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.

Scientists at Skoltech, MIPT, Prokhorov General Physics Institute of RAS, and several other institutions have demonstrated that microbubbles made of a protein called albumin are efficient carriers for the delivery of photodynamic agents, which are drugs used in advanced anti-cancer therapy that avoids many of the side effects of chemo- and radiation therapy.

The study’s principal investigator, Professor Dmitry Gorin of Skoltech Photonics, commented, “Photodynamic therapy involves injecting a photosensitizing compound into the bloodstream and illuminating the tumor with light whose wavelength matches the compound, either through the skin or with an endoscope. After light energy is absorbed by the sensitizing agent, this gives rise to free radicals and an aggressive form of oxygen, which kills off cells in a very focused manner in the region exposed to light.”

The lead author of the paper, Skoltech MSc graduate Roman Barmin, said: “What we did in this study is we showed on two popular photosensitizing agents that binding them with albumin, a bovine serum protein, results in their greater photoactivity, and whipping up the protein-drug conjugate into bubbles results in an even greater efficiency boost. This could be used to advance photodynamic cancer therapy.”

In the experiment, the scientists employed two commercial photosensitizing substances to show albumin’s covalent attachment to zinc phthalocyanine and its electrostatic binding to aluminum phthalocyanine. The former is a photodynamic treatment agent that has received clinical approval, while the latter is still in clinical testing. To “whip up” air bubbles, the phthalocyanine-albumin solution was subjected to ultrasound at the proper frequency and temperature.

Barmin noted, “While microbubble delivery is generally a popular approach, this study is the first to use albumin bubbles in the context of photodynamic therapy and to work with the two phthalocyanines we considered. The idea behind microbubble delivery is that the shell of an air bubble can densely pack the molecules of photodynamic agents, and bubbles can be conveniently ‘popped’ by medical ultrasound in a controlled manner to release the drug.”

Professor Dmitry Gorin of Skoltech Photonics commented, “Tests indicated that agent delivery to cells was successful. Our next goal is a deeper understanding of microbubble-cell interactions to enhance therapeutic performance.”

After exploring microbubble characteristics, the team concluded that photodynamically conjugated microbubbles have an improved physicochemical properties profile: The bubbles maintain the same mean diameter as their phthalocyanine-free counterparts (used in ultrasound imaging), which is advantageous for controlling their performance, while at the same time having a higher bubble concentration and storage stability.

Barmin said, “The next question was: Will microbubbles increase the efficiency of the photodynamic agents they deliver? Our colleagues from Prokhorov General Physics Institute have developed a technique using a red blood cell suspension to test just that. The result was clear: For both agents, the phthalocyanine-albumin complex proved more effective than plain phthalocyanines, and microbubble delivery further boosted drug photoactivity. This results from the dense molecule packing in the microbubble shell.”

Journal Reference:

  1. Roman A.Barmin, Elizaveta A. Makismova et al. Albumin microbubbles conjugated with zinc and aluminum phthalocyanine dyes for enhanced photodynamic activity. Colloids and Surfaces B: Biointerfaces. DOI: 10.1016/j.colsurfb.2022.112856