INTRODUCTION
During the past decade, immune checkpoint inhibitors (ICIs) have become a dazzling star in curing multiple advanced cancers for its breakthrough efficacy and manageable untoward effect(1, 2). Antibodies inhibiting Cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) and programmed death-1/ligand-1 (PD-1/PD- L1) are mostly and widely researched ICIs in the world(2, 3). Nevertheless, the gut microbiota has been verified by many researchers to be closely related to clinical outcome among patients receiving ICIs(4-7).Several medications that can alter the gut flora distribution are usually administered to patients along with ICIs thus have adverse or beneficial impact on the efficacy of ICIs, such as antibiotics (ATBs) use which have been confirmed to be related to shorter OS and PFS as well as overall response rates (ORR) compared to ATBs-unexposed cancer patients undergoing ICIs therapy(8-12).
Except for ATBs, proton pump inhibitors (PPIs), maybe one of the most commonly concomitant prescribed medication among cancer patients(13), can also significantly decrease the gut microbiota diversity by altering the PH of gastric acid and delaying gastric emptying(14, 15), thereby, affect the efficacy of ICIs. Nevertheless, the effect of PPIs exposure and clinical outcome in cancer patients undergoing ICIs therapy remains controversial according to current existing literature. Hence, we collect the relevant studies and perform a meta-analysis to investigate the relationship between PPIs exposure and efficacy of ICIs to get a better understanding of this issue.