Targeting SYK overcomes acquired resistance to inavolisib in PIK3CA- mutant HER2+ breast cancer

Abstract Background The phosphatidylinositol 3-kinase (PI3K)-protein kinase B (AKT) signaling pathway plays a pivotal role in cancer drug resistance. Thus, targeting this axis provides a vulnerability in cancer therapies, particularly for breast cancer. To this end, inavolisib, a highly selective PI3Kα inhibitor, has been recently approved for clinical use in breast cancer therapies. However, acquired resistance remains a challenge. Methods We established the acquired resistance cell lines with PIK3CA -mutation to inavolisib in both hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-positive (HER2+) conditions. Subsequently, we screened a library of 1,729 Food and Drug Administration (FDA)-approved compounds using cell viability assays. RNA-seq revealed signaling pathway differences between resistant and parental cells. The synergistic effects of candidate sensitizers with inavolisib were validated using cell viability, colony formation, apoptosis, and wound healing assays. The role of spleen tyrosine kinase (SYK) was assessed by using pharmacological inhibition, CRISPR/Cas9-mediated knockout, and siRNA-mediated knockdown. Results Screening of the FDA-approved drug library identified several inhibitors, especially fostamatinib, a well-established SYK inhibitor used for chronic immune thrombocytopenia (ITP), as potent sensitizers in HER2 + inavolisib-resistant cell lines. Further experiments showed that fostamatinib could efficiently sensitize inavolisib-resistant cells to inavolisib treatment by decreasing colony formation and increasing apoptosis. Mechanistically, inavolisib-resistant cells exhibited highly phosphorylated SYK. As a result, SYK knockout restored sensitivity to inavolisib and, when combined, synergistically enhanced both this effect and the suppression of cell migration. Conclusions These findings together indicate that targeting SYK with fostamatinib overcomes acquired resistance to inavolisib in PIK3CA -mutant HER2 + breast cancer cells, supporting further evaluation of this combination in clinical settings.