Determination of lipophilicity and ionization of fentanyl and its 3‑substituted analogs by reversed-phase thin-layer chromatography.

Lipophilicity and ionization constant of sixteen fentanyl analogues were investigated by reversed-phase thin-layer chromatography (RP-TLC). Fourteen compounds have nitrogen containing groups at C-3 position of the piperidine ring and two compounds are 3-carbomethoxy derivatives of fentanyl. Among them, five are diastereomeric cis/trans couples and six novel trans diastereomers. In addition, the lipophilicity and ionization constant of fentanyl were also determined by the same method, as a reference. The physicochemical property, lipophilicity, expressed as retention indices R, b, and C, as well as PC1, was determined and correlated with in silico values. Ionization constants were determined on the basis of the relationships between analyte's retention expressed as R and mobile phase pH.Calculated structural descriptors together with the retention indices, were subjected to the principal component analysis - PCA and hierarchical cluster analysis - HCA, in order to provide basic insights into the similarities among the studied compounds. The blood - brain barrier (BBB) permeation was investigated in the function of experimentally obtained values of lipophilicity, polar surface area and molecular weight. In general, results of the present research corroborate well with previously determined antinociceptive activities of the investigated compounds, pointing out that besides the cis/trans isomerism, another set of important parameters should be taken into account when designing new derivatives of C-3 substituted fentanyl, especially lipophilicity, voluminosity and HBD/A character of the substituents. Accordingly, RP-TLC can be considered as a valuable asset in the ligand-based drug design.