Drug delivery via the oral route is preferred over other strategies, owing to high patient compliance and the ease of home-based drug ingestion [1, 2]. However, because of the low permeability and poor enzymatic stability of large molecules in the gastrointestinal (GI) tract, the oral route is usually not appropriate when proteins/peptides are directly delivered in a free form. The effective oral administration of protein-based drugs remains a challenge [3]. Insulin, the first protein-based drug synthesized for human use, has been used for the treatment of diabetes since 1921. Insulin is a 51-polypeptide hormone comprising an A chain (21 amino acids) and B chain (30 amino acids) linked by two disulfide bonds [4]. Its large size and hydrophilic nature hinder its permeation through absorption barriers. To date, most major commercial formulations of insulin are administered by subcutaneous injection. This conventional route of protein delivery may lead to poor patient compliance because of inconvenience and pain. To improve patient compliance, the oral route is considered a better alternative. In addition, the oral delivery of insulin is advantageous because it closely imitates the physiological behavior of endogenous insulin [2, 5]. Around 80% of oral/pancreatic insulin is cleared in the liver, while the rest reaches systemic circulation. Oral administration can thus avoid subcutaneous injection-related hyperinsulinemia. Several approaches have been adopted in recent decades to improve the oral bioavailability of insulin and other proteins/peptides [2, 6, 7]. Nevertheless, the development of a commercial oral formulation for insulin remains a challenge because no strategies have been able to successfully overcome both physicochemical drawbacks (molecular size, stability, and high hydrophilicity) and biological barriers (proteolysis in the stomach, poor permeation, and membrane efflux) [8, 9].
Recently, several products have completed or are currently undergoing phase II or phase III clinical trials, including oral insulin capsules (NIDDK; NIH, Bethesda, MD, USA), rH-insulin crystals (Technische Universität München, Munich, Germany), oral ORMD-0801 (Oramed, Jerusalem, Israel), Oshadi Icp (Oshadi Drug Administration, Rehovot, Israel), IN-105 (Biocon, Bangalore, India), insulin 338 (GIPET1), insulin glargine (Novo Nordisk, Bagsværd, Denmark), and an oral formulation of insulin (Nutrinia, Ramat Gan, Israel) [10, 11, 12]. Most approaches for oral insulin administration have focused on structural modifications, absorption and penetration enhancers, complicated carriers (nanoparticles, polymer micelles, liposomes), or enzyme inhibitors. Although these products have marginally improved the oral bioavailability of insulin, they require complicated formulations and excessive bioactive additives, which result in undesirable effects, increased manufacturing costs, and a high risk of drug development. Notably, Banerjee et al. [2] developed an ionic liquid (IL)-based oral formulation of insulin (insulin-CAGE). They reported an unprecedented improvement in the oral bioavailability of insulin. The oral bioavailability of 5 U/kg IJ insulin-CAGE was found to be 51% higher than that of 2 U/kg subcutaneous injection. Nonetheless, IL preparations have their own drawbacks, such as potential long-term toxicity, negative effects on the GI tract, and low biocompatibility. Hence, there has been increasing interest in drug delivery research to develop oral carriers of insulin showing both high efficacy and safety.
Our recent study reported a technique to encapsulate and “dissolve” water-soluble chemotherapeutic agents into vegetable oil by forming “oil-soluble” reversed lipid nanoparticles (ORLN) [13, 14]. This carrier can either decrease the intestinal hydrolytic degradation of topotecan (TPT) via protection by both a phospholipid (PC) shell and oil medium, or improve the oral absorption of TPT by enhancing the intestinal lymphatic transport.
In the present study, we designed an oral insulin formulation based on the ORLN system, in which amphipathic PC molecules could self-assemble to construct an internal polar pool for insulin molecules, with non-polar tails radiating to the outer oil phase to form ORLN. ORLN-insulin dispersed in medium-chain triglyceride (MCT) was prepared as an oral formulation to evaluate its efficacy and stability both in vitro and in vivo.