The Impact of the Intestinal Microbiota in Therapeutic Responses Against Cancer

In the last years, there is a growing awareness regarding the role of the microbiota in various chronic inflammatory disorders including cancers. The intestinal microbiome is not only regulating the spontaneous course of malignancies of the digestive tract but also acts at a distant site on extraintestinal neoplasia, to play a beneficial or a detrimental role, depending on the context. By providing pathogen and damage-associated molecular patterns and by activating local and distant antigen-presenting cells, our commensals modulate not only the gut, the systemic inflammatory, and immune tonus but also the tumor microenvironment and anticancer-specific adaptive immune responses. Additionally, therapeutic interventions alter the delicate balance between the epithelium, the microbial community, and the lamina propria immunity, influencing the clinical outcome. This chapter will focus on the impact of the intestinal composition in the bioactivity and therapeutic efficacy of distinct compounds (alkylating agents, platinum salts, radiotherapy, conditioning regimen for stem cell transplantation, immunotherapies, and immune checkpoint blockers). This era of research opens novel therapeutic avenues that aim to restore gut eubiosis for a better clinical outcome in cancer patients.