Association between peripheral eosinophils and clinical outcomes in patients with non-small cell lung cancer treated with immune checkpoint inhibitors

Programmed cell death ligand 1 is considered a predictor of the therapeutic effect of immune checkpoint inhibitors (ICPIs), but a more simple and useful predictor is needed. The aim of this study was to identify the relationship between eosinophil counts and percentages and response to ICPI therapy. In 190 patients with non-small cell lung cancer (NSCLC) treated with ICPI therapy, peripheral eosinophil counts and percentages at the time of ICPI therapy initiation, the maximum counts and percentages of eosinophils during ICPI therapy, response to therapy, and time to treatment failure (TTF) were investigated. Both an increase in the peripheral eosinophil count and an elevation of eosinophil percentage following the initiation of ICPI therapy were observed, regardless of whether the patients had controlled or progressive disease. The median time to the maximum eosinophil percentage was 5 weeks in patients with controlled disease and 2 weeks in those with progressive disease. The cutoff value for the maximum eosinophil counts and percentage during ICPI therapy was set at 300/μl and 5%, respectively, to identify the presence or absence of a therapeutic effect. Time to treatment failure was longer in patients with maximum eosinophil counts exceeding 300/μl and a maximum eosinophil percentage above 5%. In a multivariable analysis, a maximum eosinophil percentage of 5% during ICPI therapy was a significant predictive factor for therapeutic efficacy. The measurement of peripheral eosinophils up to around 5 weeks following the initiation of treatment, especially the maximum eosinophils count and percentage, might provide useful information about the efficacy of ICPIs.