Intrinsic fibrillation of fast-acting insulin analogs

Monitoring insulin fibrillation kinetics using chromatographic analysis

Insulin is a small protein widely used to treat patients with diabetes and is a commonly used model for protein fibrillation studies. Under specific conditions, such as low pH and high temperature, insulin monomers aggregate to form fibrils. This aggregation is problematic for manufacturing and storage of insulin. The thioflavin T (ThT) assay is commonly used to study amyloid fibrillation but suffers from several limitations, such as the effect of protein concentration, the size of the amyloid fibrillar bundles, competitive binding, and fibril aggregation, all of which hinder precise quantitative analysis. Here, we present a method for studying the kinetics of insulin fibrillation utilizing ultra-performance liquid chromatography (UPLC). This method enables the quantitative detection of soluble insulin components, including chemically modified components. The formation of a deamidated species could be monitored at the early stage of fibrillation, and this species was likely included in the fibrils. In addition, in the presence of inhibitors known to compete with ThT for binding to fibrils, UPLC analysis showed the disappearance of soluble components even though the ThT assay did not indicate the presence of fibrils. These results suggest that the UPLC-based analysis presented here can complement the ThT assay for investigating the kinetics of protein fibrillation.

Structural basis of insulin fibrillation

Insulin is a hormone responsible for maintaining normal glucose levels by activating insulin receptor (IR) and is the primary treatment for diabetes. However, insulin is prone to unfolding and forming cross-β fibers. Fibrillation complicates insulin storage and therapeutic application. Molecular details of insulin fibrillation remain unclear, hindering efforts to prevent fibrillation process. Here, we characterized insulin fibrils using cryo-electron microscopy (cryo-EM), showing multiple forms that contain one or more of the protofilaments containing both the A and B chains of insulin linked by disulfide bonds. We solved the cryo-EM structure of one of the fibril forms composed of two protofilaments at 3.2-Å resolution, which reveals both the β sheet conformation of the protofilament and the packing interaction between them that underlie the fibrillation. On the basis of this structure, we designed several insulin mutants that display reduced fibrillation while maintaining native IR signaling activity. These designed insulin analogs may be developed into more effective therapeutics for type 1 diabetes.

A highly sensitive hemicyanine-based near-infrared fluorescence sensor for detecting toxic amyloid aggregates in human serum

The development of an accurate and sensitive sensor for detecting amyloid plaques, which are responsible for many protein disorders like Alzheimer's disease, is crucial for early diagnosis. Recently, there has been a notable increase in the development of fluorescence probes that exhibit emission in the red region (>600 nm), aiming to effectively tackle the challenges encountered when working with complex biological matrices. In the current investigation, a hemicyanine-based probe, called LDS730, has been used for the sensing of amyloid fibrils, which belong to the Near-Infrared Fluorescence (NIRF) family of dyes. NIRF probes provide higher precision in detection, prevent photo-damage, and minimize the autofluorescence of biological specimens. The LDS730 sensor emits in the near-infrared region and shows a 110-fold increase in fluorescence turn-on emission when bound to insulin fibrils, making it a highly sensitive sensor. The sensor has an emission maximum of ~710 nm in a fibril-bound state, which shows a significant red shift along with a Stokes' shift of ~50 nm. The LDS730 sensor also displays excellent performance in the complicated human serum matrix, with a limit of detection (LOD) of 103 nM. Molecular docking calculations suggest that the most likely binding location of LDS730 in the fibrillar structure is the inner channels of amyloid fibrils along its long axis, and the sensor engages in several types of hydrophobic interactions with neighboring amino acid residues of the fibrillar structure. Overall, this new amyloid sensor has great potential for the early detection of amyloid plaques and for improving diagnostic accuracy.

Inhibition of Amyloid Protein Aggregation Using Selected Peptidomimetics

Aberrant protein aggregation leads to the formation of amyloid fibrils. This phenomenon is linked to the development of more than 40 irremediable diseases such as Alzheimer's disease, Parkinson's disease, type 2 diabetes, and cancer. Plenty of research efforts have been given to understanding the underlying mechanism of protein aggregation, associated toxicity, and the development of amyloid inhibitors. Recently, the peptidomimetic approach has emerged as a potential tool to modulate several protein-protein interactions (PPIs). In this review, we discussed selected peptidomimetic-based approaches for the modulation of important amyloid proteins (Islet Amyloid Polypeptide, Amyloid Beta, α-synuclein, mutant p53, and insulin) aggregation. This approach holds a powerful platform for creating an essential stepping stone for the vital development of anti-amyloid therapeutic agents.

Insulin amyloidosis: A case report

Insulin amyloidosis is a rare form of localized amyloidosis due to insulin aggregation into subcutaneous amyloid fibrils. We describe the case of a 55 years old male with insulin-requiring type 1 diabetes presenting with two non-inflammatory intra-dermal nodules associated with local lymph node enlargement. Diagnosis was confirmed by Congo red coloration of the amyloid deposit and insulin protein identification on mass spectrometry. Insulin amyloidosis is a potential complication of repeated subcutaneous insulin injections. The main risk factor is the intrinsic characteristic of the insulin used. Insulin amyloidosis leads to systemic metabolic consequences such as chronic hyperglycemia or unpredictable hypoglycemia, as well as unesthetic cutaneous lumps or abscesses. Standard-of-care is yet to be defined but mainly rely on therapeutical education of insulin injections, while surgical excision is reported to improve glycemic control in some patients.

Insulin Derived Fibrils Induce Cytotoxicity in vitro and Trigger Inflammation in Murine Models

BACKGROUND

Effective exogenous insulin delivery is the cornerstone of insulin dependent diabetes mellitus management. Recent literature indicates that commercial insulin-induced tissue reaction and cellular cytotoxicity may contribute to variability in blood glucose as well as permanent loss of injection or infusion site architecture and function. It is well accepted that insulin formulations are susceptible to mechanical and chemical stresses that lead to insulin fibril formation. This study aims to characterize in vitro and in vivo toxicity, as well as pro-inflammatory activity of insulin fibrils.

METHOD

In vitro cell culture evaluated cytotoxicity and fibril uptake by macrophages and our modified murine air-pouch model quantified inflammatory activity. The latter employed FLOW cytometry and histopathology to characterize fibril-induced inflammation in vivo, which included fibril uptake by inflammatory phagocytes.

