Studies on the preparation and stability of samarium-153 propylene diamine tetramethylene phosphonate (PDTMP) complex as a bone seeker

Preliminary absorbed dose evaluation of two novel 153Sm bone-seeking agents for radiotherapy of bone metastases: comparison with 153Sm-EDTMP

Aim

The amount of energy deposited on any organ by ionising radiation termedabsorbed dose, plays an important role in evaluating the risks associated with the administration of radiopharmaceuticals. In this research work, the absorbed dose received by human organs for153Sm-TTHMP and153Sm-PDTMP was evaluated based on biodistribution studies on the Syrian rats.

Materials and methods

153Sm-TTHMP and153Sm-PDTMP were successfully prepared with radiochemical purity of higher than 99%. The biodistribution of the complexes was investigated within the Syrian rats up to 48 hours post injection. The human absorbed dose of the complexes was estimated by the radiation dose assessment resource method.

Results

The highest absorbed dose for153Sm-TTHMP and153Sm-PDTMP was observed in the trabecular bone with 1·085 and 1·826 mGy/MBq, respectively. The bone to other critical organ dose ratio for153Sm-PDTMP is significantly greater than153Sm-TTHMP. Also, the bone/red marrow dose ratio for these complexes is comparable with this ratio for153Sm-EDTMP, as the most clinically used Sm-153 bone pain palliative radiopharmaceutical.

Findings

According to the considerable bone absorbed dose against the insignificant absorbed dose of non-target organs, these complexes can be used as potential bone pain palliative agents in clinical applications.

Development of (166)Holmium-1,2 Propylene Di-amino Tetra (Methy1enephosphonicacid) as a Possible Bone Palliation Agent

¹⁶⁶Holmium-1,2-propylene di-amino tetra (methy1enephosphonicacid) (¹⁶⁶ Ho-PDTMP) complex was prepared successfully using an in-house synthesized PDTMP ligand and ¹⁶⁶ HoCl ₃ . Ho-166 chloride was obtained by thermal neutron irradiation (1 × 10 ¹³ n/cm ² /s) of natural Ho (NO ₃ ) ₃ samples (specific activity = 3-5 GBq/mg), dissolved in acidic media. Radiochemical purity of ¹⁶⁶ Ho-PDTMP was checked by instant thin layer chromatography (>99%). Stability studies of the complex in the final preparation and in the presence of human serum were performed up to 72 h. The biodistribution of ¹⁶⁶ Ho-PDTMP and ¹⁶⁶ HoCl ₃ in wild-type rats was checked in animal tissues up to 48 h. The produced ¹⁶⁶ Ho-PDTMP properties suggest a possible new bone palliative therapeutic to overcome the metastatic bone pains.

175Yb labeled polyaminophosphonates as potential agents for bone pain palliation

Ytterbium-175 (T1/2 = 4.2 d, Ebeta(max) = 480 keV) has radionuclidic properties suitable to be used in palliative therapy of bone metastases. 175Yb can be produced in moderate specific activity and good radionuclidic purity by thermal neutron bombardment of natural Yb target. The analysis of the neutron irradiated sample exhibited the presence of 96.2% 175Yb along with 2.1% 169Yb and 1.7% 177Lu at 6 h post-EOB. Four polyaminomethylene phosphonic acid ligands, ethylenediamine tetramethylene phosphonic acid, propylenediamine tetramethylene phosphonic acid, triethylenetetramine hexamethylene phosphonic acid and diethylenetriamine pentamethylene phosphonic acid were synthesized and radiolabeled with 175Yb. Complexation parameters were optimized to achieve maximum yields (92-99%). All complexes were found to retain their stability at room temperature even after 10d of preparation. Biodistribution studies of the complexes carried out in Wistar rats showed significant bone uptake (3-4.4%/g in tibia at 3h post-injection) with rapid clearance from blood and minimum uptake in soft tissues for all the complexes (bone/blood ratio approximately 40-150 and bone/muscles ratio approximately 40-400 at 3 h post-injection). These studies suggest that 175Yb complexes with the phosphonate ligands have potential for use in palliative treatment of painful bone metastases.