Gut Microbiota: Influence on Carcinogenesis and Modulation Strategies by Drug Delivery Systems to Improve Cancer Therapy

Abstract

Gut microbiota have close interactions with the host. It can affect cancer progression and the outcomes of cancer therapy, including chemotherapy, immunotherapy, and radiotherapy. Therefore, approaches toward the modulation of gut microbiota will enhance cancer prevention and treatment. Modern drug delivery systems (DDS) are emerging as rational and promising tools for microbiota intervention. These delivery systems have compensated for the obstacles associated with traditional treatments. In this review, the essential roles of gut microbiota in carcinogenesis, cancer progression, and various cancer therapies are first introduced. Next, advances in DDS that are aimed at enhancing the efficacy of cancer therapy by modulating or engineering gut microbiota are highlighted. Finally, the challenges and opportunities associated with the application of DDS targeting gut microbiota for cancer prevention and treatment are briefly discussed.

Keywords: cancer therapy; carcinogenesis; drug delivery system; gut microbiota; modulation.

Figure 1

The role played by pro‐tumoral bacteria in cancer development in both local and distal sites. A) Diminish the intestinal barrier integrity. Barrier breach facilitates bacterial colonization and invasion, biofilm formation, as well as persistent inflammation. B) Form biofilms which enhances the persistent and refractory bacterial infection as well as inflammation. C) Promote tumor‐associated inflammation. Chronic inflammation caused by persistent infection mediated by NF‐kB and STAT3 signaling favor carcinogenesis. D) Produce pro‐tumoral bacterial metabolites, for example, colibactin, H₂S, and polyamines. E) Induce abnormal cell life cycle. Bacterial products mediate cell life cycle‐associated signaling pathways including NF‐κB, β‐catein/Wnt, RAS/MAPK. F) Tumor‐associated bacteria (TAB) mediate carcinogenesis at both local and distal sites due to the translocation of bacteria or bacterial metabolites. Based on the role played by pro‐tumoral bacteria in cancer development, eliminating pro‐tumoral bacteria and bacterial products can be rational strategies for microbiota modulation.

Figure 2

The role played by beneficial bacteria in intestinal homeostasis maintenance and anti‐tumor immunity. A) Suppress the community of pathogenic bacteria. B) Protect the host against bacterial infection. C) Produce regulatory EVs which intervene in the bacterial communities and host physiological activities. D) Produce beneficial metabolites including butyrate and anti‐tumor phytochemical metabolites. E) Maintain immunological homeostasis and inhibit chronic inflammation. F) Alleviate GIT side effects induced by cancer therapy. G) Promote anti‐cancer immunity responses. Specific bacterial communities promote the differentiation of CD8⁺ T cells and enhance sensitivity to cancer therapy. Based on the role played by beneficial bacteria, boosting bacterial communities will favor systemic cancer therapy. Particularly, the microbiota EVs can be potential tools for microbiota modulation. Engineered bacteria based on DDS have natural advantages to be functionalized as cancer therapeutics (e.g., hypoxia tropism, anti‐cancer immune response stimulation).

Figure 3

Modern DDS for microbiota modulation to improve the outcomes of cancer therapy. A) Deliver prebiotics. Prebiotics can be used as sustained release materials to prepare nanoparticles for drug delivery, conjugated with functional groups to form self‐assembly micelles, or conjugated to nanoparticles. Prebiotics are appropriate candidates to be delivered with other cancer therapeutic agents to exert synergy effects. B) Deliver probiotics. The encapsulation of probiotics protects their bioactivity from harsh environments. C) Eliminate pro‐tumoral bacteria. Antibiotics delivered by nanosystems are promising to selectively eliminate pro‐tumoral bacteria and avoid dysbiosis, by virtue of nanotechnologies including surface modification, controlled release, and stimuli‐responsive drug release. Inorganic nanoparticles are effective for anti‐infection therapy, overcoming antibiotic resistance of bacteria to some degree. In addition, synergy of pathogen suppression and probiotic promotion based on DDS has shown its promising outlook in cancer therapy. D) Capture bacteria products. Nanoparticles can be designed to bind or adsorb the bacterial products, and release or intracellularly express bacteria products inhibitors.