RESULTS

These studies demonstrated that insulin derived fibrils are cytotoxic to cells in vitro. Furthermore, inflammation is induced in the murine air-pouch model in vivo and in response, macrophages uptake fibrils both in vitro and in vivo.

CONCLUSIONS

Administration of insulin fibrils can lead to cytotoxicity in macrophages. In vivo data demonstrate insulin fibrils to be pro-inflammatory which over time can lead to cumulative cell/tissue toxicity, inflammation, and destructive wound healing. Long term, these tissue reactions could contribute to loss of insulin injection site architecture and function.

Multi-Level High-Throughput Screening for Discovery of Ligands That Inhibit Insulin Aggregation

We have developed a multi-level virtual screening protocol to identify lead molecules from the FDA inactives database that can inhibit insulin aggregation. The method is based on the presence of structural and interaction specificity in non-native aggregation pathway protein-protein interactions. Some key challenges specific to the present problem, when compared with native protein association, include structural heterogeneity of the protein species involved, multiple association pathways, and relatively higher probability of conformational rearrangement of the association complex. In this multi-step method, the inactives database was first screened using the dominant pharmacophore features of previously identified molecules shown to significantly inhibit insulin aggregation nucleation by binding to its aggregation-prone conformers. We then performed ensemble docking of several low-energy ligand conformations on these aggregation-prone conformers followed by molecular dynamics simulations and binding affinity calculations on a subset of docked complexes to identify a final set of five potential lead molecules to inhibit insulin aggregation nucleation. Their effect on aggregation inhibition was extensively investigated by incubating insulin under aggregation-prone aqueous buffer conditions (low pH, high temperature). Aggregation kinetics were characterized using size exclusion chromatography and Thioflavin T fluorescence assay, and the secondary structure was determined using circular dichroism spectroscopy. Riboflavin provided the best aggregation inhibition, with 85% native monomer retention after 48 h incubation under aggregation-prone conditions, whereas the no-ligand formulation showed complete monomer loss after 36 h. Further, insulin incubated with two of the screened inactives (aspartame, riboflavin) had the characteristic α-helical dip in CD spectra, while the no-ligand formulation showed a change to β-sheet rich conformations.

Formulation Excipients and Their Role in Insulin Stability and Association State in Formulation

While  excipients are often overlooked as the "inactive" ingredients in pharmaceutical formulations, they often play a critical role in protein stability and absorption kinetics. Recent work has identified an ultrafast absorbing insulin formulation that is the result of excipient modifications. Specifically, the insulin monomer can be isolated by replacing zinc and the phenolic preservative metacresol with phenoxyethanol as an antimicrobial agent and an amphiphilic acrylamide copolymer excipient for stability. A greater understanding is needed of the interplay between excipients, insulin association state, and stability in order to optimize this formulation. Here, we formulated insulin with different preservatives and stabilizing excipient concentrations using both insulin lispro and regular human insulin and assessed the insulin association states using analytical ultracentrifugation as well as formulation stability. We determined that phenoxyethanol is required to eliminate hexamers and promote a high monomer content even in a zinc-free lispro formulation. There is also a concentration dependent relationship between the concentration of polyacrylamide-based copolymer excipient and insulin stability, where a concentration greater than 0.1 g/mL copolymer is required for a mostly monomeric zinc-free lispro formulation to achieve stability exceeding that of Humalog in a stressed aging assay. Further, we determined that under the formulation conditions tested zinc-free regular human insulin remains primarily hexameric and is not at this time a promising candidate for rapid-acting formulations.

An amphiphilic small molecule drives insulin aggregation inhibition and amyloid disintegration

The aggregation of proteins into ordered fibrillar structures called amyloids, and their disintegration represent major unsolved problems that limit the therapeutic applications of several proteins. For example, insulin, commonly used for the treatment of diabetes, is susceptible to amyloid formation upon exposure to non-physiological conditions, resulting in a loss of its biological activity. Here, we report a novel amphiphilic molecule called PAD-S, which acts as a chemical chaperone and completely inhibits fibrillation of insulin and its biosimilars. Mechanistic investigations and molecular docking lead to the conclusion that PAD-S binds to key hydrophobic regions of native insulin, thereby preventing its self-assembly. PAD-S treated insulin was biologically active as indicated by its ability to phosphorylate Akt, a protein in the insulin signalling pathway. PAD-S is non-toxic and protects cells from insulin amyloid induced cytotoxicity. The high aqueous solubility and easy synthetic accessibility of PAD-S facilitates its potential use in commercial insulin formulations. Notably, PAD-S successfully disintegrated preformed insulin fibrils to non-toxic smaller fragments. Since the structural and mechanistic features of amyloids are common to several human pathologies, the understanding of the amyloid disaggregation activity of PAD-S will inform the development of small molecule disaggregators for other amyloids.

Failure mode identification of Insulin drug products - Impact of relevant stress conditions on the quality of the drug

Fast-acting insulin drug products (DPs) are carried and administered by diabetic patients to maintain their blood glucose level throughout the day, exposing the DPs to stress conditions. Apidra, Novolog, and Humalog insulin DPs were tested under various stress conditions. Dynamic light scattering (DLS), and size exclusion chromatography (SEC) were used to monitor the stability and aggregation. Thermal stress alone did not influence the stability. However, 24 hr exposure to vigorous mechanical stress shifted the DLS size peaks of Novolog and Humalog from 5 ± 1 nm to > 50.9 ± 25.6 nm, and the SEC native protein peak areas decreased 52% for Novolog and 18.4% for Humalog. Combined stress accelerated protein aggregation more drastically. Novolog and Humalog size shifted (>75 nm) after 3 hr and the peak area decreased > 97.9% after 6 hr exposure, indicating that high temperature accelerated the aggregation triggered by agitation. Soluble aggregates were captured by DLS early on compared to SEC. Apidra was comparably stable indicating DP formulation plays a critical role in stability. Our study provides a greater understanding of potential failure modes patients and care givers may encounter while handling insulin DPs.

