Oral delivery of bioencapsulated exendin‐4 expressed in chloroplasts lowers blood glucose level in mice and stimulates insulin secretion in beta‐ TC 6 cells

Summary Glucagon-like peptide ( GLP -1) increases insulin secretion but is rapidly degraded (half-life: 2 min in circulation). GLP -1 analogue, exenatide ( B yetta) has a longer half-life (3.3-4 h) with potent insulinotropic effects but requires cold storage, daily abdominal injections with short shelf life. Because patients with diabetes take >60 000 injections in their life time, alternative delivery methods are highly desired. Exenatide is ideal for oral delivery because insulinotropism is glucose dependent, with reduced risk of hypoglycaemia even at higher doses. Therefore, exendin-4 ( EX4 ) was expressed as a cholera toxin B subunit ( CTB )-fusion protein in tobacco chloroplasts to facilitate bioencapsulation within plant cells and transmucosal delivery in the gut via GM 1 receptors present in the intestinal epithelium. The transgene integration was confirmed by PCR and S outhern blot analysis. Expression level of CTB - EX 4 reached up to 14.3% of total leaf protein ( TLP ). Lyophilization of leaf material increased therapeutic protein concentration by 12- to 24-fold, extended their shelf life up to 15 months when stored at room temperature and eliminated microbes present in fresh leaves. The pentameric structure, disulphide bonds and functionality of CTB - EX 4 were well preserved in lyophilized materials. Chloroplast-derived CTB - EX 4 showed increased insulin secretion similar to the commercial EX 4 in beta- TC 6, a mouse pancreatic cell line. Even when 5000-fold excess dose of CTB - EX 4 was orally delivered, it stimulated insulin secretion similar to the intraperitoneal injection of commercial EX 4 but did not cause hypoglycaemia in mice. Oral delivery of the bioencapsulated EX 4 should eliminate injections, increase patient compliance/convenience and significantly lower their cost.