Glycoconjugated Re(CO)₃⁺ complexes: syntheses, characterization, cytotoxicity and cellular localization

Rhenium tricarbonyl complexes with the general formula fac-[Re(CO)₃(N,N')L]^0/+ (where N,N' = a bidentate polypyridyl ligand and L = a monodentate axial ligand) have been recognized over the past decade as potential anticancer therapeutics. In the present study, we introduced glycoconjugated diimine ligands into such complexes to enhance their water solubility, biocompatibility and bioavailability, and to target the overexpressed glucose transporters (GLUTs) on cancer cells. We prepared a series of water-stable Re(I) complexes (1-4) with the general formula [Re(CO)₃(N,N')L](CF₃SO₃), where N,N' = a glycoconjugated diimine (GluP, GluQ, and AcGluP) and L = imidazole (HIm) or indazole (HIn), and tested their in vitro cytotoxicity against a series of cancer cell lines. All complexes were characterized using electrospray ionization mass spectrometry, NMR spectroscopy and X-ray absorption spectroscopy. The structure of the diimine ligand (N,N') and the lipophilicity of the complexes were varied, e.g. by protecting the hydroxyl (-OH) groups of the glucose moiety with acetyl groups in complex 4, fac-[Re(CO)₃(β-AcGluP)(HIn)](CF₃SO₃), to compare potential GLUT-mediated transport versus passive diffusion uptake. In vitro cytotoxicity results revealed that the more lipophilic complexes 3, fac-[Re(CO)₃(GluQ)(HIn)](CF₃SO₃), and 4 have moderate cytotoxicity. X-ray fluorescence microscopy (XFM) allowed us to evaluate the cellular distribution of complexes 3 and 4 in A2780 ovarian cancer cells. Complex 3, which has unprotected sugar -OH groups, accumulated more in the nuclear/perinuclear area. In contrast, the more lipophilic complex 4 showed lower Re accumulation and a more uniform intracellular distribution. For the non-cytotoxic hydrophilic complex 1, fac-[Re(CO)₃(GluP)(HIm)](CF₃SO₃), XFM showed a much weaker intracellular Re signal, while ICP-MS detected significant uptake by HepG2 liver cancer cells. The observations suggest partial involvement of glucose transport pathways in the uptake of complexes 1 and 3.