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

Evolution of biosynthetic human insulin and its analogues for diabetes management

Hormones play a crucial role in maintaining the normal human physiology. By acting as chemical messengers that facilitate the communication between different organs, tissues and cells of the body hormones assist in responding appropriately to external and internal stimuli that trigger growth, development and metabolic activities of the body. Any abnormalities in the hormonal composition and balance can lead to devastating health consequences. Hormones have been important therapeutic agents since the early 20th century, when it was realized that their exogenous supply could serve as a functional substitution for those hormones which are not produced enough or are completely lacking, endogenously. Insulin, the pivotal anabolic hormone in the body, was used for the treatment of diabetes mellitus, a metabolic disorder due to the absence or intolerance towards insulin, since 1921 and is the trailblazer in hormone therapeutics. At present the largest market share for therapeutic hormones is held by insulin. Many other hormones were introduced into clinical practice following the success with insulin. However, for the six decades following the introduction the first therapeutic hormone, there was no reliable method for producing human hormones. The most common source for hormones were animals, although semisynthetic and synthetic hormones were also developed. However, none of these were optimal because of their allergenicity, immunogenicity, lack of consistency in purity and most importantly, scalability. The advent of recombinant DNA technology was a game changer for hormone therapeutics. This revolutionary molecular biology tool made it possible to synthesize human hormones in microbial cell factories. The approach allowed for the synthesis of highly pure hormones which were structurally and biochemically identical to the human hormones. Further, the fermentation techniques utilized to produce recombinant hormones were highly scalable. Moreover, by employing tools such as site directed mutagenesis along with recombinant DNA technology, it became possible to amend the molecular structure of the hormones to achieve better efficacy and mimic the exact physiology of the endogenous hormone. The first recombinant hormone to be deployed in clinical practice was insulin. It was called biosynthetic human insulin to reflect the biological route of production. Subsequently, the biochemistry of recombinant insulin was modified using the possibilities of recombinant DNA technology and genetic engineering to produce analogues that better mimic physiological insulin. These analogues were tailored to exhibit pharmacokinetic and pharmacodynamic properties of the prandial and basal human insulins to achieve better glycemic control. The present chapter explores the principles of genetic engineering applied to therapeutic hormones by reviewing the evolution of therapeutic insulin and its analogues. It also focuses on how recombinant analogues account for the better management of diabetes mellitus.

Muco-Adhesive and Muco-Penetrative Formulations for the Oral Delivery of Insulin

Insulin, a pivotal anabolic hormone, regulates glucose homeostasis by facilitating the conversion of blood glucose to energy or storage. Dysfunction in insulin activity, often associated with pancreatic β cells impairment, leads to hyperglycemia, a hallmark of diabetes. Type 1 diabetes (T1D) results from autoimmune destruction of β cells, while type 2 diabetes (T2D) stems from genetic, environmental, and lifestyle factors causing β cell dysfunction and insulin resistance. Currently, insulin therapy is used for most of the cases of T1D, while it is used only in a few persistent cases of T2D, often supplemented with dietary and lifestyle changes. The key challenge in oral insulin delivery lies in overcoming gastrointestinal (GI) barriers, including enzymatic degradation, low permeability, food interactions, low bioavailability, and long-term safety concerns. The muco-adhesive (MA) and muco-penetrative (MP) formulations aim to enhance oral insulin delivery by addressing these challenges. The mucus layer, a hydrogel matrix covering epithelial cells in the GI tract, poses significant barriers to oral insulin absorption. Its structure, composition, and turnover rate influence interactions with insulin and other drug carriers. Some of the few factors that influence mucoadhesion and mucopenetration are particle size, surface charge distribution, and surface modifications. This review discusses the challenges associated with oral insulin delivery, explores the properties of mucus, and evaluates the strategies for achieving excellent MA and MP formulations, focusing on nanotechnology-based approaches. The development of effective oral insulin formulations holds the potential to revolutionize diabetes management, providing patients with a more convenient and patient-friendly alternative to traditional insulin administration methods.

