Oral delivery of insulin loaded into peptide-conjugated polymeric nanoparticles in diabetic rats

The epidemic diffusion of overweight/obesity, together with physical inactivity, aging, urbanization, and population growth are the principal responsibles of the steadily increasing global prevalence of diabetes mellitus: from 2.8% in 2000 (= 171 million people) to a projected value of 4.4% in 2030 (= 366 million people). People with type 1 (insulin-dependent) diabetes mellitus, and also a consistent number of people with type 2 (non insulin-dependent) diabetes mellitus require multiple daily injections of insulin. It can be estimated that several dozens million people self-inject insulin every day. Thus, alternative routes of administration have been extensively investigated, especially the oral route. A successful oral formulation of insulin would have to bypass the two main barriers against the oral delivery of proteins: the enzymatic barrier of the gastrointestinal tract and the physical barrier made up of the intestinal epithelium. The most effective devices so far prepared have produced a maximum oral bioavailability of insulin of 13%, but a measurable absorption could be obtained only starting from the dose of 50 I.U.kg-1. We have recently demonstrated that polymeric nanoparticles (Np) made of the polyester poly(D,L-lactide-co-glycolide)(PLGA) conjugated with the simil-opioid glycosilated heptapeptide Gly-L-Phe-D-Th-Gly-L-Phe-L-Leu-L-Ser-(O-β-D-Glucose)-CONH2 (simil-opioid peptide-conjugated polymeric nanoparticles: SOP-Np) are able to cross the blood-brain barrier (BBB) much more effectively than the other so far prepared nanoparticles: the rationale of this approach lied on the known possibility for several opioid peptides to cross the BBB and other barriers, including the intestinal wall, by the activity of selective transport systems. Thus, aim of the present research was to investigate the suitability of SOP-Np as carriers for insulin across the intestinal barrier. Method: SOP-Np were prepared (185-220 nm size; polydispersity index 0.10-0.15; -35/-39 mV surface charge) and loaded with insulin (35 I.U./100 mg nanoparticles). Different amounts of such insulin-loaded SOP-Np were administered by oral gavage to diabetic rats, so to give 1, 3, or 10 I.U.kg-1 of insulin. A group of rats received by the same route 10 I.U.kg-1 of insulin in aqueous solution. Results: The aqueous solution of insulin produced only a negligible, non-significant decrease of glycemia; on the other hand, in rats treated with insulin-loaded SOP-Np, a dose-related increase of plasma insulin levels was observed, and a consequent, dose-related, and sustained decrease of glycemia was obtained: -50%, 90-180 min after the dose of 3 I.U.kg-1, -50/-70%, 120-240 min after the dose of 10 I.U.kg-1 (ANOVA followed by Bonferroni test: P<0.002/0.0001). Conclusion: The present results show that, in diabetic rats, nanoparticles of very low size, made of PLGA conjugated with a glycosilated simil-opioid heptapeptide, are able to effectively transport insulin across the intestinal barrier, preserving the biological activity of the hormone. If replicated in humans, these results could mean that the oral administration of insulin may be a concrete possibility.