Fig.4 Drugs with gastrointestinal toxicity modulated by gut microbiota: methotrexate (A), irinotecan (B), 5-fluorouracil (C), and mycophenolate mofetil (D). Red structures signify hepatotoxic drugs. Gastrointestinal toxicity of MPA and SN-38 can be attenuated by β-glucuronidases inhibition. Abbreviation: 5-FU, 5-fluorouracil; DPD, dihydropyrimidine dehydrogenase; MMF, mycophenolate mofetil; MPA mycophenolic acid; MPAG, mycophenolic acid glucuronide; MTX, methotrexate; SN-38G, SN-38 glucuronide; UGTs, UDP-glucuronosyltransferases.
4.1 Methotrexate
Methotrexate (MTX) is an antifolic antineoplastic but brings about multiple side effects including hepatotoxicity, nephrotoxicity, pulmonary toxicity, and gastrointestinal toxicity, among which hepatotoxicity and gastrointestinal toxicity are the most common (Wang et al., 2018). In terms of gastrointestinal toxicity, it is characterized by gastrointestinal mucositis. A previous study found that MTX treatment substantially decreased gut microbial quantity and diversity of rats, with anaerobes and streptococci significantly decreased, which is proportionally associated with the severity of MTX-induced gastrointestinal mucositis (Fijlstra et al., 2015). A similar study confirmed the microbial dysbiosis caused by MTX in mice with a significantly decreased abundance of Bacteroides fragilis , and supplemental Bacteroides fragilis respited MTX-induced inflammatory reactions and macrophage polarization (Zhou et al., 2018a). Moreover, Bifidobacterium longum was also found to be a probiotic in attenuating MTX-induced intestine damage, which was discovered in the co-treatment of leucovorin with MTX for the sake of reducing its side effects (Huang et al., 2020b). Supplementation with probiotics perturbed by drugs is the most direct and explicit way in alleviating microbiota related adverse effects, but safety remains a question for long-term adhibition. In comparison, dietary control offers a safer alternative herein. It was found that dietary restriction remarkably enhanced the survival rate of mice exposed to a lethal dose of MTX and ameliorated MTX-induced intestine inflammation with Lactobacillus rhamnosusplaying a critical role (Tang et al., 2020). In this case, dietary restriction needless of specific diet, drug, or probiotics intervened achieved remarkable results in reducing MTX-induced gastrointestinal side effects, suggesting what we eat, which constitutes the major part of gastrointestinal contents, probably determines therapeutic and toxic outcomes of orally administered drugs somehow.
4.2 Irinotecan
Irinotecan (also called CPT-11) is one of the most commonly applied anticancer agents for colon carcinoma therapy but causes lethal toxicity in at least 36% of patients, most of whom undergo diarrhea, mucositis, and other gastrointestinal reactions (Encarnação et al., 2018). In the human body it is metabolized both by liver and gut microbiota (Fig.4): first converted into bioactive SN-38 by liver carboxylesterases and further transforms into SN-38G by conjugating with glucuronic acid catalyzed by hepatic UDP-glucuronosyltransferases (UGTs), which is then subject to gut bacterial β-glucuronidases to generate SN-38, also a toxicant that induces damage of intestinal epithelial cells and severe diarrhea (Yang et al., 2018b). Due to the critical role of gut bacterial β-glucuronidases plays therein, it has been highlighted in mitigating irinotecan-induced gastrointestinal toxicity. Intestinal bacterial β-glucuronidases are considered to be a predictive biomarker of irinotecan triggered diarrhea severity (Chamseddine et al., 2019) and inhibition of it has been demonstrated to be effective in abrogating irinotecan toxicity (Wallace et al., 2010; Roberts et al., 2013). However, inhibition of bacterial β-glucuronidases of different origins and structures received distinctly different outcomes: inhibition ofFirmicutes and Proteobacterium derived β-glucuronidases alleviated irinotecan-induced diarrhea in mice while inhibition ofBacteroidetes derived one did not, suggesting functional diversity in orthologous enzymes of gut microbiota (Wallace et al., 2015). Additionally, probiotics such as Saccharomyces cerevisiae UFMG A-905 (Bastos et al., 2016) and Escherichia coli Nissle 1917 (Wang et al., 2019c) treatment also has been proven to confer protective effects against irinotecan-induced gastrointestinal side effects.
