The correlations of chemotherapeutic drugs and gut microbiota are mutual. Chemotherapy drugs can directly act on the gut microbiota or damage the intestinal epithelial cells and destroy the intestinal barrier, resulting in the imbalance of the microbiota (Figure 1). Similarly, dysregulated gut microbiota may affect the absorption and metabolism of chemotherapeutic drugs, increasing their toxicity and reducing their efficacy.

A growing body of evidence demonstrated that gut microbiota is closely associated with drug efficacy and side effects, and the interaction between gut microbes and commonly used non-antibiotic drugs is mutual: the composition of the gut microbiome can be affected by drugs, and vice versa. The gut microbiome can also influence an individual’s response to a drug by enzymatically altering its structure and altering its bioavailability, bioactivity, or toxicity, which is called pharmacomicrobiology (Lucafo et al., 2020). However, gut microbiota can also directly influence an individual’s response to chemotherapeutic drugs in the treatment of AL. Methotrexate (MTX) is one of the most clinically crucial and potent anticancer drugs, which is widely used for the treatment of leukemia due to blocking folate metabolism. MTX can induce distinct intestinal impairment, including the atrophy of jejunal villi, increasing goblet cells, and collapse of muscularis mucosa. Moreover, the MTX-induced intestinal mucosal injury promotes a series of inflammatory responses, including the increase of inflammatory factors, and dysregulation of macrophages (increase of M1 phenotype and decrease of M2 phenotype) in lymph nodes and spleen. Besides, MTX can gradually induce an alteration in population, diversity, and composition of the gut microbiota in mice, especially the distinct decrease of Bacteroidales. Bacteroides fragilis also significantly decreases, whereas the Bacteroides fragilis supplementation can ameliorate the inflammatory response and polarization imbalance of macrophages induced by MTX (Zhou et al., 2018). Changes in the gut microbiota were also observed in some in vivo experiments in mice models (Huang et al., 2020; Letertre et al., 2020).

Cyclophosphamide (CTX) is another broad-spectrum antitumor drug and has a curative effect on leukemia and solid tumors. One important study demonstrated that the intestinal epithelial barrier was impaired after treatment with non-high-dose CTX in mice, which was manifested by discontinuity of epithelial barrier, shortening of villi, interstitial edema, focal accumulation of monocytes in lamina propria, and increase of goblet cells and Paneth cells (Viaud et al., 2013). Daillère et al. (2016) identified two bacterial species, Enterococcus hirae and Barnesiella intestinihominis that are involved in CTX therapy. Whereas E. hirae could be translocated from the small intestine to secondary lymphoid organs and increased the intratumoral CD8/Treg ratio. Barnesiella intestinihominis was accumulated in the colon and promoted the infiltration of interferon gamma (IFN-γ)-producing gamma delta T (γδT) cells in cancer lesions, and they can improve the efficacy of the most common alkylated immunomodulatory compounds. The above mentioned pathological changes increase the intestinal permeability, facilitate intestinal bacterial translocation, and eventually lead to bloodstream infection (BSI). Viaud et al. (2013) also found a significant change of the microbiota composition in the small intestine and a significant translocation of commensal bacteria in most mice treated with CTX, and some gram-positive bacteria species (e.g., Lactobacillus johnsonii, Lactobacillus murinus and Enterococcus hirae) were detected in lymphoid organs such as mesenteric lymph nodes and spleens. Furthermore, a previous study indicated that gut microbiota was correlated with the antitumor effect induced by chemotherapeutic agents. CTX induced the conversion of CD4⁺ T cells into type-1 T helper (Th1) and Th17 cells, which is closely associated with gram-positive bacterial species present in the small intestine and lymphatic organs. Antibiotic treatment can decrease the CTX-mediated inhibition of tumor growth, whereas adoptive transfer of pathogenic Th17 (pTh17) cells can eliminate this negative effect of antibiotics, suggesting that gut microbiota may contribute to CTX-mediated anticancer immune responses through pTh17 cells, and gram-positive bacteria may contribute to the formation of anticancer immune responses by stimulating Th17 cells (Viaud et al., 2013). A recent large-scale study (Zimmermann et al., 2019) involving 76 diverse human gut bacteria and 271 oral drugs indicated that the metabolic effects of specific gut bacteria on drugs have a significant influence on chemotherapy. In this study, dexamethasone was metabolized by Clostridium scindens in vitro, and it was metabolized (side-chain cleaving) by an intestinal microbe in vivo, which may influence the serum metabolism level. Other corticosteroids, such as prednisone, may be metabolized by Clostridium scindens. Their findings suggested that the metabolism of several drugs is chemically modified by microbiota. High-throughput gene sequencing was combined with mass spectrometry to systematically identify microbial gene products that metabolize drugs, and drug metabolic activities of human gut bacteria and microbiota can be predicted based on their genomic contents. When the interpersonal variation of microbiota is correlated with differences in drug metabolism, drug responses may be safer and more efficient through adjustment of exogenous microorganisms.

