Stability of salicylamide-caffeine complex at different temperatures and its thermodynamic parameters

Drug release kinetics from polymeric films containing propranolol hydrochloride for transdermal use

Polymeric films containing propranolol hydrochloride (PPN) were formulated and evaluated with a view to select a suitable formulation for the development of transdermal drug delivery systems. Films containing different ratios of ethyl cellulose (EC), poly(vinylpyrrolidone) (PVP), and PPN were prepared by mercury substrate method. In vitro drug release and skin permeation studies were conducted using paddle over disk and modified Franz diffusion cell, respectively. The drug release profiles from the polymeric film indicated that the drug content in the film decreased at an apparent first-order rate, whereas the quantity of drug release was proportional to the square root of time. The release rate of PPN increased linearly with increasing drug concentration and PVP fraction in the film, but was found to be independent of film thickness. The increase in release rate may be due to leaching of hydrophilic fraction of the film former, which resulted in the formation of pores. It was also observed that the release of drug from the films followed the diffusion-controlled model at low drug concentration. A burst effect was observed initially, however, at high drug loading level, which may be due to rapid dissolution of the surface drug followed by the diffusion of the drug through the polymer network in the film. The in vitro skin permeation profiles displayed increased flux values with increase of initial drug concentration in the film, and also with the PVP content. From this study, it is concluded that the films composed of EC/PVP/PPN, 9:1:3, 8:2:2, and 8:2:3, should be selected for the development of transdermal drug delivery systems using a suitable adhesive layer and backing membrane for potential therapeutic applications.

Thermodynamic parameters of molecular complexes in aqueous solution: enthalpy-entropy compensation in a series of complexes of caffeine with beta- naphthoxyacetic acid and drug-related aromatic compounds

Stability constants and thermodynamic parameters have been evaluated for the complexation reaction in aqueous solution of caffeine with beta-naphthoxy acetic acid. The values were higher than those previously reported for the complexation of other ligands with methyl xanthines. In nearly all aromatic ligands complexing with caffeine and theophylline for which data are available, both entropy and free energy of complexation were linearly related to the enthalpy, giving an isoequilibrium relationship. Salicylamide, sodium benzoate and cis-methyl cinnamate exhibited slight deviations on the delta G-delta H plot; the non-aromatic dehydroacetic acid showed the largest deviation. The isoequilibrium relationship was shown to be valid statistically (349-365 K, caffeine systems; 353-372 K, caffeine and theophylline systems) indicating underlying chemical causation. Thermodynamic equations are presented for analysis of the factor involved, which are attributed to a combination of substrate-ligand interactions and solvent effects. The substrate-ligand overlap area is considered as a common parameter through which the solvent and interaction forces might cooperate to give rise to linearity in the isoequilibrium relationship. The increasingly negative experimental values of the enthalpy and entropy with increase in ligand planar overlap area are discussed in relation to the underlying forces involved in the complexation.

Sustained release of drugs from ethylcellulose--polyethylene glycol films and kinetics of drug release

Cast films composed of different ratios of polyethylene glycol and ethylcellulose containing salicylic acid, caffeine, and tripelennamine as model dispersed drugs were prepared and exhibited sustained release. The drug content of the film declined at an apparent first-order rate initially, whereas the drug quantity released was proportional to the square root of time. Data analysis validated the latter treatment, which is in accordance with the diffusional matrix model, and disproved the validity of the apparent first-order conformity. The release rates were independent of film thickness and proportional to drug concentration in pure ethylcellulose films; in polyethylene glycol--ethylcellulose films, a positive deviation from linearity was observed. The logarithm of the rate constant was proportional to the fraction of polyethylene glycol in the film. Unlike in pure ethylcellulose films, the release rate in mixed films was altered by a change in the external fluid pH.

Estimation of dissolution rate of salicylamide in complexing media using a theoretical diffusion model

Dissolution rates of salicylamide in water and caffeine solutions under perfect sink conditions were predicted by theoretical diffusion equations applicable to dissolution in complexing media. Experimental dissolution rates were measured using a compartmentalized rotating-basket apparatus under two sets of conditions. Agreement was found between experimental and predicted rates. Use of the theoretical equation for estimating dissolution rates involves simple calculations of diffusion coefficients and diffusion layer thickness under the operative dissolution conditions. The increase in dissolution rate caused by addition of the complexant can be calculated for diffusion-controlled dissolution directly if the stability constant and the drug solubility in water are known or measured.

The dissolution mechanism in a system undergoing complexation: salicylamide in caffeine solution

The dissolution rate of compressed salicylamide discs has been measured in water and in caffeine solutions of increasing concentration at 15, 25, 37 and 45 degrees in an apparatus rotating at 48 rev min-1 or more. Dissolution rate profiles showed breaks indicative of a shift in the mechanism of dissolution from interfacial towards transport control. The shifts occurred at higher caffeine concentrations on increasing the agitation rate or temperature. The dependencies of dissolution rates on agitation rates typified the intermediate type of dissolution and Arrhenius plots indicated that interfacial and transport processes participated in salicylamide dissolution.