Introduction

Oral chemotherapy is an exemplary drug administration route with a high interest in pharmaceutical research and development. Oral drug administration has several benefits, such as i) patient compliance, ii) cost-effectiveness, iii) prevention of discomfort and pain linked with the use of needles in intravenous therapy, iv) reduction of additional hospitalization, etc. (Borner et al., 2001; Gornas and Szczylik, 2007; Liu et al., 2016). However, a drug must effectively pass through several barriers that can reduce oral bioavailability. Some of the most common issues resulting in low drug bioavailability are: i) poor drug solubility and permeability, ii) strong acidic conditions in the stomach, iii) digestive enzymes and natural flora of the gastrointestinal (GI) tract causing drug degradation, iv) mucus barriers preventing the drug absorption, and v) efflux transporters, such as P-glycoprotein (P-gp) and multidrug resistance proteins (MRPs), pumping drugs back into the intestinal lumen after they have entered intestinal cells (Huang et al., 2015; Mazzaferro et al., 2013; Pathak and Raghuvanshi, 2015). For instance, anticancer drugs such as paclitaxel and doxorubicin have an extremely low oral bioavailability of not more than 1% and 5%, respectively (Mei et al., 2013). Some rational attempts to increase bioavailability by encapsulating chemotherapeutics for oral drug delivery in a dispersion formulation include nano-emulsions, dendrimers, micelles, liposomes, solid-lipid nanoparticles, and cubosomes (Mei et al., 2013; Pimenta et al., 2023; Sohail et al., 2018). Furthermore, delivering the drug safely to a specific segment of the GI tract (e.g., the small intestine) for maximum drug absorption or site-specific delivery of chemotherapeutics remains a key interest for researchers.

Daunorubicin (a red pigment) is an anthracycline glycoside produced as a secondary metabolite by Streptomyces peucetius (Vasanthakumar et al., 2013). It is a chemotherapeutic agent used for the treatment of certain types of cancer, such as acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and chronic myelogenous leukemia (CML) (Grein, 1987; Thirumaran et al., 2007). The proposed mechanism of action is interference with DNA, topoisomerase I and II, entrapment of reactive oxygen species, and induction of apoptosis (Ahmad et al., 2019). Commercially, the drug is only available in the form of injections, e.g., DaunoXome® and VYXEOS™. However, intravenous therapy can never completely overcome severe side effects like cardiotoxicity, myelosuppression, tissue death at the site of injection, ulcers, and multi-drug resistance due to high doses (Ahmad et al., 2019). Nevertheless, oral chemotherapy is a less preferred option due to the drug's low bioavailability. Various oral formulations of daunorubicin have been tested in the past to cope with this. However, these formulations are still in the experimental phase, and to our knowledge, none have been commercially available until now. In the broader context of oral anthracycline delivery, most formulation studies have focused on doxorubicin rather than daunorubicin. These include polymeric or polysaccharide-based carriers, chitosan-coated nanoparticles or liposomes, lipid-based nanocarriers, and self-nanoemulsifying systems that have been designed to reduce premature gastric release, improve intestinal stability or permeability, and enhance oral bioavailability (Benival and Devarajan, 2012; Qin et al., 2024; Su et al., 2016; Usmani et al., 2019; Wu et al., 2020). However, direct studies on oral daunorubicin formulations remain comparatively limited (Ahmad et al., 2019; Lam et al., 2014a), which leaves scope for alternative delivery systems that can protect daunorubicin during gastric transit and release it in the intestinal environment.

To improve bioavailability by achieving the highest possible concentration of the free drug in the intestine (below the dose-limiting toxicity to the gastrointestinal epithelium), stable encapsulation is required during transit from the mouth to the stomach to prevent (acidic) degradation, together with a rapid burst release upon entering the intestine. Simultaneously, masking the drug's bad taste, and protecting the GI tract against high concentrations of chemotoxic drugs are also required. If ingested directly, the free drug can cause mouth ulcers and local tissue necrosis when contacting the mucus membranes directly (Ahmad et al., 2019; Lu et al., 2009; Wang et al., 2011).

In this study, we propose a novel encapsulation system, i.e., nanoparticle-stabilized antibubbles, for daunorubicin's oral/intestinal delivery. The antibubbles contain liquid core(s) surrounded by an air shell in a bulk liquid, i.e., in the form of a water-in-air-in-water dispersion (Zia et al., 2024). Such antibubbles are the freeze-dried and reconstituted product of particle-stabilized double emulsions (water-in-oil-in-water) produced using cryoprotecting sugars (e.g., maltodextrin) in the aqueous phases and a volatile oil (Poortinga, 2013; Zia et al., 2022). The double emulsion is frozen (preferably via quick freezing), followed by freeze-drying to remove the water and volatile oil. The freeze-dried powder, on rehydration of the sugar-containing compartments, will give the required antibubbles. The cryoprotectant prevents the collapse of the structure during freeze-drying, maintains the structure's integrity, and attracts water upon rehydration. The hydrophilic drug (such as daunorubicin) can be incorporated inside the liquid cores. Previously, we developed antibubbles that degrade and release their contents at low pH (e.g., within the stomach) (Zia et al., 2023). However, further research is needed to develop antibubbles that can release the drug in the intestine, e.g., which become unstable in response to external stimuli inside the GI tract. Therefore, the focus of this study was to produce daunorubicin-loaded antibubbles to protect the drug at low pH (i.e., stomach) and trigger its release in the intestinal environment, rather than to act as an absorbable particulate carrier.