Cause of photodynamic therapy pain identified

Image by Skin58 via Wikimedia Commons. licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

The pain associated with photodynamic therapy and porphyria appears to be associated with the TRPA1 and TRPV1 ion channels, suggesting that drugs that block these channels—already being tested for other conditions—may reduce patient discomfort.

An international team of researchers, based in the Institute of Physiology and Pathophysiology at Friedrich-Alexander University Erlangen-Nürnberg’s [FAU] in Germany, exposed the skin of healthy volunteers to a 405 nm blue light with and without pre-treatment with aminolevulinic acid. The investigators also looked at the impact of the light on heterologous expression systems and cultured sensory neurons, according to a press release from the university on June 23, 2016.

According to the findings, published in The Journal of Neuroscience (May 11, 2016; 36(19): 5264–5278), participants reported a slight, pinprick-like pain on untreated skin exposed to the blue light for approximately 30 seconds. On skin that had been treated with the photosensitizer, volunteers reported the pain became much stronger, and none of the participants was able to withstand it for longer than 40 seconds.

In the culture tests the authors found that the 405 nm light activated TRPA1 ion channels and, to a lesser extent, TRPV1 channels in nerve endings in the absence of additional photosensitization. However, pretreatment with aminolevulinic acid or with protoporphyrin IX dramatically increased the light sensitivity of both TRPA1 and TRPV1 via generation of reactive oxygen species. 

TRPA1 is known to trigger pain or itching, and TRPV1 is known as the capsaicin receptor and triggers the burning sensation caused by hot peppers, according to the release.

Identifying the involvement of these two ion channels in the pain from photodynamic therapy is interesting because antagonists to both channels are already in human trials. “Medications that block both ion channels have already been tested in the treatment of other types of pain, such as the pain experienced in diabetic neuropathy or arthritis,” said FAU researcher and study author Prof. Dr. Peter Reeh in the release. “Based on the findings of this study, these should have two benefits in this case, as they would reduce both pain and inflammation.”