeling provided the 68Ga-labeled conjugate in radiochemical yields of >95% and radiochemical purities of >98% with mean molar activities of Am ∼17 MBq nmol-1. The competitive GRPr affinity of the metal-free and 69/71Ga-labeled conjugate was determined to be IC50 = 0.41 ± 0.06 and 1.45 ± 0.06 nM, respectively. The metal-free GRPr antagonist DOTA-RM2 and its corresponding 69/71Ga complex had IC50 values of 1.42 ± 0.07 and 0.98 ± 0.19 nM, respectively. Small-animal PET imaging of mice bearing GRPr(+) PC-3 tumors revealed high radioactivity accumulation in the tumors and in the pancreas as an organ with high levels of GRPr expression. These findings were corroborated by the corresponding ex vivo biodistribution data, in which the tumors and the pancreas exhibited the highest radioactivity accumulation. By coinjection of an excess of NODIA-Me-Ahx-JMV594, uptake in the tumors and GRPr(+) organs was significantly reduced, confirming specific receptor-mediated uptake. The estrogen receptor-positive tumor of a female breast cancer patient was clearly visualized by PET imaging using 68Ga-labeled NODIA-Me-Ahx-JMV594. To summarize, the positive charge at the N-terminus of the conjugate induced by the Ga(NODIA-Me) complex resulted in high GRPr affinity comparable to that of the potent antagonist DOTA-RM2. The conjugate NODIA-Me-Ahx-JMV594 is a promising probe for imaging of GRPr tumors that warrants further evaluation in larger patient cohorts as well as in combination with other radiometals.

Gallium Complexation, Stability, and Bioconjugation of 1,4,7-Triazacyclononane Derived Chelators with Azaheterocyclic Arms.

We have recently introduced a 1,4,7-triazacyclononane (TACN) based chelating system with additional five-membered azaheterocyclic substituents for complexation of radioactive Cu2+ ions. In this work, we investigated the complexation properties of these novel chelators with Ga3+. In labeling experiments, we could show that the penta- and hexadentate imidazole derivatives NODIA-Me 4 and NOTI-Me 1 can be labeled with 68Ga in specific activities up to ∼30 MBq nmol-1, while the corresponding thiazole derivative NOTThia 2 did not label satisfactorily under identical conditions. NMR studies on the Ga complexes of 1 and the model compound NODIA-Me-NH-Me 5 revealed formation of rigid 1:1 chelates with a slow macrocyclic interconversion and inert Ga-N bonds to the methylimidazole residues on the NMR time scale. The TACN-derived bifunctional chelator NODIA-Me was furthermore conjugated to a prostate-specific membrane antigen (PSMA) targeting moiety to give the corresponding bioconjugate NODIA-Me-PSMA 7. Serum stability and ligand challenge experiments of 68Ga-7 confirmed formation of a stable complex for up to 4 h. The remaining coordination site of five-coordinate Ga complexes was found to be occupied by monodentate ligands including hydroxide and chloride anions depending on the conditions. According to density functional theory calculations, coordination of monodentate ligands as well as of the amide group for the bioconjugated ligand are energetically plausible. Finally, the labeled bioconjugate 68Ga-7 exhibited rapid renal clearance in biodistribution studies performed by small animal PET imaging with no indication of transchelation/demetalation in vivo. Altogether, our results provide strong evidence for a stable Ga complexation of our novel TACN-based chelators bearing imidazole arms. Despite the formation of two complexes incorporating different monodentate ligands in vitro, the imidazole type ligands show promise as chelating agents for the future development of gallium based radiopharmaceuticals.