Development of a novel 99mTc-chelate-conjugated bisphosphonate with high affinity for bone as a bone scintigraphic agent.
In bone scintigraphy using 99mTc with methylenediphosphonate (99mTc-MDP) and hydroxymethylenediphosphonate (99mTcHMDP), it takes 2-6 h after an injection before imaging can start. This interval could be shortened with a new radiopharmaceutical with higher affinity for bone. Here, based on the concept of bifunctional radiopharmaceuticals, we designed a 99mTc-mercaptoacetylglycylglycylglycine (MAG3)-conjugated hydroxy-bisphosphonate (HBP) (99mTc-MAG3-HBP) and a 99mTc-6-hydrazinopyridine-3-carboxylic acid (HYNIC)-conjugated hydroxy-bisphosphonate (99mTc-HYNIC-HBP).
Methods: 99mTc-MAG3-HBP was prepared by complexation of MAG3-HBP with 99mTc using SnCl2 as a reductant. The precursor of 99mTc-HYNIC-HBP, HYNIC-HBP, was obtained by deprotection of the Boc group after the coupling of Boc-HYNIC to a bisphosphonate derivative. 99mTc-HYNIC-HBP was prepared by a 1-pot reaction of HYNIC-HBP with 99mTcO4-, tricine, and 3-acetylpyridine in the presence of SnCl2. Affinity for bone was evaluated in vitro by hydroxyapatite-binding assays for 99mTc-HMDP, 99mTc-MAG3HBP, and 99mTc-HYNIC-HBP. Biodistribution experiments for the 3 99mTc-labeled compounds were performed on normal rats.
Results: 99mTc-MAG3-HBP and 99mTc-HYNIC-HBP were each prepared with a radiochemical purity of >95%. In the in vitro binding assay, 99mTc-MAG3-HBP and 99mTc-HYNIC-HBP had greater affinity for hydroxyapatite than 99mTc-HMDP. In the biodistribution experiments, 99mTc-MAG3-HBP and 99mTc-HYNIC-HBP had higher levels of radioactivity in bone than 99mTc-HMDP. 99mTc-MAG3-HBP was cleared from the blood slower than 99mTc-HMDP, whereas there was no significant difference in clearance between 99mTc-HYNIC-HBP and 99mTc-HMDP. Consequently, 99mTc-HYNIC-HBP showed a higher bone-to-blood ratio than 99mTc-HMDP.
Conclusion: We developed a novel 99mTc-chelate-conjugated bisphosphonate with high affinity for bone and rapid clearance from blood, based on the concept of bifunctional radiopharmaceuticals. The present findings indicate that 99mTc-HYNIC-HBP holds great potential for bone scintigraphy.