Nanotherapeutics for the treatment of inflammatory bowel disease

KEYWORDS:

• Nanotherapeutic
• inflammatory bowel diseases
• intravenous injection
• oral administration

1. Introduction

​Inflammatory bowel disease (IBD), which mainly consists of Crohn’s disease (CD) and ulcerative colitis (UC), is a chronic relapsing disease of the gastrointestinal tract (GIT) [1]. The clinical goals of IBD treatment are to control inflammation, achieve mucosal healing, and reduce surgeries and hospitalizations [2,3]. Traditional treatment strategies have been limited to the daily administration of high doses of medications, including biologic therapies, corticosteroids, immunomodulators, aminosalicylates, and antibiotics [4,5]. Although some of these medications have proven effective in initially alleviating IBD, their long-term application had been largely restricted by poor efficacy and serious side effects [6].

Recently, nanoparticle (NP)-based nanomedicine has been widely used in IBD therapy. NPs are characterized by their nanometer-scale dimensions, a targeted drug delivery capacity, and the ability to undergo controlled drug release. Most importantly, drug-loaded NPs confer similar or even better therapeutic efficacies at lower drug concentrations compared to the conventional formulations [7]. In practical treatment, there are two main types of administration strategies for NPs: intravenous injection and oral administration. Generally, intravenous injection is used to treat patients suffering from moderate-to-severe IBD, whereas oral administration is used to treat mild-to-moderate episodes of IBD [8].

2. Intravenous injection

In most solid tumors, vascular permeability is markedly enhanced by the defective architecture of the vessel walls and the existence of huge amounts of vascular permeability factors. This can facilitate the accumulation and penetration of macromolecular drugs or NPs to yield the so-called enhanced permeability and retention (EPR) effect [9]. A similar EPR effect can be seen in inflamed regions of the intestine, which are characterized by defective vessel architectures and poor lymphatic drainage. The rapid angiogenesis in inflamed sites may further contribute to the infiltration of immune cells [10].

In recent years, some studies have demonstrated that NPs accumulate in the inflamed colon following intravenous injection. Watanabe et al. [11] prepared several versions of lipid nano-droplets and injected them into mice that had been subjected to dextran sodium sulfate (DSS)-induced colitis. They found that most of the larger NPs (~180 nm) were accumulated in the livers and spleens of mice by 24 h post-administration, whereas the smaller NPs (~55 nm) were more often found in the blood circulation. Strikingly, however, the medium-sized NPs (~110 nm) preferred to accumulate in regions of intestinal inflammation. Lee et al. [12] fabricated bilirubin NPs (BRNPs) using polyethylene glycol (PEG)-modified bilirubin and found that they increased the water solubility of bilirubin and preferentially accumulated in inflamed colonic regions. These BRNPs accumulated at inflamed sites via the EPR effect alone; they did not have any inherent targeting capability. In 2008, in contrast, Peer et al. [13] developed innovative β₇ integrin-targeting stabilized NPs (β₇ I-tsNPs). The authors exploited the ability of β₇ integrin first to target the specific leukocyte subsets associated with gut inflammation, and then to suppress leukocyte proliferation and T helper cell 1 cytokine expression. In vivo anti-inflammation experiments in the DSS-induced mouse model of colitis demonstrated that β₇ I-tsNPs could be more effective at improving weight loss, histology, and cytokine profiles (e.g. ​interleukin (IL)-10, IL-12, tumor necrosis factor (TNF)-α, Cyclin D1) compared with nontargeted NPs or β₇ integrin-targeted NPs loaded with scrambled siRNAs.

3. Oral administration

Compared with intravenous injection, oral administration is more convenient in many aspects. For example, it avoids the pain and discomfort caused by the intravenous injection, enables patients to easily self-medicate, and greatly reduces the possibility of contamination. However, numerous obstacles have complicated the development and application of oral nanotherapeutics, including the harsh environments of the stomach (low pH and digestive enzymes), small intestine (bicarbonate and bile salts), and colon (microbial population). It is worth noting that mucus is also an important barrier lining the mucosa. It is generally accepted that mucus-penetrating NPs, characterized by size less than the mucus mesh spacing and non-mucoadhesive surface, preferentially penetrate the mucus layer deeply. In contrast, the mucoadhesive NPs tend to stick to mucus components and aggregate locally [14].

Inflamed colon tissues exhibit disruption of the enterocyte layer and accumulation of immune cells beneath this layer, resulting in increased permeability in the inflamed tissue. This phenomenon, termed as the ‘epithelial EPR’ effect, is expected to facilitate the penetration of NPs at inflamed colon sites and thereby increase the local drug concentration [15]. The targeting of drug delivery to the inflamed colon can be divided into three levels: targeting the colon (organ level), targeting inflamed colon sites (tissue level), and targeting specific cells (cell level) [16]. Most of the current oral formulations for IBD therapy, which include pellets, capsules, and tablets, are based on pH-, pressure-, or bacteria-responsive mechanisms [17], and can only perform organ-level targeting. NPs, in contrast, have the potential to accumulate in inflamed colon tissues passively, and studies have shown that reactive oxygen species (ROS)​-sensitive formulations can induce active targeted drug delivery at the tissue level [18]. For cell-level delivery of NPs, receptor-mediated drug delivery is a promising approach for achieving target specificity and avoiding nonspecific interactions.

