Synthesis, characterisation and in vitro evaluation of novel thiolated derivatives of polyallylamine and quaternised polyallylamine

Polyallylamine (Paa) was quaternised by methylation of its primary amines using methyl iodide to yield quaternised Paa (QPaa). Average level of polymer quaternisation was determined by elemental analysis and was found to be 72 +/- 2 mol%. Subsequent thiolation of Paa (15 kDa) and QPaa using two different thiolation procedures involving carbodiimide-mediated conjugation to N-acetylcysteine (NAC) and modification of the polymers using 2-iminothiolane hydrochloride yielded their respective NAC and 4-thiobutylamidine (TBA) conjugates: Paa-NAC/QPaa-NAC and Paa-TBA/QPaa-TBA. Estimation of the free thiol content of thiomers by iodometric titration showed that Paa-NAC and QPaa-NAC displayed 60 +/- 1.2 and 60 +/- 4.3 mu mol free thiol groups per gram polymer, respectively, while Paa-TBA and QPaa-TBA conjugates displayed 490 +/- 18 and 440 +/- 21 mu mol free thiol groups per gram polymer, respectively. Assessment of polymer mucoadhesion using the mucin adsorption assay method revealed that Paa-NAC and Paa-TBA had the best mucoadhesive profile both adsorbing > 20 % more mucin than the parent polymer Paa. However, thiolation of QPaa was not observed to result in a marked improvement in adsorption of mucin. Also, while other thiolated derivatives were stable in Tris/phosphate buffer pH 8 with no change in free thiol content, Paa-TBA solutions displayed in situ crosslinking of free thiol groups to a disulphide network which was evident after 2 h. These results show that thiolation of Paa enhanced its mucoadhesive properties, with Paa-TBA also exhibiting in situ gelling properties. Hence, these novel thiolated Paa derivatives exhibit properties which may be useful in facilitating transmucosal drug delivery and controlled drug release.