Molecular Aspects of Insulin Aggregation and Various Therapeutic Interventions

Protein aggregation leading to the formation of amyloid fibrils has various adverse effects on human health ranging from fatigue and numbness to organ failure and death in extreme cases. Insulin, a peptide hormone commonly used to treat diabetes, undergoes aggregation at the site of repeated injections in diabetic patients as well as during its industrial production and transport. The reduced bioavailability of insulin due to aggregation hinders the proper control of glucose levels in diabetic patients. Thus, it is necessary to develop rational approaches for inhibiting insulin aggregation, which in turn requires a detailed understanding of the mechanism of fibrillation. Given the relative simplicity of insulin and ease of access, insulin has also served as a model system for studying amyloids. Approaches to inhibit insulin aggregation have included the use of natural molecules, synthetic peptides or small molecules, and bacterial chaperone machinery. This review focuses on insulin aggregation with an emphasis on its mechanism, the structural features of insulin fibrils, and the reported inhibitors that act at different stages in the aggregation pathway. We discuss molecules that can serve as leads for improved inhibitors for use in commercial insulin formulations. We also discuss the aggregation propensity of fast- and slow-acting insulin biosimilars, commonly administered to diabetic patients. The development of better insulin aggregation inhibitors and insights into their mechanism of action will not only aid diabetic therapies, but also enhance our knowledge of protein amyloidosis.

Enhanced structural stability of insulin aspart in cholinium aminoate ionic liquids

Cholinium aminoates [Ch][AA] have gained tremendous interest as a promising ionic liquid medium for the synthesis and storage of proteins. However, high alkalinity of [Ch][AA] limits its usage with pH-sensitive proteins. Here, we probed the structure, stability, and interactions of a highly unstable therapeutic protein, insulin aspart (IA), in a range of buffered [Ch][AA] (b-[Ch][AA]) using a combination of biophysical tools and in silico pipeline including ultraviolet-visible, fluorescence, and circular dichroism spectroscopies, dynamic light scattering measurements and molecular docking. b-[Ch][AA] used in the study differed in concentrations and their anionic counterparts. We reveal information on ion and residue specific solvent-protein interactions, demonstrating that the structural stability of IA was enhanced by a buffered cholinium prolinate. In comparison to the glycinate and alaninate anions, the hydrophilic prolinate anions established more hydrogen bonds with the residues of IA and provided a less polar environment that favours the preservation of IA in its active monomeric form, opening new opportunities for utilizing [Ch][AA] as storage medium.

New Horizons: Next-Generation Insulin Analogues: Structural Principles and Clinical Goals

Design of "first-generation" insulin analogues over the past 3 decades has provided pharmaceutical formulations with tailored pharmacokinetic (PK) and pharmacodynamic (PD) properties. Application of a molecular tool kit-integrating protein sequence, chemical modification, and formulation-has thus led to improved prandial and basal formulations for the treatment of diabetes mellitus. Although PK/PD changes were modest in relation to prior formulations of human and animal insulins, significant clinical advantages in efficacy (mean glycemia) and safety (rates of hypoglycemia) were obtained. Continuing innovation is providing further improvements to achieve ultrarapid and ultrabasal analogue formulations in an effort to reduce glycemic variability and optimize time in range. Beyond such PK/PD metrics, next-generation insulin analogues seek to exploit therapeutic mechanisms: glucose-responsive ("smart") analogues, pathway-specific ("biased") analogues, and organ-targeted analogues. Smart insulin analogues and delivery systems promise to mitigate hypoglycemic risk, a critical barrier to glycemic control, whereas biased and organ-targeted insulin analogues may better recapitulate physiologic hormonal regulation. In each therapeutic class considerations of cost and stability will affect use and global distribution. This review highlights structural principles underlying next-generation design efforts, their respective biological rationale, and potential clinical applications.

Patterns of insulin therapy and insulin daily doses in children and adolescents with type 1 diabetes in Germany

AIM

To describe the different insulin therapy patterns and insulin daily doses in children and adolescents (aged 1-17 years) with type 1 diabetes.

METHODS

This cross-sectional study based on the longitudinal prescription (LRx) database (IQVIA) included children and adolescents who received at least two insulin prescriptions of the same drug from 1 January 2016 to 31 December 2019. The study outcomes included the proportion of patients with insulin pumps and multiple daily injection therapy, human insulin and insulin analogue use, as well as insulin daily doses. A multivariable linear regression model was used to study the association between age, sex, insulin drugs, and daily dose.

RESULTS

A total of 22 512 children and adolescents (mean age: 13.5 years, 47.1% female) were included. The proportion of patients using insulin pump therapy decreased with age, from 72.6% (females) and 73.0% (males) in the age group of younger than 6 years to 30.8% (females) and 26.1% (males) in adolescents. Insulin aspart was the most common short-acting insulin, with the proportion of users increasing from 56% in the age group of younger than 6 years to 69%-70% in the 13-17 years age group. The daily dose of insulin pump therapy was 10 units lower than multiple daily injection (MDI) (P < .001).

CONCLUSION

We found a marked age dependency for pump use, with a strong increase observed in the youngest age group. Insulin aspart was the most frequently used analogue insulin. A higher total daily insulin dose was shown in patients on MDI versus insulin pump, along with a significant age dependency.

Fatty Acid-Based Polymeric Micelles to Ameliorate Amyloidogenic Disorders

To develop anti-amyloidogenic inhibitors for ameliorating the treatment of diabetes, herein, we have synthesized amphiphilic block copolymers with side-chain fatty acid (FA) moieties via reversible addition fragmentation chain-transfer (RAFT) polymerization....

Subcutaneous amyloidoma models for screening potential anti-fibrillating agents in vivo

Here, we describe a robust protocol using mouse models to screen potential insulin-stabilizers and insulin moieties. We have generated a mouse model of amyloidoma, found in diabetic patients undergoing insulin therapy. This model can be used to screen potential insulin stabilizers and insulin moieties to prevent amyloidoma formation. This protocol can further be used for the preclinical validation of therapeutically relevant insulin stabilizers and formulations. The protocol highlights all the critical steps for generating amyloidoma in a preclinical model.

For complete details on the use and execution of this profile, please refer to Mukherjee et al. (2021).

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We present a detailed protocol for the generation of subcutaneous amyloidoma in mice

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This protocol facilitates screening of novel insulin-stabilizing molecules in vivo

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This approach can be adapted to study amyloidosis in other amyloid-related diseases

Hydroxy-Porphyrin as an Effective, Endogenous Molecular Clamp during Early Stages of Amyloid Fibrillization

Amyloid fibril formation of proteins is of great concern in neurodegenerative disease and can be detrimental to the storage and stability of biologics. Recent evidence suggests that insulin fibril formation reduces the efficacy of type II diabetes management and may lead to several complications. To develop anti-amyloidogenic compounds of endogenous origin, we have utilized the hydrogen bond anchoring, π stacking ability of porphyrin, and investigated its role on the inhibition of insulin amyloid formation. We report that hydroxylation and metal removal from the heme moiety yields an excellent inhibitor of insulin fibril formation. Thioflavin T, tyrosine fluorescence, Circular Dichorism (CD) spectroscopy, Field emission scanning electron microscopy (FESEM) and molecular dynamics (MD) simulation studies suggest that hematoporphyrin (HP) having hydrogen bonding ability on both sides is a superior inhibitor compared to hemin and protoporphyrin (PP). Experiments with hen egg white lysozyme (HEWL) amyloid fibril formation also validated the efficacy of endogenous porphyrin based small molecules. Our results will help to decipher a general therapeutic strategy to counter amyloidogenesis.

