Rhemium-186-1-hydroxyethylidene-1,1-diphosphonate (
186Re-HEDP) has been used for the palliation of metastatic bone pain. However, delayed blood clearance and high gastric uptake of radioactivity have been observed upon injection, due to the instability of
186Re-HEDP. We designed, synthesized and evaluated a stable
186Re-mercaptoacetylglycylglycylglycine (MAG3) complex-conjugated bisphosphonate, [[[[(4-hydroxy-4,4-diphosphonobutyl)carbamoylmethyl] carbamoylmethyl]carbamoylmethyl]carbamoylmethanethiolate] oxorhenium(V) (
186Re-MAG3-HBP). The stability of
186Re-MAG3-HBP and
186Re-HEDP in phosphate buffer were compared. No measurable decomposition of
186Re-MAG3-HBP occurred, while only approximately 30% of
186Re-HEDP remained intact 24 hours post-incubation. In biodistribution experiments, the radioactivity level of
186Re-MAG3-HBP in bone was significantly higher than that of
186Re-HEDP. Blood clearance of
186Re-MAG3-HBP was faster than that of
186Re-HEDP. In addition, the gastric accumulation of
186Re-MAG3-HBP radioactivity was lower. To evaluate the therapeutic effects of
186Re-MAG3-HBP, an animal model of bone metastasis was prepared. In the rats treated with
186Re-HEDP, tumor growth was comparable to that in untreated rats. In contrast, when
186Re-MAG3-HBP was administered, tumor growth was significantly inhibited. Bone pain was attenuated by treatment with
186Re-MAG3-HBP or
186Re-HEDP, but
186Re-MAG3-HBP tended to be more effective. These results indicate that
186Re-MAG3-HBP could be useful as a therapeutic agent of metastatic bone pain. Moreover, based on the similar concept, we designed, synthesized, and evaluated a
99mTc-6-hydrazinopyridine-3-carboxylic acid-conjugated bisphosphonate (
99mTc-HYNIC-HBP) as a bone scintigraphic agent.
99mTc-HYNIC-HBP gave higher levels of radioactivity in bone than
99mTc-HMDP. There was no significant difference in clearance from blood between
99mTc-HYNIC-HBP and
99mTc-HMDP. Consequently,
99mTc-HYNIC-HBP showed a higher bone-to-blood ratio than
99mTc-HMDP. The findings indicate that
99mTc-HYNIC-HBP holds great potential for bone scintigraphy.
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