Insulin formulation and delivery

Se-Glargine: Chemical Synthesis of a Basal Insulin Analogue Stabilized by an Internal Diselenide Bridge

Insulin has long provided a model for studies of protein folding and stability, enabling enhanced treatment of diabetes mellitus via analogue design. We describe the chemical synthesis of a basal insulin analogue stabilized by substitution of an internal cystine (A6-A11) by a diselenide bridge. The studies focused on insulin glargine (formulated as Lantus® and Toujeo®; Sanofi). Prepared at pH 4 in the presence of zinc ions, glargine exhibits a shifted isoelectric point due to a basic B chain extension (ArgB31 -ArgB32 ). Subcutaneous injection leads to pH-dependent precipitation of a long-lived depot. Pairwise substitution of CysA6 and CysA11 by selenocysteine was effected by solid-phase peptide synthesis; the modified A chain also contained substitution of AsnA21 by Gly, circumventing acid-catalyzed deamidation. Although chain combination of native glargine yielded negligible product, in accordance with previous synthetic studies, the pairwise selenocysteine substitution partially rescued this reaction: substantial product was obtained through repeated combination, yielding a stabilized insulin analogue. This strategy thus exploited both (a) the unique redox properties of selenocysteine in protein folding and (b) favorable packing of an internal diselenide bridge in the native state, once achieved. Such rational optimization of protein folding and stability may be generalizable to diverse disulfide-stabilized proteins of therapeutic interest.

Insulin Analogues with Altered Insulin Receptor Isoform Binding Specificities and Enhanced Aggregation Stabilities

Insulin is a lifesaver for millions of diabetic patients. There is a need for new insulin analogues with more physiological profiles and analogues that will be thermally more stable than human insulin. Here, we describe the chemical engineering of 48 insulin analogues that were designed to have changed binding specificities toward isoforms A and B of the insulin receptor (IR-A and IR-B). We systematically modified insulin at the C-terminus of the B-chain, at the N-terminus of the A-chain, and at A14 and A18 positions. We discovered an insulin analogue that has Cα-carboxyamidated Glu at B31 and Ala at B29 and that has a more than 3-fold-enhanced binding specificity in favor of the "metabolic" IR-B isoform. The analogue is more resistant to the formation of insulin fibrils at 37 °C and is also more efficient in mice than human insulin. Therefore, [AlaB29,GluB31,amideB31]-insulin may be interesting for further clinical evaluation.

Structure-based stabilization of insulin as a therapeutic protein assembly via enhanced aromatic-aromatic interactions

Key contributions to protein structure and stability are provided by weakly polar interactions, which arise from asymmetric electronic distributions within amino acids and peptide bonds. Of particular interest are aromatic side chains whose directional π-systems commonly stabilize protein interiors and interfaces. Here, we consider aromatic-aromatic interactions within a model protein assembly: the dimer interface of insulin. Semi-classical simulations of aromatic-aromatic interactions at this interface suggested that substitution of residue TyrB26 by Trp would preserve native structure while enhancing dimerization (and hence hexamer stability). The crystal structure of a [TrpB26]insulin analog (determined as a T3Rf3 zinc hexamer at a resolution of 2.25 Å) was observed to be essentially identical to that of WT insulin. Remarkably and yet in general accordance with theoretical expectations, spectroscopic studies demonstrated a 150-fold increase in the in vitro lifetime of the variant hexamer, a critical pharmacokinetic parameter influencing design of long-acting formulations. Functional studies in diabetic rats indeed revealed prolonged action following subcutaneous injection. The potency of the TrpB26-modified analog was equal to or greater than an unmodified control. Thus, exploiting a general quantum-chemical feature of protein structure and stability, our results exemplify a mechanism-based approach to the optimization of a therapeutic protein assembly.