Converting Short-Acting Insulin into Thermo-Stable Longer-Acting Insulin Using Multi-Layer Detachable Microneedles

The detachable dissolving microneedles (DDMNs) feature an array of needles capable of being separated from the base sheet during administration. Here they were fabricated to address delivery efficiency and storage stability of insulin. The constructed insulin-DDMN is multi-layered, with 1) a hard tip cover layer; 2) a layer of regular short-acting insulin (RI) mixed with hyaluronic acid (HA) and sorbitol (Sor) which occupies the taper tip region of the needles; 3) a barrier layer situated above the RI layer; and 4) a fast-dissolving layer connecting the barrier layer to the base sheet. RI entrapped in DDMNs exhibited enhanced thermal stability; it could be stored at 40 °C for 35 days without losing significant biological activity. Differential scanning calorimetric analysis revealed that the HA-Sor matrix could improve the denaturation temperature of the RI from lower than room temperature to 186 °C. Tests in ex vivo porcine skin demonstrated RI delivery efficiency of 91±1.59 %. Experiments with diabetic rats revealed sustained release of RI, i.e., when compared to subcutaneous injection with the same RI dose, RI-DDMNs produced slower absorption of insulin into blood circulation, delayed onset of hypoglycemic effect, longer serum insulin half-life, and longer hypoglycemic duration.

Ultra-stable insulin-glucagon fusion protein exploits an endogenous hepatic switch to mitigate hypoglycemic risk

The risk of hypoglycemia and its serious medical sequelae restrict insulin replacement therapy for diabetes mellitus. Such adverse clinical impact has motivated development of diverse glucose-responsive technologies, including algorithm-controlled insulin pumps linked to continuous glucose monitors ("closed-loop systems") and glucose-sensing ("smart") insulins. These technologies seek to optimize glycemic control while minimizing hypoglycemic risk. Here, we describe an alternative approach that exploits an endogenous glucose-dependent switch in hepatic physiology: preferential insulin signaling (under hyperglycemic conditions) versus preferential counter-regulatory glucagon signaling (during hypoglycemia). Motivated by prior reports of glucagon-insulin co-infusion, we designed and tested an ultra-stable glucagon-insulin fusion protein whose relative hormonal activities were calibrated by respective modifications; physical stability was concurrently augmented to facilitate formulation, enhance shelf life and expand access. An N-terminal glucagon moiety was stabilized by an α-helix-compatible Lys 13 -Glu 17 lactam bridge; A C-terminal insulin moiety was stabilized as a single chain with foreshortened C domain. Studies in vitro demonstrated (a) resistance to fibrillation on prolonged agitation at 37 °C and (b) dual hormonal signaling activities with appropriate balance. Glucodynamic responses were monitored in rats relative to control fusion proteins lacking one or the other hormonal activity, and continuous intravenous infusion emulated basal subcutaneous therapy. Whereas efficacy in mitigating hyperglycemia was unaffected by the glucagon moiety, the fusion protein enhanced endogenous glucose production under hypoglycemic conditions. Together, these findings provide proof of principle toward a basal glucose-responsive insulin biotechnology of striking simplicity. The fusion protein's augmented stability promises to circumvent the costly cold chain presently constraining global insulin access.

Significance Statement

The therapeutic goal of insulin replacement therapy in diabetes is normalization of blood-glucose concentration, which prevents or delays long-term complications. A critical barrier is posed by recurrent hypoglycemic events that results in short- and long-term morbidities. An innovative approach envisions co-injection of glucagon (a counter-regulatory hormone) to exploit a glycemia-dependent hepatic switch in relative hormone responsiveness. To provide an enabling technology, we describe an ultra-stable fusion protein containing insulin- and glucagon moieties. Proof of principle was obtained in rats. A single-chain insulin moiety provides glycemic control whereas a lactam-stabilized glucagon extension mitigates hypoglycemia. This dual-hormone fusion protein promises to provide a basal formulation with reduced risk of hypoglycemia. Resistance to fibrillation may circumvent the cold chain required for global access.

