Stability of Proline-Containing Peptides in Biological Media

New data on peptide drugs characterized by their high stability, which is determined by both introduction of Pro-Gly-Pro in various amino acid sequences and also modification of the glyproline fragment have been summarized. The following proline-containing peptides were used: Pro-Gly-Pro-Leu, ACTH(6-9)Pro-Gly-Pro, 5-oxo-Pro-Arg-Pro and 5-oxo-Pro-His-Pro-NH2. Tritiated peptides had the following specific radioactivity: Pro-Gly-Pro-Leu-35 Ci/mmol, ACTH(6-9)Pro-Gly-Pro-26 Ci/mmol, 5-oxo-Pro-Arg-Pro-60 Ci/mmol and 5-oxo-Pro-His-Pro-NH2-75 Ci/mmol. The labeled peptides were used for determination of their content in vivo. The concentration of Pro-Gly-Pro-Leu, ACTH(6-9)Pro-Gly-Pro, 5-oxo-Pro-Arg-Pro and 5-oxo-Pro-His-Pro-NH2 in the blood was found to be about 200 times more than in the brain for intranasal administration, and about 600 times higher in blood than in the brain after intravenous administration. The stability of proline-containing peptides in vitro experiments was determined using different commercially available peptidases (leucine aminopeptidases, dipeptidases, carboxypeptidases B and Y), and using nasal mucus, rat brain microsomal fraction (RBM) and rat blood plasma. During peptidase hydrolysis of Pro-Gly-Pro-Leu, the main metabolites were Gly-Pro-Leu, Pro-Gly-Pro, Gly-Pro, and Pro-Gly. In the case of peptidase hydrolysis of ACTH(6-9)Pro-Gly-Pro, the main metabolites were Phe-Arg-Trp-Pro-Gly-Pro and Trp-Pro-Gly-Pro. Peptidase hydrolysis of 5-oxo-Pro-His-Pro-NH2, resulted in formation of 5-oxo-Pro-His-Pro as the major metabolite. It was shown that with different methods of peptides administration the composition of the metabolites formed was different. Based on the data obtained, resistance to enzymatic cleavage of peptides and their metabolic pathways were evaluated. Thus, these new data have shown that the above approaches can be used to prolong the action of glyprolines in living organisms. Degradation of proline-containing peptides occurs mainly via degradation of amino acid constituents rather than peptidase action. In general, the data presented in the review indicate the prospect for the use of intranasal route for administration of biologically active peptides for their delivery into the brain.