High-dose treatment with Re-186-HEDP or Sm-153-EDTMP combined with amifostine in a rabbit model

The aim of this experimental study was to investigate the myeloprotective p otential of amifostine in rabbits receiving high-dose treatment with either Sm-153-ethylenediaminetetramethylene phosphonate (EDTMP) or Re-186-hydroxy ethylidene diphosphonate (HEDP) and to check for drug interactions impairin g the skeletal uptake of these radiopharmaceuticals by amifostine. Methods: To a total of 24 rabbits, we administered 1,000 MBq of either Sm-153-EDTMP (n = 12) or Re-186-HEDP (n = 12). Six animals of each group received 500 m g amifostine intravenously 10-15 min before injection of the radiopharmaceu tical, whereas the other 6 animals served as controls. Up to 8 wk after tre atment, blood samples were collected every 3-5 d to measure platelet and le ukocyte counts. Furthermore, whole-body images were acquired at 3 min, 3 h, and 24 h after injection of the radiopharmaceutical to quantify the skelet al uptake. Results: For 186Re-HEDP, the mean decrease in platelets was sign ificantly less in the amifostine group (35.5% +/- 2.4%) than in the control group (61.3% +/- 5.4%, P < 0.001). Similar results were found for 153Sm-ED TMP (36.5% +/- 8.3% vs. 52.3% +/- 14.0%, P < 0.05). No significant differen ces in leukocyte counts were found for 186Re-HEDP (75.3% +/- 12.3% in the a mifostine group and 72.5% +/- 4.1% in the control group, P > 0.05), whereas rabbits treated with Sm-153-EDTMP plus amifostine showed a significantly g reater decrease in leukocytes (69.2% 10.8%) than did the control group (56. 6% +/- 4.0%, P < 0.05). Bone uptake in percentage of initial total whole-bo dy activity was significantly decreased in animals treated with amifostine compared with the control groups for both 186Re-HEDP (15.8% +/- 3.1% vs. 30 .9% +/- 1.9%, P < 0.001) and Sm-153-EDTMP (31.7% +/- 8.9% vs. 44.0% +/- 6.5 %, P < 0.05). Conclusion: For amifostine, we found a highly significant cyt oprotective effect on platelets but no leukoprotective effect. The latter p robably relies on the intrinsic myelotoxicity of high-dose amifostine, whic h seemed to potentiate the leukodepression of the radiopharmaceuticals. The lower bone uptake in amifostine-treated animals may be caused by the chemi cal structure of amifostine, which is a potentially complex-forming compoun d that may be able to displace bisphosphonates from the rhenium- and samari um-bisphosphonate complexes, resulting in altered biodistribution patterns.