Ferrocifens labelled with an infrared rhenium tricarbonyl tag: synthesis, antiproliferative activity, quantification and nano IR mapping in cancer cells.

Antiproliferative activities of several members of the ferrocifen family, both in vitro and in vivo, are well documented although their precise location in cancer cells has not yet been elucidated. However, two different infrared imaging techniques have been used to map the non-cytotoxic cyrhetrenyl analogue of ferrociphenol in a single cell. This observation prompted us to tag two ferrocifens with a cyrhetrenyl unit [CpRe(CO)3; Cp = η5-cyclopentadienyl] by grafting it, via an ester bond, either to one of the phenols (4, 5) or to the hydroxypropyl chain (6). Complexes 4-6 retained a high cytotoxicity on breast cancer cells (MDA-MB-231) with IC50 values in the range 0.32-2.5 μM. Transmission IR spectroscopy was used to quantify the amount of cyrhetrenyl tag present in cells incubated with 5 or 6. The results show that after a 1-hour incubation of cells at 37 °C, complexes 5 and 6 are mainly present within cells while only a limited percentage, quantified by ICP-OES, remained in the incubation medium. AFM-IR spectroscopy, a technique coupling infrared irradiation with near-field AFM detection, was used to map the cyrhetrenyl unit in a single MDA-MB-231 cell, incubated at 37 °C for 1 hour with 10 μM of 6. The results show that signal distribution of the characteristic band of the Re(CO)3 entity at 1950 cm-1 matched those of amide and phosphate, thus indicating a location of the complex mainly in the cell nucleus.