Assessment of Free Drug Concentration in Cyclodextrin Formulations Is Essential to Determine Drug Potency in Functional In Vitro Assays

Cyclodextrins (CD) have the ability to form inclusion complexes with drugs and can be used as excipients to enhance solubility of poorly soluble drugs. To make accurate estimations of the potency of the drug, knowledge of the free drug concentration is important. The aim of this study was to evaluate the applicability of calculated free drug concentrations toward response measurements in a transient receptor potential vanilloid receptor-1 cell-based in vitro assay. This included accounting for potential competitive CD binding of 2 transient receptor potential vanilloid receptor-1 active entities: 1 antagonist, and 1 agonist (capsaicin). Solubility of the CD-drug complexes was measured, and the ligand to substrate affinity in CD formulations was determined according to the phase-solubility technique. The total concentration of antagonist, agonist, CD, and the binding constants between ligands and CD were used to calculate the free concentration of CD ligands. For capsaicin and 2 of the 3 investigated model drugs, the calculated free drug concentration was consistent with the experimental in vitro data while it was overestimated for one of the compounds. In conclusion, the suggested approach can be used to calculate free drug concentration and competitive binding in CD formulations for the application of cell-based drug functionality assays.

Structure-Interaction Relationships between the Bile Acid GCA and Pharmaceuticals using Multivariate Data Analysis and Capillary Electrophoresis

Capillary electrophoresis (CE) has been used in an interaction study of 66 pharmaceutical compounds with the bile acid glycocholate (GCA). The developed method proved to have a high precision in its ability to determine the mobility of drugs in buffer and buffer bile acids solutions. The relationship between solute structure and interaction with GCA was studied using two-dimensional descriptors with the in-house software SELMA and a three-dimensional model (quantum mechanical descriptors) in combination with the experimental CE-interaction data. The multivariate analysis method used was projection to latent structures by means of partial least squares (PLS). Two selections of training and test set were used for evaluation of a two-class model on interaction data. In the first selection all observations were used for training set, for example, creating a model, and re-predicting the observations on the model. A successful prediction on 85% of the drugs was observed using this model. The second selection used the 21 first tested compounds in the training set, where 78% of the compounds were correctly predicted using the two-dimensional model (SELMA) on the remaining 45 compounds and, respectively, 82% using the three-dimensional (quantum mechanical) model. Analysis of the impact of the descriptors showed that descriptors relating to hydrophobicity have a large positive effect on the interaction. Descriptors relating to polar properties have a pronounced negative effect on the interaction of drugs with bile acids.

The determination of bromide in a local anaesthetic hydrochloride by capillary electrophoresis using direct UV detection

The determination of residual amounts of bromide in a local anaesthetic hydrochloride by capillary electrophoresis was developed. Direct UV detection at 200 nm was used for the determination of the bromide content. The separation capacity of the system must be sufficient when bromide is determined in the presence of a large excess of chloride since electromigration dispersion of the highly concentrated chloride peak may impair the resolution. The background electrolyte (BGE) contained both acetonitrile and methanesulphonic acid in order to improve the selectivity and minimise the electromigration dispersion. The system was optimised with respect to resolution of the chloride and the bromide peaks by statistical experimental design using a multivariate optimisation program. The developed method was validated in accordance with the ICH guidelines and proved to be suitable for its intended use.