Promising results of photodynamic therapy

New research on photodynamic therapy (PDT) has shown the safety and efficiency of the procedure performed on baboons. The joint study was carried out by Sechenov University, Laboratory of Laser Biospectroscopy at Prokhorov General Physics Institute (Russian Academy of Sciences), and Research Institute of Medical Primatology (Adler, Russia). The results may be used to improve the diagnostics and therapy of various diseases, especially cancer.

The study was carried out on hamadryas baboons (Papio hamadryas). Three adult female primates that had chronic productive inflammation of the fingers and toes were simultaneously injected with three different photosensitisers — with different mechanisms of action and accumulation time. These compounds were chlorin e6 derivatives; protoporphyrin IX induced by 5-aminolaevulinic acid (5ALA); and methylene blue. The PDT experiment sought to investigate the safety of using these photosensitisers simultaneously — and their accumulation in the area of ​​interest (with the help of laser spectroscopy).

‘As a result of this experiment, we were able to gather some initial data, indicating that it’s possible to combine these photosensitisers and PDT’, said Igor Reshetov, Director of Levshin Institute of Cluster Oncology at Sechenov University. ‘The studies didn’t reveal any complications. There were no local or systemic adverse reactions in the baboons’.

The researchers expect that further work will provide new avenues for more effective PDT applications, particularly for the treatment of cancer in humans.

Photodynamic therapy is a minimally invasive method for local treatment of various conditions, for example, cancer and skin diseases. The technique is based on the accumulation of photosensitisers in the tissue of interest. Photochemical reactions start after exposure to laser radiation whose wavelength corresponds to the absorption peak of the photosensitiser. The photochemical reaction produces free radicals which trigger apoptosis and necrosis.

Photosensitisers are natural or synthetic substances which increase the sensitivity of biological tissues to light. One of the benefits of photosensitisers is that they can selectively accumulate in tumours but not healthy tissues. In fluorescent diagnostics, the photosensitiser’s degree of accumulation in the tissue is determined by the intensity of the recorded signal, thus facilitating the detection and accurate determination of the area occupied by the malignant tissue.

Chlorin e6 derivatives are used for PDT because of the formation of blood clots in tumour vessels and interstitial oedema, leading to the impairment of blood flow in the tumour. Protoporphyrin IX induced by 5ALA is an excellent tool for fluorescence diagnostics used for the differentiation of tumour foci — since the compound accumulates mainly in cancer cells and produces a therapeutic effect during PDT due to cellular phototoxicity.

Methylene blue is used as a photosensitiser but also has antiviral and antibacterial effects. Recent studies have shown that this chemical is active against the SARS-CoV-2 virus. In PDT, methylene blue induces macrophage apoptosis.