Figure 5. Correlations between drug properties and empirical diffusion parameters.

Diffusion coefficient (D) shows a weak negative correlation to molecular weight in both systems (A,D). In the reservoir system (A,B,C), the combined empirical parameter Dk [M = ADkC_St] is inversely correlated to API solubility (aq, 25°C) and proportional to the partition coefficient “P” as illustrated by the logarithmic correlation between LogP and Dk (B). D and k can not be independently determined from reservoir system release data. In the matrix system, described by the empirical equations $[\raisebox{1ex}{$M$}\!\left/ \!\raisebox{-1ex}{$M_T$}\right.=({(\mathit{D}*\mathit{Y}\prime )}^{\frac{1}{2}}*A*{((2C_A-C_S)C_S)}^{\frac{1}{2}}*t^{\frac{1}{2}})/f_d*m;\raisebox{1ex}{$M$}\!\left/ \!\raisebox{-1ex}{$M_T$}\right.<\mathbf{0.6}][\raisebox{1ex}{$M$}\!\left/ \!\raisebox{-1ex}{$M_T$}\right.=1-\raisebox{1ex}{$8$}\!\left/ \!\raisebox{-1ex}{${\pi }^2$}\right.\text{exp}[\raisebox{1ex}{$-{\pi }^2\mathit{D}\mathit{t}$}\!\left/ \!\raisebox{-1ex}{$L^2$}\right.;\raisebox{1ex}{$M$}\!\left/ \!\raisebox{-1ex}{$M_T$}\right.>0.6]$ Y’ shows similar yet opposite correlations to Dk in the reservoir system in that it is inversely correlated to drug solubility (F) and negatively proportional to the partition coefficient “P” (E).