Sulfated polysaccharide ascophyllan prevents amyloid fibril formation of human insulin and inhibits amyloid-induced hemolysis and cytotoxicity in PC12 cells

We found that ascophyllan significantly inhibited the fibrillation of human insulin and was the most effective among the sulfated polysaccharides tested. Gel-filtration analysis suggested that ascophyllan was capable of forming a complex with insulin through a weak interaction. Secondary structure transition from native α-helix to β-sheet predominant structure of insulin under the fibrillation conditions was suppressed in the presence of ascophyllan. Interestingly, ascophyllan attenuated insulin fibril-induced hemolysis of human erythrocytes. Moreover, ascophyllan attenuated insulin amyloid-induced cytotoxicity on rat pheochromocytoma PC12 cells and reduced the level of intracellular reactive oxygen species. This is the first report indicating that a sulfated polysaccharide, ascophyllan, can suppress the insulin amyloid fibril formation and inhibit the fibril-induced detrimental bioactivities.

Structural principles of insulin formulation and analog design: A century of innovation

BACKGROUND

The discovery of insulin in 1921 and its near-immediate clinical use initiated a century of innovation. Advances extended across a broad front, from the stabilization of animal insulin formulations to the frontiers of synthetic peptide chemistry, and in turn, from the advent of recombinant DNA manufacturing to structure-based protein analog design. In each case, a creative interplay was observed between pharmaceutical applications and then-emerging principles of protein science; indeed, translational objectives contributed to a growing molecular understanding of protein structure, aggregation and misfolding.

SCOPE OF REVIEW

Pioneering crystallographic analyses-beginning with Hodgkin's solving of the 2-Zn insulin hexamer-elucidated general features of protein self-assembly, including zinc coordination and the allosteric transmission of conformational change. Crystallization of insulin was exploited both as a step in manufacturing and as a means of obtaining protracted action. Forty years ago, the confluence of recombinant human insulin with techniques for site-directed mutagenesis initiated the present era of insulin analogs. Variant or modified insulins were developed that exhibit improved prandial or basal pharmacokinetic (PK) properties. Encouraged by clinical trials demonstrating the long-term importance of glycemic control, regimens based on such analogs sought to resemble daily patterns of endogenous β-cell secretion more closely, ideally with reduced risk of hypoglycemia.

MAJOR CONCLUSIONS

Next-generation insulin analog design seeks to explore new frontiers, including glucose-responsive insulins, organ-selective analogs and biased agonists tailored to address yet-unmet clinical needs. In the coming decade, we envision ever more powerful scientific synergies at the interface of structural biology, molecular physiology and therapeutics.

Structural Lessons From the Mutant Proinsulin Syndrome

Insight into folding mechanisms of proinsulin has been provided by analysis of dominant diabetes-associated mutations in the human insulin gene (INS). Such mutations cause pancreatic β-cell dysfunction due to toxic misfolding of a mutant proinsulin and impairment in trans of wild-type insulin secretion. Anticipated by the "Akita" mouse (a classical model of monogenic diabetes mellitus; DM), this syndrome illustrates the paradigm endoreticulum (ER) stress leading to intracellular proteotoxicity. Diverse clinical mutations directly or indirectly perturb native disulfide pairing leading to protein misfolding and aberrant aggregation. Although most introduce or remove a cysteine (Cys; leading in either case to an unpaired thiol group), non-Cys-related mutations identify key determinants of folding efficiency. Studies of such mutations suggest that the hormone's evolution has been constrained not only by structure-function relationships, but also by the susceptibility of its single-chain precursor to impaired foldability. An intriguing hypothesis posits that INS overexpression in response to peripheral insulin resistance likewise leads to chronic ER stress and β-cell dysfunction in the natural history of non-syndromic Type 2 DM. Cryptic contributions of conserved residues to folding efficiency, as uncovered by rare genetic variants, define molecular links between biophysical principles and the emerging paradigm of Darwinian medicine: Biosynthesis of proinsulin at the edge of non-foldability provides a key determinant of "diabesity" as a pandemic disease of civilization.

Determination of amyloid core regions of insulin analogues fibrils

A rapid-acting insulin lispro and long-acting insulin glargine are commonly used for the treatment of diabetes. Clinical cases have described the formation of injectable amyloidosis with these insulin analogues, but their amyloid core regions of fibrils were unknown. To reveal these regions, we have analysed the hydrolyzates of insulin fibrils and its analogues using high-performance liquid chromatography and mass spectrometry methods and found that insulin and its analogues have almost identical amyloid core regions that intersect with the predicted amyloidogenic regions. The obtained results can be used to create new insulin analogues with a low ability to form fibrils.

Abbreviations

a.a., amino acid residues; HPLC-MS, high-performance liquid chromatography/mass spectrometry; m/z, mass-to-charge ratio; TEM, transmission electron microscopy.

In Silico study of Rosmarinic Acid Derivatives as Novel Insulin Fibril Inhibitors

The self-assembly of human insulin (HI) plays a crucial role in regulating amyloid fibrils. Therefore, it is a significant problem for the medical management of diabetes therapy and these findings have led us to investigate the amyloid formation and its inhibition. Few potential inhibitors have been identified to inhibit amyloid fibrils. Rosmarinic acid (RA) is one of the things that inhibits amyloid formation completely by increasing the resistivity of the amyloidogenic insulin (dimer) protein to thermal unfolding. Here, we choose different tested derivative compounds for designing amyloid inhibitors by substituting various functional groups of RA. These derivative compounds were subjected to in silico studies to determine the best drug candidates. In comparison to RA, 14 molecules have higher binding affinity and interactions with the target receptor. After frontier molecular orbitals study, ADME and toxicity analysis, the eight best compounds may act as the best inhibitors. The stability of the docked complexes was visualized by molecular dynamics (MD) simulations. This finding opens a new proposal to explore future studies with these best compounds to increase the thermal stability of the insulin dimers.