Figure 4

DDS for eliminating pro‐tumoral H. pylori. A) Schematic representation of the preparation of AMX‐PLGA/UCCs‐2 nanoparticles. B) The strategy and process to eradicate H. pylori using the pH‐sensitive targeting AMX‐PLGA/UCCs‐2 nanoparticles. C) Scanning electron microscopy images of gastric tissue of normal mice or H. pylori‐infected mice after treated with PBS, AMX, AMXPLGA/Cs nanoparticles, and AMX‐PLGA/UCCs‐2 nanoparticles. Reproduced with permission.^{[ 198 ]} Copyright 2018, Elsevier Ltd. D) Schematic representation of OM‐NPs to inhibit H. pylori adhesion on the stomach lining. OM‐NPs were prepared by coating polymeric cores made from poly(lactic‐co‐glycolic acid) (PLGA) with H. pylori outer membranes containing the adhesins that are critical for bacterial colonization. By mimicking the adhesion of H. pylori onto gastric epithelium, OM‐NPs occupy the binding sites and hence inhibit the colonization of the bacteria. E) Fluorescence images of H. pylori on AGS cells before and after incubation with PBS or OM‐NPs (40 mg  mL⁻¹). Green represents FITC‐labeled H. pylori. Scale bar = 25 µm. F) Remaining H. pylori on AGS cells when incubated with OM‐NPs (10, 20, 30, and 40 mg mL⁻¹) at various time points. Error bars represent standard deviations (n = 3). Reproduced with permission.^{[ 203 ]} Copyright 2019, Wiley‐VCH. G) Schematic preparation of pH‐sensitive GNS@Ab and application for targeted imaging and photothermal therapy of H. pylori with antibiotic resistance. H) Enzyme‐linked immunosorbent assay for detecting H. pylori antigen in stool samples. Data represent mean ± SD (n = 5, ^* p > 0.05, and ^*** p < 0.01). I) The phyla‐level richness of PBS and GNS@Ab groups at different time (0 h, 4 h, 8 h, 1 day, 3 days, 5 days, and 15 days). Reproduced with permission.^{[ 177d ]} Copyright 2019, Elsevier Ltd.

Figure 5

Eliminating pro‐tumoral bacteria with a bioinorganic hybrid system for CRC therapy. A) Schematic illustration of phage‐based bio/abiotic hybrid system (M13@Ag) to regulate gut microbes for cancer‐specific immune therapy. M13@Ag reversed immunosuppressive TME for activation of anti‐tumor immune responses. B) Representative In Vivo Imaging System images presenting the phage accumulation in orthotopic CT26‐luc tumors after treatment with wild phages (without Fn‐binding ability), M13 phages (with Fn affinity), and M13@Ag (n = 3). C) In vivo bioluminescence imaging of orthotopic CT26‐luc tumor‐bearing mice after receiving PBS, M13 phages, AgNP, M13@Ag, and M13@Ag combined with α‐PD1 or FOLFIRI [5‐FU (30 mg kg⁻¹), leucovorin (90 mg kg⁻¹), and IRT (16 mg kg⁻¹)] treatments (n = 5). Reproduced with permission.^{[ 209 ]} Copyright 2020, AAAS.

