Synthesis, characterization and biological evaluation of tricarbonyl M(i) (M = Re, 99mTc) complexes functionalized with melanin-binding pharmacophores

Abstract

Aiming to evaluate their potential as radioactive probes for in vivo targeting of melanotic melanoma and its metastases, we have synthesized ^99mTc(I) tricarbonyl complexes (Tc1–Tc8) anchored by pyrazolyl-containing chelators with (N₃) or (N₂O) donor atom sets and functionalized with 2-aminoethyldiethylamine and 4-amino-N-(2-diethylaminoethyl)benzamide groups as melanin-binding pharmacophores. The chemical identification of the several ^99mTc complexes has been accomplished by HPLC comparison with the Re congeners (Re1–Re8), which were synthesized at the macroscopic level and fully characterized by common analytical techniques. The biological evaluation of the ^99mTc(I) complexes comprised the determination of their in vitro binding to synthetic melanin, measurement of cellular uptake in B16F1 murine melanoma cells, as well as biodistribution studies in B16F1 melanoma-bearing mice. All the tested complexes have shown a moderate to high in vitro affinity to melanin, with percentages of binding spanning between 60 and 94%. In agreement with the poor cellular uptake measured in vitro, the in vivo tumor uptake of the complexes was in general relatively low, ranging between 0.12 and 1.69% ID g⁻¹ at 4 h p.i. However, some complexes have shown favorable tumor-to-organ ratios with values as high as 28 and 5.3 for tumor–muscle and tumor–blood ratios, respectively. This seems to indicate that some selectivity towards melanoma tissue was conserved, and encourages further optimization of the in vitro/in vivo biological properties of this type of complexes aimed at finding novel radioactive probes for non-invasive imaging of melanoma.