Photodynamic Therapy

Photodynamic theraphy is a newly emerging modality against cancer in
which photosensitizing molecules acts against tumors under the
influence of light.
It all began centuries ago,ancient Egyptians knew well that chemicals
from orally ingested plants can accumulate in skin .This 4000 years old
treatment is still used today with Psoralins and UV light
exposure,representing the best treatment for vitiligo,widespread skin
disease.Raab in 1900 showed that acridine dyes and light exposure
efficiently killed paramecia.In 1925 Policard examined the ability of
porphyrins-a class of tetrapyrrolic pigments which form the core of
active sites of hemoglobin(figure 1),chloroplast(figure
2),cytochromes(figure 3) and related biomolecules to produce
phototoxic effects.Mayer-Betz, in 1933,injected with around 200 mg of
hematoporphyrin and suffered no illness until he exposed to sunlight.In
1964, Lipson showed that hematoporphyrin(figure 4) preferentially
accumulated in cancerous tissue.In 1980s Dougherty showed
combination of a photosensitizer and light can indeed be used in cancer


figure 1figure 2



figure 3figure 4

Cancer is characterized by uncontrolled growth and spread of abnormal
cells and is often detected by the formation of a malignant tumors.So
important steps to prevent cancer is to kill malignant cells.Current
modes for the treatment are surgery and chemotheraphy.
In the 1 st step ,the patient receives an intravenous injection of a
photosensitizer ,a dye molecule that sensitizes chemical or bio-chemcal
reactions upon absorption of light.In the 2 nd step, after some days or
month the dye molecule is accumulated in particular malignant cells
and the cells are brought to a light of particular wavelength ,which is
provided by the argon pumped dye lasers or solid state diode lasers
coupled with optical fibre by the clinician.

figure 5

When the sensitizers are illuminated with light of a particular
wavelength ,they absorb energy(figure 5) and in turn transfer it to
molecular oxygen(figure 6).Oxygen in its ground state is a triplet.The
energy transferred by the photosensitizer converts triplet Oxygen to
singlet Oxygen,an extremely reactive species that ultimately leads to
the destruction of the target cells.The biochemical reaction of singlet
Oxygen include lipid peroxidation,oxidation of unsaturated fatty
acid,proteins with oxidisable Amino acids and bases in DNA.(figure 7)

figure 6figure 7

However photosensitizer can stay in skin for approx. 30 days after
treatment making the patient sensitive to light.So on a sunny day the
patient either has to stay indoor or need to wear hat and
gloves.Nonetheless with severe side effects that accompany chemo and
radiation theraphy ,the problems are benign.
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Image sources: Google.

Soumyadip Jana.


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