Combination therapy with levofloxacin and the probiotic Clostridium butyricumMIYAIRI 588 enhances immune checkpoint inhibitor efficacy

Abstract The gut microbiome influences immune checkpoint inhibitor (ICI) efficacy. In this study, we explored the effects of combined levofloxacin (LVFX) and Clostridium butyricum MIYAIRI 588 (CBM588) on ICI outcomes using a CT26 tumor model in BALB/c mice. When compared with the control, the LVFX+CBM588 combination enhanced anti‐programmed cell death (PD)‐1 therapy, with CD8 + T cells playing a key role. Gut microbiota analysis showed reduced Lactobacillus relative abundance and increased Oscillibacter and Muribaculaceae in the LVFX+CBM588 group. A broad‐spectrum antibiotic cocktail (ampicillin, neomycin, vancomycin, and metronidazole) with CBM588 diminished antitumor effects and reduced survival in mice when compared with the control, demonstrating the importance of microbiota‐targeted therapeutic combinations. However, while LVFX+CBM588 improved ICI efficacy, it worsened dextran sulfate sodium (DSS)‐induced colitis, suggesting immune activation contributes to inflammation. These findings emphasize the potential of customized antibiotic–probiotic combinations in cancer immunotherapy, while also stressing the necessity to manage immune‐related adverse effects.