Photochemotherapy

Generality

There photochemotherapy (or photodynamic therapy) is a minimally invasive and low toxicity therapeutic strategy, which involves the use of photosensitive drugs (sensitive to light); these drugs are selectively exposed to light radiation, which is capable of transforming them into toxic substances for cancer cells or other specific types of cells.

Two key elements are therefore needed in photochemotherapy: the photosensitive compound and the light radiation. The latter must have an adequate wavelength in order to activate the photosensitive compound, which will thus be able to carry out a cytotoxic action (toxic for the cells) towards the target cells.
Photochemotherapy has been shown to be effective in killing bacteria, fungi and viruses, and is also used in the treatment of acne. It can also be used in the treatment of diseases such as age-related macular degeneration, psoriasis, vitiligo and various types of malignant tumors.

Photochemotherapy for cancer treatment

As mentioned above, this type of therapy is also used in the treatment of tumors. The photosensitive compound must be selective for malignant cells, so that - once activated - it kills only the diseased cells, sparing the healthy ones.
Photochemotherapy can be used, for example, for the treatment of skin cancers or internal cancers.
For the treatment of skin cancers, the photosensitive compound is usually administered locally and only the skin affected by the disease is irradiated.

An example of skin cancer treated with photochemotherapy is cutaneous T-cell lymphoma. In this case, the photosensitive compound used is a psoralen derivative (P) and the light source is long wavelength ultraviolet radiation (UVA rays); this strategy is defined PUVA therapy. Psoralen is the progenitor of a family of natural molecules called furanocoumarins. When psoralen or its derivatives are irradiated by UVA rays, they form bonds with the DNA of the cells in which they are found, causing their death.
For what concern treatment of internal tumors, the photosensitive compound can be administered intravenously, while the irradiation with light occurs through endoscopes or optical fiber catheters. Among the various photosensitive compounds that can be used we find the porphyrin derivatives, which are activated by light radiations with a wavelength between 600 and 1,000 nm. Porphyrins are natural molecules, this class includes cellulose and the heme group; the former is essential for plant organisms to carry out chlorophyll photosynthesis, while the heme group - present in the hemoglobin of red blood cells - is essential for the transport of oxygen in the organism.

Side effects

The side effects can be due both to the intake of the photosensitive compound and to the exposure to the light source; in this case the possible side effects depend on the area that has been irradiated.
Below are some of the adverse effects that can occur:

  • Headache;
  • Nausea and vomit;
  • Skin irritation and rashes;
  • Color changes of the skin in the vicinity of the treated area;
  • Itching;
  • Dry mouth
  • Sleep disorders.

Photochemotherapy could also induce the development of a secondary tumor. This, however, rarely happens and in order to occur it requires several factors that concur with each other in order to favor the onset of the pathology.


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