Photodynamic therapy: the hopeful treatment that uses light to cure cancer

By Enrique Ortega Forte, biochemist and researcher at the University of Murcia.

According to the World Health Organization, one in six people will suffer from cancer in their lifetime. It is a devastating statistical estimate that highlights the dimensions, difficulties and, above all, the challenges posed by this prevalent disease.

The term “cancer” encompasses more than a hundred different diseases, which are classified according to the tissue of the body in which it appears. The main characteristic of cancer cells is that they divide rapidly and end up producing tumors.

Therefore, one of the most common tools to treat cancer is chemotherapy, drugs that block cell division. However, many of the anticancer drugs in use today have a major drawback: selectivity, or rather the lack of it.

In our body there are cells that are dividing continuously in a normal way, such as hair cells or stomach cells. So, when administered, chemotherapy drugs not only prevent the division of cancer cells, but also that of these healthy cells.

This is when the so-called side effects appear, such as hair loss, vomiting, diarrhea and nausea, among others.

As an alternative to conventional chemotherapy, the scientific community has been joining forces for decades to develop treatments that improve this selectivity in order to minimize side effects.

Among the many strategies that are being investigated, one of the most interesting is one that seeks to fight cancer with light. There is a treatment modality called “photodynamic therapy.” It consists of applying light radiation as a method to stop the proliferation of cancer cells.

But how is it possible that light can treat cancer?

Photodynamic therapy is a non-invasive technique that has been used successfully in hospitals since 1970, mainly for skin cancer and for very localized superficial tumors where the application of light is accessible.

The technique is based on a very particular type of molecules called “photosensitizers”: substances capable of capturing the energy of light. These do not usually have pharmacological activity by themselves, but when they are irradiated with light they activate and trigger a series of chemical reactions that end up inducing the death of cancer cells.

Photodynamic therapy requires the simultaneous presence of three elements: the photosensitizer, light and oxygen.

While the first two are external agents, oxygen is an endogenous factor that is present inside our cells.

Its operation is as follows:

The photosensitizer is administered to the patient first. This allows it to accumulate inside the cell. Next, with a laser or LED lights of the indicated color —depending on the photosensitizer, green, yellow or red lamps are used—, the area where the tumor is located is illuminated.

The photosensitizer then captures the light energy and is activated, moving to a higher energy state. From this state, the photosensitizer transfers the light energy to oxygen in the cells.

When cellular oxygen receives this energy transfer, a reactive oxygen is generated that is extremely toxic to the cancer cell and ends up destroying it. Meanwhile, the photosensitizer returns to its original state, to the starting point, ready to receive a second photon of light and start the cycle again.

In summary, the antitumor action of photodynamic therapy is not due to the drug or light, but to the reactive oxygen that is formed in these reactions. In fact, both the light and the drug and oxygen itself are harmless. It is the combination of the three elements that becomes lethal for tumor cells.

The main advantage of photodynamic therapy lies in the fact that – unlike conventional chemotherapy – the use of light allows the pharmacological action to be controlled in time and space, as if it were a switch, since only where it is applied these photochemical processes will take place.

In this way, by illuminating only the tumor area, this therapy would selectively destroy that localized region. This would reduce the side effects on the rest of the body.

Despite the advantages that photodynamic therapy offers, this strategy has a weak point. It is an Achilles heel that often prevents its therapeutic effect. It’s about the third element: oxygen.

In general, the amount of oxygen inside tumors is really low. Internal tumor cells are able to survive with little oxygen, so photodynamic therapy reactions cannot occur in them and, consequently, treatment fails. Furthermore, the low penetration of light into the innermost tissues also limits the effect of this treatment.

However, phototherapy for cancer has a promising future.

On the one hand, current scientific research is focused on the design of new photosensitizing drugs with improved effects: from greater accumulation in tumor tissue or greater activation when applying light to the development of molecules that act independently of oxygen.

On the other hand, advances in optical technology are allowing the development of probes and fibers capable of conducting light to internal areas of the human body, which would allow its application in other types of cancer still unexplored by this therapy.

Photodynamic therapy does not aspire to displace conventional chemotherapy, which is still essential and effective, but we are getting closer to seeing how this therapeutic tool illuminates the approach to that group of diseases that we know as cancer.

This article was originally published on The Conversation. You can read it here.