Chiral HPLC analysis of formoterol stereoisomers and thermodynamic study of their interconversion in aqueous pharmaceutical formulations.

A chiral HPLC method was validated and successfully applied for the determination of formoterol stereoisomers and their inversion products in an aqueous matrix stored at 5-70 degrees C up to 3 weeks. Analysis was performed on a Chiral-AGP column (100 x 4-mm, 5-microm) using a variable mixture of mobile phase A (50-mM sodium phosphate buffer, pH 7.0) and B (10% v/v IPA) at a flow rate of 1.3 ml min(-1), and UV detection at 242 nm. All four formoterol stereoisomers were adequately resolved with acceptable detection and quantitation limits varying from 0.01-0.04 microg/ml and 0.04-0.1 microg/ml, respectively. The method showed acceptable accuracy (> or = 88%), precision (RSD < or = 8.5%) and good linearity (r(2) > or = 0.9999) over the concentration range investigated. While interconversion at 5+/-3 degrees C and 25+/-2 degrees C/60% RH +/-5% RH was too low to be determined accurately within the study period, chiral inversion of formoterol stereoisomers measured at high temperatures followed the first order rate kinetics and occurred at a single chiral center, resulting in the reversible formation of diastereoisomers, (R,R)<-->(S,R) and (S,S)<-->(R,S). No enantiomerization or diastereomerization occurred. There was no significant difference in inversion of the active components in racemic (R,R/S,S)-formoterol fumarate and the single isomer (R,R)-arformoterol tartrate drug formulations, and both drugs are expected to maintain their stereochemical integrity throughout the proposed shelf-life at the recommended storage condition (5+/-3 degrees C).