Preparation and evaluation of a radiogallium complex-conjugated bisphosphonate as a bone scintigraphy agent

(68)Ga is a radionuclide of great interest as a positron emitter for positron emission tomography (PET). To develop a new bone-imaging agent with radiogallium, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was chosen as a chelating site and Ga-DOTA complex-conjugated bisphosphonate, which has a high affinity for bone, was prepared and evaluated. Although we are interested in developing (68)Ga-labeled bone imaging agents for PET, in these initial studies (67)Ga was used because of its longer half-life.

DOTA-conjugated bisphosphonate (DOTA-Bn-SCN-HBP) was synthesized by conjugation of 2-(4-isothiocyanatebenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid to 4-amino-1-hydroxybutylidene-1,1-bisphosphonate (alendronate). (67)Ga-DOTA-Bn-SCN-HBP was prepared by coordination with (67)Ga, and its in vitro and in vivo evaluations were performed.

(67)Ga-DOTA-Bn-SCN-HBP was prepared with a radiochemical purity of over 95% without purification. (67)Ga-DOTA-Bn-SCN-HBP had great affinity for hydroxyapatite in binding assay. In biodistribution experiments, (67)Ga-DOTA-Bn-SCN-HBP accumulated in bone rapidly but was hardly observed in tissues other than bone. Pretreatment of an excess amount of alendronate inhibited the bone accumulation of (67)Ga-DOTA-Bn-SCN-HBP.

(67)Ga-DOTA-Bn-SCN-HBP showed ideal biodistribution characteristics as a bone-imaging agent. These findings should provide useful information on the drug design of bone imaging agents for PET with (68)Ga.