Many b 2 -adrenoceptor agonists possess a benzylalcohol moiety with chirality at the b-carbon atom, and the pharmacological activity resides predominantly in the (R)-(Ϫ)-enantiomer.1) It is well known that the racemization of b 2 -adrenoceptor agonists such as catecholamines occurs in aqueous solution, 2,3) though many studies were conducted only in the acidic region. 4,5) Meluadrine tartrate ( Fig. 1) is a novel b 2 -adrenoceptor agonist which inhibits spontaneous rhythmic contraction of the uterus.6) The drug is used as the optically pure (R)-enantiomer. This paper deals with a kinetic study of meluadrine tartrate racemization over the pH range from 1.2 to 12.
ExperimentalMaterials Meluadrine ((R)-enantiomer) L-tartrate and its (S)-enantiomer L-tartrate were synthesized at Hokuriku Seiyaku Co., Ltd. All other reagents were of analytical grade.HPLC Conditions for Enantiomer Assay HPLC (Jasco, Tokyo, Japan) with a ULTRON ES-OVM (4.6 i.d.ϫ150 mm, 5 mm particle size) column was used to determine the ratio of (R)-enantiomer and (S)-enantiomer. The mobile phase was a mixture of pH 6.8 phosphate buffer-methanol (9 : 1, v/v). The detector wavelength was set at 220 nm, the flow rate of the mobile phase was 0.6 ml min Ϫ1 and the column temperature was 30°C. The retention times of (R)-enantiomer and (S)-enantiomer were about 8 and 18 min, respectively.HPLC Conditions for Assay of Total Residual Percent of Meluadrine HPLC (Jasco, Tokyo, Japan) with a TSKgel ODS-80T M (4.6 i.d.ϫ150 mm, 5 mm particle size) column was used to determine the total residual percent of (R)-enantiomer and (S)-enantiomer. The mobile phase was a mixture of pH 2.5 phosphate buffer containing sodium 1-heptanesulfonate-acetonitrile (3 : 1, v/v). The detector wavelength was set at 220 nm, the flow rate of mobile phase was 1.0 ml min Ϫ1 and the column temperature was 30°C. The retention times of meluadrine and internal standard (clorprenaline) were about 6 and 8 min, respectively.
Calculation of Residual Percent of (R)-Enantiomer and Formed Amount of (S)-EnantiomerThe residual percent of (R)-enantiomer and the formed amount of (S)-enantiomer were calculated from enantiomer ratio of the above enantiomer assay and total residual percent.Kinetic Procedure The sample ((R)-enantiomer or (S)-enantiomer) was dissolved in Britton-Robinson buffer or diluted hydrochloric acid adjusted to the appropriate pH from 1.2 to 12, to give a concentration of 10 mg ml Ϫ1 . Aliquots of 10 ml were put in glass ampules, which were then sealed. The ampules were placed in a chamber heated at 40, 60 or 80°C. At intervals, samples were removed from the chamber and assayed.
Determination of Ionization ConstantsThe apparent ionization constants of meluadrine were determined by potentiometry and spectrophotometry. The apparent ionization constants pK a1 and pK a2 , which refer to the dissociation of the phenolic group and the amino group, were 8.35 and 9.99, respectively (Chart 1).
Results and DiscussionAs shown in Fig. 2, semilogarithmic plots of the residual percent of total meluad...