Osimertinib (AZD9291) is a third-generation epidermal growth factor receptor 
(EGFR) tyrosine kinase inhibitor (TKI) that has been approved for the treatment 
of EGFR-mutated non-small cell lung cancer (NSCLC). In NSCLC patients, an EGFR 
mutation is likely to be correlated with high levels of expression of programmed 
death ligand-1 (PD-L1). Here, we showed that osimertinib decreased PD-L1 
expression in human EGFR mutant NSCLC cells in vitro. Osimertinib (125 nmol/L) 
markedly suppressed PD-L1 mRNA expression in both NCI-H1975 and HCC827 cells. 
Pretreatment with the N-linked glycosylation inhibitor tunicamycin, osimertinib 
clearly decreased the production of new PD-L1 protein probably due to a 
reduction in mRNA. After blocking transcription and translation processes with 
actinomycin D and cycloheximide, respectively, osimertinib continued to reduce 
the expression of PD-L1, demonstrating that osimertinib might degrade PD-L1 at 
the post-translational level, which was confirmed by a cycloheximide chase 
assay, revealing that osimertinib (125 nmol/L) decreased the half-life of PD-L1 
from approximately 17.8 h and 13.8 h to 8.6 h and 4.6 h, respectively, in 
NCI-H1975 and HCC827 cells. Pretreatment with the proteasome inhibitors (MG-132 
or bortezomib) blocked the osimertinib-induced degradation of PD-L1, but an 
inhibitor of autophagy (chloroquine) did not. In addition, inhibition of GSK3β 
by LiCl prevented osimertinib-induced PD-L1 degradation. The results demonstrate 
that osimertinib reduces PD-L1 mRNA expression and induces its protein 
degradation, suggesting that osimertinib may reactivate the immune activity of T 
cells in the tumor microenvironment in EGFR-mutated NSCLC patients.