Enzymatic semisynthesis of human insulin: an update

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.

A novel peptide design aids in the expression and its simplified process of manufacturing of Insulin Glargine in Pichia pastoris

Manufacturing of insulin and its analogues relied upon in vitro enzymatic cleavages of its precursor forms (single chain precursor, SCP) at both ends of a connecting peptide (C-peptide) that links the respective B-chain and A-chains to corresponding final forms. We have demonstrated a simplified approach of cleaving P. pastoris expressed SCP, distinctly at one site for conversion to insulin glargine. The design of the precursor was made in such a way that there is no C-peptide in the precursor which needs to be removed in the final product. Instead of traditional both side cleavage of the C-peptide and removing the C-peptide (by trypsin), followed by 2nd enzyme reaction (typically carboxipeptidase B), present work established only one side cleavage of the sequence by only trypsin converts the precursor to final insulin glargine product. The novel design of the precursor helped in producing insulin glargine in a single step with an application of single enzyme brought high degree of process efficiencies. Highly purified product was generated through two reversed phase high pressure chromatographic steps. Purified product was compared with the reference product Lantus®, for various physico-chemical and biological properties. Primary, secondary and tertiary structures as well as biological pharmaco-dynamic effects were found comparable. High cell density fermentation that gave a good yield of the SCP, a single step conversion to insulin glargine, enabled by a unique design of SCP and a distinct purification approach, has led to a simplified and economical manufacturing process of this important drug used to treat diabetes. KEY POINTS: • Novel concept for processing single chain precursor of insulin glargine • Simple and economic process for insulin glargine • Physicochemical characterization and animal Pharmacodynamics show similarity to Lantus.

Equivalent Recombinant Human Insulin Preparations and their Place in Therapy

Recombinant human insulin was one of the first products of biotechnology. It was developed in response to the need for a consistent and sufficient worldwide supply. Recombinant human insulin replaced the animal insulins and semisynthetic insulins obtained by modification of animal insulins. Bioequivalence studies were required for regulatory approval. Three reference products were independently established during these procedures: Humulin® (Eli Lilly and Co), Novolin® (NovoNordisk) and Insuman® (Sanofi). Numerous brand names have been used during the commercial development of recombinant human insulin formulations. In this review, three current brand names are used for consistent identification. Human insulin for Humulin and Insuman are produced by fermentation in bacteria (Escherichia coli) and for Novolin in yeast (Saccharomyces cerevisiae). The bioequivalence of recombinant human insulin products was investigated in euglycaemic clamp studies. An overview of such bioequivalence studies is provided here. This paper will consider the relevance of human insulin formulations today and their place in therapy.

Two-step transpeptidation of the insulin precursor expressed in Pichia pastoris to insulin ester via trypsin-catalyzed cleavage and coupling

Insulin precursor fusion protein expressed in Pichia pastoris is a single-chain protein with a spacer peptide (EEAEAEAEPK) localized at its N-terminal. Currently, the one-step transpeptidation reaction with low yield and high cost is generally employed to convert the insulin precursor fusion protein into human insulin ester. In this study, a two-step transpeptidation reaction was proposed separating the cleavage step from the coupling step so that each reaction was performed under its optimal conditions. Using this method, the total efficiency doubled and the reaction time was shortened compared with the one-step method. In addition, the amount of O-t-butyl-l-threonine t-butyl ester and trypsin dosages were reduced by 50% and 75%, respectively. This two-step transpeptidation strategy was simple and efficient and could be used for the pharmaceutical production of human insulin.

Backbone cyclic insulin

Backbone cyclic insulin was designed and prepared by reverse proteolysis in partial organic solvent of a single-chain precursor expressed in yeast. The precursor contains two loops to bridge the two chains of native insulin. The cyclisation method uses Achromobacter lyticus protease and should be generally applicable to proteins with C-terminal lysine and proximal N-terminal. The presence of the ring-closing bond and the native insulin disulfide patterns were documented by LC-MS peptide maps. The cyclic insulin was shown to be inert towards degradation by CPY, but was somewhat labile towards chymotrypsin. Intravenous administration of the cyclic insulin to Wistar rats showed the compounds to be equipotent to HI despite much lower insulin receptor affinity.

Rearrangement of terminal amino acid residues in peptides by protease-catalyzed intramolecular transpeptidation

Protease-catalyzed rearrangements of amino acid residues in peptides are observed during enzymatic digestion of proteins. When two enzyme-specific cleavage sites are within one or two residues of each other in the protein sequence, only one of the two sites usually is hydrolyzed by the protease, resulting in a peptide that contains an extra cleavage site near one of its termini. It is observed that in this type of peptide, the residues between the two cleavage sites often rearrange from one terminus of the peptide to the other terminus, catalyzed by the protease that created the peptide. It is proposed that the rearrangement is caused by protease-catalyzed intramolecular transpeptidation through a cyclic peptide intermediate. Several cases of this type of rearrangement were observed for different peptides generated by different proteases, indicating that this type of rearrangement is a general phenomenon occurring during enzymatic digestion of proteins.

Subtiligase: a tool for semisynthesis of proteins

A variant of subtilisin BPN', which we call subtiligase, has been used to ligate esterified peptides site-specifically onto the N termini of proteins or peptides in aqueous solution and in high yield. We have produced biotinylated or heavy-atom derivatives of methionyl-extended human growth hormone (Met-hGH) by ligating it onto synthetic peptides containing biotin or mercury. Polyethylene glycol (PEG)-modified atrial natriuretic peptide (ANP) was produced by ligating ANP onto peptides containing sites for PEG modification. We have established the N-terminal sequence requirements for efficient ligation onto proteins, using either synthetic substrates or pools of filamentous phage containing Met-hGH with random N-terminal sequences (substrate phage). To facilitate ligations involving proteins with highly structured or buried N termini, a more stable subtiligase was designed that effectively ligates peptides onto Met-hGH even in 4 M guanidine hydrochloride. The use of subtiligase should expand the possibilities for protein semisynthesis and rational protein design.