Targeted inhibition of FBXL2 confers susceptibility of HER2-negative breast cancer to trastuzumab deruxtecan.

Trastuzumab deruxtecan (T-DXd), an anti-HER2-drug conjugate, has transformed treatment for HER2-expressing solid tumors. However, heterogeneous intratumoral HER2 expression, particularly high densities of HER2-immunohistochemistry score 0 (HER2-IHC 0) cells, limits its clinical efficacy. Here, we discovered that targeted inhibition of F-box protein FBXL2 elevates HER2 expression on the plasma membrane of HER2-IHC 0 triple-negative breast cancer (TNBC) cells, thereby sensitizing them to T-DXd. Mechanistically, FBXL2 promotes HER2 polyubiquitination at K747 and proteasomal degradation. Notably, small molecules GGTi-2418 and ketoconazole effectively elevate HER2 expression via blocking FBXL2 membrane localization. We further developed lipid nanoparticles (LNPs) to encapsulate GGTi-2418 and ketoconazole, enabling their enrichment in TNBC tumors. Strikingly, GGTi-2418@LNP or ketoconazole@LNP combined with T-DXd induced robust and durable tumor regression in HER2-IHC 0 TNBC xenograft models in female mice. Together, this study highlights that targeted inhibition of FBXL2 combined with T-DXd may be a viable therapeutic strategy for treating HER2-IHC 0 solid tumors.