3008 Background: Significant advances have been made in cancer therapy with immune checkpoint blockade. However, responses in pts with MM are variable, and insights are needed to identify biomarkers of response and strategies to overcome resistance. There is a growing appreciation of the role of the microbiome in cancer, and evidence in murine models that modulation of the gut microbiome may enhance responses to immune checkpoint blockade, though this has not been well studied in pts. Thus we evaluated the microbiome in a large cohort of pts with MM, focusing on responses to anti-PD-1. Methods: We collected oral (n = 234) and gut microbiome samples (n = 120) on a large cohort of of MM patients (n = 221). Of note, the majority of pts were treated with PD-1 based therapy (n = 105). Pts on anti-PD1 were classified as either responders (R) or non-responders (NR) based on RECIST criteria, and 16S rRNA and whole genome shotgun (WGS) sequencing were performed. Immune profiling (via immunohistochemistry, flow cytometry, cytokines and gene expression profiling) was also done in available pre-treatment tumors at baseline. Results: Significant differences in diversity and composition of the gut microbiome were noted in R vs NR to anti-PD-1, with a higher diversity of bacteria in R vs NR (p = 0.03). Differences were also noted in the composition of gut bacteria, with a higher abundance of Clostridiales in R and of Bacteroidales in NR. Immune profiling demonstrated increased tumor immune infiltrates in R pts , with a higher density of CD8+T cells; this correlated with abundance of specific bacteria enriched in the gut microbiome (r = 0.59, 0.014). Other features of enhanced immunity were also noted, and WGS revealed differential metabolic signatures in R vs NR. Furthermore, diversity (p = 0.009; HR = 7.67) and abundance of specific bacteria in R (p = 0.007; HR = 3.88) was associated with improved PFS to anti-PD-1 therapy. Conclusions: Diversity and composition of the gut microbiome differ in R vs NR pts with MM receiving anti-PD-1 therapy. These have potentially far-reaching implications, though results need to be validated in larger cohorts across cancer types.
LBA10 Critical role of eosinophils during response to immune checkpoint blockade in breast cancer and other cancer types