The idea is to use antimicrobial photodynamic therapy (aPDT), using an already approved photosensitizer (methylene blue), as a new approach to treat COVID-19 infections.

Antimicrobial photodynamic therapy (aPDT) is a non-antibiotic approach regarded as a promising new procedure of antimicrobial treatment. It combines a nontoxic photosensitizer (PS) with visible light in the presence of oxygen to generate highly reactive oxygen species (ROS) that cause irreversibly lethal damage to microorganisms vital constituents. The main advantages of aPDT are broad spectrum of action, low probability of induction of resistance and rapid action. Also, multidrug-resistant (MDR) microorganisms are as susceptible to aPDT as their native counterparts.

The main cellular targets of aPDT are external structures such as viral capsid and envelope. Therefore, the PS does not need to enter the microorganism since the specific adhesion to external structures is sufficient for their inactivation. This mechanism reduces the probability of resistance development by uptake blockage, increased metabolic detoxification or enhancement of drug export.

With the exception of sporadic uses of hematoporphyrin and aminolevulinic acid (ALA) for the treatment of viral lesions, acne, cutaneous leishmaniasis, routine application of aPDT for the treatment of microbial infections is still unavailable.  Although hematoporphyrin and ALA are used in the clinical field, they have limiting factors in practice as pain associated with the treatment and time of incubation necessary for uptake and metabolism of the precursor. However, the current position concerning PS for clinical introduction suggest phenothiazinium dyes (such as methylene blue – MB) as likely candidate. MB although being the most studied PS and having a broad spectrum of microbial action have no clinical approval for photodynamic therapy. MB has been used for diagnosis and therapy, for what is considered safe.

Although has been used to treat superficial localized infections, the recent development of light sources optimized for the antimicrobial application makes possible to apply this tecnhology to treat infections in other parts of the human body, such as in lungs.

The University of Aveiro (UA) (partnership between Departments of Biology and Chemistry) has invested in the synthesis and evaluation of PSs with broad-spectrum of action, capable to efficiently inactivate bacteria, including MDR strains, bacterial endospores, viruses and fungi, having a vast experience in aPDT.