Ultra‐High to Ultra‐Low Drug‐Loaded Micelles: Probing Host–Guest Interactions by Fluorescence Spectroscopy

• Published in: Chemistry : a European journal 2019-09, Vol.25 (54), p.12601-12610
• Main Authors: Lübtow, Michael M., Marciniak, Henning, Schmiedel, Alexander, Roos, Markus, Lambert, Christoph, Luxenhofer, Robert
• Format: Article
• Language: English
• Subjects: Antineoplastic Agents - chemistry; Chemistry; Coloring Agents - chemistry; Control stability; Curcumin; Curcumin - chemistry; Drug Carriers - chemistry; drug delivery; Drug delivery systems; Drug Liberation; Emissions; Fluorescence; Fluorescence spectroscopy; Formulations; Hydrogen-Ion Concentration; Hydrophobic and Hydrophilic Interactions; Kinetics; Light; Micelles; Particle Size; poly(2-oxazine); poly(2-oxazoline); Polymers; Polymers - chemistry; polymer–drug interaction; Solubility; Spectrometry, Fluorescence; Temperature; Upconversion
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.201902619

Polymer micelles are an attractive means to solubilize water insoluble compounds such as drugs. Drug loading, formulations stability and control over drug release are crucial factors for drug‐loaded polymer micelles. The interactions between the polymeric host and the guest molecules are considered critical to control these factors but typically barely understood. Here, we compare two isomeric polymer micelles, one of which enables ultra‐high curcumin loading exceeding 50 wt.%, while the other allows a drug loading of only 25 wt.%. In the low capacity micelles, steady‐state fluorescence revealed a very unusual feature of curcumin fluorescence, a high energy emission at 510 nm. Time‐resolved fluorescence upconversion showed that the fluorescence life time of the corresponding species is too short in the high‐capacity micelles, preventing an observable emission in steady‐state. Therefore, contrary to common perception, stronger interactions between host and guest can be detrimental to the drug loading in polymer micelles. Don't come too close! Two isomeric triblock copolymers show differences in their interaction with curcumin. Unexpectedly, the polymer that leaves curcumin more freedom allows for much higher drug loading while the one that binds more tightly can incorporate much less drug.