Water-Soluble [Tc(N)(PNP)] Moiety for Room-Temperature ^99mTc Labeling of Sensitive Target Vectors

The incorporation of bioactive molecules into a water-soluble [^99mTc][Tc(N)(PNP)]-based mixed compound is described. The method, which exploits the chemical properties of the new [^99mTc][Tc(N)(PNP3OH)]²⁺ synthon [PNP3OH = N,N-bis(di-hydroxymethylenphosphinoethyl)methoxyethylamine], was successfully applied to the labeling of small, medium (cysteine-functionalized biotin and c-RGDfK pentapeptide), and large molecules. Apomyoglobin was chosen as a model protein and derivatized via site-specific enzymatic reaction catalyzed by transglutaminase (TGase) with the H-Cys-Gly-Lys-Gly-OH tetrapeptide for the insertion in the protein sequence of a reactive N-terminal Cys for ^99mTc chelation. Radiosyntheses were performed under physiological conditions at room temperature within 30 min. They were reproducible, highly specific, and quantitative. Heteroleptic complexes are hydrophilic and stable. Biodistributions of the selected compounds show favorable pharmacokinetics within 60 min post-injection and predominant elimination through the renal-urinary pathway. In a wider perspective, these data suggest a role of the [^99mTc][Tc(N)(PNP)] technology in the labeling of temperature-sensitive biomolecules, especially targeting proteins for SPECT imaging.