Albumin microbubbles conjugated with zinc and aluminum phthalocyanine dyes for enhanced photodynamic activity

Gas-liquid interfaces are reaching a particular interest in biomedicine. Microbubbles, ultrasound contrast agents of clinical routine, gained increasing attention as theranostic platforms due to the preserved acoustic response, drug conjugation capabilities, and applicability in biological barrier opening. A combination of microbubbles and photodynamic therapy agents can enhance the photodynamic effect, yet the evaluation of agent conjugation on microbubble stabilization and photodynamic effect is needed. Hence, two commercially available phthalocyanine photosensitizers - Holosens (R) (ZnPc) and Photosens (R) (AlPc) - were coupled with bovine serum albumin before microbubble synthesis. We demonstrated an albumin: phthalocyanine ratio of 1:1 and covalent attachment for ZnPc, a ratio of 1:3 with electrostatic binding for AlPc. Submicron-sized microbubbles (air- and SF6-filled) had a diameter of 0.8 mu m. Albumin-phthalocyanine conjugates increased the microbubble concentration and shelf-life stability compared to plain ones. We hypothesized that phthalocyanine fluorescence lifetime values decreased after conjugation with microbubbles due to narrow distance between conjugates in the shell. Agents based on AlPc demonstrated higher photodynamic activity than agents based on ZnPc, and microbubbles preserved acoustic stability in human blood plasma. The biodistribution of AlPc-conjugated microbubbles was evaluated. We conclude that our microbubble platforms demonstrate greater photodynamic activity and prolonged stability for further applications in photodynamic therapy.

Automated summary

Gas-liquid interfaces have gained interest in biomedicine, particularly in the use of microbubbles as theranostic platforms. In this study, two commercially available photosensitizers were conjugated to bovine serum albumin before microbubble synthesis to enhance photodynamic therapy. The conjugates increased microbubble concentration and stability compared to plain microbubbles. Agents based on one of the photosensitizers demonstrated higher photodynamic activity and microbubbles showed acoustic stability in human blood plasma. The study concludes that these microbubble platforms have potential for further applications in photodynamic therapy.