Microbiome Landscape and Association with Response to Immune Checkpoint Inhibitors in Advanced Solid Tumors: A SCRUM-Japan MONSTAR-SCREEN Study.

Although the gut microbiome is associated with cancer development and progression, little is known about the effects of the gut microbiome landscape and the efficacy of immune checkpoint inhibitors (ICI) across cancer types. We investigated the association between the microbiome, clinical features, and ICI efficacy across cancer types in a large nationwide screening project for solid tumors. Among 2,180 patients with advanced solid tumors enrolled in the SCRUM-Japan MONSTAR-SCREEN between October 2019 and September 2021, in the chemotherapy-naïve cohort (n = 817), a high prevalence of oral bacteria was observed in patients using proton pump inhibitors (PPI) and those with upper gastrointestinal cancers, particularly postoperative patients with gastric or pancreatic cancer. Among patients treated with ICIs (n = 333), a high abundance of sequence variants in the gut microbiome was not significantly associated with ICI efficacy across cancer types (HR = 0.94; 95% confidence interval, 0.73-1.21). However, high oral bacteria in feces significantly correlated with a shorter progression-free survival compared with low oral bacteria (median, 4.34 vs. 6.97 months; HR = 1.38; 95% confidence interval, 1.07-1.78). Notably, in patients using PPIs, a higher proportion of oral bacteria influenced progression-free survival outcomes of ICI treatment (median, 3.15 vs. 2.04 months; P = 0.08), unlike in PPI nonusers (median, 7.13 vs. 5.55 months; P = 0.74). This study of the gut microbiome has unveiled significant insights into its landscape and potential impact on ICI efficacy. It highlights that the abundance of oral bacteria in feces may play a critical role in diminishing ICI efficacy among patients using PPIs.

Significance: As part of the MONSTAR-SCREEN, a prospective nationwide project for patients with solid tumors, we found that although gut microbiome diversity does not consistently predict ICI efficacy across cancer types, a high level of oral bacteria in the gut is linked to reduced ICI effectiveness, especially in patients using PPIs. These findings highlight the potential clinical impact of microbiome variations on cancer treatment outcomes.