Isoflavones impair response to anti-PD1 therapy in murine breast cancer models, irrespective of dietary fiber and fecal short chain fatty acid levels.

Background: Fermentable dietary fibers, also called microbiota-accessible carbohydrates (MAC), and the consequent increase in fecal short-chain fatty acids (SCFAs) are linked to improved responsiveness to immune checkpoint blockade (ICB) therapy in human and mouse studies. However, experimental diets high in MAC also often contain estrogenic isoflavones, which may counter fiber's beneficial effects by causing immunosuppression. Methods: We studied the effects of feeding female C57BL/6Tac mice low-MAC (AIN93G), low-MAC supplemented with isoflavone genistein, high-MAC (5V5M) or high-MAC isoflavone (high-MACi; 5058D) diet on their gut microbiome and response to anti-PD1 therapy against E0771 allografted triple negative breast cancer (TNBC) and 7,12-dimethylbenz[a]anthracene (DMBA)-initiated estrogen receptor α positive (ERα+) mammary tumors. We also determined whether blocking ERα with tamoxifen (TAM) impacted responsiveness to anti-PD1 therapy in mice fed different diets. The effect of diet and treatments on immune cell signaling pathways was investigated using NanoString PanCancer Immune Profiling Panel. Results: High-MAC and high-MACi diets increased fecal microbial alpha-diversity and the abundances of SCFA producing families Conclusions: Our results highlight the role of diet in impacting the effectiveness of ICB therapies. We found that increased SCFA levels alone are not predictive of response to anti-PD1, but if tumor expresses ERα or if diet contains ERα activating compounds, such as isoflavones, blocking ERα+ might convert unresponsive tumors responsive to anti-PD1. What is already known on this topic: Dietary fiber is proposed to improve response to checkpoint inhibitor therapy against melanoma, but this has been challenged by a recent preclinical study in which different mouse tumor models were used. None of the studies have been done in breast cancer or preclinical breast cancer models. What this study adds: Our study showed, using triple negative breast cancer (TNBC) and estrogen receptor positive (ER+) breast cancer models, that indeed high levels of microbiota accessible carbohydrates (MACs) in diet did not alone determine responsiveness to anti-PD1 therapy, but diet high in plant isoflavones/ hormones impaired anti-PD1 effectiveness, regardless of whether diet contained high fiber levels or not. We also found that the adverse effects of isoflavones were counteracted by tamoxifen, partial estrogen receptor antagonist. How this study might affect research practice or policy: Our findings could indicate that breast cancer patients, both those with TNBC and ER+ disease, should not consume diets high in isoflavones when treated with anti-PD1.