Post by Vijayalakshmi Kalyanaraman
Cancer takes almost 7.9 million lives around the world every year, 1500 per day in the US alone. Even when treatment is successful, it is likely to be exhausting and painful. However, cancer is also one of the most studied diseases in medical history. One less well-known treatment is photodynamic therapy.
Photodynamic therapy (PDT) combines a light-sensitive chemical compound (a photosensitizer) with light exposure to kill cancer cells. The treatment works like this: The photosensitizer is either injected into the bloodstream via a vein or is applied to the skin, as needed to reach the cancer cells. The compound is absorbed, but stays in cancer cells longer than normal cells. After a few hours or days, these areas are treated with a light of the wavelength that the compound is sensitive to. With this exposure, the photosensitizer produces singlet oxygen that kills nearby cancer cells. PDT can be less painful and grueling than chemotherapy or surgery.
PDT has a few limitations that need to be overcome. The light used to activate the photosensitizer cannot penetrate more than 1 centimeter of tissue, so only areas just below the skin and the linings of inner organs or cavities can be treated this way. However, recently researchers have identified a way to tackle this difficulty using scintillation nanoparticles with attached photosensitizers. The nanoparticles would emit light when they are exposed to x-rays, and the emitted light would then be used by the photosensitizer to produce the active oxygen that kills cancer cells. This would take advantage of x-rays’ deep penetration to affect cancer cells anywhere in the body. Dr. Wei Chen at the University of Texas, Arlington, published the first research on this technique in 2006.
Of course, the addition of scintillation nanoparticles has led to new challenges. For practical applications, the nanoparticles must be water soluble and at the same time emit the required level of light (high quantum efficiency) when exposed to x-rays. The higher the quantum efficiency, the higher would be the efficiency of the treatment. Several research groups around the world are now working to develop this technique for practical use.
Nanoparticle–porphyrin conjugates to be used for X-ray stimulated photodynamic therapy. Annexin V is a molecule that can target specific markers found only on tumor cells. Adapted from Wei Chen’s article published in Nanowerk.
PDT can cure or treat cancers and precancers. It is less expensive and invasive than surgical or chemotherapy procedures. Only a fraction of the radiation exposure is required and it is often done as an outpatient treatment. Patients treated with PDT experience relative few side effects. However, they do need to avoid light just after treatment, or photosensitizer still present in healthy tissue will cause rashes and burns.