Computational Approach to Elucidating Insulin-Protamine Binding Interactions and Dynamics in Insulin NPH Formulations

Insulin NPH is an intermediate-acting insulin. Its protracted action profile is due to the formation of microcrystalline suspensions when insulin is complexed with a basic peptide protamine, zinc ion, and phenolic ligands. Despite advancements in analytical techniques, the binding epitope and binding mode of the protamine in the insulin-protamine complex are still unknown. In this study, we used bioinformatics tools such as molecular docking and molecular dynamics (MD) simulations to compute the binding sites and energetics of the insulin-protamine complex. We have taken four naturally occurring protamine peptides that are independently docked with the insulin R6 hexamer and subjected them to 200 ns MD simulations to observe the dynamics of the complexes and estimate the binding energies. The arginine-rich protamine peptides were found to bind on the surface of the insulin hexamer through hydrogen bonding, hydrophobic, and electrostatic interactions well supported by the calculated negative binding energies. The overall structure of the insulin hexamer was retained upon binding, highlighting its dynamic stability in the complex. Furthermore, the residues at the termini of the protamine peptides in the complex were seen to be highly dynamic, which stabilize toward the end of the simulation.

Lipidization as a tool toward peptide therapeutics

Peptides, as potential therapeutics continue to gain importance in the search for active substances for the treatment of numerous human diseases, some of which are, to this day, incurable. As potential therapeutic drugs, peptides have many favorable chemical and pharmacological properties, starting with their great diversity, through their high affinity for binding to all sort of natural receptors, and ending with the various pathways of their breakdown, which produces nothing but amino acids that are nontoxic to the body. Despite these and other advantages, however, they also have their pitfalls. One of these disadvantages is the very low stability of natural peptides. They have a short half-life and tend to be cleared from the organism very quickly. Their instability in the gastrointestinal tract, makes it impossible to administer peptidic drugs orally. To achieve the best pharmacologic effect, it is desirable to look for ways of modifying peptides that enable the use of these substances as pharmaceuticals. There are many ways to modify peptides. Herein we summarize the approaches that are currently in use, including lipidization, PEGylation, glycosylation and others, focusing on lipidization. We describe how individual types of lipidization are achieved and describe their advantages and drawbacks. Peptide modifications are performed with the goal of reaching a longer half-life, reducing immunogenicity and improving bioavailability. In the case of neuropeptides, lipidization aids their activity in the central nervous system after the peripheral administration. At the end of our review, we summarize all lipidized peptide-based drugs that are currently on the market.

Effects of glargine on hyperglycemia in patients with diabetes mellitus type II undergoing off-pump coronary artery bypass graft: A randomized, controlled, double-blind clinical trial

Introduction: In this trial, effects of glargine on hyperglycemia in patients with diabetes mellitus type II who were undergoing off-pump coronary artery bypass graft (CAGB), were examined. Methods: Seventy diabetic patients who were candidate for off-pump CABG were randomly divided into the following two groups (1) Control group who were treated with normal saline+regular insulin and (2) Glargine group who received glargine+regular insulin. Normal saline and glargine were administered subcutaneously 2 hours before surgery, and regular insulin was injected before, during and after the surgery in the intensive care unit (ICU) in both groups. Finally, levels of blood sugar before, 2 hours after starting the surgery and at the end of the surgery, were recorded. Blood sugar levels during ICU stay were also measured every 4 hours for 36 hours. Results: There were no significant differences in blood sugar levels between the groups at the three time points (i.e. before, 2 hours after starting the surgery and at the end of the surgery). In addition, during 36 hours of ICU stay, blood sugar levels did not show significant variations between the groups; however, 20 hours after ICU admission, blood sugar level was significantly higher in the glargine group (P=0.04). Conclusion: The results indicated that both glargine and regular insulin effectively control the blood glucose in diabetic patients undergoing CABG. However, the blood sugar fluctuation was less in the glargine group than control group.

Evolution of the diagnostic value of "the sugar of the blood": hitting the sweet spot to identify alterations in glucose dynamics

In this paper, we provide an overview of the evolution of the definition of hyperglycemia during the past century and the alterations in glucose dynamics that cause fasting and postprandial hyperglycemia. We discuss how extensive mechanistic, physiological research into the factors and pathways that regulate the appearance of glucose in the circulation and its uptake and metabolism by tissues and organs has contributed knowledge that has advanced our understanding of different types of hyperglycemia, namely prediabetes and diabetes and their subtypes (impaired fasting plasma glucose, impaired glucose tolerance, combined impaired fasting plasma glucose, impaired glucose tolerance, type 1 diabetes, type 2 diabetes, gestational diabetes mellitus), their relationships with medical complications, and how to prevent and treat hyperglycemia.

