Abstract P1-04-02: Gut microbiome diversity correlates with tumor PD-L1 status in metastatic triple negative breast cancer (mTNBC): correlative analysis of gut microbiome and tumoral biomarkers
Authors: Chun Brie , Goodyear Shaun , Rice-Stitt Travis , Karstens Lisa , Dahl Erin , Li Allen , Hobbs Evthokia , Zahi Mitri 
Published: March 2023

Abstract
Background: Greater gut microbiome diversity is observed in patients with a clinical response to immune checkpoint inhibitor (ICI) therapy in solid tumors and in preclinical models targeting the gut microbiome is a promising strategy to improve ICI efficacy. The investigation of the immunomodulatory effects of the gut microbiome and tumor biology is in its infancy and whether interindividual microbiome variations correlate with tumor biology in breast cancer is unknown. Here we report a correlative analysis of gut microbiome diversity with tumor biomarkers: PD-L1, tumor immune cell density, tumor infiltrating lymphocytes (TILs), MutSig3, and interferon (IFN) gene signatures in a cohort of mTNBC patients treated with olaparib and durvalumab of the Adaptive Multi-Drug Treatment of Evolving Cancers (AMTEC) trial (NCT03801369). Methods: AMTEC participants undergo a baseline biopsy (bx) and fecal sample collection, then start olaparib monotherapy on a 28-day cycle. Following the first cycle, an on-treatment (on-tx) bx and fecal sample are collected, then as part of enrollment to Arm 1, durvalumab is added to the treatment regimen. Ten patients with stool collections and tissue biopsies were used in this analysis, eight had paired pre- and on-tx collections. At interim-analysis, participants were categorized as responders (PR/SD) or non-responders (PD). Gut microbiome composition in fecal samples collected pre- and on-olaparib monotherapy was assessed by 16S rRNA sequencing and alpha diversity was quantified by observed diversity, Fisher, Inverse Simpson, Pielou, and Shannon indices. Response, PDL1 expression (Positive CPS &gt;1), multiplex immunohistochemistry (mIHC) immune cell density and immune cell signature, TILs, RNA signatures including interferon (INF) family signatures, and mutational significance (MutSig3) were correlated to alpha diversity by T-test. Results: Average baseline alpha diversity of the fecal microbiome did not vary based on response (6 non-responses and 4 responses). However, in the on-tx samples, Fisher (CB: mean = 21.81, std. dev. = 3.6; PD: mean = 17.04, std. dev. = 2.3; p=0.048) and Observed alpha diversity (CB: mean = 260, std. dev. = 38.8; PD: mean = 209, std. dev. = 25.8; p = 0.049) were higher in responders. Average baseline alpha diversity of the gut microbiome as measured across all examined indices was significantly lower in participants with PD-L1 expressing tumors (p &lt; 0.05 for each, T-test). Among on-tx samples, there was a trend toward lower alpha diversity in PD-L1 expressing tumors, but PD-L1 expression did not correlate with clinical response. There was a notable trend toward greater alpha diversity in on-tx bx and high TILs. Alpha diversity did not differ by immune cell densities by mIHC (CD8+ T cell, B cell, and regulatory T cells), hypo-inflamed/pro-inflammatory signature, IFN gene signature, nor MutSig3 score. Conclusion: Our preliminary data reveal that higher alpha diversity following olaparib monotherapy correlated with response to therapy with combination olaparib and durvalumab and correlated with tumor PD-L1 status. Whether gut microbiome features represent a meaningful biomarker for ICI treatment of mTNBC warrants further study. Enrollment to stage 2 of AMTEC Arm 1 is ongoing.
Citation Format: Brie Chun, Shaun Goodyear, Travis Rice-Stitt, Lisa Karstens, Erin Dahl, Allen Li, Evthokia Hobbs, Mitri Zahi. Gut microbiome diversity correlates with tumor PD-L1 status in metastatic triple negative breast cancer (mTNBC): correlative analysis of gut microbiome and tumoral biomarkers [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-04-02.