4.3 5-Fluorouracil
5-Fluorouracil (5-FU), used for colorectal cancer therapy, is frequently related to mucosal side effects such as diarrhea and mucositis. 5-FU in the human body is mainly detoxified by hepatic dihydropyrimidine dehydrogenase (DPD), inhibition of which induces 5-FU accumulation and consequently aggravates adverse effects. In 1993, a combination of Sorivudine and 5-FU resulted in 18 deaths with symptoms of diarrhea and decreased blood cells and platelets in Japan, 40 days after Sorivudine was approved in clinical use. It was found that gut microbial metabolite of Sorivudine, bromovinyluracil inactivated DPD and thus limited detoxification of 5-FU (Klaassen and Cui, 2015). It is an example that gut microbiota mediates drug-drug interactions and further studies illustrate that gut microbiota regulates 5-FU toxicity directly. A study relating 5-FU-induced colon inflammation to gut microbiota suggested that 5-FU caused alterations in gut microbiota and fecal transplantation from 5-FU treated mice to normal ones diminished functional performance and altered colon inflammatory markers of the latter (Sougiannis et al., 2019). Another study focusing on fecal transplantation from the therapeutic side revealed transplantation of fecal microbiota from health donor mice attenuated 5-FU-based chemotherapy regimen, FOLFOX (5-FU, leucovorin, and oxaliplatin)-induced intestinal mucositis by downregulating expression levels of toll-like receptors (TLRs), MyD88, and serum IL-6 with verified safety (Chang et al., 2020a). Similarly, probiotics were also found to palliate 5-FU-triggered intestinal injury, including Lactobacillus casei rhamnosu (Chang et al., 2018), Saccharomyces Boulardii (Justino et al., 2014), and LactobacillusGG (Holma et al., 2013), in animal experiments and human trials respectively. The human trial also showed that methanogenic archaea made a difference in whether patients underwent diarrhea or constipation after 5-FU treatment (Holma et al., 2013). Additionally, prebiotics such as the herbal pair, Atractylodes Macrocephala and Panax Ginseng conferred a protective effect against 5-FU by restoring gut microbiota disturbed by it and especially inhibited 5-FU-induced release of intestinal inflammatory cytokines (Wang et al., 2019a). As we put, strategies for tackling 5-FU-induced diarrhea and mucositis are restricted within conventional ones, mainly including fecal microbial transplant, probiotics, and prebiotics supplementation. The discovery of intestinal microbial metabolism of the drug proffers different and effective methods in coping with its toxicity, suggesting microbial metabolism of drugs plays a significant role in determining toxic outcomes of drugs.
4.4 Mycophenolate mofetil
Mycophenolate mofetil (MMF) is an immunosuppressive agent prescribed to patients after transplantation and also exerts severe gastrointestinal side effects which limit its application. As a prodrug, it is hydrolyzed to mycophenolic acid (MPA) to confer efficacy and is converted into glucuronidated MPA (MPAG) by hepatic enzymes. MPAG is primarily excreted via urine but approximately 10% of it is transported into the gastrointestinal tract and is metabolized by gut microbial β­glucuronidase back into MPA (Fig.4), whose accumulation in the colon is associated with MMF-induced colonic inflammation (Lamba et al., 2014). A study revealed that MMF disturbed gut microbiota of which Proteobacteria (specifically Escherichia/Shigella ) and gene expression of LPS biosynthesis were significantly enhanced while overall diversity was diminished in mice, which could be reversed by broad-spectrum antibiotics treatment (Flannigan et al., 2018). Consistent with the metabolic pathway of MPA in the gut, it was found that MMF induced active β­glucuronidase expression and in turn exacerbated its gastrointestinal adverse effect, which could be eliminated by vancomycin (Taylor et al., 2019), suggesting that β­glucuronidase also can be an efficient target in alleviating MMF toxicity.