Amyloid Fibrillation of Insulin: Amelioration Strategies and Implications for Translation

Insulin is a therapeutically relevant molecule with use in treating diabetes patients. Unfortunately, it undergoes a range of untoward and often unpredictable physical transformations due to alterations in its biochemical environment, including pH, ionic strength, temperature, agitation, and exposure to hydrophobic surfaces. The transformations are prevalent in its physiologically active monomeric form, while the zinc cation-coordinated hexamer, although physiologically inactive, is stable and less susceptible to fibrillation. The resultant molecular reconfiguration, including unfolding, misfolding, and hydrophobic interactions, often results in agglomeration, amyloid fibrillogenesis, and precipitation. As a result, a part of the dose is lost, causing a compromised therapeutic efficacy. Besides, the amyloid fibrils form insoluble deposits, trigger immunologic reactions, and harbor cytotoxic potential. The physical transformations also hold back a successful translation of non-parenteral insulin formulations, in addition to challenges related to encapsulation, chemical modification, purification, storage, and dosing. This review revisits the mechanisms and challenges that drive such physical transformations in insulin, with an emphasis on the observed amyloid fibrillation, and presents a critique of the current amelioration strategies before prioritizing some future research objectives.

Insulin Hexamer-Caged Gadolinium Ion as MRI Contrast-o-phore

High-relaxivity protein-complexes of Gd^III are being pursued as MRI contrast agents in hope that they can be used at much lower doses that would minimize toxic-side effects of Gd^III release from traditional contrast agents. We construct here a new type of protein-based MRI contrast agent, a proteinaceous cage based on a stable insulin hexamer in which Gd^III is captured inside a water filled cavity. The macromolecular structure and the large number of "free" Gd^III coordination sites available for water binding lead to exceptionally high relaxivities per one Gd^III ion. The Gd^III slowly diffuses out of this cage, but this diffusion can be prevented by addition of ligands that bind to the hexamer. The ligands that trigger structural changes in the hexamer, SCN^- , Cl^- and phenols, modulate relaxivities through an outside-in signaling that is allosterically transduced through the protein cage. Contrast-o-phores based on protein-caged metal ions have potential to become clinical contrast agents with environmentally-sensitive properties.

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 T(3)R(3) conformation in presence of phenol and zinc, changing to T(6) 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.