Anti-inflammation is widely recognized as the main goal in IBD therapy, and thus various macrophage-specific NPs have been developed. The mannose receptor (MR), which is a transmembrane protein of the C-type lectin family, is expressed exclusively on the surfaces of macrophages in inflamed tissue. It can bind specifically to terminal MR-bearing NPs, enabling them to be internalized into macrophages through receptor-mediated endocytosis. For example, Xiao et al. [19] synthesized a mannosylated bioreducible cationic polymer, and further assembled NPs with tripolyphosphate (TPP)​ and TNF-α siRNA (siTNF). Ex vivo experiments demonstrated that these NPs markedly inhibited TNF-α secretion in DSS-induced colitis tissues. Similar to the MR, galactose receptor is also overexpressed on the surface of macrophages. Thus, it can be used as a target site for the interaction of galactose-functionalized NPs. Recently, Zhang et al. [20] developed galactosylated trimethyl chitosan-cysteine (GTC) NPs to treat DSS-induced colitis via the oral delivery of a mitogen-activated protein kinase kinase kinase kinase 4 siRNA (siMap4k4) to the activated macrophages. The authors found that siMap4k4-loaded GTC/TPP NPs effectively decreased TNF-α production and improved DSS-induced body weight loss, colon length shortening, and myeloperoxidase (MPO) activity in an in vivo experimental setting. In addition, F4/80 glycoprotein is also a common macrophage marker. Laroui et al. [21]​ fabricated siTNF-loaded NPs grafted with the fragment antigen-binding (Fab’)​ fragment of the F4/80 antibody. In an in vivo experiment, Fab’-bearing NPs loaded with siTNF accumulated efficiently in colitis regions and improved many physiological signs (e.g. weight loss, MPO activity, and ​ accumulation of nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor-α) compared with nontargeted NPs, thereby showing better therapeutic potential and targeted drug delivery.

The second goal of clinical IBD therapy is to achieve mucosal healing [2]. Toward this end, the intestinal epithelial cell is another target cell in IBD therapy. CD98 glycoprotein is a type II membrane glycoprotein heterodimer that comprises a non-glycosylated light chain and a glycosylated heavy chain. Previous studies showed that CD98 is aberrantly overexpressed in intestinal macrophages and the apical plasma membrane of epithelial cells [22]. Moreover, CD98 expression is highly upregulated in colonic epithelial cells from mice with active colitis [23]. Xiao et al. [24] fabricated CD98 Fab’-bearing quantum dot-loaded NPs (Fab’-NPs) to investigate whether CD98 could be used as a colitis-targeting delivery receptor. Their results demonstrated that Fab’-NPs showed better internalization into colonic epithelial cell lines and Raw 264.7 macrophages, and accumulated more in colitis tissues in an ex vivo experiment, compared with their nontargeted counterparts (PEG-NPs). The CD44 receptor, which was recently found to be overexpressed on the surface of epithelial cells and activated inflammatory cells in colitis tissues [25], has also been applied as a targeting receptor for drug delivery. Hyaluronic acid (HA) is a natural polyanionic polysaccharide that has a high affinity for the CD44 receptor. Xiao et al. [26,27] generated HA-functionalized NPs, and found that they effectively guided the drug delivery to UC-related target cells (colonic epithelial cells and macrophages).

4. Challenges and prospects

Nanotherapeutic seems to be very promising in IBD therapy. Although some progress has been made in recent years, there are still many challenges ahead. Most studies have concentrated on inhibiting inflammatory responses, such by decreasing the synthesis/secretion of pro-inflammatory cytokines.​ Very few nanotherapeutic studies have pursued mucosal healing, which is the second main goal for IBD therapy. Future studies should seek to achieve both anti-inflammation and mucosal healing. We should also strengthen the ability of NPs to target inflamed sites while taking full advantage of the different environmental parameters (e.g. the levels of ROS and matrix metalloproteinases) to develop stimulus-responsive NPs. For a translational implementation of the nanotherapeutics, numerous issues have to be addressed in the future, including the following: (1) the in vivo safety of nanotherapeutics after administration; (2) the in vivo stability of nanotherapeutics during transit in blood or GIT; (3) the drug retention amount and drug residence time, which have to be further optimized; and (4) simplification of nanotherapeutics, which facilitate the large-scale manufacture.

Declaration of interest

The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Additional information

Funding

This work was supported by grants from the National Institute of Diabetes and Digestive and Kidney Diseases​ [RO1-DK-071594], the National Natural Science Foundation of China [51503172, 81571807], the Fundamental Research Funds for the Central Universities [SWU114086], the Young Core Teacher Program of the Municipal Higher ​Educational Institution of Chongqing, and the Department of Veterans Affairs [Merit Award BX002526]. D Merlin is a recipient of a Career Scientist Award from the Department of Veterans Affairs.​