Excited-State Proton Transfer on the Surface of a Therapeutic Protein, Protamine

Proton transfer reactions on biosurfaces play an important role in a myriad of biological processes. Herein, the excited-state proton transfer reaction of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) has been investigated in the presence of an important therapeutic protein, Protamine (PrS), using ground-state absorption, steady-state, and detailed time-resolved emission measurements. HPTS forms a 1:1 complex with Protamine with a high association constant of 2.6 × 10⁴ M^-1. The binding of HPTS with Protamine leads to a significant modulation in the ground-state prototropic equilibrium causing a downward shift of 1.1 unit in the acidity constant (pK_a). In contrast to a large number of reports of slow proton transfer of HPTS on biosurfaces, interestingly, HPTS registers a faster proton transfer event in the presence of Protamine as compared to that of even the bulk aqueous buffer medium. Furthermore, the dimensionality of the proton diffusion process is also significantly reduced on the surface of Protamine that is in contrast to the behavior of HPTS in the bulk aqueous buffer medium, where the proton diffusion process is three-dimensional. The effect of ionic strength on the binding of HPTS toward PrS suggests a predominant role of electrostatic interaction between anionic HPTS and cationic Protamine, which is further supported by molecular docking simulations which predict that the most preferable binding site for HPTS on the surface of Protamine is surrounded by multiple cationic arginine residues.

Assessment of the permeability and toxicity of polymeric nanocapsules using the zebrafish model

Aim: To assess the capacity of a new drug delivery nanocapsule (NC) with a double shell of hyaluronic acid and protamine to overcome biological barriers using the zebrafish model. Materials & methods: NCs were prepared by the solvent displacement method, tagged with fluorescent makers and physicochemically characterized. Toxicity was evaluated according to the Fish Embryo Acute Toxicity test, and permeability was tested by exposing zebrafish, with and without chorion, to the fluorescent NCs. Results: Toxicity of NCs was very low as compared with that of a control nanoemulsion. Double-shell NCs were able to cross chorion and skin. Conclusion: Beyond the potential value of hyaluronic acid:protamine NCs for overcoming epithelial barriers, this works highlights the utility of zebrafish for fast screening of nanocarriers.

Silver triangular nanoplates as an high efficiently FRET donor-acceptor of upconversion nanoparticles for ultrasensitive "Turn on-off" protamine and trypsin sensor

Silver triangular nanoplates (STNPs) as a high efficient fluorescence quenching reagent of upconversion nanoparticles (UCNPs) was used to constract a novel label-free fluorescence nanosensor for ultrasensitive detection of protamine and trypsin based on fluorescence resonance energy transfer (FRET) between STNPs and UCNPs. In this assay, the negatively charged STNPs can bind with positively charged UCNPs through electrostatic interaction, and then quenched the fluorescence of UCNPs. When protamine was added to the mixture of UCNPs-STNPs, the STNPs interacted with protamine and then detached from the surface of UCNPs and aggregated, which result in the recovery of the fluorescence of UCNPs. Trypsin could catalyze the hydrolysis of protamine and effectively quench the fluorescence recovered by protamine. By measuring the changes of the fluorescence of UCNPs, the concentrations of protamine and trypsin were determined. Under the optimized conditions, the linear response range was obtained from 10 to 500ng/mL, 5-80ng/mL and with the low detection limit of 3.1ng/mL and 1.8ng/mL for protamine and trypsin, respectively. Meanwhile, the nanosensor shows good selectivity, sensitivity and can be successfully applied to detection of protamine and trypsin in serum samples.

Why most oral insulin formulations do not reach clinical trials

Oral insulin able to induce an efficient antihyperglycemic effect either to replace or complement diabetes mellitus therapy is the major goal of health providers, governments and diabetic patients. Oral therapy is associated not only with the desire to exclude needles from the daily routine of diabetic patient but also with the physiological provision of insulin they would get. Despite numerous efforts over the past few decades to develop insulin delivery systems, there is still no commercially available oral insulin. The reasons why the formulations developed to administer insulin orally fail to reach clinical trials are critically discussed in this review. The principal features of nanoformulations used so far are also addressed as well as the undergoing clinical trials.

Insulin fibrillation and protein design: topological resistance of single-chain analogs to thermal degradation with application to a pump reservoir

