Verena M. Leitner

Mucoadhesive and cohesive properties of poly(acrylic acid)-cysteine conjugates with regard to their molecular mass.

The objective of this study was to evaluate the influence of the molecular mass and accordingly the polymer chain length on mucoadhesion and cohesion of thiolated polymers. Linear poly(acrylic acid)-cysteine (PAA-Cys) conjugates of 2-, 45-, 250- and 450 kDa (PAA(2)-Cys, PAA(45)-Cys, PAA(250)-Cys and PAA(450)-Cys) and polycarbophil-cysteine (PCP-Cys, 750-3000 kDa), all displaying on average 404.1+/-65.5 microMol thiol groups per gram polymer were compressed into tablets to perform disintegration tests, mucoadhesion studies and viscosity measurements. Moreover, the influence of free unbound cysteine on mucoadhesion was evaluated. Disintegration tests showed a stability of the tablets as following: PAA(2)-Cys<PAA(45)-Cys<PAA(250)-Cys<PAA(450)-Cys=PCP-Cys. According to tensile studies and tests on the rotating cylinder the following rank order in mucoadhesive properties could be established: PAA(2)-Cys<PAA(45)-Cys<PCP-Cys<PAA(250)-Cys<PAA(450)-Cys. Evidence for the formation of disulphide bonds between thiolated polymers and mucin could be provided by the addition of free cysteine resulting in strongly decreased mucoadhesion and by viscosity studies showing comparatively higher viscosity of conjugate/mucin mixtures than of unthiolated polymer/mucin mixtures. The results of the present study contribute to the development of new polymers displaying further improved mucoadhesive properties.

Synthesis and In Vitro Characterization of a Poly(Acrylic Acid)‐Homocysteine Conjugate

It was the aim of this study to improve our knowledge on thiolated polymers by the synthesis and in vitro characterization of a poly(acrylic acid)‐homocysteine conjugate. Mediated by a carbodiimide, homocysteine was therefore covalently attached to poly(acrylic acid) via the formation of an amide bond. The isolated conjugate displayed 930 µmol ± 83 µmol sulfur atoms per gram polymer. Of these thiol groups, 80.1% were oxidized to disulfide bonds during the coupling reaction. In aqueous solutions the conjugate was rapidly oxidized by the formation of disulfide bonds at pH 8, whereas it remained stable at pH 7 and below during the observation period of 4 hours. Due to the immobilization of thiol groups on the polymer, the mucoadhesive and cohesive properties of poly(acrylic acid) were strongly improved. Furthermore, the thiolated polymer exhibited a significantly (p < 0.05) improved permeation enhancing effect in comparison to the unmodified polymer. Because of these features the poly(acrylic acid)‐homocysteine conjugate seems to represent a promising novel tool, which might be useful in particular for aqueous formulations based on thiomers.

Polymer-enzyme inhibitor conjugates: influence of the molecular mass on the inhibition of membrane-bound aminopeptidase N activity

Conjugates of mucoadhesive polymers and protease inhibitors have been developed to protect incorporated (poly)peptides from enzymatic attack by luminally secreted or membrane-bound proteases. Within the present study, the influence of the molecular mass of such polymer conjugates on the inhibition of membrane-bound enzyme activity was evaluated. The aminopeptidase N inhibitor bacitracin (Bac) was covalently immobilised on the mucoadhesive polymer model poly(acrylic acid) with a molecular mass of 2 kDa (PAA 2 ) and 3000 kDa (polycarbophil, PCP). The resulting conjugates PAA 2 -Bac and PCP-Bac exhibited comparable amounts of bacitracin (24.4 and 25.3% (m/m), respectively) and were used to perform inhibition studies on isolated and membrane-bound aminopeptidase N. The conjugates showed no differences in the inhibition of isolated aminopeptidase N, whereas PAA 2 -Bac exhibited a significantly higher inhibitory activity versus membrane-bound aminopeptidase than PCP-Bac. Due to its lower molecular mass, PAA 2 -Bac could more easily diffuse into the mucus layer to get in contact with the enzyme. Obtained results should provide helpful basic knowledge for the development of multifunctional polymers as auxiliary agents for the oral administration of peptide drugs.