The aim of this work was to investigate the mechanism of action of
ferrocifen (Fc-
OH-TAM), the ferrocenyl analog of
4-hydroxy-tamoxifen (
OH-TAM), which is the active metabolite of
tamoxifen, the
drug most widely prescribed for treatment of
hormone-dependent breast
cancers. Fc-
OH-TAM showed an anti-proliferative effect on the six
breast cancer cell lines tested, 3
ERalpha positive (MCF-7, T-47D, ZR-75-1) and 3
ERalpha negative (MDA-MB-231, SKBR-3, Hs578-T) whatever their ER (
estrogen receptor) status. However, the mechanism of action of the ferrocenyl derivative appeared to differ depending on the status of the
ERalpha. Analysis of cell cycle distribution revealed that Fc-
OH-TAM first recruits cells in the S phase in both
ERalpha positive and
ERalpha negative cells. In the presence of
ERalpha, Fc-
OH-TAM allowed cell cycle progression, with a subsequent blockade in G0/G1, whereas in the absence of
ERalpha, cells remained in the S phase. Significant production of ROS was observed only in the presence of Fc-
OH-TAM in both
ERalpha positive and negative
breast cancer cell lines. Within our experimental conditions, this ROS production is associated with cell cycle arrest and senescence rather than apoptosis. In the presence of
ERalpha, Fc-
OH-TAM seems to mainly act in the same way as
OH-TAM but also induces an additional cytotoxic effect not mediated by the receptor. Our data suggest that this cytotoxic effect of Fc-
OH-TAM is expressed via a mechanism of action distinct from the non-genomic pathway observed with high doses of
OH-
Tamoxifen.