ERα-Independent Activity of Tamoxifen-Based Transition Metal Hybrids in Triple-Negative Breast Cancer Models In Vitro and In Vivo

Multiple studies have demonstrated that the conjugation of various metal cores to a modified tamoxifen vector amplifies its antitumor activity, rendering such engineered structures effective even in triple-negative breast cancer (TNBC), a tumor subtype traditionally considered irrelevant for endocrine therapy. With a focus on TNBC cell line, this study shows that hybrids with Pd- and Cu- in comparison to Pt-based counterparts exerted an advanced cytotoxic profile in terms of sustained cytotoxicity throughout all tested periods, well synchronized with an intensive and prolonged oxidative burst measured by 4-amino-5-methylamino-2′,7′-difluorofluorescein diacetate (DAF-FM), dihydroethidium (DHE), and dihydrorhodamine 123 (DHR-123) in the background. Translation to the orthotopic syngeneic mouse in vivo model confirmed their superiority toward Pt-based conjugates, as well as tamoxifen alone, with a more profound tumor-reducing potential of Cu-tamoxifen, which was finally restricted by its toxicity. Surprisingly, the tamoxifen vector per se, with an approx. 2-fold lower cytotoxic potential than Pt- and Cu-hybrids in vitro, showed exceptional tumor-reducing potential in vivo, profiled in the last days of the treatment period. Intensive infiltration of immune cells, preferentially lymphocytes, was observed in tumor samples from animals exposed to the tamoxifen vector, underscoring the ligand’s immune potential and again suggesting that cytotoxicity is not a measure of successful treatment.