Figure 6

Delivering prebiotics to boost probiotics for cancer therapy. A) Schematic illustration of drug loading and glutathione‐responsive drug release from drug‐loaded cross‐linked (CR) micelle at a high concentration of glutathione in cancer cells. The micelles will eventually be degraded to nontoxic products, inulin, and lipoic acid. B) Effect of IN, TAN‐loaded NCR, and CR micelles on the proliferative activity of B. longum. C) Toxicity of TAN‐loaded NCR and CR micelles and free TAN in HT29 cells for 24 h. D) Toxicity of TAN‐loaded NCR and CR micelles and free TAN in HT29 cells for 48 h. Data are presented as mean ± SD. Reproduced with permission.^{[ 172b ]} Copyright 2018, Elsevier Ltd. E) Schematic of 5‐FU‐loaded guar gum and xanthan gum nanoparticles. The natural gums play a dual role of achieving colon‐targeted release of 5‐FU and maintain the integrity of gut microbiota along with probiotics supplementation.^{[ 177a ]} F) The molecular structure of NpRg3. Fe@Fe₃O₄ nanoparticles were synthesized with a metal salt reduction and redox process via a programmed microfluidic process, and conjugated with ginsenoside Rg3 by our invented coupling process sequence. G) The ultrastructures of Fe@Fe₃O₄ and NpRg3 were observed by transmission electron microscopy. The apparent core–shell structures of Fe@Fe₃O₄ (I and II) and NpRg3 (III and IV) were shown, and NpRg3 had significant organic coupling compared to that of Fe@Fe₃O₄. H) Compared with the DEN‐30W group, 15 genera were decreased in the DEN‐42W group, but increased after NpRg3 application. I) The ratio of HCC metastasis to the lung among the four groups. ^* p < 0.05, ^*** p < 0.001, and ^**** p < 0.0001. Reproduced with permission.^{[ 177c ]} Copyright 2019, Wiley‐VCH.

Figure 7

Delivering probiotics for microbiota modulation. A) The efficacy of the microencapsulated live L. acidophilus cells on animal tumorigenesis using histochemical analysis. The number of tumors in Apc^Min/+ mice was scored according to 0–5 scale. Control (n = 24) animals were gavaged 0.3 mL of 0.85% saline solution and treatment (n = 24) animals were gavaged with APA microencapsulated L. acidophilus bacterial cells blended in 2% M.F. yogurt. Reproduced with permission.^{[ 194 ]} Copyright 2016, Taylor & Francis. B) Schematic illustration for the preparation of CMCB by extruding bacteria with cell membranes. C) In vivo blood reservation of bacteria. EcN or CMCB (1 × 10⁷ CFUs) were injected through the tail vein and blood was withdrawn intraorbitally at the indicated time points, diluted and spread onto LB agar plates. Plates were incubated at 37 °C for 24 h prior to enumeration. Significance was assessed using Student's t‐test, giving p‐values, ^* p < 0.05, ^** p < 0.01, ^*** p < 0.005. D) Tumor imaging of 4T1 tumor‐bearing mice at 3, 5, 7, and 12 days post‐injection of EcN or CMCB expressing LuxCDABE (1 × 10⁷ CFUs). Reproduced with permission.^{[ 210 ]} Copyright 2019, Springer Nature. E) Schematic illustration of bioinspired oral delivery of gut microbiota with superior oral bioavailability and mucoadhesion by self‐coating with biofilms. F) Total amounts of Bacillus subtilis (BS), biofilm‐coated BS, and biofilm fragments‐coated BS retained in the GI tract. Error bars represent SD (n = 5). ^* p < 0.05, ^** p < 0.01, ^*** p < 0.001, ^**** p < 0.0001, Student's t‐test. Reproduced with permission.^{[ 216 ]} Copyright 2020, AAAS.

