LAUSR

Triple-negative breast cancer (TNBC) has a poor prognosis and new drugs are urgently needed. The CYP-derived ω-3 epoxyeicosapentaenoic acid and its saturated analogues were found to activate apoptosis in MDA-MB-231 TNBC cells [1] , [2] , but in vivo instability limits their drug potential. We prepared stable bioisosteres of ω-3 epoxy-fatty acids for testing in TNBC cells in vitro and in vivo. Cell viability was assessed by ATP formation, cell killing by caspase-3/7 activity and annexin V/7AAD staining, mitochondrial membrane potential with JC-1, gene profiling by RNA-seq and real-time RT-PCR, and protein expression by immunoblotting. In vivo tumour growth was evaluated in nude mice carrying intramammary xenografts. Of 11 bioisosteres, one agent – CTU – impaired ATP formation, activated apoptosis and disrupted the mitochondrion in TNBC cells. RNA-seq profiling identified 382 differentially-expressed genes (⩾2-fold) in CTU-treated cells with selective enrichment of genes for endoplasmic reticulum (ER) stress (XBP-1 and CHOP). The ER-stress inhibitors, AEBSF and toyocamycin, attenuated caspase-3/7 activation by CTU. MDA-MB-231 xenografts in nude mice were decreased to 42 ± 13% of control after CTU (40 mg/kg i.p., 7 weeks). Increased TUNEL and Ki-67 staining in tumours is consistent with CTU-mediated apoptosis and decreased proliferation in vivo. CTU acts by a unique mechanism to target the mitochondrion and activate ER-stress, and is the prototype of a new class of agents based on naturally occurring lipids with activity in TNBC.