Physical properties and pharmacological activity: antihistaminics

Prediction of pKa Values for Druglike Molecules Using Semiempirical Quantum Chemical Methods

Rapid yet accurate pK_a prediction for druglike molecules is a key challenge in computational chemistry. This study uses PM6-DH+/COSMO, PM6/COSMO, PM7/COSMO, PM3/COSMO, AM1/COSMO, PM3/SMD, AM1/SMD, and DFTB3/SMD to predict the pK_a values of 53 amine groups in 48 druglike compounds. The approach uses an isodesmic reaction where the pK_a value is computed relative to a chemically related reference compound for which the pK_a value has been measured experimentally or estimated using a standard empirical approach. The AM1- and PM3-based methods perform best with RMSE values of 1.4-1.6 pH units that have uncertainties of ±0.2-0.3 pH units, which make them statistically equivalent. However, for all but PM3/SMD and AM1/SMD the RMSEs are dominated by a single outlier, cefadroxil, caused by proton transfer in the zwitterionic protonation state. If this outlier is removed, the RMSE values for PM3/COSMO and AM1/COSMO drop to 1.0 ± 0.2 and 1.1 ± 0.3, whereas PM3/SMD and AM1/SMD remain at 1.5 ± 0.3 and 1.6 ± 0.3/0.4 pH units, making the COSMO-based predictions statistically better than the SMD-based predictions. For pK_a calculations where a zwitterionic state is not involved or proton transfer in a zwitterionic state is not observed, PM3/COSMO or AM1/COSMO is the best pK_a prediction method; otherwise PM3/SMD or AM1/SMD should be used. Thus, fast and relatively accurate pK_a prediction for 100-1000s of druglike amines is feasible with the current setup and relatively modest computational resources.

pKa Values of Medicinal Compounds in Pharmacy Practice

A brief review of pKa values and their uses in pharmacy practice is presented. Tables of pKa values for 400 medicinal compounds, of pH values of 18 body fluid sites, and of electron-inducing chemical substituents are provided. Adequate literature sources are included to permit further reference to the more detailed aspects of conditions associated with pKa value determinations or uses.

Observed and modeled effects of pH on bioconcentration of diphenhydramine, a weakly basic pharmaceutical, in fathead minnows

A need exists to better understand the influence of pH on the uptake and accumulation of ionizable pharmaceuticals in fish. In the present study, fathead minnows were exposed to diphenhydramine (DPH; disassociation constant = 9.1) in water for up to 96 h at 3 nominal pH levels: 6.7, 7.7, and 8.7. In each case, an apparent steady state was reached by 24 h, allowing for direct determination of the bioconcentration factor (BCF), blood-water partitioning (PBW,TOT), and apparent volume of distribution (approximated from the whole-body-plasma concentration ratio). The BCFs and measured PBW,TOT values increased in a nonlinear manner with pH, whereas the volume of distribution remained constant, averaging 3.0 L/kg. The data were then simulated using a model that accounts for acidification of the gill surface caused by elimination of metabolically produced acid. Good agreement between model simulations and measured data was obtained for all tests by assuming that plasma binding of ionized DPH is 16% that of the neutral form. A simpler model, which ignores elimination of metabolically produced acid, performed less well. These findings suggest that pH effects on accumulation of ionizable compounds in fish are best described using a model that accounts for acidification of the gill surface. Moreover, measured plasma binding and volume of distribution data for humans, determined during drug development, may have considerable value for predicting chemical binding behavior in fish.

Differences in preparative loadability between the charged and uncharged forms of ionizable compounds

Ionizable compounds experience a drastic difference in preparative loadability as a function of pH. It can be shown that the preparative loadability of a compound in the ionic form is by a factor of 20 or more inferior to the loadability of the same compound in the unionized form. In this paper, we demonstrate the reason for this behavior, and show practical applications of the principle.

Binding of hydrophobic drugs to lipid bilayers and to the (Ca2+ + Mg2+)-ATPase

Microelectrophoretic studies of the binding of a number of commonly used hydrophobic amine drugs to liposomes demonstrated the existence of relatively large surface potentials associated with binding of the protonated forms of the drugs. A theoretical treatment based on Langmuir adsorption isotherms and the Gouy-Chapman theory of the diffuse double layer allows estimation of drug-binding constants from electrophoretic mobility data. Such constants allow calculation of the charge effects arising from drug binding in more complex membrane systems, and it is shown that shifts in the apparent Ca+ affinity of the (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum in the presence of hydrophobic amine drugs are readily explicable in terms of the electrostatic effects of drug binding.

pKa determination of benzhydrylpiperazine antihistamines in aqueous and aqueous methanol solutions

The pKa1 and pKa2 values of three benzhydrylpiperazine antihistamines, cyclizine (I), chlorcyclizine (II), and hydroxyzine (III), were determined at 24.5 +/- 0.5 degrees by potentiometric titration in aqueous solution to be 2.16 +/- 0.02 and 8.05 +/- 0.03, 2.12 +/- 0.04 and 7.65 +/- 0.04, and 1.96 +/- 0.05 and 7.40 +/- 0.03, respectively. The pKa2 values were also determined by titration in seven aqueous methanol solutions in the range of 11.5-52.9% (w/w) methanol. The apparent dissociation constants of I-III in the aqueous methanol solutions, psKa2, were plotted according to two linear regression equations from which the values in water, p omega Ka2, were extrapolated. The plotted variables were psKa2 versus methanol concentration (%w/w) and psKa2 + log (water concentration, M) versus 1000/epsilon, where epsilon is the dielectric constant of the aqueous methanol solution. The maximum difference between pKa2 and p omega Ka2 was observed in the case of II where p omega Ka2 was 5.23% higher. Statistical analysis of the linear regression data obtained from the plots showed that slightly better accuracy (p less than 0.13) and correlation (p less than 0.16) were obtained, but the precision was essentially equal with both methods. The observed ratio of Ka1/Ka2 in I-III, 2.75 X 10(5)-7.76 X 10(5), was attributed to solven- and space-mediated field effects and electrostatic induction between nitrogen atoms in the piperazine ring.