Separation of Paclitaxel and Related Taxanes by Micellar Electrokinetic Capillary Chromatography

Micellar electrokinetic capillary chromatography is used to separate paclitaxel (trade name Taxol, Bristol-Myers Squibb) and 14 related taxanes in 11.5 min. An aqueous acetonitrile (ACN) buffer containing sodium dodecyl sulfate (SDS) surfactant allows resolution of the 15 taxanes from each other and from the principal matrix ingredient in the injectable dosage form of the drug, Cremophor EL (polyethoxylated castor oil). Although the precise effect of high concentrations of ACN on micelle size and structure is not clear, the separation achieved is not possible in the absence of the organic modifier. The migration order is apparently that of decreasing aqueous-phase solubility, which is related to the presence of a side chain at the carbon 13-position, to the addition of a xylosyl group at the 7-position, and to the number of acetylated hydroxyl groups; it is also related to the increasing affinity of the taxane side chain for the micellar phase. The retention factors, k‘, increase linearly with SDS concentration above the critical micelle concentration. With increasing concentration of ACN, k‘ values for the taxanes without side chain decrease; for the most hydrophobic taxanes, k‘ increases up to 20% (v/v) ACN and then decreases sharply at higher concentrations; for the others, k‘ varies little up to 10% ACN, and then decreases rapidly with added ACN. Similar behavior was observed with methanol, but the break in k‘ occurred between 30% and 40% (v/v). Resolution was unacceptably poor if the samples dissolved in methanol were injected; samples dissolved in buffer containing SDS were well-behaved, probably because of stacking of the micelles during injection. Van't Hoff plots (ln k‘ vs 1/T) were linear and allowed calculation of the ΔH° of transfer of taxane from aqueous to micelle phase. However, similar to the situation in HPLC, uncertainty in the micelle/aqueous phase ratio precludes reliable determination of the corresponding ΔS°.