Repurposing butoconazole as a GRP78 substrate-binding domain (SBD) inhibitor to induce endoplasmic reticulum stress-mediated apoptosis in triple-negative breast cancer.

Triple-negative breast cancer (TNBC) is aggressive with limited therapies and poor prognosis. Butoconazole, a clinical topical imidazole antifungal for vulvovaginal candidiasis, has not previously been investigated for TNBC treatment. Here, we repurposed butoconazole as a novel allosteric inhibitor of glucose-regulated protein 78 (GRP78) for TNBC treatment. It suppressed TNBC cell proliferation (IC50: 13.61 ± 1.07 μM for MDA-MB-231, 26.17 ± 1.17 μM for MDA-MB-468), inhibited migration, and induced apoptosis, resulting in 68.28% tumor growth inhibition in MDA-MB-231 xenograft mouse models without evident toxicity. Mechanistically, we combined RNA-seq and limited proteolysis-mass spectrometry (LiP-MS) to identify GRP78 as its potential target, validated by surface plasmon resonance (SPR), biolayer interferometry (BLI), cellular thermal shift assay (CETSA), drug-affinity-responsive target-stability (DARTS) assay and molecular dynamics simulations. Unlike existing GRP78 modulators, butoconazole allosterically attaches to the helical bundle at the distal tip of GRP78 substrate-binding domain α (SBD-α), rearranges its binding pocket and blocks GRP78 chaperone activity. This triggers endoplasmic reticulum (ER) stress, elevates the expression levels of ATF4 and CHOP and induces apoptosis. Rescue assays with 4-phenylbutyrate (4-PBA), ATF4/CHOP knockdown or GRP78 overexpression attenuated butoconazole's anti-tumor activity. Collectively, butoconazole suppresses TNBC through allosteric GRP78 inhibition to trigger ER stress-mediated ATF4/CHOP apoptosis, and represents a promising lead compound for further development as a systemic anti-TNBC agent through formulation optimization.