This article is protected by German copyright law. You may copy and distribute this article for your personal use only. Other use is only allowed with written permission by the copyright holder. Radiochim. Acta 99, 43–51 (2011) / DOI 10.1524/ract.2011.1791  by Oldenbourg Wissenschaftsverlag, München Comparison of different phosphorus-containing ligands complexing 68 Ga for PET-imaging of bone metabolism By M. Fellner 1 , P. Riss 1 , N. Loktionova 1 , K. Zhernosekov 1 , O. Thews 2 , C. F. G. C. Geraldes 3 , Z. Kovacs 4 , I. Lukeˇ s 5 and F. Rösch 1 , ∗ 1 Institute of Nuclear Chemistry, Johannes Gutenberg University of Mainz, Fritz-Strassmann-Weg2, 55128 Mainz, Germany 2 Institute of Pathophysiology, Johannes Gutenberg University of Mainz, Duesbergweg 6, 55128 Mainz, Germany 3 Department of Life Science, Faculty of Science and Technology, and Center of Neurosciences and Cell Biology, University of Coimbra, 3001-401 Coimbra, Portugal 4 Advanced Imaging Research Center, University of Texas Southwestern Medical Center at Dallas, 5801 Forest Park Rd., Dallas, TX 75235-9085, USA 5 Department of Inorganic Chemistry, Charles University, Hlavova 2030, 12840 Prague, Czech Republic (Received June 22, 2009; accepted in final form August 19, 2010) Ga-68 / Macrocylic ligands / Phosphonates / Complex formation / Bone metastases Summary. 99m Tc-phosphonate structures are well estab- lished tracers for bone tumour imaging. Our objective was to investigate different 68 Ga-labelled phosphonate ligands concerning labelling kinetics, binding to hydroxyapatite and bone imaging using μ -PET. Seven macrocyclic phosphorus- containing ligands and EDTMP were labelled in nanomolar scale with n.c.a. 68 Ga in Na-HEPES buffer at pH ∼ 4. Ex- cept for DOTP, all ligands were labelled with > 92% yield. Binding of the 68 Ga-ligand complexes on hydroxyapatite was analysed to evaluate the effect of the number of the phos- phorus acid groups on adsorption parameters. Adsorption of 68 Ga-EDTMP and 68 Ga-DOTP was > 83%. For the 68 Ga- NOTA-phosphonates an increasing binding with increasing number of phosphonate groups was observed but was still lower than 68 Ga-DOTP and 68 Ga-EDTMP. μ -PET studies in vivo were performed with 68 Ga-EDTMP and 68 Ga-DOTP with Wistar rats. While 68 Ga-EDTMP-PET showed uptake on bone structures, an excess amount of the ligand (> 1.5 mg EDTMP/kg body weight) had to be used, otherwise the 68 Ga 3+ is released from the complex and forms gallium hy- droxide or it is transchelated to 68 Ga-transferrin. As a result, the main focus of further phosphonate structures has to be on complex formation in high radiochemical yields with macro- cyclic ligands with phosphonate groups that are not required for complexing 68 Ga. 1. Introduction Besides lung and liver, the bones are most frequently af- fected by metastases. About 60–80% of these metastases are caused by breast or prostate carcinoma. However, their symptoms (bone pain, pathological fracture, spinal cord compression, hypercalcaemia, bone marrow suppression) are recognized rather late [1, 2]. Hence a diagnosis of bone *Author for correspondence (E-mail: frank.roesch@uni-mainz.de). metastases in an early state together with a subsequent ther- apy is of great importance for patients. 99m Tc-phosphonates are well established tracers for the diagnosis of bone metastases using planar imaging or sin- gle photon emission tomography (SPECT) [3]. However, due to the higher spatial resolution of positron emission to- mography (PET), adequate pharmaceuticals with positron emitters would be of great potential. The superior imaging quality in the case of PET/CT imaging is clearly demon- strated by using 18 F-fluoride [4]. However, non-cyclotron dependent PET-tracers, i.e. radionuclide generator-based derivatives, would provide the required availability for in- stant tracer synthesis and PET/CT diagnosis. For this at- tempt the Germanium-68/Gallium-68 generator with the positron emitter 68 Ga (T 1/2 = 67.7 min) represents a promis- ing system. Using our recently developed generator post- processing, it is an excellent source for synthesizing and evaluating new tracers [5]. In the context of imaging bone metastases, several pa- pers have recently discussed the use of 68 Ga-EDTMP as a chemical analogue of e.g. 153 Sm-EDTMP, utilized for the palliative treatment of bone diseases [6 – 9]. In this study, we investigated several macrocylic phosphonate ligands in terms of 68 Ga-ligand complex formation and binding be- haviour to hydroxyapatite. The aim of the study was also to systematically study the efficiency of 68 Ga 3+ labelling of a series of phosphonate ligands of different structural char- acteristics (linear and triaza or tetraza macrocyclic ligands). In vivo bone uptake of some of those 68 Ga-phosphonates was also evaluated in a preliminary μ -PET imaging study on healthy Wistar rats. 2. Materials and methods 2.1 Generator Germanium-68 (T 1/2 = 270.8 d) provides the positron emit- ter Gallium-68 (T 1/2 = 67.7 min; 89% positron branching) as an easily available and relatively inexpensive source of