**Abstract**

Nanoparticles (NPs) offer a promising solution for orally delivering therapeutic substances due to their capability to surpass traditional drug delivery system (DDS) limitations like low solubility, bioavailability, and stability. However, the possible toxic effects of using NPs for oral therapeutic delivery raise significant concerns, as they might interact with biological systems unexpectedly. This chapter aims to comprehensively understand the potential toxicity of NPs employed in oral therapeutic delivery. Factors such as size, surface area, surface charge, and surface chemistry of NPs can impact their toxicity levels. Both in vitro and in vivo models have been utilised to evaluate NPs toxicity, with in vivo models being more suitable for anticipating human toxicity. The possible toxic consequences of different NPs varieties, including polymer, lipid, and metal NPs, have been documented. Ultimately, grasping the potential toxicity of NPs in oral therapeutic delivery is essential for creating safe and effective DDS.

**Keywords:** nanoparticles, oral drug delivery, cytotoxicity, inflammation, intestinal microbiota

### **1. Introduction**

The use of nanoparticles (NPs) for oral drug delivery is a ground-breaking and rapidly growing area of research [1]. NPs are solid colloidal drug delivery systems (DDS) with sizes ranging from 1 nm to 1000 nm. They comprise polymers, lipids, carbon, silica, or metal encapsulating the drug moiety [2–5]. Encapsulating drugs within NPs can increase stability, solubility, and bioavailability, ultimately boosting their therapeutic effectiveness [6]. Recent advances in biomedical research have led to the successful enhancement of therapeutic agents for treating various diseases. However, a significant challenge remains in efficiently delivering these agents to the target site [7]. NPs can be engineered with specific characteristics to optimise the DDS [4]. For example, biocompatible polymer NPs are coated to enhance their presence in the bloodstream or be equipped with unique ligands or antibodies that focus on specific target cells or tissues [2]. By directing drug delivery to specific locations within the body, NPs can reduce negative side effects and increase the effectiveness of treatments [8].

Oral drug administration is the most prevalent method and the favoured option for patients, as it is non-invasive, user-friendly, and well-accepted [9]. However, traditional forms like tablets and capsules may have a rapid and inadequately

regulated drug release, potentially leading to drug degradation and alteration due to the gastrointestinal tract (GIT) environment (like, changes in pH, digestive enzymes, and microbiota) [10]. NPs can address these issues by safeguarding drugs from degradation and promoting their absorption through the intestinal barrier. Moreover, oral drugs can be directed to specific areas within the GIT to provide localised treatment for stomach and colorectal cancers, infections, inflammation, bowel disorders, gastro-duodenal ulcers, and gastroesophageal reflux issues [11]. GIT fluid consists of enzymes, acids, and other compounds that quickly decompose drugs, resulting in inadequate absorption and decreased effectiveness. Furthermore, a mucus layer in the GI tract can hinder drug absorption by acting as a barrier, restricting their access to the intestinal epithelium [12, 13]. Thus, using NPs as an oral DDS holds great promise due to their unique properties (which include good therapeutic properties, selfassembly, enhanced biocompatibility, serving as vehicles for antimicrobial agents, controlled drug release, reducing off-target effects, etc.) [2, 4, 14].

Although NPs-based DDS offer promising benefits, their safety must be thoroughly evaluated. The small size of NPs can result in interactions with biological systems that are not yet fully understood, possibly causing toxic consequences. Furthermore, NPs can accumulate in specific tissues or organs, leading to long-lasting damage. A recent study by Cabellos et al. [15] showed that mice exposed to silica NPs suffered inflammation and epithelial damage in the intestines and showed the expression of autophagy proteins [4, 16]. Various mechanisms can contribute to the potential toxicity of NPs. For example, they can cause oxidative stress by generating reactive oxygen species (ROS) that harm cell components [17]. NPs can also trigger inflammatory reactions by activating immune cells, causing tissue injury and malfunction [18]. Moreover, certain NPs might exhibit genotoxic effects by damaging DNA, potentially resulting in mutations and cancer [19]. However, it is crucial to meticulously assess the possible toxicity of NPs to ensure their safety and effectiveness [20, 21]. By refining the development and analysis of NPs, one can reduce toxicity and increase therapeutic advantages by modifying functional groups, coating with cell membranes, or using nanorobots [22]. For example, DDS can be developed using stable intestinal cell-derived exosomes that target colonic cells. It is even known that these exosomes can be found in faces. Lipid NPs derived from plants can also be used as an alternative strategy, thus providing a safe NPs delivery [23, 24]. Thereby, conducting comprehensive pre-clinical and clinical research is vital to evaluate the safety of NPs in the context of orally administered treatments. Such investigations should examine the biodistribution, pharmacokinetics, and toxicity of NPs in both animal models and human subjects [21, 25]. Specifically, close observation of NPs accumulation in different organs and tissues is necessary, along with monitoring any negative impacts on cellular and molecular functions [26]. Therefore, the present chapter delves into the diverse toxicological issues of orally administered NPs (from polymer, lipid, protein, carbon, silica, and metal-based NPs) and summarises the in vitro and *in vivo* assessments of the toxicity of oral NPs.