Although chemotherapy and hematopoietic stem cell transplantation for AL have achieved a remarkable therapeutic efficacy, some patients still encounter with troubles, such as primary or secondary drug-resistance, relapse after remission, and serious complications. In recent years, novel cancer immunotherapeutic methods with a high specificity and a low toxicity have shown a great potential for the treatment of refractory or relapsed AL. At present, the main immunotherapy for AL contains chimeric type of antigen receptor (CAR) T cell therapy, antibody-drug conjugate (ADC), bispecific T cell engager (BiTE), immune checkpoint inhibitors (ICIs), natural killer (NK) cell therapy, dendritic cell (DC) therapy, etc. A growing body of evidence showed that microbiota may alter the response to cancer immunotherapy. ICIs prevent the binding of inhibitory molecules (e.g., CTLA4, PD-1/PD-L1, LAG-3, and TIM-3) to their ligand receptors to activate the suppressed immune system and promote the removal of malignant cells (Seidel et al., 2018). A remarkable study demonstrated that the composition of gut microbiota (distinct Bacteroides species) affected the anti-tumor efficacy of anti-CTLA4 in mice with tumor. It was found that in mice and patients, T cell responses specific for B. thetaiotaomicron or B. fragilis were associated with the efficacy of CTLA-4 blockade. Tumors in antibiotic-treated or germ-free mice did not respond to CTLA blockade. This defect was overcome by gavage with B. fragilis, by immunization with B. fragilis polysaccharides, or by adoptive transfer of B. fragilis–specific T cells. Fecal microbes from individuals to mice transplanted confirmed that treatment of melanoma patients with anti-CTLA-4 antibody is beneficial to the growth of B. fragilis and has anticancer properties (Vétizou et al., 2015). Another study reported that special taxa can promote the anti-tumor efficacy of anti-PD-L1 in mice with melanoma (Sivan et al., 2015). Studies confirmed that a higher diversity of gut microbiota and a higher abundance of particular taxa were associated with positive responses to ICIs in patients with different types of cancer, such as melanoma, non-small cell lung carcinoma, renal cell carcinoma, and urothelial carcinoma (Dubin et al., 2016; Frankel et al., 2017; Routy et al., 2018; Jin et al., 2019). Kuczma et al. found that the loss of gut microbiota caused by administration of antibiotics reduced the therapeutic efficacy of adoptive T cell therapy in mice with colorectal tumors, while did not affect the efficacy of CD19-targeted CAR-T cell therapy in mice with B-cell lymphoma (Kuczma et al., 2017). Abid et al. speculated that gut microbiota may potentially enhance CAR-T response without clear evidence (Abid et al., 2019). Additional relevant studies are required to confirm the above-mentioned assumption. Establishment of a mouse model of liver cancer revealed that the altered gut microbiota induced by vancomycin might have anti-cancer effects by increasing the number of hepatic NKT cells and reducing the level of secondary bile acid metabolism (Ma et al., 2018). To explore the role of the gut microbiota in response to CD19-targeted CAR-T cell therapy, Smith et al. (2022) first conducted a multicenter retrospective study, and it was found that the antibiotic exposure of patients receiving CAR-T cell therapy was associated with worse overall survival (OS). Then, they prospectively collected stool samples from 48 patients with leukemia or lymphoma, and demonstrated that patients receiving CAR-T cell therapy with an abnormally low alpha diversity and a higher abundance of Ruminococcus, Bacteroides and Faecalibacterium were associated with the higher efficacy of CD19-targeted CAR-T cell therapy. Moreover, additional microbiota data at different treatment time points (e.g., after CAR-T cell therapy and during follow-up) may assist clinicians to better define the role of gut microbiota in CAR-T cell therapy. Further studies should also investigate specific mechanisms of fecal microbiota regulation in preclinical models, such as animal and cell experiments, to explore the interaction of bacterial taxa and bacterial metabolites in the immune system, in order to improve patient outcomes after immunotherapy.

