Thiolated polymers: synthesis and in vitro evaluation of polymer-cysteamine conjugates

The purpose of the present study was to synthesize and characterize novel thiolated polymers. Mediated by a carbodiimide cysteamine was covalently linked to sodium carboxymethylcellulose (CMC) and polycarbophil (PCP). The resulting CMC-cysteamine conjugates displayed 77.9+/-6.7 and 365.1+/-8.7 micromol thiol groups per gram of polymer, whereas the PCP-cysteamine conjugates showed 26.3+/-1.9 and 122.7+/-3.8 micromol thiol groups per gram of polymer (mean+/-S.D.; n=3). In aqueous solutions above pH 5.0 both modified polymers were capable of forming inter- and/or intra-molecular disulfide bonds. The reaction velocity of this oxidation process was accelerated with a decrease in the proton concentration. The oxidation proceeded more rapidly within thiolated CMC than within thiolated PCP. Permeation studies carried out in Ussing-type chambers with freshly excised intestinal mucosa from guinea pigs utilizing sodium fluorescein as model drug for the paracellular uptake revealed an enhancement ratio (R=P(app) (conjugate)/P(app) (control)) of 1.15 and 1.41 (mean+/-S.D.; n=3) for the higher thiolated CMC-cysteamine (0.5%; m/v) and PCP-cysteamine conjugate (1.0%; m/v), respectively. The decrease in the transepithelial electrical resistance values was in good correlation with the enhancement ratios. Due to a high crosslinking tendency by the formation of disulfide bonds stabilizing drug carrier systems based on thiolated polymers and a permeation enhancing effect, CMC- and PCP-cysteamine conjugates represent promising excipients for the development of novel drug delivery systems.