Design of the novel protraction mechanism of insulin degludec, an ultra-long-acting basal insulin

Purpose Basal insulins with improved kinetic properties can potentially be produced using acylation by fatty acids that enable soluble, high-molecular weight complexes to form post-injection. A series of insulins, acylated at B29 with fatty acids via glutamic acid spacers, were examined to deduce the structural requirements. Methods Self-association, molecular masses and hexameric conformations of the insulins were studied using size exclusion chromatography monitored by UV or multi-angle light scattering and dynamic light scattering, and circular dichroism spectroscopy (CDS) in environments (changing phenol and zinc concentration) simulating a pharmaceutical formulation and changes following subcutaneous injection. Results With depletion of phenol, insulin degludec and another fatty diacid–insulin analogue formed high molecular mass filament-like complexes, which disintegrated with depletion of zinc. CDS showed these analogues adopting stable T3R3 conformation in presence of phenol and zinc, changing to T6 with depletion of phenol. These findings suggest insulin degludec is dihexameric in pharmaceutical formulation becoming multihexameric after injection. The analogues showed weak dimeric association, indicating rapid release of monomers following hexamer disassembly. Conclusions Insulins can be engineered that remain soluble but become highly self-associated after injection, slowly releasing monomers; this is critically dependent on the acylation moiety. One such analogue, insulin degludec, has therapeutic potential.