The hydrophobic nature of the photosensitizer pheophorbide-a poses a challenge for its effective delivery in photodynamic therapy applications. Encapsulation within nanocarriers, such as block copolymer-based micelles, is essential to address this limitation. However, understanding the interaction dynamics between these nanovectors and biological membranes is crucial. Our study aimed to explore the mechanisms governing the interaction between copolymer micelles and membranes. We conducted physico-chemical investigations on biomimetic membranes and performed biological experiments on cell cultures. Our investigation centered on block copolymer-based micelles, particularly poly(ethyleneoxide)-block-poly(e-caprolactone) PEO-PCL, poly(ethyleneoxide)-block-poly(lactide) PEO-PLA, and poly(ethyleneoxide)-block-poly(styrene) PEO-PS, with liposomes.
Using the fluorescence properties of pheophorbide-a, we determined its affinity constants with both micelles and lipid vesicles, facilitating the assessment of its transfer from micelles to vesicles. We evaluated the relative production of singlet oxygen during the irradiation of pheophorbide-a, based on the type of micelles. Additionally, we monitored the leakage of a fluorescent probe from the liposomes to evaluate membrane permeability and the impact of singlet oxygen on membrane integrity. Lipid oxidation was tracked using mass spectrometry. Interestingly, although no significant differences were observed in the abilities of PEO-PCL and PEO-PS micelles to deliver pheophorbide-a to model membranes, higher concentrations of pheophorbide-a were detected in cells treated with PEO-PCL micelles. This underscored subtle differences in the delivery of pheophorbide-a through cell membranes by PEO-PCL and PEO-PS micelles.
An intriguing finding was the profound morphological transitions observed in giant unilamellar lipid vesicles upon irradiation with pheophorbide-a. This endocytosis-like process was observed exclusively when the photoactive species were encapsulated in a copolymer nanocarrier and was highly dependent on the chemical nature of the copolymer.
In summary, our study offers novel insights into the complex mechanisms governing the interaction between block copolymer-based nanocarriers and biological membranes, providing valuable perspectives on photochemical internalization mediated by nanoassemblies.