Colorectal cancer (CRC), characterized with a high incidence rate (∼ 10% of all cancer) and a high mortality rate (47.8% worldwide), ranks as the third most prevalent cancer and induces a majority of worldwide cancer related death. [1] In spite of the tremendous progress achieved in cancer diagnose, CRC management continues to face challenges, leading to persistently high incidence and mortality rates. Especially for stage 4 CRC patients, the overall 5-year survival rate is <10%. [2] Besides，it is worth to mention that about 35% of CRC patients have metastatic tumor in distal organ such as liver at the time of first diagnosis, which is a major cause of CRC related death. [3] The difficulty in diagnosis and treatment of CRC should attribute to the complex interactions between CRC with intestinal microbiota, incompletely digested food residues and feces. [4,5] Therefore, the development of effective treatments for CRC is an urgent unmet clinical need.

The treatment of CRC now focuses on surgery, radiotherapy and chemotherapy. As for immunotherapy based on immune checkpoint inhibitors (αPD-1 and αPD-L1 antibodies), only <10% of patients with high microsatellite instability have treatment responsiveness to this therapy. [6,7] Surgery and radiotherapy can only solve non-metastatic local CRC, and only achieve little improvement in the overall prognosis of patients with advanced CRC. [8] Therefore, systematic chemotherapeutics remains essential for eradicating residual and metastatic cancer cells. [9] While, the rapid clearance of chemotherapeutics from the bloodstream via intravenous administration hinders their adequate accumulation in CRC tissues. [10] Although strategies like single high-dosage or multiple low-dosage intravenous administrations may enhance drug accumulation in CRC tissues, their therapeutic efficacy is constrained by severe off-target toxicities, potential drug resistance, and metastasis. [11] Drug delivery system based on nanotechnology provides a powerful tool for improving drug biodistribution and bioavailability in treating many kinds of cancers, especially in mice with primary hepatocellular carcinoma and some subcutaneous tumors, due to the enhanced permeability and retention effect of cancer tissues. [[12], [13], [14], [15]] Nevertheless, intravenous nanomedicine still suffers from the challenge of primary CRC accumulation, which should attribute to the indirectly reaching the lesion and the clearance of reticuloendothelial system in circulation.

Given that primary CRC major occurs in the large intestine, oral administration represents a potentially more efficient and noninvasive delivery route for CRC management over intravenous administration. [16] Oral aspirin have been proved to reduce the risk of CRC through modulating CRC related inflammation. [17] Nevertheless, significant challenges caused by physicochemical and biological barriers of the digestive (GI) tract restrict using oral aspirin for CRC treatment. [18,19] Daily oral high dosage of free aspirin (325 mg) shows promising potential in regulating the composition of gut microbiota and microbial-associated inflammation, while, such high dosage of free aspirin may cause severe immune system disorders. [20,21] Orally administered nanomedicine offers multi advantages over free drugs in CRC treatment, including improved target delivery and accumulation, controlled drug release, enhanced drug solubility and stability, and increased cellular uptake. These benefits rely on optimizing the nanocarriers types and the physicochemical properties of nanomedicines, such as size and surface charge. [[22], [23], [24]] Indeed, the nano size of oral nanomedicine acts as an important factor in enhancing drug therapeutic efficacy for treating colonic disease, including inflammatory bowel disease and CRC. Compared with free small molecular drugs, nano-sized carriers have been proved to improve colonic residence time in inflamed intestinal regions, facilitating active drug accumulation into pathological colon tissues, due to the epithelial enhanced permeability and retention (eEPR) effect. [25,26] Moreover, nano-sized carriers can evade rapid elimination by being readily taken up into inflamed tissue and cells, a benefit lacking in conventional formulations designed for regional drug deposition in the gastrointestinal tract. This preferential accumulation in inflamed tissues increases the local concentration of therapeutics against CRC. [27] Nano-sized carriers also penetrate biological barriers more efficiently, including the intestinal mucosa, facilitating deeper drug delivery into tumor tissues. [28] Oral nanomaterials also hold promising potential in modulating immune cells in the gut and affecting systemic immunity. [29,30] Elaborately constructed oral nanomedicine has emerged as a promising candidate to not only overcome these barriers but also to modulate immunity and gut microbiota. For example, oral inulin gel has been demonstrated to increase the relative abundances of short-chain-fatty-acid associated microorganisms, resulting in enhanced interferon-γ+CD8+ T cells infiltration in tumor tissues. [31]

In our previous work, we developed a reactive oxygen species (ROS)-sensitive dextran-aspirin conjugate named P3C-Asp, which demonstrated the capability to improve the immunosuppressive tumor microenvironment (TME) in a subcutaneous tumor model after intravenous administration [13]. We hypothesized that P3C-Asp might be very suitable for oral treatment of primary CRC as it could realize the following functions: 1) protecting aspirin from absorption in the small intestine and hydrolysis by gastric juice with self-assembled nanostructures, and enhancing its enrichment in the primary CRC tissue; 2) releasing salicylic acid (SA) in response to high ROS level of primary CRC tissue; 3) scavenging ROS to synergistically reduce inflammation with SA; 4) modulating gut microbiota to promote the proliferation of probiotics under the synergistical effect of prebiotic (dextran) and inflammation relief (Scheme 1). We verified the therapeutic effect of oral P3C-Asp in the primary CRC model, and comprehensively evaluated its effect on inflammation modulation in CRC tissue, gut microbiota composition and immune TME. Additionally, we further demonstrated that oral P3C-Asp could enhance the response rate of primary CRC to αPD-L1 immunotherapy. Noninvasive and function-rich oral prebiotic-drug nanomedicine presents a promising route for designing novel treatment in gastrointestinal diseases.