Prion-derived tetrapeptide stabilizes thermolabile insulin via conformational trapping

Unfolding followed by fibrillation of insulin even in the presence of various excipients grappled with restricted clinical application. Thus, there is an unmet need for better thermostable, nontoxic molecules to preserve bioactive insulin under varying physiochemical perturbations. In search of cross-amyloid inhibitors, prion-derived tetrapeptide library screening reveals a consensus V(X)YR motif for potential inhibition of insulin fibrillation. A tetrapeptide VYYR, isosequential to the β2-strand of prion, effectively suppresses heat- and storage-induced insulin fibrillation and maintains insulin in a thermostable bioactive form conferring adequate glycemic control in mouse models of diabetes and impedes insulin amyloidoma formation. Besides elucidating the critical insulin-IS1 interaction (R4 of IS1 to the N24 insulin B-chain) by nuclear magnetic resonance spectroscopy, we further demonstrated non-canonical dimer-mediated conformational trapping mechanism for insulin stabilization. In this study, structural characterization and preclinical validation introduce a class of tetrapeptide toward developing thermostable therapeutically relevant insulin formulations.

An ultrafast insulin formulation enabled by high-throughput screening of engineered polymeric excipients

Insulin has been used to treat diabetes for almost 100 years; yet, current rapid-acting insulin formulations do not have sufficiently fast pharmacokinetics to maintain tight glycemic control at mealtimes. Dissociation of the insulin hexamer, the primary association state of insulin in rapid-acting formulations, is the rate-limiting step that leads to delayed onset and extended duration of action. A formulation of insulin monomers would more closely mimic endogenous postprandial insulin secretion, but monomeric insulin is unstable in solution using present formulation strategies and rapidly aggregates into amyloid fibrils. Here, we implement high-throughput-controlled radical polymerization techniques to generate a large library of acrylamide carrier/dopant copolymer (AC/DC) excipients designed to reduce insulin aggregation. Our top-performing AC/DC excipient candidate enabled the development of an ultrafast-absorbing insulin lispro (UFAL) formulation, which remains stable under stressed aging conditions for 25 ± 1 hours compared to 5 ± 2 hours for commercial fast-acting insulin lispro formulations (Humalog). In a porcine model of insulin-deficient diabetes, UFAL exhibited peak action at 9 ± 4 min, whereas commercial Humalog exhibited peak action at 25 ± 10 min. These ultrafast kinetics make UFAL a promising candidate for improving glucose control and reducing burden for patients with diabetes.

Synthesis and Evaluation of Arylamides with Hydrophobic Side Chains for Insulin Aggregation Inhibition

Insulin, a peptide hormone, forms fibrils under aberrant physiological conditions leading to a reduction in its biological activity. To ameliorate insulin aggregation, we have synthesized a small library of oligopyridylamide foldamers decorated with different combination of hydrophobic side chains. Screening of these compounds for insulin aggregation inhibition using a Thioflavin-T assay resulted in the identification of a few hit molecules. The best hit molecule, BPAD2 inhibited insulin aggregation with an IC₅₀ value of 0.9 μM. Mechanistic analyses suggested that BPAD2 inhibited secondary nucleation and elongation processes during aggregation. The hit molecules worked in a mechanistically distinct manner, thereby underlining the importance of structure-activity relationship studies in obtaining a molecular understanding of protein aggregation.

Phenolic Preservative Removal from Commercial Insulin Formulations Reduces Tissue Inflammation while Maintaining Euglycemia

Background: Exogenous insulin therapy requires stabilization of the insulin molecule, which is achieved through the use of excipients (e.g., phenolic preservatives (PP)) that provide protein stability, sterility and prolong insulin shelf life. However, our laboratory recently reported that PP, (e.g., m-creosol and phenol) are also cytotoxic, inducing inflammation and fibrosis. Optimizing PP levels through filtration would balance the need for insulin preservation with PP-induced inflammation. Method: Zeolite Y (Z-Y), a size-exclusion-based resin, was employed to remove PP from commercial insulin formulations (Humalog) before infusion. Results: PP removal significantly decreased cell toxicity in vitro and inflammation in vivo. Infusion site histological analysis after a 3 day study demonstrated that leukocyte accumulation increased with nonfiltered preparations but decreased after filtration. Additional studies demonstrated that a Z-Y fabricated filter effectively removed excess PP such that the filtered insulin solution achieved equivalent glycemic control in diabetic mice when compared to nonfiltered insulin. Conclusion: This approach represents the proof of concept that using Z-Y for in-line PP removal assists in lowering inflammation at the site of insulin infusion and thus could lead to extending the functional lifespan of insulin infusion sets in vivo.

Engineering biopharmaceutical formulations to improve diabetes management

Insulin was first isolated almost a century ago, yet commercial formulations of insulin and its analogs for hormone replacement therapy still fall short of appropriately mimicking endogenous glycemic control. Moreover, the controlled delivery of complementary hormones (such as amylin or glucagon) is complicated by instability of the pharmacologic agents and complexity of maintaining multiple infusions. In this review, we highlight the advantages and limitations of recent advances in drug formulation that improve protein stability and pharmacokinetics, prolong drug delivery, or enable alternative dosage forms for the management of diabetes. With controlled delivery, these formulations could improve closed-loop glycemic control.

Insulin Storage: A Critical Reappraisal

Insulin, as a peptide hormone drug, is susceptible to changes in stability when exposed to environmental factors under storage. Proper storage according to the manufacturer's recommendations is important to maintain its potency and enable precise dosing for people with diabetes (PwD). While it is reasonable to assume that transport conditions and temperature are well controlled during the supply chain, little is known about insulin storage after dispensing and insulin potency at the moment of administration. Insulin is exposed to various environmental factors when carried by PwD and storage recommendations are often not met when it is stored in household refrigerators. It is difficult to assess changes in insulin potency in clinical practice, and there is a gap in the current scientific literature on insulin stability. Package leaflet recommendations only give limited information on the impact of improper storage conditions on insulin stability and guidelines by health organizations are inconsistent. Given the importance of precise dosing in diabetes care, there is a need for more transparency on insulin stability, awareness for proper storage among health care professionals and PwD as well as clear guidelines and practical storage recommendations from manufacturers and health organizations.

