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Preparation and Biological Distribution of Technetium Diphosphonate Radiotracers Synthesized Without Stannous Ion

University of Cincinnati and Miami Valley Laboratories, Procter & Gamble Co., Cincinnati, Ohio

Correspondence: For reprints contact: Edward Deutsch, PhD, Dept. of Chemistry, Univ. of Cincinnati, Cincinnati, OH 45221.

ABSTRACT

Two HEDP complexes of technetium (either Tc-99 or a mixture of Tc-99 and Tc-99m) have been prepared without the use of stannous ion. The first, Tc(NaBH₄)-HEDP, is synthesized by reduction of TcO₄^- with NaBH₄ in the presence of excess HEDP; this is analogous to the preparation of Tc(Sn)-HEDP in commercial kits wherein Sn(II) functions as the reductant. The second, Tc-HEDP, is prepared by substitution of HEDP onto the pre-formed, pre-reduced, technetium center TcBr₆^2-. The HEDP-to-Tc ratio in Tc-HEDP was found to be 1.0 by double-labeling procedures (Tc-99 and [³H]HEDP), implying that in solution this material is polymeric or at least dimeric. Preparations of Tc(NaBH₄)-HEDP and Tc-HEDP with Tc-99m are excellent bone-imaging agents in both rats and dogs. Tissue distribution studies in rats show that uptake of Tc(NaBH₄)-HEDP and Tc-HEDP by the bone is at least equivalent to that achieved by Tc(Sn)-HEDP prepared in commercial kits with Sn(II) as the reductant. Tin is therefore not necessary for the bone-seeking properties of Tc(Sn)-HEDP, and the in vivo distribution of a given HEDP radiotracer seems to depend primarily on the presence of the HEDP ligand and not on the exact nature of the technetium complex itself. Synthesis of technetium radiotracers by a substitution route, rather than by redox, is practicable; this route has the potential of introducing hitherto unattainable flexibility and subtlety into the preparation of technetium radiotracers.