Expanding the therapeutic potential of TXNRD inhibitors for triple-negative breast cancer

Triple negative breast cancer (TNBC) has the lowest 5-year survival of all breast cancersubtypes, and for many patients the standard of care is limited to a multiagent chemotherapyregimen. Therefore, there is a need to explore improved therapeutic targets and strategies. We havepreviously demonstrated that elevated levels of both thioredoxin reductase 1 (TXNRD1) andthioredoxin reductase 2 (TXNRD2) correlates with lower recurrence-free and overall survival inTNBC and are both relevant targets for TNBC.We have developed new non-covalent TXNRD inhibitors (TXNRD(i)s) that successfullyinhibit TXNRD1 and TXNRD2 in TNBC models as demonstrated through fluorescent biochemicalassays and qPCR of TXNRD related genes. Additionally, these TXNRD(i)s show pleiotropic anti-cancer activity against aggressive phenotypes in both in vitro and in vivo TNBC models. Treatmentwith TXNRD(i)s reduces 2D viability, mammosphere formation, collagen invasion, and primaryxenograft growth in TNBC models.Mechanistically, we have identified ribonucleotide reductase (RNR) dysfunction causedby our TXNRD(i)s. Given that RNR provides dNTPs, the building block for both DNA synthesisand repair, this led to the hypothesis that TXNRD(i)s induce DNA damage and replicative stress.Furthermore, this is aggravated by poly (ADP-ribose) polymerase (PARP) inhibition, leading toa synergistic effect in BRCA wild-type TNBC cell lines. Using COMET and fork stall assays,microscopy, and western blotting, we concluded there is evidence of replicative stress and DNAdamage induced by our TXNRD(i)s and validated the inhibitors are acting on target by using agenetic approach with inducible TXNRD1 and TXNRD2 knockdowns of our model cell line.Using our TXNRD(i)s in combination with Olaparib, we observed synergistic effects with the twoinhibitors in BRCA wild-type TNBC models. The synergy shown between our TXNRD(i)s andPARP inhibitors in BRCA wild-type TNBC cell lines expands the therapeutic potential of ourTXNRD(i)s as well as opening the possibility to an expansion of PARP inhibitors.