Probiotics are active microorganisms that confer health benefits on host by decreasing intestinal dysbiosis, promoting nutrient absorption, protecting intestinal mucosal barrier, and modulating immunity or inhibiting intestinal inflammation. Probiotics have been widely used to manage intestinal side effects caused by antibiotics or other factors. A study demonstrated that a probiotic (Lactobacillus) can regulate the diversity and abundance of gut microbiota in mice that received 5-fluorouracil (5-FU; Yeung et al., 2020). In a randomized controlled trial involving 60 children with AL who received chemotherapy, the group of patients who took oral probiotics (Lactobacillus rhamnosus) daily showed a significant reduction in most gastrointestinal side effects, including vomiting, nausea, abdominal distension, flatulence, constipation, and abdominal pain (Reyna-Figueroa et al., 2019). In a systematic review, the analysis of 11 studies including 1,557 patients has shown that probiotics reduce both frequency and severity of diarrhea in cancer patients (Redman et al., 2014). However, a meta-analysis of 1,091 patients showed that prophylactic probiotics were not effective in reducing diarrhea induced by any cancer therapy (Wardill et al., 2018). Therefore, there is no consistent conclusion about the side effects of probiotics in cancer patients, and further research needs to be conducted to draw definitive conclusions.

Limited supplementation of probiotics may not be enough to restore gut microbiota dysbiosis secondary to AL, while according to the patient’s specific chemotherapy regimen and changes in the gut microbiota, the selection of individualized probiotics may be beneficial to patients, and the prognosis of the patient’s disease and treatment-related side effects may be improved.

Fecal microbiota transplantation (FMT) is an infusion of fecal suspension containing healthy donor microbiota into the gastrointestinal tract of the recipient. FMT has been given as self-administered or medically administered enemas, oral capsules, via nasogastric tube, or instilled into the duodenum by upper endoscopy or the right colon by colonoscopy or cecostomy. It can directly change the intestinal microbiota of the recipient and restore its normal composition, thus obtaining therapeutic effects. FMT has been highly regarded since it was approved by the US Food and Drug Administration in 2013 for the treatment of recurrent and refractory Clostridium difficile infections, and since then, its application has expanded rapidly and widely, not only for gastrointestinal related diseases, but also for parenteral diseases. For different hosts and diseases, FMT can be personalized to different patients and conditions (Vindigni and Surawicz, 2017; Wang J. W. et al., 2019; Tkach et al., 2022).

