Comparative study of three structurally related acid polyelectrolytes as carriers of basic drugs: Carbomer, Eudragit L-100 and S-100

A detailed description of equilibrium and drug release properties of aqueous dispersions of complexes of model basic drugs (D) [lidocaine (Ld), atenolol and metoclopramide (Me)] with three structurally related acid polyelectrolytes (PE) is reported. Thus, affinity constants for ionic pair formation (Kip) of dispersions of polymetacrylates, Eudragit L-100 and Eudragit S-100, neutralised with increasing proportions of Ld and Me, were determined and compared with those of Carbomer previously reported. Affinity constants were calculated from the concentration of D condensed with PE (RCOO-DH+) and those of free species (D and DH+). In agreement with the high degree of counterionic condensation observed, the three PE-D complexes placed on Franz-type cells released D at slow rates as water was placed as receptor medium. Rates increased over three times as water was replaced by 0.9% NaCl solution. Similar average of diffusional exponent n (water, 0.61 and NaCl, 0.69) was found in both media. The overall kinetic behaviour suggests that, under the conditions assayed, the dissociation of RCOO-DH+ is the factor that controls releasing rates. Structure-related properties of the PE-D systems were identified in order to expand their potential uses as drug carriers.