Insulin is susceptible to thermal fibrillation, a misfolding process that leads to nonnative cross-β assembly analogous to pathological amyloid deposition. Pharmaceutical formulations are ordinarily protected from such degradation by sequestration of the susceptible monomer within native protein assemblies. With respect to the safety and efficacy of insulin pumps, however, this strategy imposes an intrinsic trade-off between pharmacokinetic goals (rapid absorption and clearance) and the requisite physical properties of a formulation (prolonged shelf life and stability within the reservoir). Available rapid-acting formulations are suboptimal in both respects; susceptibility to fibrillation is exacerbated even as absorption is delayed relative to the ideal specifications of a closed-loop system. To circumvent this molecular trade-off, we exploited structural models of insulin fibrils and amyloidogenic intermediates to define an alternative protective mechanism. Single-chain insulin (SCI) analogs were shown to be refractory to thermal fibrillation with maintenance of biological activity for more than 3 months under conditions that promote the rapid fibrillation and inactivation of insulin. The essential idea exploits an intrinsic incompatibility between SCI topology and the geometry of cross-β assembly. A peptide tether was thus interposed between the A- and B-chains whose length was (a) sufficiently long to provide the "play" needed for induced fit of the hormone on receptor binding and yet (b) sufficiently short to impose a topological barrier to fibrillation. Our findings suggest that ultrastable monomeric SCI analogs may be formulated without protective self-assembly and so permit simultaneous optimization of pharmacokinetics and reservoir life.

Osteoconductive protamine-based polyelectrolyte multilayer functionalized surfaces

The integration of orthopedic implants with host bone presents a major challenge in joint arthroplasty, spinal fusion and tumor reconstruction. The cellular microenvironment can be programmed via implant surface functionalization allowing direct modulation of osteoblast adhesion, proliferation, and differentiation at the implant--bone interface. The development of layer-by-layer assembled polyelectrolyte multilayer (PEM) architectures has greatly expanded our ability to fabricate intricate nanometer to micron scale thin film coatings that conform to complex implant geometries. The in vivo therapeutic efficacy of thin PEM implant coatings for numerous biomedical applications has previously been reported. We have fabricated protamine-based PEM thin films that support the long-term proliferation and differentiation of pre-osteoblast cells on non-cross-linked film-coated surfaces. These hydrophilic PEM functionalized surfaces with nanometer-scale roughness facilitated increased deposition of calcified matrix by osteoblasts in vitro, and thus offer the potential to enhance implant integration with host bone. The coatings can make an immediate impact in the osteogenic culture of stem cells and assessment of the osteogenic potential of new therapeutic factors.

Insulin: a small protein with a long journey

Insulin is a hormone that is essential for regulating energy storage and glucose metabolism in the body. Insulin in liver, muscle, and fat tissues stimulates the cell to take up glucose from blood and store it as glycogen in liver and muscle. Failure of insulin control causes diabetes mellitus (DM). Insulin is the unique medicine to treat some forms of DM. The population of diabetics has dramatically increased over the past two decades, due to high absorption of carbohydrates (or fats and proteins), lack of physical exercise, and development of new diagnostic techniques. At present, the two largest developing countries (India and China) and the largest developed country (United States) represent the top three countries in terms of diabetic population. Insulin is a small protein, but contains almost all structural features typical of proteins: α-helix, β-sheet, β-turn, high order assembly, allosteric T®R-transition, and conformational changes in amyloidal fibrillation. More than ten years' efforts on studying insulin disulfide intermediates by NMR have enabled us to decipher the whole picture of insulin folding coupled to disulfide pairing, especially at the initial stage that forms the nascent peptide. Two structural switches are also known to regulate insulin binding to receptors and progress has been made to identify the residues involved in binding. However, resolving the complex structure of insulin and its receptor remains a challenge in insulin research. Nevertheless, the accumulated knowledge of insulin structure has allowed us to specifically design a new ultra-stable and active single-chain insulin analog (SCI-57), and provides a novel way to design super-stable, fast-acting and cheaper insulin formulations for DM patients. Continuing this long journey of insulin study will benefit basic research in proteins and in pharmaceutical therapy.

An Achilles' heel in an amyloidogenic protein and its repair: insulin fibrillation and therapeutic design

