Enhancement of Breast Cancer Chemotherapy and Radiotherapy Responses by Modulation of Ferroptosis

Breast cancer is the most common cancer in women in the UK. Surgery, radiotherapy, hormone therapy and chemotherapy are all current therapies. The success rate of chemotherapy and radiotherapy are affected by intrinsic insensitivity, and acquired resistance, and novel chemo-sensitisation and radio-sensitisation strategies are currently being researched. Ferroptosis is an iron-dependent program cell death pathway characterised by massive accumulation of reactive oxygen species. Reactive oxygen species accumulation and subsequent lipid peroxidation-mediated death is Fe2+-dependent, leading to a form of cell death that is distinct from apoptosis. Since it is known that both chemotherapy and radiotherapy exert some ferroptosis-mediated effects, it was hypothesised that chemotherapy responses and radiotherapy responses would be enhanced by co-treatment with ferroptosis inducers. To test the co-treatment with ferroptosis inducers in a disease relevant model, both 2D standard cell culture and 3D tumour spheroids in two breast cancer cell lines were assessed. Ferroptosis inducers affected MDA-MB-231, but not MCF-7. Ferroptosis inducers did not have a robust enhancement of Doxorubicin responses, although more promising responses were observed with Cisplatin but only in MDA-MB-231 and only in 2D cell culture, not in spheroids. Since ferroptosis is controlled in-part by Nrf2, the Nrf2-inhibitor ML385 was tested, which partially enhanced chemotherapy responses, and also identified a specific drug combination vulnerability when combined with the ferroptosis inducer RSL3, whereby MDA-MB-231 cells did not respond potently to RSL3 plus Doxorubicin, but were sensitive when ML385 was added. This was not observed in MCF-7. In spheroids, responses to RSL3 were heterogeneous in that adjacent spheroids showed differential responses to RSL3, which is a novel finding and may identify a resistance mechanism. Radiotherapy responses were not robustly enhanced by ferroptosis inducers. Despite the being a strong theoretical case for combining ferroptosis inducers with chemotherapy or radiotherapy, responses in these two cell lines were promising at times, but data does not strongly support further study in pre-clinical models.