Advancing continuous subcutaneous insulin infusion in vivo: New insights into tissue challenges

Continuous Subcutaneous Insulin Infusion (CSII) is superior to conventional insulin therapy as it improves glycemic control thus reducing the probability of diabetic complications. Notwithstanding CSII's benefits, insulin dependent diabetic patients rarely achieve optimal glucose control. Moreover, CSII is only FDA approved for 3 days and often fails prematurely for reasons that have not been fully elucidated. We hypothesize that phenolic compounds, such as m-cresol and phenol, which are present in all commercial insulin formulations are responsible for the tissue reaction occurring at the insulin infusion site. This hypothesis was examined with in vitro cell cultures and a mouse air-pouch model to determine cellular and tissue reactions following infusions with saline, phenolic compounds, (i.e., commercial diluent), and insulin. We demonstrated that diluent and insulin were cytotoxic to cells in culture at sub-clinical concentrations (e.g., >1:10 of commercial insulin). Air pouch studies demonstrated that infusion of either diluted insulin or diluent itself induced three to five-fold level of recruited leukocytes as compared to saline. At both 3- and 7-days post infusion, these were predominantly neutrophils and macrophages. We conclude that phenolic compounds in commercial insulin preparations are cell and tissue toxic, which contributes to the failure of effective insulin infusion therapy.

Amphiphilic Polyacrylamide Excipients Lead to a Record-Breaking Fast-Acting Insulin

Fast-acting insulins are central to the regulation of prandial glucose in diabetic patients. Current fast-acting insulins require 20-30 min for the onset and longer for the peak blood concentrations. The recent work by Mann et al. used high-throughput synthesis and screening of polyacrylamide-based excipients to yield a formulation with pharmacokinetics that is faster than the currently available fast-acting insulins.

Biopolymer-coated gold nanoparticles inhibit human insulin amyloid fibrillation

Deposits of protein misfolding and/or aggregates are a pathological hallmark of amyloid-related diseases. For instance, insulin amyloid fibril deposits have been observed in patients with insulin-dependent diabetes mellitus after insulin administration. Here, we report on the use of AuNPs functionalized with linear- (i.e. dextrin and chitosan) and branched- (i.e. dextran-40 and dextran-10) biopolymers as potential agents to inhibit insulin fibril formation. Our dynamic light scattering analyses showed a size decrease of the amyloid fibrils in the presence of functionalized AuNPs. Circular dichroism spectroscopy as well as enzyme-linked immunosorbent assay data demonstrated that the secondary structural transition from α-helix to β-sheet (which is characteristic for insulin amyloid fibril formation) was significantly suppressed by all biopolymer-coated AuNPs, and in particular, by those functionalized with linear biopolymers. Both transmission electron microscopy and atomic force microscopy analyses showed that the long thick amyloid fibrils formed by insulin alone become shorter, thinner or cluster when incubated with biopolymer-coated AuNPs. Dextrin- and chitosan-coated AuNPs were found to be the best inhibitors of the fibril formation. Based on these results, we propose a mechanism for the inhibition of insulin amyloid fibrils: biopolymer-coated AuNPsstrongly interact with the insulin monomers and inhibit the oligomer formation as well as elongation of the protofibrils.Moreover, cytotoxicity experiments showed that AuNP-insulin amyloid fibrils are less toxic compared to insulin amyloid fibrils alone. Our results suggest that both dextrin- and chitosan-AuNPs could be used as therapeutic agents for the treatment of amyloid-related disorders.

Small-Molecule Inhibitor Prevents Insulin Fibrillogenesis and Preserves Activity

Amyloidosis is a well-known but poorly understood phenomenon caused by the aggregation of proteins, often leading to pathological conditions. For example, the aggregation of insulin poses significant challenges during the preparation of pharmaceutical insulin formulations commonly used to treat diabetic patients. Therefore, it is essential to develop inhibitors of insulin aggregation for potential biomedical applications and for important mechanistic insights into amyloidogenic pathways. Here, we have identified a small molecule M1, which causes a dose-dependent reduction in insulin fibril formation. Biophysical analyses and docking results suggest that M1 likely binds to partially unfolded insulin intermediates. Further, M1-treated insulin had lower cytotoxicity and remained functionally active in regulating cell proliferation in cultured Drosophila wing epithelium. Thus, M1 is of great interest as a novel agent for inhibiting insulin aggregation during biopharmaceutical manufacturing.

A co-formulation of supramolecularly stabilized insulin and pramlintide enhances mealtime glucagon suppression in diabetic pigs

Treatment of patients with diabetes with insulin and pramlintide (an amylin analogue) is more effective than treatment with insulin only. However, because mixtures of insulin and pramlintide are unstable and have to be injected separately, amylin analogues are only used by 1.5% of people with diabetes needing rapid-acting insulin. Here, we show that the supramolecular modification of insulin and pramlintide with cucurbit[7]uril-conjugated polyethylene glycol improves the pharmacokinetics of the dual-hormone therapy and enhances postprandial glucagon suppression in diabetic pigs. The co-formulation is stable for over 100 h at 37 °C under continuous agitation, whereas commercial formulations of insulin analogues aggregate after 10 h under similar conditions. In diabetic rats, the administration of the stabilized co-formulation increased the area-of-overlap ratio of the pharmacokinetic curves of pramlintide and insulin from 0.4 ± 0.2 to 0.7 ± 0.1 (mean ± s.d.) for the separate administration of the hormones. The co-administration of supramolecularly stabilized insulin and pramlintide better mimics the endogenous kinetics of co-secreted insulin and amylin, and holds promise as a dual-hormone replacement therapy.

Resveratrol as a nontoxic excipient stabilizes insulin in a bioactive hexameric form

Insulin aggregation is the leading cause of considerable reduction in the amount of active drug molecules in liquid formulations manufactured for diabetes management. Phenolic compounds, such as phenol and m-cresol, are routinely used to stabilize insulin in a hexameric form during its commercial preparation. However, long term usage of commercial insulin results in various adverse secondary responses, for which toxicity of the phenolic excipients is primarily responsible. In this study we aimed to find out a nontoxic insulin stabilizer. To that end, we have selected resveratrol, a natural polyphenol, as a prospective nontoxic insulin stabilizer because of its structural similarity with commercially used phenolic compounds. Atomic force microscopy visualization of resveratrol-treated human insulin revealed that resveratrol has a unique ability to arrest hINS in a soluble oligomeric form having discrete spherical morphology. Most importantly, resveratrol-treated insulin is nontoxic for HepG2 cells and it effectively maintains low blood glucose in a mouse model. Cryo-electron microscopy revealed 3D morphology of resveratrol-stabilized insulin that strikingly resembles crystal structures of insulin hexamer formulated with m-cresol. Significantly, we found that, in a condition inductive to amyloid fibrillation at physiological pH, resveratrol is capable of stabilizing insulin more efficiently than m-cresol. Thus, this study describes resveratrol as an effective nontoxic natural molecule that can be used for stabilizing insulin in a bioactive oligomeric form during its commercial formulation.