Insulin fibrillation provides a model for a broad class of amyloidogenic diseases. Conformational distortion of the native monomer leads to aggregation-coupled misfolding. Whereas beta-cells are protected from proteotoxicity by hexamer assembly, fibrillation limits the storage and use of insulin at elevated temperatures. Here, we have investigated conformational distortions of an engineered insulin monomer in relation to the structure of an insulin fibril. Anomalous (13)C NMR chemical shifts and rapid (15)N-detected (1)H-(2)H amide-proton exchange were observed in one of the three classical alpha-helices (residues A1-A8) of the hormone, suggesting a conformational equilibrium between locally folded and unfolded A-chain segments. Whereas hexamer assembly resolves these anomalies in accordance with its protective role, solid-state (13)C NMR studies suggest that the A-chain segment participates in a fibril-specific beta-sheet. Accordingly, we investigated whether helicogenic substitutions in the A1-A8 segment might delay fibrillation. Simultaneous substitution of three beta-branched residues (Ile(A2) --> Leu, Val(A3) --> Leu, and Thr(A8) --> His) yielded an analog with reduced thermodynamic stability but marked resistance to fibrillation. Whereas amide-proton exchange in the A1-A8 segment remained rapid, (13)Calpha chemical shifts exhibited a more helical pattern. This analog is essentially without activity, however, as Ile(A2) and Val(A3) define conserved receptor contacts. To obtain active analogs, substitutions were restricted to A8. These analogs exhibit high receptor-binding affinity; representative potency in a rodent model of diabetes mellitus was similar to wild-type insulin. Although (13)Calpha chemical shifts remain anomalous, significant protection from fibrillation is retained. Together, our studies define an "Achilles' heel" in a globular protein whose repair may enhance the stability of pharmaceutical formulations and broaden their therapeutic deployment in the developing world.

Supramolecular protein engineering: design of zinc-stapled insulin hexamers as a long acting depot

Bottom-up control of supramolecular protein assembly can provide a therapeutic nanobiotechnology. We demonstrate that the pharmacological properties of insulin can be enhanced by design of "zinc staples" between hexamers. Paired (i, i+4) His substitutions were introduced at an alpha-helical surface. The crystal structure contains both classical axial zinc ions and novel zinc ions at hexamer-hexamer interfaces. Although soluble at pH 4, the combined electrostatic effects of the substitutions and bridging zinc ions cause isoelectric precipitation at neutral pH. Following subcutaneous injection in a diabetic rat, the analog effected glycemic control with a time course similar to that of long acting formulation Lantus. Relative to Lantus, however, the analog discriminates at least 30-fold more stringently between the insulin receptor and mitogenic insulin-like growth factor receptor. Because aberrant mitogenic signaling may be associated with elevated cancer risk, such enhanced specificity may improve safety. Zinc stapling provides a general strategy to modify the pharmacokinetic and biological properties of a subcutaneous protein depot.

Circulating insulin antibodies: influence of continuous subcutaneous or intraperitoneal insulin infusion, and impact on glucose control

The purification of animal insulin preparations and the use of human recombinant insulin have markedly reduced the incidence, but not completely suppressed, the development of anti-insulin antibodies (IAs). Advances in technologies concerning the mode of delivery of insulin, i.e. continuous subcutaneous insulin infusion (CSII), continuous peritoneal insulin infusion (CPII) and more recently inhaled insulin administration, appear to significantly increase circulating levels of immunoglobulin G (IgG) anti-IAs in diabetic patients. However, the increase is usually moderate and mostly transient as compared to previous observations with poorly purified animal insulin preparations. The clinical impact of these circulating anti-IAs remains unclear. Nevertheless, several studies have suggested that antibodies could retard insulin action, leading to a worsening of postprandial hyperglycaemia and/or serve as a carrier, thus leading to unexpected hypoglycaemia. CPII may be associated with more marked and sustained increase in IAs levels, possibly related to the use of an unstable insulin and the formation of immunogenic aggregates of insulin. The possible clinical consequences of these high levels of IAs remain to be evaluated because a low-glucose morning syndrome or severe insulin resistance with ketone bodies production have been reported in some cases. In conclusion, even if CSII and CPII may promote the development of circulating IAs, this increase does not lead to immunological insulin resistance, compared to that previously described with animal non-purified insulin preparations, and seems to have only marginal influence on blood glucose control or complications in most diabetic patients.

Basal insulin supplementation in Type 1 diabetic children: a long-term comparative observational study between continuous subcutaneous insulin infusion and glargine insulin

