Assessment of Re chelate-conjugated bisphosphonate for the development of new radiopharmaceuticals for bones

Uehara, Tomoya*; Jin, Z. L.*; Ogawa, Kazuma*; Akizawa, Hiromichi*; Hashimoto, Kazuyuki; Nakayama, Morio*; Arano, Yasushi*

In this study, a key factor affecting the pharmacokinetics of a chelate-conjugated BP was investigated to estimate the validity and the applicability of molecular design. Chemically inert and well-characterized [Re]CpTR-Gly was conjugated with 3-amino-1-hydroxypropylidene-1,1-bisphosphonate and purified by HPLC to prepare [Re]CpTR-Gly-APD. Plasma stability, plasma protein binding, hydroxyapatite (HA) binding and the pharmacokinetics of [Re]CpTR-Gly-APD were compared with those of Re 1-hydroxyethylidene-1,1-diphosphonate (HEDP). The HPLC-purified [Re]CpTR-Gly-APD showed higher plasma stability, higher HA binding, higher bone accumulation and lower plasma protein binding than did Re -HEDP. Although Re -HEDP possessed HA binding and bone accumulation similar to those of [Re]CpTR-Gly-APD, the specific activity of Re -labeled BPs was found to play a crucial role in bone accumulation and blood clearance. Thus, the molecular design of chelate-conjugated BP would be useful for the development of bone-seeking radiopharmaceuticals with a variety of radionuclides by selecting chelating molecules that provide high specific activities.