e19003 Background: Silibinin (SLB), a bioactive flavonolignan isolated from the dried fruits of milk thistle (Silybum marianum), suppresses the resistance to erlotinib (ERL) caused by the EGFR T790M mutation in NSCLC xenograft models. Poor water solubility and low bioavailability might severely limit the efficacy of SLB. The ability of SLB to overcome ERL resistance attributed to mechanisms other than the second-site EGFR mutations remains completely unexplored. Methods: Using PC-9 cells harboring the ERL-sensitizing EGFR exon 19-mutation ΔE746-A750, we developed ERL-refractory derivatives (PC-9 ERL-R) that lacked EGFR T790M mutation. An oral formulation of milk thistle extract enriched (30%) with a water-soluble form of SLB complexed with the excipient amino-sugar meglumine (SLB-m) was explored for its ability to inhibit the growth of PC-9 ERL-R xenografts. Results: In the absence of second-site EGFR mutations or the alternative activation of MET or AXL, acquired cross-resistance to ERL and GEF correlated with enhanced epithelial-to-mesenchymal transition (EMT) features. Single-agent SLB-m significantly decreased the overall tumor volumes of PC-9 ERL-R xenografts, which exhibited enhanced engraftment and faster growth compared with the ERL-/GEF-sensitive PC-9 parental xenografts. Combination treatment with ERL + SLB-m or GEF + SLB-m significantly suppressed tumor growth (> 90%) compared with single-agent treatment. The SLB-m treatment fully inhibited the development of an EMT phenotype in the ERL-/GEF-refractory cells in vitro. Systemic SLB-m treatment significantly affected the expression of EMT-related transcription factors in the ERL-/GEF-refractory xenografts. Conclusions: The oral administration of SLB-m delays tumor progression in ERL-/GEF-refractory EGFR-mutant NSCLC mouse xenografts. Given that the EMT phenomenon is required by a multiplicity of mechanisms of acquired resistance mechanisms to ERL, our data might facilitate the design of clinical trials to test the administration of SLB-m in combination with ERL or GEF in patients with EGFR-mutated tumors that progress to these drugs.