Improved kinetic stability of DTPA-dGlu as compared with conventional monofunctional DTPA in chelating indium and yttrium: preclinical and initial clinical evaluation of radiometal labelled minigastrin derivatives

The development of monofunctional DTPA derivatives has been a major breakthrough in the labelling of proteins or peptides with a variety of radiometals. Although this methodology is simple and useful for indium-111 labelling, the stability of these conjugates is too low for most therapeutic nuclides. Cyclic chelators, such as DOTA, have shown excellent kinetic stability with a variety of radiometals, but the labelling procedure is more difficult, requiring ultra-pure reagents and a heating step that sometimes endangers the biomolecule's integrity. The aim of this work was twofold: (a) to develop a novel, open chain chelator which can be easily labelled with various radiometals, displaying higher kinetic stability than monofunctional DTPA, and (b) to evaluate this chelator in vitro and in vivo when conjugated to a CCK-B receptor ligand as a detection modality for receptor-(over-)expressing tumours. DTPA derivatives of Leu¹- and dGlu¹-minigastrin were synthesised. All conjugates could be labelled with ¹¹¹In or ^88/90Y at high specific activities (8.5–44.4 GBq/μmol) and with high radiochemical purity. Serum stability testing was performed, and the labelled conjugates were compared concerning their stability against DTPA challenge. The biodistribution of the radiolabelled Leu¹- and dGlu¹-minigastrin derivatives was studied in tumour-bearing nude mice, in one healthy human volunteer and in three patients with metastatic medullary thyroid carcinoma. The transchelation of all tested radiometals to serum proteins was significantly slower with the DTPA-Glu conjugates as compared with their Leu analogues (e.g. transchelation t _1/2 of DTPA-dGlu¹-minigastrin vs its Leu¹ analogue at 37°C in human serum for ¹¹¹In: 239 h vs 91 h; for ⁹⁰Y: 130 h vs 53 h). In animals, all labelled CCK-B receptor ligands showed fast and specific uptake in CCK-B-receptor-positive tissues, such as the stomach and tumour, as well as a fast renal clearance pattern. However, DTPA-Leu¹-minigastrin showed higher background activity in the whole body and those organs known to accumulate the respective free radiometal (e.g. ⁸⁸Y-DTPA-Leu¹-minigastrin had bone uptake of 22%ID/g as compared to only 1.2%ID/g with its dGlu¹ analogue). In humans, fast tumour and stomach uptake was observed for both ¹¹¹In-labelled compounds, but DTPA-dGlu¹-minigastrin lacked the liver, spleen and bone marrow uptake observed with its Leu¹ analogue. In conclusion, anionic amino acid derivatives of DTPA may display improved metabolic stability as compared with monofunctional DTPA conjugates. DTPA-dGlu¹-minigastrin is preferred to "monofunctional" DTPA-Leu¹-minigastrin for diagnostic application with ¹¹¹In for the in vivo detection of CCK-B receptor-expressing tissues.