Introduction To Photodynamic Medicine

The light diffusing tip delivers light to the treatment area, powering the photodynamic reaction without heat or generating resistance

The light diffusing tip delivers light to the treatment area, powering the photodynamic reaction without heat or generating resistance

“There are two kinds of light — the glow that illumines, and the glare that obscures…”

In 1963, James Thurber was almost certainly unaware of photodynamic medicine when he authored that quote, or else he might have added a 3rd type of light – the glimmer that heals. Loosely put, the word photodynamic means using light to cause an action or effect.  This definition, though simple, succinctly summarizes the concept of photodynamic medicine – a set of therapies that leverages visible light to create a targeted, potent effect. So how can simple light in the range of red to violet (the kind we can see) generate a clinical effect?  Save for a select group of therapies, some more reputable than others, it cannot…on its own.

If, however, one could harness the energy of visible light and convert it into a controlled and useful form, then innocuous visible light would become a valuable resource.  Nature has accomplished this feat many times, most famously in chlorophyll’s signature process, photosynthesis.  Chlorophyll’s ability to convert visible light energy into fuel for growth has formed much of the base for life on earth.  For medicine to harness the power of visible light, it needs chlorophyll equivalents that can open the door to innovative new therapies.

Photosensitizer has been the moniker attached to the active molecules in photodynamic medicine.  Photosensitizers are aptly named: they associate with cells of interest (the targets), and sensitize them to light.  Photosensitizers are usually specific to certain cell types, and readily absorb light in a portion of the visible spectrum.  Targets have included tumor cells, bacteria, viruses, fungi, toxins, and certain diseased tissues.

The mechanism of action can most easily be described as an energy transformation and transfer. Visible light is absorbed from the environment by the photosensitizer, and is transformed into energy that is capable of oxidizing nearby molecules.   With the excited photosensitizer (the electrons are in a high energy state) already in close proximity to the target, damage is inflicted on the desired cells. The light diffusing tip delivers light to the treatment area, powering the photodynamic reaction in a safe and predictable manner.

The catch-all term for these processes has been Photodynamic Therapy (PDT), though that term has more recently come to represent applications in the field of oncology.  Antimicrobial PDT (aPDT), boasts many of the same advantages as PDT, including its minimally invasive technique and relatively simple application.  aPDT, commercially known as photodisinfection, is supported by years of research from institutions such as University College London and Harvard, and by a multitude of studies from Ondine Biomedical.  aPDT has not been shown to generate resistant strains in bacteria[1], even with 25 repeat exposures to the king of resistance, MRSA (Methicillin-resistant Staphylococcus aureus) [2].

The potential applications of photodynamics in medicine are too far reaching to completely cover in this post – not to mention established success stories like Visudyne, QLT’s revolutionary treatment for wet age-related macular degeneration.Visit to learn about the progress in the fight against infectious disease.

  1. Tavares, A et al. Antimicrobial Photodynamic Therapy: Study of Bacterial Recovery Viability and Potential Development of Resistance after Treatment
  2. Pedigo, LA et al. Absence of bacterial resistance following repeat exposure to photodynamic therapy
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