Comparison of myelotoxicities between ²²⁷Th-EDTMP and ²²³RaCl₂ in rats

Abstract

Thorium-227 (t_1/2 = 18.72 d) is a promising alpha-emitting radiotherapeutic nuclide for treatment of bone metastases. The daughter nuclide ²²³Ra, which have been used in clinical study for human, also decays via several alpha-emitting nuclides to stable ²⁰⁷Pb. The decay and growth patterns of radioactivity of ²²⁷Th and ²²³Ra after accumulation in bone are specific for each nuclide. We attempt here to evaluate and compare the myelotoxicities of ²²⁷Th-EDTMP and ²²³Ra in rats. Seven-week-old Sprague-Dawley rats were injected ²²⁷Th-EDTMP or ²²³RaCl₂. The body weight and blood cells (RBCs, WBCs, platelets) of each rat were counted before injection and more than 8 weeks after injection. A rat injected 8 MBq/kg b.w. of ²²³Ra died 9 day postinjection. The rats injected more than 250 kBq/kg b.w. of ²²³Ra showed decrease of WBCs and platelets. In the case of ²²⁷Th injection, a rat injected 8 MBq/kg b.w. died 8 day postinjection. On the other hand, the rats injected 4 and 2 MBq/kg b.w. died 4 and 6 weeks postinjection, respectively. The rats injected more than 500 kBq/kg b.w. showed weight loss and decrease of RBCs, WBCs, and platelets. The myelotoxicity of ²²⁷Th-EDTMP showed specific pattern and may correlate to the decay and growth pattern of ²²⁷Th and its progeny radioactivity.