Triple combination targeting PI3K, ER, and CDK4/6 inhibits growth of ER‐positive breast cancer resistant to fulvestrant and CDK4/6 or PI3K inhibitor

List of abbreviations CDK4/6 Cyclin-dependent kinase 4/6 CDK4/6i CDK4/6-inhibitor ER Estrogen receptor ERBB2 Erb-B2 receptor tyrosine kinase 2 ES Enrichment score ESR1 Estrogen receptor 1 FDR False discovery rate GSEA Gene Set Enrichment Analysis mTOR Mammalian target of rapamycin mTORC1 Mammalian target of rapamycin complex 1 NES Normalized enrichment score p-PRAS40 Phosphorylated proline-rich Akt substrate of 40 kDa p-S6 Phosphorylated ribosomal protein S6 PARP Poly (ADP-ribose) polymerase PDX Patient-derived xenograft PFS Progression-free survival PI3K Phosphoinositide 3-kinase PI3Ki PI3K-inhibitor PIK3CA phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha PDK-1 3-phosphoinositide dependent protein kinase-1 PTEN Phosphatase and Tensin homolog Rb Retinoblastoma protein RB1 Retinoblastoma 1 Dear Editor, Despite the improved outcome of advanced estrogen receptor-positive (ER+) breast cancer patients treated with endocrine therapy in combination with either a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) or a phosphoinositide 3-kinase inhibitor (PI3Ki), the disease will eventually progress, and the optimal treatment strategy upon progression remains undefined [ 1–4]. The Gene Set Enrichment Analysis (GSEA) showed that alterations in the regulators of various pathways, including the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, apoptosis, cholesterol homeostasis, and interferon alpha response were significantly enriched gene datasets in MPF-R cells compared to M-S cells (Supplementary Table S1, Supplementary Figure S1A). [...]it is unknown whether the clinical combination of CDK4/6i and fulvestrant therapy is efficacious following progression on combined PI3Ki and fulvestrant. [...]we tested the effects of the PI3Ki alpelisib, CDK4/6i palbociclib, and ER degrader fulvestrant as single drugs and in various dual and triple combinations on growth and viability of breast cancer cells resistant to the combination of palbociclib or alpelisib and fulvestrant. By studying the underlying mechanisms of the inhibitory effect caused by the triple combination, we observed a marked increase in apoptosis and cleaved poly(ADP-ribose) polymerase (PARP) levels in both MPF-R and M-S cells treated with the triple combination compared to the palbociclib and fulvestrant combination (Supplementary Figure S4A-B), but these changes were not observed in TPF-R and T-S cells (Supplementary Figure S4C).