Tamoxifen is a long-known anti-tumor drug, which is the gold standard therapy in estrogen receptor (ER) positive breast cancer patients. According to previous studies, the conjugation of the original tamoxifen molecule with different functional groups can significantly improve its antitumor effect. The purpose of this research was to uncover the molecular mechanisms behind the cytotoxicity of different ferrocene-linked tamoxifen derivates. Tamoxifen and its ferrocene-linked derivatives, T5 and T15 were tested in PANC1, MCF7, and MDA-MB-231 cells, where the incorporation of the ferrocene group improved the cytotoxicity on all cell lines. PANC1, MCF7, and MDA-MB-231 express ERα and GPER1 (G-protein coupled ER 1). However, ERβ is only expressed by MCF7 and MDA-MB-231 cells. Tamoxifen is a known agonist of GPER1, a receptor that can promote tumor progression. Analysis of the protein expression profile showed that while being cytotoxic, tamoxifen elevated the levels of different tumor growth-promoting factors (e.g., Bcl-XL, Survivin, EGFR, Cathepsins, chemokines). On the other hand, the ferrocene-linked derivates were able to lower these proteins. Further analysis showed that the ferrocene-linked derivatives significantly elevated the cellular oxidative stress compared to tamoxifen treatment. In conclusion, we were able to find two molecules possessing better cytotoxicity compared to their unmodified parent molecule while also being able to counter the negative effects of the presence of the GPER1 through the ER-independent mechanism of oxidative stress induction.

Combination antitumor treatments are essential parts of modern tumor therapy as—compared to monotherapies—(i) they are more effective; (ii) the dose of the compounds can be reduced; and (iii) therefore the side effects are improved. Our research group previously demonstrated the antitumor character of bortezomib (BOZ) in A2058 melanoma cells. Unfortunately, dose-related side effects are common during BOZ therapy, which could be prevented by reducing the dose of BOZ. This study aimed to characterize synergistic combinations of BOZ with a TRAIL (TNF-related apoptosis-inducing ligand) -inducing compound (TIC10), where the doses can be cut down but the efficacy is preserved. Endpoint cell viability assays were performed on A2058 cells, and synergism of BOZ and TIC10 was observed after 72 h. Synergism was further validated in a real-time impedimetric assay, and our results showed that BOZ-treated melanoma cells survived the treatment, an effect not registered in the co-treatments. Treatment with the combinations resulted in increased apoptosis, which was not accompanied by enhanced LDH release. Nevertheless, the expression of death receptor 5 (DR5) was increased on the cell surface without transcriptional regulation. In summary, our findings support the theory that the application of BOZ and TIC10 in combination could provide higher efficacy in vitro.