Studies have shown that the post-antibiotic intestinal microbiota of the human and murine can recover more quickly with autologous FMT than with multi-strain probiotics (Suez et al., 2018). Therefore, FMT is a faster and more effective method to restore the altered gut microbiota, and many clinical trials have confirmed the therapeutic effect of FMT. In a clinical trial including four acute leukemia patients undergoing allo-HSCT, who suffered from refractory diarrhea because of refractory intestinal infection or intestinal GVHD, FMT was a successful and safe treatment (Wang Q. et al., 2019). In addition, FMT is recommended for the treatment of multiple recurrent Clostridioides difficile infection (CDI) in adult and children who have failed appropriate antimicrobial therapy (McDonald et al., 2018). A case report presented a successful FMT treatment in a 27 year-old AML patient with prolonged and severe neutropenia. The patient developed fulminant CDI refractory to maximal antibiotic treatment, then appeared high fever, diffuse abdominal pain, watery diarrhea, colitis and other serious clinical symptoms, whereas his clinical symptoms improved after FMT therapy without further recurrence.(Lee et al., 2020) Two other similar case studies also reported that FMT was successful in eliminating multidrug-resistant pathogens and treating recurrent CDI after allo-HSCT in ALL patients (de Castro et al., 2015; Innes et al., 2017). In addition, FMT intervention achieved good results in restoring microbial diversity and reconstructing gut microbiota composition after HSCT (DeFilipp et al., 2018; Taur et al., 2018), as well as in treating four patients with steroid-resistant gut aGVHD (Kakihana et al., 2016). In order to evaluate the efficacy of autologous fecal microbiota transfer (AFMT) in dysbiosis correction and multidrug-resistant bacteria eradication, 25 AMl patients treated with (de Vos et al., 2022) intensive chemotherapy and antibiotics were recruited in a multicenter study, they showed that AFMT appears to be safe and effective for gut microbiota restoration in patients receiving intensive induction chemotherapy and antibiotic for AML (Malard et al., 2021).

In 2017, the International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus panel defined a prebiotic as follows: a substrate that is selectively utilized by host microorganisms conferring a health benefit (Gibson et al., 2017). Some substances present in the diet, such as soybean extracts, koji glycosylceramides, grape extracts, tea polyphenols, and seaweed extracts, can be considered as potential prebiotics, because they can selectively stimulate the proliferation of beneficial bacteria in the intestine (Cheng et al., 2022). The effects of prebiotics are indirect, and their positive effects are exerted by affecting the composition of gut microbiota.

A recent study suggested that the use of appropriate prebiotics during chemotherapy may reduce the therapeutic side effects. Establishment of a mouse model revealed that short-term dietary restriction significantly alleviated the intestinal fatal damage in mice after high-dose MTX exposure by increasing the Lactobacillus genus to reduce intestinal inflammation and protect PCNA-positive cells in basal crypt, as well as the function of intestinal stem cells (Tang et al., 2020). Importantly, Lactobacillus rhamnosus supplementation plays the same role as dietary restriction, suggesting that dietary regulation is a viable method to restore the altered microbiota after treatment. Taskin et al. (2017) found in a study on mice that lactulose significantly improved MTX-induced liver damage. Similarly, a recent study conducted by Zhou et al. (2021) pointed out that polysaccharides and ginsenosides extracted from American ginseng had a certain protective effect on CTX therapy. They restored the composition of gut microbiota and increased various beneficial mucosa-associated bacterial taxa (Clostridiales, Bifidobacterium, and Lachnospiraceae), while decreased harmful bacteria (Escherichia-Shigella and Peptococcaceae). Besides, specific nutrients may play an important role in adjusting gut microbiota. For instance, a study found that melatonin supplementation increased the diversity of gut microbiota and regulated the composition, such as enrichment of the abundance of Lactobacillus in mice (Ren et al., 2018), indicating that supplementing food enriched in melatonin is conducive to the early reconstruction of gut microbiota after chemotherapy. Another study confirmed that soy-whey blended protein can significantly increase the diversity of gut microbiota and effectively improve muscle status in AL survivors (Ren et al., 2020). In addition, enteral nutrition was given to promptly restore the gut microbiota post-HSCT (D'Amico et al., 2019). Therefore, a proper diet and supplementation of prebiotics are also an alternative treatment for the altered gut microbiota.