Figure 8

Combination cancer therapy with DDS modulating gut microbiota. A) Illustration of the phage‐guided biotic‐abiotic hybrid nanosystem and its therapeutic effects. B) Ex vivo fluorescence imaging of the intratumoral accumulation of D‐IDNPs 24 h after intravenous injection of D‐IDNPs. Images are representative of three biological replicates. Circles indicate tumors. C) Changes in gut microbiota after the treatments. The identified reads of each bacteria and total sequenced 16S rDNA were compared. Three biological replicates are shown. Sequencing was performed at day 14. Reproduced with permission.^{[ 46 ]} Copyright 2019, Springer Nature. D) Illustration of the prebiotics‐encapsulated probiotics to regulate gut microbiota and suppress colon cancer. Bacteria specifically enriching in tumor tissues were screened. C. butyricum was then modified with prebiotic dextran by host–guest chemistry. The system that carried the chemotherapeutic drugs was used orally by mice for colon cancer treatment. E) Schematic illustration for the possible mechanism by which spores‐dex regulate gut microbiota and suppress tumor growth. F) In vivo therapeutic effect of DC@ spore, spores, bacteria, DC, and PBS in mice bearing subcutaneous CT26 tumors. Five biological replicates are shown. G) Fecal SCFA levels after the different treatments. Three biological replicates are shown. Gas chromatography‐mass spectrometry analysis was used to analyze fecal SCFA levels at day 14 of treatments. Three biological replicates are shown. Reproduced with permission.^{[ 211 ]} Copyright 2020, Wiley‐VCH.

Figure 9

Binding bacteria products to promote immunotherapy against CRC. A) The schematic of the trimeric LPS trap and multivalent interaction of the LPS trap with the lipid A region of LPS. The red region is the LPS‐binding moiety. B) Preparation of LPS trap plasmid (pTrap)‐loaded LPD nanoparticles. C) LPS trap protein expression in serum, major organs, and CT26‐FL3 tumor. LPS trap plasmid‐loaded LPD was given on day 0, and the LPS trap expression was measured using enzyme‐linked immunosorbent assay (ELISA) on day 2 (n = 3). ^*** p < 0.001. D) Kaplan–Meier survival curves; the difference between different groups is significant by the log‐rank test. Reproduced with permission.^{[ 177f ]} Copyright 2018, Wiley‐VCH.

Figure 10

Engineered bacteria‐based DDS for cancer therapy. A) E. coli with SLC reach a quorum and induce the phage‐lysis protein ϕX174E, leading to bacterial lysis and release of a constitutively produced, CD47‐blocking nanobody that binds to CD47 on the tumor cell surface. B) Tumor growth curves in BALB/c mice (n = 7 per group) bearing A20 B cell lymphoma tumors received intratumoral injections every 3–4 days with PBS, E. coli with SLC (eSLC) or eSLC undergoes intratumoral quorum lysis to locally release an encoded nanobody antagonist of CD47 (eSLC‐CD47nb). (^**** p < 0.0001, two‐way ANOVA with Tukey's multiple comparisons test). Data are representative of two independent experimental replicates. C) Percentages of intratumoral GzmB⁺CD8⁺ T cells (n  =  5 per group, ^** p =  0.0011, unpaired two‐tailed t‐test). All data are shown as mean  ±  s.e.m. Reproduced with permission.^{[ 212 ]} Copyright 2019, Springer Nature. D) Preparation procedure of YB1‐INPs. Synthesized INPs with single‐step sonication were attached to YB1 through amide bonds. E) YB1‐INPs with hypoxia‐targeting and photothermal‐assisted bioaccumulation for tumor penetrative therapy. After migrating into tumor hypoxic cores and subsequently irradiated with NIR laser, the loosening of tumor tissue and tumor lysis generated bacteria‐attracting nutrients, which further enhanced the accumulation and coverage of YB1‐INPs in large solid tumors. Ultimately, the enriched YB1‐INPs under NIR laser irradiation completely ablated the large solid tumor without relapse. F) In vivo FL imaging of INPs, YB1‐INPs, and YB1‐INPs (+) at different time points. (+) refers to laser irradiation at 12 h for promoting bioaccumulation of YB1‐INPs. The ex vivo NIR FL images of tumors at 72 h. G) Infrared thermal images of MB49 tumor‐bearing mice exposed to laser irradiation after intravenous injection of PBS, INPs or YB1‐INPs at 72 h. Reproduced with permission. ^{[ 213 ]} Copyright 2019, Elsevier Ltd.