Nitroimidazole conjugates of bis(thiosemicarbazonato)64Cu(II) – Potential combination agents for the PET imaging of hypoxia

Abstract

Combination agents comprising two different pharmacophores with the same biological target have the potential to show additive or synergistic activity. Bis(thiosemicarbazonato)copper(II) complexes (e.g. 64Cu-ATSM) and nitroimidazoles (e.g. 18F-MISO) are classes of tracer used for the delineation of tumor hypoxia by positron emission tomography (PET). Three nitroimidazole-bis(thiosemicarbazonato)copper(II) conjugates were produced in order to investigate their potential as combination hypoxia imaging agents. Two were derived from the known bifunctional bis(thiosemicarbazone) H2ATSM/A and the third from the new precursor diacetyl-2-(4-N-methyl-3-thiosemicarbazone)-3-(4-N-ethylamino-3-thiosemicarbazone) – H2ATSM/en. Oxygen-dependent uptake studies were performed using the 64Cu radiolabelled complexes in EMT6 carcinoma cells. All the complexes displayed appreciable hypoxia selectivity, with the nitroimidazole conjugates displaying greater selectivity than a simple propyl derivative used as a control. Participation of the nitroimidazole group in the trapping mechanism is indicated by the increased hypoxic uptake of the 2- vs. the 4-substituted 64Cu-ATSM/A derivatives. The 2-nitroimidazole derivative of 64Cu-ATSM/en demonstrated superior hypoxia selectivity to 64Cu-ATSM over the range of oxygen concentrations tested. Biodistribution of the radiolabelled 2-nitroimidazole conjugates was carried out in EMT6 tumor-bearing mice. The complexes showed significantly different uptake trends in comparison to each other and previously studied Cu-ATSM derivatives. Uptake of the Cu-ATSM/en conjugate in non-target organs was considerably lower than for derivatives based on Cu-ATSM/A.

A Comparison of the Behavior of 64Cu-Acetate and 64Cu-ATSM In Vitro and In Vivo

“…After 64 Cu-ATSM was incubated in serum in vitro, 10%-15% of the radioactivity was associated with precipitated protein after 0-2 h and not extracted into ethanol. This was in agreement with the reported results for 64 Cu-ATSM and related 64 Cu-bis(thiosemicarbazones) after incubation with serum, for which typically approximately 20% of activity was protein-bound (24,27,31). Radio-thin-layer chromatography (radio-TLC) and high-performance liquid chromatography confirmed that the ethanolextractable radioactivity contained only intact 64 Cu-ATSM in agreement with previous in vitro findings (24,27), indicating stability of the complex in serum in vitro.…”

Bifunctional Metal – Nitroimidazole Complexes for Hypoxia Theranosis in Cancer

“…Similar tumour uptakes were observed for 2 and 3, with corresponding values of 2.97 %ID/g and 2.56 %ID/g at 60 min post-injection [33]. The tumour/blood (T/B) and tumour/muscle (T/M) ratios at 60 min for complex 2 were 1.08 and 2.14 while those of complex 3 were 1.17 and 1.87, respectively [33].…”

“…In vitro studies showed rapid uptake of this compound by hypoxic cells, reaching a maximum at 5 min and remaining constant for up to 60 min [39]. Related compounds include conjugates derived from the diacetyl-bis(N 4 -methylthiosemicarbazone) (H2ASTM/A) platform linked to 2-or 4-nitroimidazole, and diacetyl-2-(4-N-methyl-3-thiosemicarbazone)-3-(4-N-ethylamino-3-thiosemicarbazone) (H2ASTM/en; 3, Figure 4) [33]. Cyclic voltammograms displayed quasi-reversible reduction waves centred at -0.60 V, which was attributed to the reduction of Cu II to Cu I .…”

“…Specifically, the transition metals (Cu, Re, Tc) can undergo facile oxygen-sensitive bioreduction to lower oxidation states under mild cellular hypoxia. These reductions occur at higher potentials (i.e., -0.6 V range) compared to the nitroimidazole moiety (> -1 V range) [33], and reduction of the metal core may occur before reduction of nitro group, an effect that may increase sensitivity but decrease selectivity for hypoxic tissue. Not many reports describing bifunctional conjugates containing 2-nitroimidazole and Re or Cu are available, but the roles of these metal cores in hypoxia-selective imaging and the therapy is valid, especially Re-186 and Re-188 which are used in the cancer patients' clinical management.…”

Neutral 99mTc(CO)3 complexes of “clicked” nitroimidazoles for the detection of tumor hypoxia

“…Nitroimidazoles which show selective accumulation in hypoxic cells are the most widely explored molecules for delineating hypoxic tumor cells from normoxic cells [10]. Several nitroimidazole radiopharmaceuticals based on PET as well as SPECT isotopes are reported [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. Presently, [ 18 F]fluoromisonidazole ([ 18 F]FMISO), a 2-nitroimidazole-radiotracer, is the radiopharmaceutical of choice for clinical imaging of tumor hypoxia [29].…”