Pleiotropic anti-cancer activities of novel non-covalent thioredoxin reductase inhibitors against triple negative breast cancer.

Mounting evidence shows that tumor growth and progression rely on thioredoxin reductase 1 (TXNRD1)-mediated detoxification of oxidative stress that results from deregulated metabolism and mitogenic signaling in tumors. TXNRD1 levels are significant higher in triple negative breast cancer (TNBC) compared to normal tissue, making TXNRD1 a compelling TNBC therapeutic target. Despite the many attempts to generate TXNRD1 inhibitors, all known and reported compounds inhibiting TXNRD1 are problematic; they interact with TXNRD1 irreversibly and non-specifically resulting in numerous adverse side effects. Recently, a series of breakthrough studies identified a novel regulatory site, the 'doorstop pocket', in Schistosoma mansoni thioredoxin glutathione reductase, a TXNRD-like enzyme and an established drug target for the human parasitic infection, schistosomiasis. This discovery underpins the development of new first-in-class non-covalent inhibitors for this family of enzymes. Our data show that novel non-covalent TXNRD inhibitors (TXNRD(i)s) are potent dose-dependent inhibitors of viability in cellular models of TNBC. TXNRD(i)s attenuate several aggressive cancer phenotypes such as, clonogenic survival, mammosphere forming efficiency, invasion, and TXNRD-related gene expression in TNBC cells. TXNRD(i)s engage and inhibit TXNRD1 in live TNBC cells and xenograft tumors, thus supporting the mechanism of action at a cellular level. More importantly, TXNRD(i)s attenuated tumor growth in a preclinical MDA-MB-231 TNBC xenograft mouse model. Although additional optimization for TXNRD(i)s' potency is warranted, these results may open a new avenue for the development of novel small molecule therapeutics for TNBC.