Peptides enzymatic degradation

In this work we will focus on the use of the cubic phase as a delivery system for oligopeptides - Desmopressin, Lysine Vasopressin, Somatostatin and the Renin inhibitor H214/03. The amino acid sequences of these peptides are given in Table I. The work focuses on the cubic phase as a subcutaneous or intramuscular depot for extended release of peptide drugs, and as a vehicle for peptide uptake in the Gl-tract. Several examples of how the peptide drugs interact with this lipid-water system will be given in terms of phase behaviour, peptide self-diffusion, in vitro and in vivo release kinetics, and the ability of the cubic phase to protect peptides from enzymatic degradation in vitro. Part of this work has been described elsewhere (4-6). 

Enzymatic Degradation of H214/03. The degradation experiments were carried out with molecular mixtures of H214/03 or LVP and MO. These were obtained from mixtures of the peptide and MO dissolved in 99.5% ethanol, from which the ethanol was evaporated under reduced pressure at room temperature for about two days. Appropriate amounts of melted mixtures were then weighed in small Erlenmeyer flasks. Care was taken to insure that the bottom of each flask was covered with the mixture, which was then left to freeze. In this way the area and the thickness of the lipid/peptide layer was comparable in the flasks. Typically five flasks were used in each experiment... 

Figure 5. Enzymatic degradation of H214/03 in simulated intestinal fluid. S(t)/S(0) is the fraction of remaining peptide at time t. The compositions of the samples are listed in Table HI.

Many observations document that modifications of peptides by substitution with artificial amino acids or D-amino acids can often protect against enzymatic degradation. This is especially true when more than one modification is made. We begin here with two examples in which replacement of a single natural amino acid with an artificial one had a favorable impact on biological stability. Then, the influence of substitution with D-amino acids is described. Examples involving two or more modifications with artificial amino acids and D-amino acids combined are presented in the following section. 

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. 

Replacement of the amide by a thioamide bond in peptides represents a minimal modification that may be of interest as it increases resistance against enzymatic degradation. 483,484 Such modification has been reported to both increase14 5 486 and decrease biological activities, and the effect is not readily predictable. 487-489 ... 

The natriuretic peptides have a short half-life in the circulation. They are metabolized in the kidneys, liver, and lungs by the neutral endopeptidase NEP 24.11. Inhibition of this endopeptidase results in increases in circulating levels of the natriuretic peptides, natriuresis, and diuresis. The peptides are also removed from the circulation by binding to ANPc receptors in the vascular endothelium. This receptor binds the natriuretic peptides with equal affinity. The receptor and bound peptide are internalized, the peptide is degraded enzymatically, and the receptor is returned to the cell surface. 

Formation of cyclic peptides (e.g. DPDPE) less prone to enzymatic degradation by aminopeptidases... 

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