Interactions of non-charged tadalafil stereoisomers with cyclodextrins: Capillary electrophoresis and nuclear magnetic resonance studies

• Over 20 cyclodextrins studied for the enantioseparation of four tadalafil isomers.
• Cavity size- and substituent-dependent enantiomer migration order reversal observed.
• Sulfobutyl-ether-α-cyclodextrin provided the desired enantiomer migration order.
• 1H NMR studies confirmed theories behind enantiomer migration order reversals.

The single isomer drug R,R-tadalafil (Cialis®) contains two chiral centers thus four stereoisomers (R,R-, S,S-, S,R- and R,S-tadalafil) exist, however, only the most potent inhibitor, the R,R-tadalafil is in clinical use.In our study, over 20 charged cyclodextrin (CD) derivatives were studied for enantiospecific host–guest type interactions in CD-modified capillary electrophoresis. Tadalafil stereoisomers are non-charged; therefore, their electrophoretic separation poses a challenge. Several candidates of both positively and negatively charged hosts were found to be effective for the enantioseparation. Eight out of the beta derivatives and three of alpha derivatives (including sulfated, sulfoalkylated, carboxyalkylated and amino derivatives) resolved all four stereoisomers partially or completely. Cavity size-dependent absolute enantiomer migration order (EMO) reversals were observed in the case of sulfopropyl-alpha (EMO: R,S; S,R; R,R; S,S) and sulfopropyl-beta (S,S; R,R; S,R; R,S) derivatives, while substituent-dependent partial EMO reversals were detected for sulfobutyl-ether-alpha (R,S; S,R; S,S; R,R) and sulfated-alpha-CD (R,R; S,S; R,S; S,R) selectors. Complexation-induced 1H NMR chemical shift changes reflected that the benzodioxole moiety plays a major role in cavity size-dependent EMO reversal.Sulfobutyl-ether-alpha-CD was the only selector that provided the desired EMO in which the clinically applied eutomer R,R-tadalafil migrates last. Finally, an electrophoretic method applying a background electrolyte (BGE) containing 75 mM Tris–acetic acid buffer (pH 4.75) and 7 mM sulfobutyl-ether-alpha-CD was developed for the baseline resolution of all isomers at 25 °C and +25 kV applied voltage.