Non-invasive quantitative chemical measurements of liposomal formulations using Raman spectroscopy†

Abstract

With a growing interest towards low batch-volume personalised medicines and continuous manufacturing of pharmaceuticals, the need for robust non-invasive quality control analytical methods is becoming increasingly important. Current methods for the quantification of total and encapsulated drug in a liposomal formulation include reversed-phase high-performance liquid chromatography with ultraviolet or fluorescence spectroscopy, which requires sample consumption after procedures such as ultrafiltration to separate the free drug from the encapsulated drug. We have developed and tested a method to perform non-invasive Raman spectroscopy measurements on liposomal doxorubicin. Raman spectroscopy provides chemically specific, potentially quantitative information, with measurements able to be performed on the contents of a sealed glass vial. We developed and validated the method by using a model system of polystyrene (PS) nanospheres and produced calibration curves for the concentration of PS at sizes of 40 nm, 125 nm and 200 nm. We then applied the same method to a liposomal doxorubicin formulation to measure the concentration of lipidic and drug components, and differences in the percentage of encapsulated drug. Our results show that by this method we can measure differences in doxorubicin concentration of 0.25 mg ml⁻¹ and distinguish between free and encapsulated doxorubicin down to a minimal relative concentration of 2.3%.