Physicochemical Properties of a New Multicomponent Cosolvent System for the pKa Determination of Poorly Soluble Pharmaceutical Compounds

Determination of the aqueous pKa of very insoluble drugs by capillary electrophoresis: Internal standards for methanol-water extrapolation

A fast determination of acidity constants (pK_a) of very insoluble drugs has become a necessity in drug discovery process because it often produces molecules that are highly lipophilic and sparingly soluble in water. In this work the high throughput internal standard capillary electrophoresis (IS-CE) method has been adapted to the determination of pK_a of water insoluble compounds by measurement in methanol/aqueous buffer mixtures. For this purpose, the reference pK_a values for a set of 46 acid-base compounds of varied structure (internal standards) have been established in methanol-water mixtures at several solvent composition levels (with a maximum of 40% methanol). The IS-CE method has been successfully applied to seven test drugs of different chemical nature with intrinsic solubilities lower than 10^-6 M. pK_a values have been determined at different methanol/aqueous buffer compositions and afterwards Yasuda-Shedlovsky extrapolation method has been applied to obtain the aqueous pK_a. The obtained results have successfully been compared to literature ones obtained by other methods. It is concluded that the IS-CE method allows the determination of aqueous pK_a values using low proportions of methanol, becoming then more accurate in the extrapolation procedure than other reference methods.

Using Theory To Extend the Scope of Azobenzene Drugs in Chemotherapy: Novel Combinations for a Specific Delivery

Gut microorganisms metabolize azobenzene compounds (Ph¹ -N=N-Ph² ) into free aniline products (Ph¹ -NH₂ +H₂ N-Ph² ), a process that has been largely investigated to reduce dyes residues in the textile industry. However, the action of bacterial core enzymes such as azoreductases (AzoR) might also help to deliver prodrugs that become active when they reach the colonic region, a mechanism with potential applications for the treatment of inflammatory bowel disease (IBD) and colorectal cancer. So far, three azo-bonded prodrugs of 5-aminosalicylic acid (5-ASA), for example, sulfasalazine, olsalazine and balsalazide, have been used for colon-targeted delivery. The present contribution describes the first rational design of a novel azobenzene prodrug thanks to a computational approach, with a focus on linking 5-ASA to another approved anti-inflammatory drug. The resulting prodrugs were assessed for their degradation upon AzoR action. Replacing the original carriers by irsogladine is found to improve action.

A single rhodamine spirolactam probe for localization and pH monitoring of mitochondrion/lysosome in living cells

Rh-BMDZ with neutral pK_a6.9 succeeds in indicating and discriminating mitochondria and lysosomes simultaneously in MCF-7 cells.

Tools for Early Prediction of Drug Loading in Lipid-Based Formulations

Identification of the usefulness of lipid-based formulations (LBFs) for delivery of poorly water-soluble drugs is at date mainly experimentally based. In this work we used a diverse drug data set, and more than 2,000 solubility measurements to develop experimental and computational tools to predict the loading capacity of LBFs. Computational models were developed to enable in silico prediction of solubility, and hence drug loading capacity, in the LBFs. Drug solubility in mixed mono-, di-, triglycerides (Maisine 35-1 and Capmul MCM EP) correlated (R² 0.89) as well as the drug solubility in Carbitol and other ethoxylated excipients (PEG400, R² 0.85; Polysorbate 80, R² 0.90; Cremophor EL, R² 0.93). A melting point below 150 °C was observed to result in a reasonable solubility in the glycerides. The loading capacity in LBFs was accurately calculated from solubility data in single excipients (R² 0.91). In silico models, without the demand of experimentally determined solubility, also gave good predictions of the loading capacity in these complex formulations (R² 0.79). The framework established here gives a better understanding of drug solubility in single excipients and of LBF loading capacity. The large data set studied revealed that experimental screening efforts can be rationalized by solubility measurements in key excipients or from solid state information. For the first time it was shown that loading capacity in complex formulations can be accurately predicted using molecular information extracted from calculated descriptors and thermal properties of the crystalline drug.

Tautomeric and Microscopic Protonation Equilibria of Anthranilic Acid and Its Derivatives

The acid-base chemistry of three zwitterionic compounds, namely anthranilic (2-aminobenzoic acid), N-methylanthranilic and N-phenylanthranilic acid has been characterized in terms of the macroconstants K_a1, K_a2, the isoelectric point pH_I, the tautomerization constant K_z and microconstants k₁₁, k₁₂, k₂₁, k₂₂. The potentiometric titration method was used to determine the macrodissociation constants. Due to the very poor water solubility of N-phenylanthranilic acid the dissociation constants pK_a1 and pK_a2 were determined in MDM-water mixtures [MDM is a co-solvent mixture, consisting of equal volumes of methanol (MeOH), dioxane and acetonitrile (MeCN)]. The Yasuda-Shedlovsky extrapolation procedure has been used to obtain the values of pK_a1 and pK_a2 in aqueous solutions. The pK_a1 and pK_a2 values obtained by this method are 2.86 ± 0.01 and 4.69 ± 0.03, respectively. The tautomerization constant K_z describing the equilibrium between unionized form ⇌ zwitterionic form was evaluated by the K_z method based on UV-VIS spectrometry. The method uses spectral differences between the zwitterionic form (found at isoelectric pH in aqueous solution) and the unionized form (formed in an organic solvent of low dielectric constant). The highest value of the K_z constant has been observed in the case of N-methylantranilic acid (log₁₀K_z = 1.31 ± 0.04). The values of log₁₀K_z for anthranilic and N-phenylanthranilic acids are similar and have values of 0.93 ± 0.03 and 0.90 ± 0.05, respectively. The results indicate that the tested compounds, in aqueous solution around the isoelectric point pH_I, occur mainly in the zwitterionic form. Moreover, the influence of the type of substituent and pH of the aqueous phase on the equilibrium were analyzed with regard to the formation and the coexistence of different forms of the acids in the examined systems.