Therapeutic peptide, insulin

The human intestine has evolved as a highly efficient organ to digest (i.e. hydrolyse) practically all the macromolecules in the human diet (albeit with the help of a few trillion bacteria ) with the exception of some plant fibres. To do this it possesses a formidable array of enzymes. This is particularly true for the digestion of proteins and peptides where peptidases are found in the stomach, are secreted by the pancreas in considerable quantities and are found on the surface of and inside intestinal epithelial cells. These enzymes work in a co-ordinated fashion to rapidly hydrolyse proteins. They present the major difficulty for designing oral delivery systems for therapeutic peptides, which may explain why 86 years after the first attempt to orally administer insulin (Bliss 1982), there is still not an oral insulin product available for diabetics. 

Chiu, Y-Y.H. and Sobel, S. (1988). Scientific review of the safety of peptide hormones insulin, growth hormone, and LHRH and its analogs. In Therapeutic Peptides and Proteins Assessing the New Technologies. D.R.Marshak and D.T.Liu, eds. (New York Cold Spring Harbor Laboratory), pp. 219 228. 

Farr, S.J. Gonda, 1. Licko, V. Physicochemical and physiological factors influencing the effectiveness of inhaled insulin, pulmonary absorption of therapeutic peptides and proteins. In Respiratory Drug Delivery VP, Dalby, R.N., Byron, P.R., Farr, S.J., Eds. Interpharm Press, Inc. Buffalo Grove, IL, 1998 25-33. 

Therapeutic peptides and proteins are far more stable in the solid state compared to the liquid state. Delicate proteins often significantly degrade within hours when held at room temperature in the liquid state. All FDA approved therapeutic proteins for human use are injectable products, except Exubera the first inhaleable insulin. A recent survey has shown that 12 of the 30 commercial products are available only as dry powder and 28 of the 30 require refrigeration (49). Not having to refrigerate a pharmaceutical product greatly increases convenience and significantly reduces transportation and distribution costs, this is particularly true for vaccines, where an alternative to break the so called distribution cold chain is badly needed in third world countries. 

Biologies may be considered a class of therapeutics directly purified from an original biological source. This category would include blood products, vaccines, and some hormones. Insulin, a peptide hormone (51 amino acids), is an example of a biologic that has been commercially available since about 1923 [ 1 ]. With the advent of... 

An excellent paper by Sanz-Nebot et al. [1303] focuses on the optimization of separations for therapeutic peptide hormones (e.g., lypressin, oxytocin, bradykinin, triptorelin, buserelin, bovine insulin, salmon calcitonin, met-enkephalin and leu-enkephalin), The optimal mobile phase for a 25°C Cjg column was 35/65/0.1 acetonitrile/water/TFA. The effects of changing solution pH and percent acetonitrile on both k and resolution were plotted. The final separation took 28 min with all analytes but calcitonin eluting prior to 8 min. 

Imniunoaffinity separations performed on a matrix-assisted laser desorption ionization-mass spectrometer (MALDI-MS) probe tip have given rise to the term PAMS (probe affinity MS) [134]. Antibiotin covalently bound to the surface of the probe allowed MS discrimination of bovine insulin with different degrees of biotinylation. The method has also been used to detect lysozyme in human tears by binding antilysozyme to the MS probe. Based on the same PAMS principle, modifications aimed to obtain faster and improved retention of Abs have been carried out by immobilizing the Abs on the probe via a nitrocellulose film instead of binding them directly to the MALDI probe surface [135]. The procedure allowed determination of a therapeutic peptide and one metabolite. 

Although a high degree of homology is evident between insulins from various species, the same is not true for proinsulins, as the C peptide sequence can vary considerably. This has therapeutic implications, as the presence of proinsulin in animal-derived insulin preparations can potentially elicit an immune response in humans. 

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