Stable Monomeric Insulin Formulations Enabled by Supramolecular PEGylation of Insulin Analogues

Current "fast-acting" insulin analogues contain amino acid modifications meant to inhibit dimer formation and shift the equilibrium of association states toward the monomeric state. However, the insulin monomer is highly unstable and current formulation techniques require insulin to primarily exist as hexamers to prevent aggregation into inactive and immunogenic amyloids. Insulin formulation excipients have thus been traditionally selected to promote insulin association into the hexameric form to enhance formulation stability. This study exploits a novel excipient for the supramolecular PEGylation of insulin analogues, including aspart and lispro, to enhance the stability and maximize the prevalence of insulin monomers in formulation. Using multiple techniques, it is demonstrated that judicious choice of formulation excipients (tonicity agents and parenteral preservatives) enables insulin analogue formulations with 70-80% monomer and supramolecular PEGylation imbued stability under stressed aging for over 100 h without altering the insulin association state. Comparatively, commercial "fast-acting" formulations contain less than 1% monomer and remain stable for only 10 h under the same stressed aging conditions. This simple and effective formulation approach shows promise for next-generation ultrafast insulin formulations with a short duration of action that can reduce the risk of post-prandial hypoglycemia in the treatment of diabetes.

Insulin fibrillation: toward strategies for attenuating the process

The environmental factors affecting the rate of insulin fibrillation. The factors are representative.

Investigation of factors that cause insulin precipitation and/or amyloid formation in insulin formulations

Background

Multiple daily subcutaneous injections (MDSIs) are mainly used for formulating an insulin therapy for diabetic patients; however, they also cause insulin-derived amyloidosis (IDA) and lead to poor glycemic control. In addition, for the continuous subcutaneous insulin infusion system (CSII), precipitation frequently causes catheter occlusion and, if the precipitate in the formulations is amyloid, the injection of the insoluble amyloid into the subcutaneous tissue leads to IDA. The aim of this study was to conduct in vitro experiments and present a situation where insulin formulations cause precipitation and amyloid formation.

Methods

Humulin®R and NovoRapid® were used as model formulations for MDSIs and CSII, respectively. The generation of the precipitation was evaluated by measuring turbidity, and amyloid formation was evaluated by using Thioflavin T. Humulin®R was mixed with saline buffer solutions and glucose solutions to evaluate the effect of dilution. In addition, we created an experimental system to consider the effect of the time course of condition changes, and investigated the effects of insulin concentration, m-cresol existence, and pH change on the generation of the precipitate and amyloid in the formulation.

Results

In both the original and diluted formulations, physical stimulation resulted in the formation of a precipitate, which in most cases was an amyloid. The amyloid was likely to be formed at a near neutral pH. On the contrary, although a precipitate formed when the pH was decreased to near the isoelectric point, this precipitate was not an amyloid. Further decreases in pH resulted in the formation of amyloids, suggesting that both the positive and negative charged states of insulin tended to form amyloids. The formulation additive m-cresol suppressed amyloid formation. When additives were removed from the formulation, the amyloid-containing gel was formed in the field of substance exchange.

Conclusions

To consider changes in conditions that may occur for insulin formulations, the relationship between the formation of precipitates and amyloids was demonstrated in vitro by using insulin formulations. From the in vitro study, m-cresol was shown to have an inhibitory effect on amyloid formation.

The quaternary structure of insulin glargine and glulisine under formulation conditions

The quaternary structures of insulin glargine and glulisine under formulation conditions and upon dilution using placebo or water were investigated using synchrotron small-angle X-ray scattering. Our results revealed that insulin glulisine in Apidra® is predominantly hexameric in solution with significant fractions of dodecamers and monomers. Upon dilution with placebo, this equilibrium shifts towards monomers. Insulin glargine in Lantus® and Toujeo® is present in a stable hexamer/dimer equilibrium, which is hardly affected by dilution with water down to 1 mg/ml insulin concentration. The results provide exclusive insight into the quaternary structure and thus the association/dissociation properties of the two insulin analogues in marketed formulations.

Probing the correlation between insulin activity and structural stability through introduction of the rigid A6-A11 bond

The development of fast-acting and highly stable insulin analogues is challenging. Insulin undergoes structural transitions essential for binding and activation of the insulin receptor (IR), but these conformational changes can also affect insulin stability. Previously, we substituted the insulin A6-A11 cystine with a rigid, non-reducible C=C linkage ("dicarba" linkage). A cis-alkene permitted the conformational flexibility of the A-chain N-terminal helix necessary for high-affinity IR binding, resulting in surprisingly rapid activity in vivo Here, we show that, unlike the rapidly acting Lys^B28Pro^B29 insulin analogue (KP insulin), cis-dicarba insulin is not inherently monomeric. We also show that cis-dicarba KP insulin lowers blood glucose levels even more rapidly than KP insulin, suggesting that an inability to oligomerize is not responsible for the observed rapid activity onset of cis-dicarba analogues. Although rapid-acting, neither dicarba species is stable, as assessed by fibrillation and thermodynamics assays. MALDI analyses and molecular dynamics simulations of cis-dicarba insulin revealed a previously unidentified role of the A6-A11 linkage in insulin conformational dynamics. By controlling the conformational flexibility of the insulin B-chain helix, this linkage affects overall insulin structural stability. This effect is independent of its regulation of the A-chain N-terminal helix flexibility necessary for IR engagement. We conclude that high-affinity IR binding, rapid in vivo activity, and insulin stability can be regulated by the specific conformational arrangement of the A6-A11 linkage. This detailed understanding of insulin's structural dynamics may aid in the future design of rapid-acting insulin analogues with improved stability.

