Synthesis and evaluation of a novel 68Ga-chelate-conjugated bisphosphonate as a bone-seeking agent for PET imaging

Abstract

(68)Ga is a positron-emitting nuclide that has significant imaging potential given that, unlike cyclotron-produced (18)F, the isotope can be produced on-site utilizing a (68)Ge/(68)Ga generator. We recently synthesized a novel bone-seeking agent by coupling a bisphosphonate with the (68)Ga chelator 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). This study presents a first report on the potential of this (68)Ga bone-seeking radiopharmaceutical in the detection of bone metastases. 4-Amino-1-hydroxybutylidene-1,1-bisphosphonate was conjugated with 2-[4,7-di(carboxymethyl)-1,4,7-triazonan-1-yl]pentanedioic acid, yielding 2-[4,7-di(carboxymethyl)-1,4,7-triazonan-1-yl]-5-[(4-hydroxy-4,4-diphosphonobutyl)amino]-5-oxopentanoic acid (NOTA-BP). (68)Ga-labeled NOTA-BP ([(68)Ga]NOTA-BP) was prepared by complexation of NOTA-BP with [(68)Ga] gallium chloride and evaluated in in vitro experiments, biodistribution experiments and micro-positron emission tomography (PET) imaging experiments. The labeling of NOTA-BP with (68)Ga was completed by heating for 10 min. [(68)Ga]NOTA-BP was determined to have a radiochemical purity of over 95%, a high affinity for hydroxyapatite and a high stability in plasma. In in vivo biodistribution experiments, [(68)Ga]NOTA-BP demonstrated high bone uptake potential. Compared with (99m)Tc-labeled methylene diphosphonate ([(99m)Tc]MDP) and [(18)F]fluoride, [(68)Ga]NOTA-BP exhibited faster blood clearance and a higher bone-to-blood ratio. In addition, mouse model bone metastasis was detected by micro-PET imaging at 1 h postinjection of [(68)Ga]NOTA-BP. We have developed a novel (68)Ga-radiolabeled bone-seeking agent. This [(68)Ga]NOTA-BP complex was found to have a high bone affinity and rapid blood clearance, and may thus prove to be useful as a bone-seeking agent for clinical PET.