Photodynamic Therapy

In Chapter 14 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss photodynamic therapy.

Photodynamic therapy (PDT) uses a drug called a photosensitizerthat is activated by light (Zhu and Finlay 2008; Wilson and Patterson 2008). PDT can treat accessible solid tumors such as basal cell carcinoma, a type of skin cancer(see Sect. 14.10.4). An example of PDT is the surface application of 5-aminolevulinic acid, which is absorbed by the tumor cells and is transformed metabolically into the photosensitizer protoporphyrin IX. When this molecule interacts with light in the 600–800-nm range (red and near infrared), often delivered with a diode laser, it converts molecular oxygeninto a highly reactive singlet state that causes necrosis, apoptosis (programmed cell death), or damage to the vasculaturethat can make the tumor ischemic. Some internal tumors can be treated using light carried by optical fibersintroduced through an endoscope.

The photosensitizer molecule interacts with near infrared light to damage tissue, kill cells, and harm blood vessels. A photon of infrared light doesn’t have much energy, and I’m surprised it can trigger all this destruction. What’s the structure of this molecule that causes so much carnage?

Let’s start with 5-aminolevulinic acid, which is an endogenous nonproteinogenic amino acid. By “endogenous” I mean it occurs naturally in the body. It’s part of the biochemical pathway that leads to the production of heme in animals, and chlorophyll in plants. By “amino acid” I mean it has an amine group (-NH₂) on one end and a carboxylic acid group (-COOH) on the other end. The amino acids that make up proteins have a single carbon atom connecting the amine to the carboxylic acid, like in glycine. 5-aminolevulinic acid, on the other hand, has several carbons linking the two groups. By “nonproteinogenic” I mean that this amino acid is not one that is encoded by our genome, and therefore it never occurs in proteins. Below is a drawing of the structure of 5-aminolevulinic acid.

Protoporphyrin IX is a complicated molecule that appears in those same pathways leading to heme and chlorophyll. It contains four pyrrole subunits, each of which is a five-membered ring composed of four carbon atoms and one nitrogen atom. It is nearly planar, and has its four nitrogen atoms facing a central hole. I show its structure below. 

In heme, an iron atom occupies the central hole, and is where oxygen binds in the protein hemoglobin found in red blood cells. In chlorophyll, a magnesium atom sits in the central hole.

Most molecules (for instance, water, carbon dioxide, methane, ammonia, urea, and glucose) don’t react when exposed to visible or infrared light, but protoporphyrin IX does. It’s closely related to chlorophyll, which is a key molecule in photosynthesis. When sunlight interacts with chlorophyll, it triggers a series of chemical reactions that leads to the production of carbohydrates from water and carbon dioxide.

I guess I’m not so surprised after all that protoporphyrin IX can wreak so much havoc when exposed to light.