Mechanistic Studies of the Inhibition of Insulin Fibril Formation by Rosmarinic Acid

The self-assembly of insulin to form amyloid fibrils has been widely studied because it is a significant problem in the medical management of diabetes and is an important model system for the investigation of amyloid formation and its inhibition. A few inhibitors of insulin fibrillation have been identified with potencies that could be higher. Knowledge of how these work at the molecular level is not known but important for the development of more potent inhibitors. Here we show that rosmarinic acid completely inhibits amyloid formation by dimeric insulin at pH 2 and 60 °C. In contrast to other polyphenols, rosmarinic acid is soluble in water and does not degrade at elevated temperatures, and thus we were able to decipher the mechanism of inhibition by a combination of solution-state 1H NMR spectroscopy and molecular docking. On the basis of 1H chemical shift perturbations, intermolecular nuclear Overhauser effect enhancements between rosmarinic acid and specific residues of insulin, and slowed dynamics of rosmarinic acid in the presence of insulin, we show that rosmarinic acid binds to a pocket found on the surface of each insulin monomer. This results in the formation of a mixed tetramolecular aromatic network on the surface of insulin dimer, resulting in increased resistance of the amyloidogenic protein to thermal unfolding. This finding opens new avenues for the design of potent inhibitors of amyloid formation and provides strong experimental evidence for the role of surface aromatic clusters in increasing the thermal stability of proteins.

Inhibition of Insulin Amyloid Fibrillation by a Novel Amphipathic Heptapeptide: MECHANISTIC DETAILS STUDIED BY SPECTROSCOPY IN COMBINATION WITH MICROSCOPY

The aggregation of insulin into amyloid fibers has been a limiting factor in the development of fast acting insulin analogues, creating a demand for excipients that limit aggregation. Despite the potential demand, inhibitors specifically targeting insulin have been few in number. Here we report a non-toxic and serum stable-designed heptapeptide, KR7 (KPWWPRR-NH2), that differs significantly from the primarily hydrophobic sequences that have been previously used to interfere with insulin amyloid fibrillation. Thioflavin T fluorescence assays, circular dichroism spectroscopy, and one-dimensional proton NMR experiments suggest KR7 primarily targets the fiber elongation step with little effect on the early oligomerization steps in the lag time period. From confocal fluorescence and atomic force microscopy experiments, the net result appears to be the arrest of aggregation in an early, non-fibrillar aggregation stage. This mechanism is noticeably different from previous peptide-based inhibitors, which have primarily shifted the lag time with little effect on later stages of aggregation. As insulin is an important model system for understanding protein aggregation, the new peptide may be an important tool for understanding peptide-based inhibition of amyloid formation.

Comparison of rechargeable versus battery-operated insulin pumps: temperature fluctuations

The role of continuous subcutaneous insulin infusion (insulin pumps) has become increasingly important in diabetes management, and many different types of these systems are currently available. This exploratory study focused on the reported heating issues that lithium-ion battery-powered pumps may have during charging compared with battery-operated pumps. It was found that pump temperature increased by 6.4°C during a long charging cycle of a lithiumion battery-operated pump under ambient temperatures. In an environmental-chamber kept at 35°C, the pump temperature increased by 4.4°C, which indicates that the pump temperature was above that of the recommended safety limit for insulin storage of 37°C. When designing new pumps, and when using currently available rechargeable pumps in warmer climates, the implications of these temperature increases should be taken into consideration. Future studies should also further examine insulin quality after charging.

Total Chemical Synthesis of an Intra-A-Chain Cystathionine Human Insulin Analogue with Enhanced Thermal Stability

Despite recent advances in the treatment of diabetes mellitus, storage of insulin formulations at 4 °C is still necessary to minimize chemical degradation. This is problematic in tropical regions where reliable refrigeration is not ubiquitous. Some degradation byproducts are caused by disulfide shuffling of cystine that leads to covalently bonded oligomers. Consequently we examined the utility of the non-reducible cystine isostere, cystathionine, within the A-chain. Reported herein is an efficient method for forming this mimic using simple monomeric building blocks. The intra-A-chain cystathionine insulin analogue was obtained in good overall yield, chemically characterized and demonstrated to possess native binding affinity for the insulin receptor isoform B. It was also shown to possess significantly enhanced thermal stability indicating potential application to next-generation insulin analogues.

Insulin Aspart in the Management of Diabetes Mellitus: 15 Years of Clinical Experience

Limiting excessive postprandial glucose excursions is an important component of good overall glycemic control in diabetes mellitus. Pharmacokinetic studies have shown that insulin aspart, which is structurally identical to regular human insulin except for the replacement of a single proline amino acid with an aspartic acid residue, has a more physiologic time-action profile (i.e., reaches a higher peak and reaches that peak sooner) than regular human insulin. As expected with this improved pharmacokinetic profile, insulin aspart demonstrates a greater glucose-lowering effect compared with regular human insulin. Numerous randomized controlled trials and a meta-analysis have also demonstrated improved postprandial control with insulin aspart compared with regular human insulin in patients with type 1 or type 2 diabetes, as well as efficacy and safety in children, pregnant patients, hospitalized patients, and patients using continuous subcutaneous insulin infusion. Studies have demonstrated that step-wise addition of insulin aspart is a viable intensification option for patients with type 2 diabetes failing on basal insulin. Insulin aspart has shown a good safety profile, with no evidence of increased receptor binding, mitogenicity, stimulation of anti-insulin antibodies, or hypoglycemia compared with regular human insulin. In one meta-analysis, there was evidence of a lower rate of nocturnal hypoglycemia compared with regular human insulin and, in a trial that specifically included patients with a history of recurrent hypoglycemia, a significantly lower rate of severe hypoglycemic episodes. The next generation of insulin aspart (faster-acting insulin aspart) is being developed with a view to further improving on these pharmacokinetic/pharmacodynamic properties.

Insulin amyloid at injection sites of patients with diabetes

The formation of insulin amyloid can dramatically impact glycemic control in patients with diabetes, making it an important therapeutic consideration. In addition, the cost associated with the excess insulin required by patients with amyloid is estimated to be $3K per patient per year, which adds to the growing financial burden of this disease. Insulin amyloid has been observed with every mode of therapeutic insulin administration (infusion, injection and inhalation), and the number of reported cases has increased significantly since 2002. The new cases represent a much broader demographic, and include many patients who have used exclusively human insulin and human insulin analogs. The reason for the increase in case reports is unknown, but this review explores the possibility that changes in patient care, improved differential diagnosis and/or changes in insulin type and insulin delivery systems may be important factors. The goal of this review is to raise key questions that will inspire proactive measures to prevent, identify and treat insulin amyloid. Furthermore, this comprehensive examination of insulin amyloid can provide insight into important considerations for other injectable drugs that are prone to form amyloid deposits.