Engineered Probiotic Carriers Emerge as Living Drug Factories for Tumor-Targeted Therapy

Engineered probiotic strains are advancing from simple drug carriers toward programmable "living drug factories" capable of homing to hypoxic tumor tissue, sensing the tumor microenvironment and releasing therapeutic payloads on demand, according to a comprehensive review published in the International Journal of Nanomedicine.

The review argues that probiotics offer a biological solution to a persistent problem in oncology drug delivery. Conventional nanomedicines relying on the enhanced permeability and retention effect are cleared rapidly by the mononuclear phagocyte system in the liver and spleen, with delivery efficiency to solid tumors typically below 1% — approximately 0.7%. Dense extracellular matrix and elevated interstitial fluid pressure further impede penetration and homogeneous distribution of drug carriers.

Probiotics, by contrast, exploit the hypoxic regions created by excessive cell proliferation, abnormal angiogenesis and insufficient perfusion. Obligate and facultative anaerobes such as Escherichia coli and Clostridium exhibit tropism toward these niches and can selectively accumulate and proliferate within tumors, traversing abnormal vascular regions and reducing drug leakage.

Dual Role as Carrier and Immunomodulator

Beyond delivery, the review emphasizes that probiotics can participate directly in immune regulation. During surface functionalization and payload loading, probiotics stimulate dendritic cells, lymphocytes and macrophages to produce cytokines and chemokines, promote IgA-related immune responses and induce IgA secretion. Some strains, including Clostridium butyricum查看药品详情, have been shown to enhance the efficacy of immune checkpoint inhibitors — suggesting that rational use of probiotic immunomodulatory properties may help convert "cold tumors" into "hot tumors," though the authors caution about the risk of excessive immune activation and systemic inflammation.

Escherichia coli Nissle 1917 as the Leading Chassis

The review identifies E. coli Nissle 1917 (EcN) as the most widely used chassis strain in engineered probiotic delivery systems, owing to its well-developed genetic toolbox and amenability to complex regulatory circuit construction. One study constructed a dual-responsive EcN strain by introducing lactate-responsive and temperature-responsive operators and decorating the bacterial surface with gold nanoparticles, achieving spatiotemporally controlled release of cytolysin while simultaneously inducing bacterial release of lysozyme and self-destruction.

More recently, Liao and colleagues genetically and nanoengineered EcN to generate IL-12 (搜索)搜索该药品 nanoparticle-loaded EcN (INP-EcN), enabling intratumoral biosynthesis of anti-PD-1查看药品详情 antibodies together with tumor-microenvironment-responsive release of an IL-12 agonist. Western blot analysis confirmed the biosynthesized anti-PD-1 was detected as a 28 kDa protein, consistent with the expected molecular weight of the murine PD-1 (搜索)搜索该靶点 inhibitor, and competition assays with commercial anti-PD-1 antibodies confirmed antagonist capacity. Coculture of mouse T cells with B16F10 melanoma (搜索)搜索该疾病 cells, combined with confocal immunofluorescence imaging and flow cytometry, demonstrated that the biosynthesized antibody effectively blocked PD-1 surface exposure and inhibited PD-1/PD-L1 (搜索)搜索该靶点 interaction. ELISA verified that INP-EcN could rapidly initiate and stably produce anti-PD-1, indicating that EcN's proliferation and protein-production capacity were retained after engineering.

The authors note that EcN can be rapidly eliminated in vivo using antibiotics if adverse events occur, and that an oral microgel delivery system based on electrostatic interactions between chitosan and sodium alginate preserved the beneficial features of therapeutic E. coli while synergistically enhancing the antitumor efficacy of the TGF-β inhibitor galunisertib查看药品详情.

Lactic Acid Bacteria as In Situ Vaccines

Lactic acid bacteria, including Lactobacillus and Lactococcus lactis (搜索)搜索该药品, are generally recognized as safe and well suited for oral administration. Zhu constructed an engineered L. lactis strain (FOLactis) secreting a Flt3L–OX40L fusion protein; following intratumoral administration, the strain remodeled the immune microenvironment in both injected tumors and tumor-draining lymph nodes, promoted dendritic cell-mediated cross-presentation, and enhanced natural killer cell and cytotoxic T lymphocyte responses, driving the canonical cold-to-hot tumor transition.

A separate study simultaneously anchored α-OX40 costimulatory signals and the TLR7/8 (搜索)搜索该靶点 agonist resiquimod查看药品详情 (R848) on the LAB surface, achieving localized costimulation plus innate immune activation within the tumor microenvironment. The immunobacteria-based living platform efficiently promoted dendritic cell maturation and rescued functional exhaustion of cytotoxic T lymphocytes.

