Correlation of Molecular State and Drug Release in Irinotecan-Loaded Liposomes Using Different Trapping Agents

Liposomes have been extensively studied as carriers for delivering anticancer agents, and a formulation that encapsulates irinotecan (IRT) is currently available on the market under the name Onyvide. This study examined the molecular state of IRT within liposomes prepared with ammonium sulfate (AS) or triethylammonium sucrose octasulfate (TEA-SOS) as trapping agents using various analytical techniques. Cryogenic transmission electron microscopy revealed distinct morphologies: solid IRT precipitates in IRT/TEA-SOS liposomes, and dot-like microstructures in IRT/AS liposomes. Small-angle and wide-angle X-ray scattering (SAXS/WAXS), Raman spectroscopy, and 1H NMR analyses indicated that IRT was in a dissolved state within the inner aqueous phase of IRT/AS liposomes. In contrast, it precipitated in an amorphous form within IRT/TEA-SOS liposomes. In situ NMR measurements, conducted while varying the temperature, revealed a molecular state change of IRT during the encapsulation process. Detailed analysis of the NMR spectrum, supported by the magic-angle spinning (MAS) technique, enhanced our understanding of the molecular state, including the mobility and fraction of IRT between monomers and aggregates within the liposome. These differences in molecular states elucidated the distinct release profiles of IRT from each liposome. The solubility of IRT salts within the inner aqueous phase of the liposome played a key role in controlling the release properties of IRT. The findings highlight that the molecular state of IRT, determined by the choice of trapping agent, significantly influences release behavior and provides valuable insights for the rational design of advanced liposomal drug formulations.