De Novo Transcriptome Assembly and Analysis of Longevity Genes Using Subterranean Termite (Reticulitermes chinensis) Castes

The longevity phenomenon is entirely controlled by the insulin signaling pathway (IIS-pathway). Both vertebrates and invertebrates have IIS-pathways that are comparable to one another, though no one has previously described de novo transcriptome assembly of IIS-pathway-associated genes in termites. In this research, we analyzed the transcriptomes of both reproductive (primary kings “PK” and queens “PQ”, secondary worker reproductive kings “SWRK” and queens “SWRQ”) and non-reproductive (male “WM” and female “WF” workers) castes of the subterranean termite Reticulitermes chinensis. The goal was to identify the genes responsible for longevity in the reproductive and non-reproductive castes. Through transcriptome analysis, we annotated 103,589,264 sequence reads and 184,436 (7G) unigenes were assembled, GC performance was measured at 43.02%, and 64,046 sequences were reported as CDs sequences. Of which 35 IIS-pathway-associated genes were identified, among 35 genes, we focused on the phosphoinositide-dependent kinase-1 (Pdk1), protein kinase B2 (akt2-a), tuberous sclerosis-2 (Tsc2), mammalian target of rapamycin (mTOR), eukaryotic translation initiation factor 4E (EIF4E) and ribosomal protein S6 (RPS6) genes. Previously these genes (Pdk1, akt2-a, mTOR, EIF4E, and RPS6) were investigated in various organisms, that regulate physiological effects, growth factors, protein translation, cell survival, proliferation, protein synthesis, cell metabolism and survival, autophagy, fecundity rate, egg size, and follicle number, although the critical reason for longevity is still unclear in the termite castes. However, based on transcriptome profiling, the IIS-pathway-associated genes could prolong the reproductive caste lifespan and health span. Therefore, the transcriptomic shreds of evidence related to IIS-pathway genes provide new insights into the maintenance and relationships between biomolecular homeostasis and remarkable longevity. Finally, we propose a strategy for future research to decrypt the hidden costs associated with termite aging in reproductive and non-reproductive castes.

A Comprehensive Review of the Evolution of Insulin Development and Its Delivery Method

The year 2021 marks the 100th anniversary of the momentous discovery of insulin. Through years of research and discovery, insulin has evolved from poorly defined crude extracts of animal pancreas to recombinant human insulin and analogues that can be prescribed and administered with high accuracy and efficacy. However, there are still many challenges ahead in clinical settings, particularly with respect to maintaining optimal glycemic control whilst minimizing the treatment-related side effects of hypoglycemia and weight gain. In this review, the chronology of the development of rapid-acting, short-acting, intermediate-acting, and long-acting insulin analogues, as well as mixtures and concentrated formulations that offer the potential to meet this challenge, are summarized. In addition, we also summarize the latest advancements in insulin delivery methods, along with advancement to clinical trials. This review provides insights on the development of insulin treatment for diabetes mellitus that may be useful for clinicians in meeting the needs of their individual patients. However, it is important to note that as of now, none of the new technologies mentioned have superseded the existing method of subcutaneous administration of insulin.

Insulins for the long term management of diabetes mellitus in dogs: a review

The year 2021 marked the centenary of the isolation of a therapeutic form of insulin and its successful use in dogs. This was a landmark moment that subsequently and rapidly led to the commercial manufacture of insulin for use in humans. The impact of insulin was almost miraculous as those destined to die from their diabetes mellitus returned to life. Over the past 100 years, insulin formulations have been modified to attempt to provide a predictable and prolonged duration of action while avoiding the development of hypoglycaemia. This has led to an ever-growing variety of insulin types in human medicine, many of which have subsequently been used in dogs. The purpose of this review article is to provide an overview of available insulin types and their application to the chronic management of canine diabetes mellitus.

