Structure-activity relationships having a basis in regular solution theory

Predicting the solubility of sulfamethoxypyridazine in individual solvents. I: Calculating partial solubility parameters

Sulfamethoxypyridazine, a representative model of a drug molecule, is used to test the extended Hansen method for estimating partial solubility parameters of solid compounds. Solubilities are determined in polar and nonpolar solvents. The method provides reasonable partial parameters for the sulfonamide, and it may be useful in obtaining partial parameters for other drug molecules. A four-parameter extended Hansen approach involving proton donor and acceptor parameters is used in fitting the data to a theoretical model. A term, Wh, is introduced as an empirical measure of solute-solvent interactions due to hydrogen bonding. The use of the empirical term Wh allows the researcher to fit experimental solubilities and thus design regression models and equations which provide a reasonable prediction of solubilities of a polar drug in a number of very different solvents. A Flory-Huggins size correction term improves the prediction of sulfamethoxypyridazine solubilities in these irregular solutions.

Use of solubility parameters of drug and vehicle to predict flux through skin

The solubilities of theophylline in, and fluxes through skin from, isopropyl myristate, octanol, dimethylformamide, propylene glycol, ethylene glycol, and formamide have been determined experimentally. Values for experimental permeability coefficients (Kp) corresponding to the respective fluxes were determined from, flux/solubility = Kp, which were then compared with values for the respective theoretical partition coefficients (PC) calculated from the known solubility parameters for the vehicles (delta v), theophylline (delta i) and skin (delta s). There was a good correlation for theoretical log PC - 2.52 = experimental log Kp for vehicles exhibiting solubility parameters in the range of delta v = 12-18 (cal/cm3)1/2. This allows relative fluxes to be determined from calculated theoretical partition coefficients and experimentally determined solubilities in that range. For vehicles or mixtures of vehicles exhibiting solubility parameters in the range of delta v = 8-12 (cal/cm3)1/2 large increases in fluxes and permeability coefficients, compared with those predicted from the results in the delta v = 12-18 (cal/cm3)1/2 range, were observed because of vehicle effects on the skin caused by the similarity in solubility parameters of those vehicles to that of skin. Qualitatively, fluxes and permeability coefficients were found to be inversely dependent on drug solubility in the vehicles with a minimum that corresponded approximately to the point where delta i = delta v.

Relationship between the solubility parameter and the binding of drugs by plasma proteins

An equation, based on regular solution theory, was used to relate the solubility parameter to the binding of drugs by plasma proteins. The equation was tested on a homologous series and a good correlation was found. Sulfonamides showed maximum binding when their solubility parameters were similar to the solubility parameters of the amino acids situated in a sequence with one tryptophan residue. This observation supports the assumption that this sequence is the primary binding site for the sulfonamides. Binding peaks were also found at solubility parameters in other drug series corresponding to the solubility parameters of human serum albumin (HSA) or bovine serum albumin (BSA) amino acids. It is suggested that the solubility parameter could be used to predict the binding of drugs to plasma proteins.

Membrane solubility parameter and in situ release of theophylline

The solubility of theophylline in polyethylene glycol 400-water binary mixtures was analyzed in terms of solute-solvent interaction using the solubility parameter principle of Hildebrand. Preliminary in situ (rat gut permeation) studies with the solvent mixtures having varying polarity as expressed by the solubility parameter showed that (a) the more alike the solubility parameter of theophylline (delta 2 = 14, as obtained from solubility studies) and the solubility parameter of the solvent mixture, the greater was the attraction of solvent for the drug (b) sufficient similarity must exist between the solubility parameter of theophylline and that of the intestinal mucosa to promote bioabsorption, and (c) from the above, it follows that there exists a competition between solvent and rat gut membrane for the drug. The solubility parameter of the solvent vehicle must be such that it does not impede absorption of the molecules into the intestinal mucosa.

Chromatographic hydrophobic parameters in correlation analysis of structure-activity relationships

The use of RM and delta RM parameters as indices of hydrophobicity for inclusion in quantitative structure activity relationships has been studied. The relationship between these parameters and other free-energy related parameters is illustrated theoretically and experimentally. It is suggested that the chromatographically obtained parameters could find a wider applicability in structure-activity relationships, and that their use would result in improved correlation of data.