inhibitory agent

Journal of Controlled Release52(1998)1–16

Invited review

The use of inhibitory agents to overcome the enzymatic barrier to perorally administered therapeutic peptides and proteins

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Andreas Bernkop-Schnurch

Institute of Pharmaceutical Technology,Center of Pharmacy,University of Vienna,Althanstr.14,A-1090Vienna,Austria Abstract

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The peroral administration of peptide and protein drugs is a major challenge to pharmaceutical science.In order to provide a suf?cient bioavailability of these therapeutic agents after oral dosing,several barriers encountered with the gastrointestinal (GI)tract have to be overcome by a suitable galenic.One of these barriers is caused by proteolytic enzymes,leading to a severe presystemic degradation in the GI tract.Beside some other strategies to overcome the so-called enzymatic barrier,the use of inhibitory agents has gained considerable scienti?c interest,as various in vivo studies could demonstrate a signi?cantly improved bioavailability of therapeutic peptides and proteins,due to the co-administration of such excipients.In vitro techniques to evaluate the actual potential of inhibitory agents include incubation with pure proteases,freshly collected gastric or intestinal?uids,mucosal homogenates,brush border vesicles and freshly excised mucosa.In situ techniques are based on single-pass perfusion studies cannulating different intestinal segments and determining the amount of undegraded model drug in perfusion solutions or blood.For in vivo studies,insulin is mostly used as a model drug,offering the advantage of a well-established method to evaluate the biological response after oral dosing by determining the decrease in blood glucose level.Generally,inhibitory agents can be divided into:inhibitors which are not based on amino acids(I),such as p-aminobenzamidine,FK-448and camostat mesilate;amino acids and modi?ed amino acids(II),such as a-aminoboronic acid derivatives;peptides and modi?ed peptides(III),e.g.bacitracin,antipain,chymostatin and amastatin;and polypeptide protease inhibitors(IV),e.g.aprotinin,Bowman–Birk inhibitor and soybean trypsin inhibitor.Furthermore,complexing agents and some mucoadhesive polymers also display enzyme inhibitory activity.Drawbacks of inhibitory agents,such as the risk of toxic side effects or high production costs,might be excluded by the development of advanced drug delivery systems.Initial steps in this direction can be seen in the development of delivery systems containing mucoadhesive polymers providing an intimate contact to the mucosa,thereby reducing the drug degradation between delivery system and absorbing membrane,controlled release systems which provide a simultaneous release of drug and inhibitor and in the immobilisation of enzyme inhibitors on delivery systems.©1998Elsevier Science B.V.

Keywords:Enzyme inhibitors;Complexing agents;Proteolytic enzymes;Peroral administration;(Poly)peptide drugs

1.Introduction ing a large number of potential therapeutic peptides

and proteins in commercial quantities.Moreover, Due to the great progress in biotechnology,as well these new techniques also allow the production of as gentechnology,the industry is capable of produc-peptide and protein vaccines based on antigens found

on the surface of various infectious microorganisms *and viruses.The majority of such drugs and vaccines Tel.:1431313368476;fax:143131336779;e-mail:

andreas.bernkop-schnuerch@univie.ac.at is most commonly administered by the parenteral

0168-3659/98/$19.00©1998Elsevier Science B.V.All rights reserved.

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