Engineered acetylated inulin nanoparticles for enhanced oral insulin delivery: sustained release, structural stability, and in vivo efficacy

Abstract

Oral insulin administration is limited by enzymatic degradation and poor gastrointestinal absorption. This study aimed to develop a biopolymer-based nanocarrier using acetylated inulin (InAc) to improve the structural stability and oral bioavailability of insulin. Inulin was produced from Salinivibrio sp. GM01 and chemically modified via acetylation. Insulin-loaded InAc (InAc-Ins) nanoparticles were prepared and characterized for morphology, size, zeta potential, and encapsulation efficiency. In vitro insulin release was evaluated under simulated gastric (SGF) and small intestinal (SSIF) conditions. In vivo efficacy was determined through oral glucose tolerance tests (OGTT) in mice. The InAc-Ins nanoparticles were spherical with mean diameter of 349 ± 38 nm and high encapsulation efficiency (92.14%). Insulin release half-life were observed in 37.1 hours in SGF and 24.3 hours in SSIF conditions. Biophysical analysis revealed enhanced structural stability of encapsulated insulin, with increased half-life and activation energy for the secondary and tertiary structure denaturation. The secondary structure denaturation half-life increased to 195 min (SGF) and 231 min (SSIF), with denaturation enthalpy of 4.03 kcal mol^-1 and 1.83 kcal mol^-1, respectively. Tertiary structure denaturation half-life were 765 min (SGF) and 919 min (SSIF), and denaturation enthalpy of 18.67 kcal mol^-1 and 4.58 kcal mol^-1, respectively. OGTT results showed that orally administered InAc-Ins nanoparticles reduced blood glucose levels more effectively than free insulin, achieving 42.8% of subcutaneous insulin efficacy. InAc nanoparticles offer effective protection and sustained release of insulin under gastrointestinal conditions, enhancing its structural integrity and hypoglycemic efficacy. This platform presents a promising strategy for non-invasive oral insulin delivery.

Fig. 1. (A) FTIR spectra of inulin from chicory root (black line), In (red line), and InAc (green line). (B) Schematic representation of the acetylation reaction of inulin.

Fig. 2. The tem images of (A) In-B NPs, (B) In-Ins NPs, (C) InAc-B NPs, and (D) InAc-Ins NPs, and the particle sizes distribution of (E) In-B NPs and In-Ins NPs, and (F) InAc-NPs and InAc-Ins NPs.

Fig. 3. EDS result for (A) InAc-B NPs and (B) InAc-Ins NPs.

Fig. 4. Insulin release profiles from (A) In-Ins NPs and (B) InAc-Ins NPs in SGF (pH 1) and SSIF (pH 7) at 37 °C over a four-hours incubation period.

Fig. 5. (A) CD spectra comparing FS insulin and insulin encapsulated within InAc NPs (InAc-Ins NPs). (B) α-Helix content reduction over time for FS insulin and InAc-Ins NPs in simulated gastric fluid (SGF; pH 1, 37 °C) and (C) α-helix content reduction over time for FS insulin and InAc-Ins NPs in simulated small intestinal fluid (SSIF; pH 7, 37 °C). FS insulin shows significantly faster α-helix structure denaturation in SGF compared to InAc-Ins NPs (p-value < 0.05).

Fig. 6. The decrease of fluorescence intensity over time for FS insulin and encapsulated insulin: (A) InAc-Ins NPs in SGF (pH 1) and (B) InAc-Ins NPs in SSIF (pH 7) while incubated at 37 °C. Tertiary structure denaturation of FS insulin is significantly faster in SGF compared to InAc-Ins NPs (p-value < 0.05).

Fig. 7. The effects of inulin-insulin nanoparticles on normal mice. (A) Glucose levels of different treatment groups, (B) glucose levels of different treatment dosage groups, and (C and D) quantification of the AUC, depicted as percentage relative to the control value. *p-value < 0.05 compared to control group.