Bifidobacterium and Clostridium Platforms

Bifidobacterium exhibits outstanding hypoxic tumor targeting specificity. A biohybrid system termed Bif@BDC-NPs, consisting of B. infantis conjugated with doxorubicin查看药品详情-loaded albumin nanoparticles coated with chitosan, achieved efficient targeting and a tumor inhibition rate of 94% in an in vivo breast cancer (搜索)搜索该疾病 model while significantly reducing the typical side effects of doxorubicin therapy.

A genetically modified cytosine deaminase-expressing Bifidobacterium strain (APS001F), combined with 5-fluorocytosine and anti-PD-1查看药品详情 monoclonal antibody in a CT26 colorectal cancer (搜索)搜索该疾病 mouse model, showed stronger antitumor efficacy and significantly prolonged survival compared with monotherapy, while reducing CD4+ T cells and maintaining a stable proportion of CD8+ T cells.

For Clostridium butyricum查看药品详情, the review cites findings that its surface protein secD binds to the GRP78查看药品详情 receptor on colorectal cancer (搜索)搜索该疾病 cells, thereby abrogating IL-6-mediated immunosuppression of CD8+ T cells and enhancing anti-PD-1查看药品详情 efficacy. However, a separate study evaluating levofloxacin combined with C. butyricum MIYAIRI 588 found that although the combination enhanced PD-1 (搜索)搜索该靶点 therapy, it could also aggravate dextran sulfate sodium-induced colitis, indicating that immune activation may be accompanied by inflammatory risk.

Bacillus Spores as Oral Delivery Vehicles

Spores of Bacillus species are dormant life forms that withstand extreme temperatures, acidic and alkaline environments, desiccation and toxic chemicals, allowing them to transit the stomach safely and germinate in the intestine. Owing to their hydrophobicity and large specific surface area, they can carry substantial amounts of hydrophobic drugs through adsorption or covalent conjugation. One biomimetic spore nanoplatform (SS@Lipo/Hb/GOx/JQ1) associated the B. coagulans spore shell with liposomes co-loaded with hemoglobin, glucose oxidase and JQ1, integrating glucose consumption and Fenton reaction-mediated oxidative damage with JQ1-enhanced immunomodulation.

Combination Strategies Across Therapeutic Modalities

The review details extensive integration of probiotic carriers with existing treatment modalities. In chemotherapy, EcN conjugated with doxorubicin查看药品详情 via an acid-sensitive linker (EcN-ca-Dox) enables tumor accumulation and pH-responsive drug release. In a nasopharyngeal carcinoma model, Lactobacillus rhamnosus-containing next-generation probiotics combined with chemoradiotherapy reduced severe oral mucositis from 14.7% to 2.8% and restored CD4+ T cell proportions.

For radiotherapy, butyric acid was shown to enhance radiosensitivity in colorectal cancer (搜索)搜索该疾病 patient-derived organoids by activating FOXO3A and inducing cell-cycle arrest. In photothermal therapy, Hu and colleagues introduced upconversion nanoparticles that convert near-infrared light into blue light and genetically engineered EcN to express and secrete α-hemolysin under blue light stimulation, inducing in situ thrombosis within tumor tissue that acts as an endogenous photothermal agent.

In sonodynamic therapy, an HMME@BiL complex using B. longum exploited the anaerobic properties of the strain to achieve tumor-targeted delivery of hematoporphyrin monomethyl ether (搜索)搜索该药品. Combined with the STING (搜索)搜索该靶点 agonist SR717, the system promoted infiltration of cytotoxic T cells and natural killer cells into tumor tissue; in a CT26 colorectal cancer (搜索)搜索该疾病 metastasis model, the combination effectively inhibited primary tumors.

Biosafety and Translation Barriers

The review is explicit about unresolved challenges. Genetic or functional modification may alter the native behavior of probiotic strains, potentially enhancing epithelial adhesion or reducing susceptibility to physiological clearance, thereby increasing the risk of unintended colonization and even pathogenicity. Some engineered strains may transfer exogenous genes to resident gut microorganisms, potentially contributing to the dissemination of antibiotic resistance.

Researchers are developing safe delivery platforms incorporating "kill switches," such as auxotrophic or temperature-sensitive self-destruction switches, to ensure timely elimination of probiotics after therapeutic effect. The review also flags that colonization kinetics within the body remain incompletely understood and that quantifying distribution across tissues — especially within the tumor microenvironment — remains a major challenge.

Translating probiotic formulations into clinically compliant products requires standardized workflows and stringent quality control covering strain stability, viability and purity, as well as production environment, scale and final product consistency. Interindividual differences in gut microbiota and their complex interactions with the host immune system may lead to substantial variability in therapeutic outcomes, making personalized probiotic strategies and precise microbiome analyses an important future direction.

The work was supported by the Natural Science Foundation of Jilin Province. The authors declare no competing interests.