Preparation Of And Animal Studies Involving A Novel Skeletal Imaging Agent,^99mTc-Labeled Ibandronate

Objectives: In this study,Tc-labeled ibandronate was preparedand its optimal labeling condition explored. The possibility of using^99mTc-labeled ibandronate as a skeletal imaging agent was investigated viacomparing^99mTc-labeled with^99mTc-MDP in terms of the in vivo distribution inmice and skeletal imaging in rabbits.Methods:^99mTc labeling of IBN was performed by using stannous chloride（SnCl₂·2H₂O） as a reducing agent; the radiochemical purity （RCP） of theproduct were determined by thin-layer paper chromatography （TLC）. Theeffects of pH value, the SnCl₂·2H₂O content, the ibandronic acid content, the^99mTcO4-activity, and the reaction time on the radioactive chemical purity of^99mTc-IBN, as well as the in vitro stability of^99mTc-IBN were observed. SixtyKunming mice were randomized into twelve groups of five. Each mouse wasinjected with0.2ml^99mTc-IBN solution （approximately0.74MBq） via thecaudal vein. At5min,10min,30min,60min,120min, and240min afterinjection, each group was subjected to blood sampling via the orbital vein anddecapitation; the heart, liver, spleen, lung, kidneys, muscle, and bone wereharvested, weighed, and counted for radioactivity with a γ counter. Then, thepercentage of tissue radioactivity count per gram compared to the total injecteddose （%ID/g） was calculated. The plasma protein binding rate and then-octanol-water partition coefficient of^99mTc-IBN were determined. Six adultrabbits were randomized into two groups of three. After successful anesthesiaand immobilization,0.5ml^99mTc-IBN （approximately185MBq） was injectedinto the rabbit’s ear rim vein, followed by bone scintigraphy. Dynamic imageswere acquired within60min after injection （5min/frame;12in a row）, and thewhole skeleton was imaged at1h,2h,3h, and4h after injection. A controlgroup was injected with^99mTc-MDP in all animal studies.Results:^99mTc-IBN was prepared successfully, with a RCP of up to97.67%± 0.24%. The optimal labeling conditions were determined as follows: pH=6.0,SnCl₂·2H₂O=100μg, IBN=5mg,^99mTcO-4=37MBq, room temperature=（25±2）°C, and reaction time=10min. Under the optimal labeling conditions,the prepared^99mTc-IBN was highly stable in vitro. After6h storage at roomtemperature, its radiochemical purity was>95%; therefore, the product met thestandards of a radioactive drug. A mouse study showed^99mTc-IBN uptake bybone to be specific and rapid, and the peak uptake （9.85±0.51）%ID/g wasobserved at30min after injection. The radioactivity was retained in bone for along time period. The uptake by the liver and spleen was low, and theradioactivity was eliminated rapidly from the blood and soft tissues,demonstrating a high bone/soft tissue ratio of radioactivity. The plasma proteinbinding rate of^99mTc-IBN was37.86%±0.39%. When pH values were7.0and7.4, the lipid-water partition coefficients were0.0024and0.0028, respectively.Rabbit skeletal imaging study showed that the background was eliminatednearly completely and the images of the spinal column, costal bones, andextremity bones were very distinct at2h after^99mTc-IBN injection. Comparedto^99mTc-MDP,^99mTc-IBN was characterized by rapid and high uptake by bone,rapid elimination from soft tissues, and low uptake by the liver and spleen. Theimaging performance of^99mTc-IBN at2h after injection was comparable to thatof^99mTc-MDP at3-4h after injection in rabbit skeletons.Conclusions:^99mTc-IBN can be prepared easily; it is highly stable and has goodbiological properties. Its imaging performance was comparable to that of^99mTc-MDP, while shortening the wait time after injection. Thus,^99mTc-IBNmay become a novel diagnostic and therapeutic skeletal imaging agent.