Proteomic Changes to the Updated Discovery of Engineered Insulin and Its Analogs: Pros and Cons

The destruction of β-cells of the pancreas leads to either insulin shortage or the complete absence of insulin, which in turn causes diabetes Mellitus. For treating diabetes, many trials have been conducted since the 19th century until now. In ancient times, insulin from an animal's extract was taken to treat human beings. However, this resulted in some serious allergic reactions. Therefore, scientists and researchers have tried their best to find alternative ways for managing diabetes with progressive advancements in biotechnology. However, a lot of research trials have been conducted, and they discovered more progressed strategies and approaches to treat type I and II diabetes with satisfaction. Still, investigators are finding more appropriate ways to treat diabetes accurately. They formulated insulin analogs that mimic the naturally produced human insulin through recombinant DNA technology and devised many methods for appropriate delivery of insulin. This review will address the following questions: What is insulin preparation? How were these devised and what are the impacts (both positive and negative) of such insulin analogs against TIDM (type-I diabetes mellitus) and TIIDM (type-II diabetes mellitus)? This review article will also demonstrate approaches for the delivery of insulin analogs into the human body and some future directions for further improvement of insulin treatment.

Diabetes-Associated Mutations in Proinsulin Provide a "Molecular Rheostat" of Nascent Foldability

PURPOSE OF REVIEW

Diabetes mellitus (DM) due to toxic misfolding of proinsulin variants provides a monogenic model of endoplasmic reticulum (ER) stress. The mutant proinsulin syndrome (also designated MIDY; Mutant INS-gene-induced Diabetes of Youth or Maturity-onset diabetes of the young 10 (MODY10)) ordinarily presents as permanent neonatal-onset DM, but specific amino-acid substitutions may also present later in childhood or adolescence. This review highlights structural mechanisms of proinsulin folding as inferred from phenotype-genotype relationships.

RECENT FINDINGS

MIDY mutations most commonly add or remove a cysteine, leading to a variant polypeptide containing an odd number of thiol groups. Such variants are associated with aberrant intermolecular disulfide pairing, ER stress, and neonatal β-cell dysfunction. Non-cysteine-related (NCR) mutations (occurring in both the B and A domains of proinsulin) define distinct determinants of foldability and vary in severity. The range of ages of onset, therefore, reflects a "molecular rheostat" connecting protein biophysics to quality-control ER checkpoints. Because in most mammalian cell lines even wild-type proinsulin exhibits limited folding efficiency, molecular barriers to folding uncovered by NCR MIDY mutations may pertain to β-cell dysfunction in non-syndromic type 2 DM due to INS-gene overexpression in the face of peripheral insulin resistance. Recent studies of MIDY mutations and related NCR variants, combining molecular and cell-based approaches, suggest that proinsulin has evolved at the edge of non-foldability. Chemical protein synthesis promises to enable comparative studies of "non-foldable" proinsulin variants to define key steps in wild-type biosynthesis. Such studies may create opportunities for novel therapeutic approaches to non-syndromic type 2 DM.

Progress in Simulation Studies of Insulin Structure and Function

Insulin is a peptide hormone known for chiefly regulating glucose level in blood among several other metabolic processes. Insulin remains the most effective drug for treating diabetes mellitus. Insulin is synthesized in the pancreatic β-cells where it exists in a compact hexameric architecture although its biologically active form is monomeric. Insulin exhibits a sequence of conformational variations during the transition from the hexamer state to its biologically-active monomer state. The structural transitions and the mechanism of action of insulin have been investigated using several experimental and computational methods. This review primarily highlights the contributions of molecular dynamics (MD) simulations in elucidating the atomic-level details of conformational dynamics in insulin, where the structure of the hormone has been probed as a monomer, dimer, and hexamer. The effect of solvent, pH, temperature, and pressure have been probed at the microscopic scale. Given the focus of this review on the structure of the hormone, simulation studies involving interactions between the hormone and its receptor are only briefly highlighted, and studies on other related peptides (e.g., insulin-like growth factors) are not discussed. However, the review highlights conformational dynamics underlying the activities of reported insulin analogs and mimetics. The future prospects for computational methods in developing promising synthetic insulin analogs are also briefly highlighted.