Chiral Stability and Photostability Evaluation of Bepotastine Besylate and Levocetirizine Dihydrochloride Formulations by Chiral HPLC and Estimation of Its Chiral Inversion Mechanism by Tautomerism

Chiral stability and photostability of bepotastine besylate (BB, S (+)-form) and levocetirizine dihydrochloride (LCTZ, R (-)-form), both are clinically useful antihistamines, were investigated by HPLC with an ovomucoid (OVM) or a β-cyclodextrin (β-CD) immobilized enantioselective columns under the reversed-phase mode. Enantiomers of BB were successfully separated by an OVM column with resolution (Rs) 3.6 within 7 min and a β-CD column within 13 min with Rs 2.7. Optical purity testing of BB has been included in the monograph of BB in the Japanese Pharmacopoeia 18th edition (JP18), where a β-CD column is used. Enantiomers eluted within 20 min with Rs 2.4 by the JP18 method. Through changing buffer pH of the mobile phase in the official JP method, faster enantiopurity evaluation of BB was achieved without loss of Rs value. Enantiomers of LCTZ, i.e., cetirizine (CTZ) were also successfully separated by HPLC with an OVM column with Rs 1.8 within 10 min. Using validated chiral HPLC methods, photostability and chiral inversion studies of BB orally disintegrated (OD) tablets and its suspensions, and LCTZ tablets and syrups under UV irradiation and the heating stress conditions were performed in addition to its quality evaluation. In photostability studies, formulations of both BB and LCTZ were stable for UV irradiation. Under heat stress testing, chiral inversion of BB was observed in OD tablets (70 ºC, 1 week, 0.5 % R(-)-form) and its suspensions (70 ºC, 1 week, 6 % R (-)-form). On the other hand, chiral inversion of LCTZ formulations by heat (70 ºC, 80 ºC, 1 week) was not observed, although chemical structure of LCTZ is very similar to BB. In our previous chiral inversion study of d-chlorpheniramine maleate (d-CPM, S (+)-form), d-CPM was quite easily chiral inverted under the heating conditions. From these chiral inversion studies of d-CPM, BB and LCTZ, chiral inversion mechanism is proposed by comparing three chemical structures. Imine-enamine tautomerism seems to be a key step for chiral inversion of these antihistamines.