No long-term data are available on the efficacy of glargine insulin in comparison with continuous sc insulin infusion (CSII) in children and adolescents affected by Type 1 diabetes (T1D). Our aim was to compare the 2-yr efficacy of the 2 insulin approaches, in order to know how to best supply basal insulin in these patients. Thirty-six 9 to 18-yr-old consecutive children with at least 3 yr previous T1D diagnosis were enrolled. As part of routine clinical care, the patients consecutively changed their previous insulin scheme (isophane insulin at bedtime and human regular insulin at meals) and were randomly selected in order to receive either multiple daily injections (MDI) treatment with once-daily glargine and human regular insulin at meals, or CSII with aspart or lispro insulin. Both groups showed a significant decrease in glycosylated hemoglobin (HbA1c) values during the 1st year of therapy, though only in the CSII treated children was the decrease also observed during the 2nd year. The overall insulin requirement significantly decreased only in the CSII group and exclusively during the 1st year, while no significant differences were observed concerning body mass index SD score, severe hypoglycemic episodes and basal insulin supplementation. The work illustrates the first long-term study comparing the efficacy of CSII to MDI using glargine as basal insulin in children. Only with CSII were better HbA1c values obtained for prolonged periods of time, so that CSII might be considered the gold standard of intensive insulin therapy also for long-term follow-ups.

An observational study comparing continuous subcutaneous insulin infusion (CSII) and insulin glargine in children with type 1 diabetes

OBJECTIVE

The advantages of continuous subcutaneous insulin infusion (CSII) or insulin glargine have been demonstrated both in adult and paediatric diabetic patients; however, as no data comparing these two approaches during childhood are available, we have examined the efficacy of these two intensive approaches.

RESEARCH DESIGN AND METHODS

We retrospectively evaluated data from 36 diabetic children, who had changed their previous insulin regimen [with isophane insulin (NPH) at bedtime] because of HbA1c levels >8.0%. Twenty patients underwent CSII, while the other 16 (significantly younger for age) started insulin glargine at bedtime.

RESULTS

At 6 and 12 months, CSII-treated patients showed a significant reduction in HbA1c values from 8.5 +/- 1.8 to 7.4 +/- 1.1% and to 7.6 +/- 1.2%, respectively. The insulin requirement significantly decreased from 0.93 +/- 0.2 IU/kg to 0.73 +/- 0.2 IU/kg of body weight and to 0.74 +/- 0.15 IU/kg of body weight, respectively, while no significant differences were observed for BMI SDS, fructosamine and severe hypoglycaemic events. The patients treated with glargine showed a small decline in HbA1c values from 8.9 +/- 1.7 to 8.3 +/- 0.9% (not significant) in the first 6 months of treatment and to 8.2 +/- 0.9% after 12 months.

CONCLUSION

The basal insulin supplementation can be supplied effectively in children with type 1 diabetes by either CSII or insulin glargine. As previously reported for adults, it is confirmed that CSII is the best current intensive approach aimed to the improvement of glycaemic control.

Diamonds in the rough: protein crystals from a formulation perspective

The focus of the present review is to address the use of protein crystals in formulation design. Although this idea has been present for some time, i.e., insulin crystals were first reported back in 1920s, macromolecular crystallization has not received as much attention as the other methods for stabilizing protein drug candidates. The prospective potential of crystalline protein formulations in light of new advances in the field of macromolecular crystallization was reviewed, and the basic concepts and the tools now available for developing protein crystals into drug formulations are introduced. In addition, formulation challenges and regulatory demands, along with examples of current applications of protein crystals, are presented.

Iontophoresis of monomeric insulin analogues in vitro: effects of insulin charge and skin pretreatment

The aim of this study was to investigate the influence of association state and net charge of human insulin analogues on the rate of iontophoretic transport across hairless mouse skin, and the effect of different skin pretreatments on said transport. No insulin flux was observed with anodal delivery probably because of degradation at the Ag/AgCl anode. The flux during cathodal iontophoresis through intact skin was insignificant for human hexameric insulin, and only low and variable fluxes were observed for monomeric insulins. Using stripped skin on the other hand, the fluxes of monomeric insulins with two extra negative charges were 50-100 times higher than that of hexameric human insulin. Introducing three additional charges led to a further 2-3-fold increase in flux. Wiping the skin gently with absolute alcohol prior to iontophoresis resulted in a 1000-fold increase in transdermal transport of insulin relative to that across untreated skin, i.e. to almost the same level as stripping the skin. The alcohol pretreatment reduced the electrical resistance of the skin, presumably by lipid extraction. In conclusion, monomeric insulin analogues with at least two extra negative charges can be iontophoretically delivered across hairless mouse skin, whereas insignificant flux is observed with human, hexameric insulin. Wiping the skin with absolute alcohol prior to iontophoresis gave substantially improved transdermal transport of monomeric insulins resulting in clinically relevant delivery rates for basal treatment.