Preparation and Biodistribution of [67Ga]-labeled- oxytocin for SPECT purposes

Oxytocin (OT) is a paracrine hormone with various biological activities and many sex organs in both sexes, as well as many tumor cells have shown to have related receptors. In this study the development of a receptor imaging tracer for possible tumor imaging has been described. OT was successively labeled with [67Ga]-gallium chloride after conjugation with freshly prepared cyclic DTPA-dianhydride. The best results of the conjugation were obtained by the addition of 1 ml of a OT pharmaceutical solution (2 mg/ml, in phosphate buffer, pH=8) to a glass tube pre-coated with DTPA-dianhydride (0.02 mg) at 25°C with continuous mild stirring for 30 min. Radiochemical purity (RCP) of the labeled compound was determined, using RTLC and ITLC followed by stability tests and animal biodistribution studies. Radiolabeling took about 60 minutes with a RCP higher than 98 % at optimized conditions (specific activity = 1000 Ci/mM, labeling efficiency 80%). The stability of the tracer at room temperature was significant, up to an hour. Preliminary in vivo studies in normal female rat model showed ovary/blood and ovary/muscle ratio uptake of the tracer in 60 minutes to be 4.53 and 9.18, respectively. The result was consistent with the reported OT receptor distribution in normal female mammals. The radiolabeled oxytocin, prepared in this study, was a possible fast acting tracer for OT receptor imaging; studies however, more studies are required to determine the best imaging conditions especially in larger mammal animals.