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New generation of porphyrinoids for mitochondria-targeted photodynamic therapy

Periodic Reporting for period 1 - PROMPT (New generation of porphyrinoids for mitochondria-targeted photodynamic therapy)

Reporting period: 2019-01-01 to 2020-12-31

Cancer is the leading cause of death worldwide. The Global Cancer Observatory (GLOBOCAN) has estimated that the global burden of cancer has risen to 19.3 million new cases and 10.0 million deaths in 2020, also predicting that the new cancer cases will increase to 32.2 million in 2040. Due to these alarming facts, it is necessary to improve cancer diagnostic methods and treatments. Every cancer type is handled with a specific treatment regimen that might encompass one or more modalities, such as surgery, radiotherapy, chemotherapy, and the more recently explored photodynamic therapy (PDT).

PDT is a non-invasive treatment strategy for different types of cancer, based on the use of light. It requires a photosensitizer (PS) which, upon light irradiation in the UV-Vis region, undergoes intersystem crossing to its triplet excited state. Eventually, the latter, reacting with molecular oxygen, generates cytotoxic species that induce cell death. The development of PSs has been recently focussed on targeting subcellular organelles, as a powerful tool to reduce side effects in healthy tissues and organs. Moreover, the possibility of two-photon (TP) excitation of the photosensitizer leads to several advantages, inherent to the TP absorption phenomenon: excitation in the biological window (700-1000 nm) where tissues have negligible absorption and reduced scattering, combined with a spatially confined and less invasive treatment.

To address these issues, the overall objective of PROMPT is the development of novel porphyrin one- and two-photon photosensitizers to selectively target mitochondria in cancer cells.

The target photosensitizer should be chemically pure and have low dark toxicity, high quantum yield of reactive oxygen species (ROS), high cellular uptake, and rapid clearance from the body to avoid phototoxic side effects. The PROMPT project is expected to deliver improvements in:
1. Absorption and emission properties of the PSs (high molar absorption coefficients and enhanced fluorescence quantum yields);
2. High singlet oxygen quantum yields;
3. Selective targeting of mitochondria in cancerous cells for apoptotic cell death;
4. Improved efficacy of one and two-photon photodynamic therapy.
Two novel porphyrin-based photosensitizers were synthesized by the Lindsey method. To achieve the project goals and to improve the photophysical properties of the molecules, heterocyclic groups were introduced at the meso position of the porphyrin core.

A complete photophysical characterization of the new porphyrins has been performed in three different solvents (dichloromethane, dimethyl sulfoxide and toluene) and the results have been supported by theoretical investigations. Singlet oxygen production quantum yields have been measured in all the solvents where the compounds showed a reasonable solubility. In dichloromethane and toluene the yield was measured by collecting the singlet oxygen phosphorescence at 1274 nm, upon excitation of the compound with a He-Cd laser, with reference to a suitable standard. Conversely, an indirect method was employed for the compounds soluble in dimethyl sulfoxide, that made use of a singlet oxygen trap which is degraded by the presence of singlet oxygen. Overall, the singlet oxygen generation was found to be particularly high for both compounds, rendering them promising photosensitizers.

Cellular uptake and subcellular localization assays were performed using the MDA-MB-231 human breast cancer cell line. Among the synthesized photosensitizers, one of the compounds showed higher cellular uptake and selective accumulation in the mitochondria. Two-photon absorption analysis and TP-PDT assays are ongoing.

PROMPT objectives and outcomes have been presented at five national and international conferences. Participation of the ER and the Supervisor to the European Researchers’ Night events in 2019 (with stand and activities) and 2020 (online event) allowed dissemination to the large public of the use of light to treat tumors and of the specific goals of the project.
Photodynamic therapy is a clinically approved treatment for localized cancer, but current PDT suffers from treating deep and large tumors and from lack of specificity for cancer cells with significant side effects.

PROMPT combines different key approaches in cancer therapy, such as targeted PDT and two-photon excitation, developing a new class of porphyrinoids with high one- and two-photon photoinduced cytotoxic activity. Mitochondria have been selected as targets since they represent the cell’s powerhouse and they are able to exert lethal functions in physiological and pathological conditions. By combining the use of light for the treatment of cancer with specific targeting of mitochondria in cancer cells, thus, PROMPT will develop an innovative approach for a highly efficient and minimally invasive treatment of cancer.