A racemosin B derivative, C25, suppresses breast cancer growth via lysosomal membrane permeabilization and inhibition of autophagic flux.

Breast cancer is the most common malignancy among women worldwide. As conventional therapies are only partially successful in eradicating breast cancer, the development of novel strategies is a top priority. We previously showed that C25, a new racemosin B derivative, exerts its anti-cancer activity through inhibition of autophagy, but the underlying mechanism remained unknown. Here we show that C25 inhibits the growth of diverse breast cancer cell subtypes and effectively suppresses tumor progression in a xenotransplantation model of triple negative breast cancer. C25 acts as a lysosomotropic agent to induce lysosomal membrane permeabilization and inhibit autophagic flux, resulting in cathepsin release and cell death. In accordance, RNA sequencing and gene set enrichment analysis revealed that C25 induces pathways consistent with autophagy inhibition, cell cycle arrest and senescence. Interestingly, knockdown of TFEB or SQSTM1 reduced cell death induced by C25 treatment. Finally, we show that C25 synergizes with the chemo-therapeutics etoposide and paclitaxel to further limit breast cancer cell growth. Thus, C25 alone or in combination with other anti-neoplastic agents offers a novel therapeutic strategy for aggressive forms of breast cancer and possibly other malignancies.