Characterization of the physicochemical interactions between exenatide and two intestinal permeation enhancers: Sodium caprate (C) and salcaprozate sodium (SNAC).

A common approach to tackle the poor intestinal membrane permeability of peptides after oral administration is to formulate them with a permeation enhancer (PE). Increased oral bioavailability for oral peptide candidates has been reported from clinical trials when either salcaprozate sodium (SNAC) or sodium caprate (C) is incorporated in the formulation. However, little is known about how they physically interact with peptides in solution. Our objective was to compare the biophysical interactions between the GLP-1 analogue exenatide (Byetta®, Lilly), and C or SNAC using a variety of advanced analytical techniques. First, critical micelle concentration was measured in different buffers for both PEs. Dynamic light scattering (DLS) measurements revealed specific supramolecular structures arising from exenatide-PE association. Surface plasmon resonance (SPR) indicated the formation of exenatide-PE complexes with a high contribution from non-specific interactions and rapid binding kinetics, resulting in overall low affinities. DLS and isothermal titration calorimetry (ITC) were used to examine the supramolecular organization of the PEs, and revealed thermodynamic signatures characterized by unfavourable enthalpic contributions compensated by favourable entropic ones, but with low-affinity estimates in water (K in the 10-100 µM range). With affinity capillary electrophoresis (ACE), weak interactions between exenatide and SNAC or C were confirmed in saline, with a dissociation constant around 10 µM and 30 µM respectively. In biorelevant intestinal media, the bile salts in FaSSIF and FeSSIF further reduced the binding of both agents to exenatide (K ≈ 100 µM), indicating that the interaction between the PEs and exenatide might be inhibited by bile salts in the GI lumen. This study suggests that the interactions of both PEs with exenatide follow a similar non-covalent mechanism and are of low affinity.