Research On ～(99)Tc～m-labeled Zoledronic Acid And Its Derivative As The New Bone-imaging Agent

Zoledronic acid (ZL)[2-(imidazol-1-yl)-1-hydroxyethane-1,1-diphosphonic acid], belongs tothe third generation bisphosphonates. It has been widely used to cure hypercalcemia ofmalignancy(HCM), bone metastases, osteoporosis, and to prevent skeletal complication. It ispossible that ⁹⁹Tc^m-Zoledronate(⁹⁹Tc^m-ZL), which has not been reported at home and abroad, maybe an ideal bone-imaging agent. ⁹⁹Tc^m-ZL was prepared and evaluated as a bone-imaging agent inorder to find a new bone-imaging agent of our intellectual property.1H-imidazol-1-ylacetic acid, a key intermediate for the synthesis of ZL, was synthesized fromthe starting material ethyl bromoacetate. The optimum production condition of1H-imidazol-1-ylacetic acid was explored through orthogonal test. The results showed thatn(imidazole)/n(ethyl bromoacetate), W(catalyst) and reaction temperature were 0.8, 3% and 300Crespectively. ZL was synthesized by hydrolysis after reaction of 1H-imidazol-1-ylacetic acid withphosphorus trichloride and phosphoric acid. The total yield for synthesis of ZL from ethylbromoacetate was 28.76%.⁹⁹Tc^m-ZL was prepared by ZL reacted with Na⁹⁹Tc^mO4 in the presence of stannous chlorideSnCl₂Ë™2H₂O. The optimum labeling conditions were investigated. The results showed that when pHwas between 3 and 9, the amounts of ZL and SnCl2Ë™2H2O were more than 2mg and 40μgrespectively, and the labeling reaction continued 8min at room temperature, the radiochemicalpurity and labeling yield of ⁹⁹Tc^m-ZL were over 90%. After six hours at room temperature, theradiochemical purity of ⁹⁹Tc^m-ZL was still over 90%.Plasma protein binding ratio and partition coefficients of ⁹⁹Tc^m-ZL were determined. Plasmaprotein binding ratio of ⁹⁹Tc^m-ZL was 17.40%. Partition coefficients of ⁹⁹Tc^m-ZL were 18.13 and16.45 at pH=7.0 and 7.4, respectively.The biodistribution in mice and bone scan in rabbit of ⁹⁹Tc^m-ZL were studied. The resultsshowed that at 30min after injection of ⁹⁹Tc^m-ZL, the uptake in bone was up to maximum13.45%I.D/g and the bone-to-muscle and bone-to-blood uptake ratios were 28.01 and 16.96respectively;and at 180min after injection the uptake in bone was decreased to 8.55%I.D/g and thebone-to-muscle and bone-to-blood uptake ratios were 50.29 and 38.86 respectively. The clearanceof ⁹⁹Tc^m-ZL from blood had a biexponential pattern. The pharmacokinetic equation wasC=30.789e(-0.460 t)+1.553e(-0.0130 t) . The bone-imaging of rabbit can be obtained at 75min afterinjection, but the clearance of ⁹⁹Tc^m-ZL from liver was very slow.On the basis of the studies on ⁹⁹Tc^m-ZL,2-(2-methyl-imidazol-1-yl)-1-hydroxyethane-1,1-diphosphonic acid (ZLM), a derivative of ZL,was synthesized. ⁹⁹Tc^m-ZLM was also prepared and evaluated as a bone-imaging agent.ZLM was synthesized by two step reactions. Ethyl chloroacetate reacted with2-methyl-1H-imidazole to yield 2-methyl-1H-imidazole-1-ylacetic acid.2-methyl-1H-imidazole-1-ylacetic acid reacted with phosphorus trichloride and phosphoric acid togain ZLM. The total yield for synthesis of ZLM was 34.3%.99Tcm-ZLM was prepared by the reduction of Na99TcmO4 with SnCl2Ë™2H2O 80μg in thepresence of ZLM 5mg in aqueous solution of pH 7.0, the radiochemical purity and labeling yieldof 99Tcm-ZLM were over 90%. Plasma protein binding ratio of 99Tcm-ZLM was 18.88%. Partitioncoefficients of 99Tcm-ZLM were 9.32 and 7.44 at pH 7.0 and 7.4, respectively.The biodistribution in mice and bone scan in rabbit of 99Tcm-ZLM were also evaluated. Theresults indicated that at 120min after injection of 99Tcm-ZLM, the uptake in bone was up tomaximum 13.45%I.D/g and the bone-to-muscle and bone-to-blood uptake ratios were 47.74 and61.83 respectively;and at 180min after injection the uptake in bone decreased to 14.19%I.D/g andthe bone-to-muscle and bone-to-blood uptake ratios were 94.60 and 54.57 respectively. Theclearance of 99Tcm-ZLM from blood had a biexponential pattern. The pharmacokinetic equation wasC=18.849e(-0.254 t)+1.553e(-0.019 t). The clear bone-imaging of rabbit was obtained at 3h afterinjection.The biological character of 99Tcm-ZLM was better compared with 99Tcm-ZL. The preparationof 99Tcm-ZLM was convenient. 99Tcm-ZLM exhibited high uptake in bone, and will be a potentialbone-imaging agent.