Interaction of verapamil hydrochloride with Carbopol 934P and its effect on the release rate of the drug and the water uptake of the polymer matrix

In the present study, investigation of the possibility of interaction of verapamil hydrochloride with Carbopol 934P using differential scanning calorimetric analysis and Fourier transform infrared analysis was performed. The effect of the drug-to-polymer ratio, the electrolyte concentration, and the pH of the medium on the extent of interaction of the drug with the polymer using 2(3) factorial design was investigated. The study also investigated the effect of this interaction on the rate of water uptake of the matrix or the rate of release of verapamil hydrochloride from the swelling polymer matrix. Results revealed that the drug-to-polymer ratio had the most influential effect on both the extent of interaction between the drug and the polymer and the rate of water uptake of the polymer matrix. On the other hand, the pH of the medium had the most significant effect on the rate of drug release. Interaction of the tertiary amine nitrogen of the drug with the anionic carboxyl group on the polymer, forming an insoluble complex, reduced the rate of drug release. This interaction also led to neutralization of the carboxyl group and suppression of the electrostatic repulsion between the anionic groups, which reduced the uncoiling and chain relaxation of the polymer and consequently decreased the swelling of the matrix. The application of the designed experiment allowed the quantification of the effect of each of the studied variables on the investigated responses through the calculation of their coefficient in the response surface equation and checking of their significance.