Determination of free concentration of Paclitaxel in liposome formulation.

PURPOSE An important step in the development of liposome-based formulations is estimating the free drug concentration in the aqueous solution surrounding liposomes. This research presents a new method for determination of free concentrations, based on membrane-protected solid-phase microextraction (SPME). METHODS For effective direct extraction of low molecular weight compounds from complex liquid samples, a hollow membrane was used to form a concentric sheath around a coated SPME fiber. The membrane blocked the access of large particles, like liposomes, to the coating surface, while target analytes with low molecular weight diffused through the membrane and reached the extraction phase. Quantification was conveniently performed by reversed-phase liquid chromatography coupled to electrospray ionization mass spectrometry. RESULTS The carbowax/templated resin SPME fiber was determined to be the most suitable for these assays, providing enough sensitivity when an extraction time of one hour was used. The free concentration of paclitaxel was found to be 0.36 microg/mL, significantly below the solubility limit of paclitaxel in water. CONCLUSION The method was successfully applied for determining free paclitaxel in liposome formulations based on dioleyl-trimethyl-ammonium-propane, with good linearity over the range of concentrations of interest. The method was faster and more practical than equilibrium dialysis, as the SPME approach provided preconcentration and convenient delivery to the analytical system.