New Bioconjugated Technetium and Rhenium Folates Synthesized by Transmetallation Reaction with Zinc Derivatives

The zinc dithiocarbamates functionalized with folic acid 2 and 3 were synthesized with a simple straightforward method, using an appropriated folic acid derivative and a functionalized zinc dithiocarbamate (1). Zinc complexes 2 and 3 show very low solubilities in water, making them useful for preparing Tc-99m radiopharmaceuticals with a potentially high molar activity. Thus, the transmetallation reaction in water medium between the zinc complexes 2 or 3 and the cation fac-[^99mTc(H₂O)₃(CO)₃]⁺, in the presence of the monodentate ligand TPPTS, leads to the formation of the 2 + 1 complexes fac-[^99mTc(CO)₃(SS)(P)] bioconjugated to folic acid (2 and 3). In spite of the low solubility of 2 and 3 in water, the reaction yield is higher than 95%, and the excess zinc reagent is easily removed by centrifugation. The Tc-99m complexes were characterized by comparing their HPLC with those of the homologous rhenium complexes (2 and 3) previously synthesized and characterized by standard methods. Preliminary in vivo studies with 2 and 3 indicate low specific binding to folate receptors. In summary, Tc-99m folates 2 and 3 were prepared in high yields, using a one-pot transmetallation reaction with low soluble zinc dithiocarbamates (>1 ppm), at moderate temperature, without needing a subsequent purification step.