Characterization of novel histidine-tagged tat-peptide complexes dual-labeled with 99mTc-tricarbonyl and fluorescein for scintigraphy and fluorescence microscopy

To enable concurrent whole body scintigraphy and direct imaging of subcellular localization of permeation peptides, dual-labeled Tat-peptides useful for both radiometric analysis and fluorescence microscopy are desired for molecular imaging applications. Thus, novel dual-labeled D-Tat-peptides comprising Tat-basic domain (hgrkkrrqrrrgc), C-terminus conjugated with fluorescein-5-maleimide (FM) and N-terminus chelated with [Tc-99m(CO)(3)] via histidine coordination, were synthesized and characterized. In human Jurkat cells, radiotracer uptake and washout studies revealed concentration-dependent accumulation of the dual-labeled Tat-peptide within cells. Subcellular localization of Tatpeptide was confirmed by fluorescence microscopy using an analogous [Re(CO)(3)] dual-labeled Tatpeptide. As seen with C-terminus single-labeled Tat-peptides, localization to the nucleoli was observed with the dual-labeled Tat-peptide, suggesting that the mechanism of Tat-peptide uptake and localization was not dependent on free peptide termini at either end. In Balb/c mice, biodistribution studies performed with the dual-labeled Tat-peptide showed fluorescence intensity by microscopic analysis that visually confirmed and correlated directly with scintigraphic and radiometric data. Of note, following intravenous administration, little brain penetration of these permeation sequences was observed in vivo. His[Tc-99m(CO)(3)]-, DTPA[Tc-99m(CO)(3)]-, and (is an element of)-lys-gly-cys [Tc-99m(O)]-labeled Tatpeptides showed significant pharmacokinetic differences in liver and kidney depending on labeling strategy, indicating that Tat-peptide biodistribution can be impacted by the chelation moiety coordinated with Tc-99m. Thus, we have shown that dual-labeled Tc-99m-tricarbonyl Tat-peptide-FM conjugates can be conveniently synthesized and enable direct comparison of quantitative radiometric and qualitative fluorescence data both in vitro as well as in vivo.