Insulin enzymatic degradation

M. K. Marschiitz, A. Bernkop-Schniirch, Oral Peptide Drug Delivery Polymer-Inhibitor Conjugates Protecting Insulin from Enzymatic Degradation in vitro, Biomaterials 2000, 21, 1499-1507. 

The enzymatic degradation of insulin was also shown to occur in the cytosol of alveolar cells, the pH optimum of the proteases being 7.4 [38]. To what extent intracellular proteases play a significant role in limiting the absorption of insulin is not clear, since the size of insulin likely allows paracellular transport over the alveolar epithelium. However, for proteins of higher molecular weight, that require transcellular transport, these proteases might certainly limit bioavailability. 

Morishita, M., et al. 1992. Novel oral microspheres of insulin with protease inhibitor protecting from enzymatic degradation. Int J Pharm 78 1. 

Marschutz, M. K., and Bernkop-Schniirch, A. Oral peptide drug delivery Polymer-inhibitor conjugates protecting insulin from enzymatic degradation in vitro. Biomaterials 21 1499—1507, 2000. 

Luessen HL, Verhoef JC, Borchard G, Lehr CM, de Boer AG, Junginger HE (1995) Mucoadhesive polymers in peroral peptide drug delivery. II. Carbomer and polycarbophil are potent inhibitors of the intestinal proteolytic enzyme trypsin. Pharm Res 12 1293-1298 Marschutz MK, Bernkop-Schnurch A (2000) Oral peptide drug delivery polymer-inhibitor conjugates protecting insulin from enzymatic degradation in vitro. Biomaterials 21 1499-1507... 

Drags that structurally resemble nutrients such as polypeptides, nucleotides, or fatty acids may be especially susceptible to enzymatic degradation. For example, the proteolytic enzymes chymotrypsin and trypsin can degrade insulin and other peptide drags. In the case of insulin, proteolysis was shown to be reduced by the coadmmistration of carbopol polymers at 1% and 4% (w/v%), which presumably shifted the intestinal pH away from the optimal pH for proteolytic degradation. 

The opposite approach, namely, the creation of additional lysine-like sites for cleavage by trypsin has been used in the conversion of cysteine bonds to /S-( -aminoethyl)-cysteine residues by reaction of the thiol groups with jS-bromoethylamine (Lindley, 1959). Enzymatic degradation of such modified proteins, e.g., reduced and /3-aminoethylated insulin, by the ac-... 

In this chapter, we summarize the general approaches that have been used to successfully achieve the formulation goals for oral delivery minimize enzymatic degradation enhance intestinal absorption maximize blood level reproducibility deliver drug through the gut wall and produce a palatable and acceptable dosage form. Then insulin will be used as an example to show how oral bioavailability has been achieved through chemical modification. 

Intemasal delivery of peptide and protein drugs is severely restricted by pre-systemic elimination due to enz5miatic degradation or mucociliary clearance and by the limited extent of mucosal membrane permeability. a-CyD has been shown to remove some fatty acids from nasal mucosa and to enhance the nasal absorption of leuprolide acetate in rats and dogs. The utility of chemically modified CyDs as absorption enhancers for peptide drugs in rats has been demonstrated. For example, DM-P-CyD was shown to be a potent enhancer of insulin absorption in rats, and a minimal effective concentration of DM-(3-CyD for absorption enhancement exerted only a mild effect on the in vitro ciliary movement.The scope of interaction of insulin with CyDs is limited, because CyDs can only partially include the hydrophobic amino acid residues in peptides with small stability constants. Under in vivo conditions, these complexes will readily dissociate into separate components, and hence the displacement by membrane lipids may further destabilize the complexes. The direct interaction of peptides with CyDs is therefore of minor importance in the enhancement of nasal absorption. Of the hydrophilic CyDs tested, DM- 3-CyD had the most prominent inhibitory effect on the enzymatic degradation of both BLA and insulin in rat nasal tissue homogenates. Because of the limited interaction between peptides and CyDs,... 

Noninvasive delivery of insulin via most mucosal membranes requires the use of chemical enhancement for notable insulin absorption (see Section 3.3 and Table II). However, most permeation enhancers have, in addition to their effect on the mucosal membrane, an often pronounced influence on insulin three-dimensional structures. Thus, sodium salicylate (Touitou et al, 1987) as well as bile salts (Gordon etal, 1985) have been shown to dissociate insulin oligomers into monomers. This effect improves membrane permeability, but it may also reduce the physical stability and increase the susceptibility of insulin to enzymatic degradation. The exposure of new epitopes may also influence the immunological properties of the insulin formulation. 

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