The long-standing interest in thiosemicarbazones (TSCs)
has been
largely driven by their potential toward theranostic applications
including cellular imaging assays and multimodality imaging. We focus
herein on the results of our new investigations into: (a) the structural
chemistry of a family of rigid mono(thiosemicarbazone) ligands characterized
by extended and aromatic backbones and (b) the formation of their
corresponding thiosemicarbazonato Zn(II) and Cu(II) metal complexes.
The synthesis of new ligands and their Zn(II) complexes was performed
using a rapid, efficient and straightforward microwave-assisted method
which superseded their preparation by conventional heating. We describe
hereby new microwave irradiation protocols that are suitable for both
imine bond formation reactions in the thiosemicabazone ligand synthesis
and for Zn(II) metalation reactions. The new thiosemicarbazone ligands,
denoted HL, mono(4-R-3-thiosemicarbazone)quinone,
and their corresponding Zn(II) complexes, denoted ZnL2,
mono(4-R-3-thiosemicarbazone)quinone, where R = H,
Me, Ethyl, Allyl,
and Phenyl, quinone = acenapthnenequinone (AN), aceanthrenequinone
(AA), phenanthrenequinone (PH), and pyrene-4,5-dione (PY) were isolated
and fully characterized spectroscopically and by mass spectrometry.
A plethora of single crystal X-ray diffraction structures were obtained
and analyzed and the geometries were also validated by DFT calculations.
The Zn(II) complexes presented either distorted octahedral geometry
or tetrahedral arrangements of the O/N/S donors around the metal center.
The modification of the thiosemicarbazide moiety at the exocyclic
N atoms with a range of organic linkers was also explored, opening
the way to bioconjugation protocols for these compounds. The radiolabeling
of these thiosemicarbazones with64Cu was achieved under
mild conditions for the first time: this cyclotron-available radioisotope
of copper (t1/2= 12.7 h; β+ 17.8%;
β– 38.4%) is well-known for its proficiency in positron
emission tomography (PET) imaging and for its theranostic potential,
on the basis of the preclinical and clinical cancer research of established
bis(thiosemicarbazones), such as the hypoxia tracer64Cu-labeled
copper(diacetyl-bis(N4-methylthiosemicarbazone)],
[64Cu]Cu(ATSM). Our labeling reactions proceeded in high
radiochemical incorporation (>80% for the most sterically unencumbered
ligands) showing promise of these species as building blocks for theranostics
and synthetic scaffolds for multimodality imaging probes. The corresponding
“cold” Cu(II) metalations were also performed under
the mild conditions mimicking the radiolabeling protocols. Interestingly,
room temperature or mild heating led to Cu(II) incorporation in the
1:1, as well as 1:2 metal: ligand ratios in the new complexes, as
evident from extensive mass spectrometry investigations backed by
EPR measurements, and the formation of Cu(L)2-type species
prevails, especially for theAN-Phthiosemicarbazone
ligand (L–). The cytotoxicity levels of a selection
of ligands and Zn(II) complexes in this class were further tested
in commonly used human cancer cell lines (HeLa, human cervical cancer
cells, and PC-3, human prostate cancer cells). Tests showed that their
IC50levels are comparable to that of the clinical drug
cis-platin, evaluated under similar conditions. The cellular internalizations
of the selected ZnL2-type compounds Zn(AN-Allyl)2, Zn(AA-Allyl)2, Zn(PH-Allyl)2, and Zn(PY-Allyl)2were evaluated
in living PC-3 cells using laser confocal fluorescent spectroscopy
and these experiments showed exclusively cytoplasmic distributions.