The process of PDT can essentially be divided into two stages. Photosensitizers are initially
administered, usually systemically (i.v. or i.p.), and given time to localize into target
tissue(s).
Then using a strong monochromatic light source or laser, a specific wavelength of non-thermal,
visible light (in the red or infrared region of the electromagnetic spectrum) is subsequently
delivered to excite the sensitizer. The sensitizer in turn undergoes a sequence of
photooxidation reactions that culminate in the generation of highly cytotoxic,
oxygen-derived species, that cause tumor cell death. In 2-4 weeks after irradiation the
tumor necrotizes and tissue lesion repairs.
Photochemical processes relating to PDT are as follows:
Photosensitisers have a stable electronic configuration which is in a singlet state in their
lowest or ground state energy level. Following absorption of a photon of light of specific
wavelength a molecule is promoted to an excited state, which is also a singlet state and which
is short lived. The photosensitiser returns to the ground state by emitting a photon
(fluorescence) or by internal conversion with energy loss as heat. It is also possible
that the molecule may convert to the triplet state via intersystem crossing which involves
a change in the spin of an electron. The triplet state photosensitiser has lower energy
than the singlet state, but has a longer lifetime (typically > 500 ns for photosensitisers)
and this increases the probability of energy transfer to other molecules.
The tendency of a photosensitiser to reach the triplet state is measured by the triplet
state quantum yield, which measures the probability of formation of the triplet state per
photon absorbed (depending on the interaction of the singlet species with other substrates
producing fluorescent quenching). The triplet state lifetime influences the amount of
cytotoxic species produced by collision-induced energy transfer to molecular oxygen and
other cellular components. A high intersystem crossing probability will produce an
effective population of excited triplet state photosensitiser molecules whose energy
can then be transferred by the two mechanisms described below. In addition, the
photosensitiser is not destroyed but returns to its ground state without chemical alteration
and is able to repeat the process of energy transfer to oxygen many times.
Type I and II reaction mechanisms
There are two mechanisms by which the triplet state photosensitiser can react with biomolecules;
these are known as the Type I and Type II reactions. Type I involves electron/hydrogen
transfer directly from the photosensitiser, producing ions, or electron/hydrogen abstraction
from a substrate molecule to form free radicals. These radicals then react rapidly, usually
with oxygen, resulting in the production of highly reactive oxygen species (e.g. the superoxide
and the peroxide anions). These radicals then attack cellular targets as described below.
Type II reactions produce the electronically excited and highly reactive state of oxygen
known as singlet oxygen. Direct interaction of the excited triplet state photosensitiser
with molecular oxygen (which, unusually, has a triplet ground state) results in the
photosensitiser returning to its singlet ground state and the formation of singlet oxygen.
In PDT, it is difficult to distinguish between the two reaction mechanisms. There is
probably a contribution from both Type I and II processes indicating the mechanism of
damage is dependent on oxygen tension and photosensitiser concentration.
Mechanisms of PDT-induced cell death
Targets of 1O2 -induced oxidative damage include membranes which may undergo lipid peroxidation,
protein cross-linking and/or loss of ionic homeostasis, cytoskeletal elements such as
nonpolymerized tubulin or actin, mitochondria; resulting in respiratory failure, endoplasmic
reticulum and golgi apparatus; disrupting protein synthesis/routing and lysosomes; which
rupture releasing harmful acidic hydrolases into the cytosol.
Ultimately, the physico-chemical properties of a given photosensitizer will determine its
pharmacokinetic behaviour and hence its phototherapeutic potential to kill cancer cells,
as well as the mechanism of cell killing. Death of PDT-targeted cell(s) can occur via one
or more of three very distinct and well defined mechanisms: direct cell kill; indirect cell
kill or immunological-induced cell kill.
1. Direct Cell Kill.
A direct cell death may occur in which the photosensitizer generates sufficient
1O2 during PDT to result in irreparable intracellular damage to
the target cell(s). Treated cells subsequently undergo apoptotic cell death or necrosis.
The propensity for direct cell kill correlates with the enhanced ability of cationic,
lipophilic sensitizers to bind the plasma membrane and gain access to the cell interior.
Once inside the cell, cationic compounds localize mainly in the mitochondria, lysosomes and/or
intracellular membranes. It is conceivable therefore, that death occurs due to mitochondrial
dysfunction (disruption of the electron transport chain and mitochondrial electrochemical
gradient), oxidative phosphorylation, or rupturing of lysosomes.
2. Indirect Cell Kill - Vascular Damage.
The effects of PDT can be far more dynamic in the presence of a functioning vascular system,
as in the case of a growing solid tumor. PDT-induced vascular damage usually results in an
indirect cell kill as cells are deprived of life-sustaining nutrients and oxygen. A number
of different cell types, are sensitive to PDT and contribute to PDT-induced vascular shutdown
including endothelium, macrophages, neutrophils and platelets. A physiological cascade of
events involving endothelial cell damage, vascular leakage and/or vasoconstriction,
mirocagglutination of blood cells (leukocytes, neutrophils) and platelet aggregation,
all combine to result in blood flow stasis.
3. Immunological response of PDT.
A third means by which of PDT can result in cell death involves the release of cytokines,
chemokines and related mediators of the inflammatory response. Damaged cells and host cells
involved in immunity such as macrophages and lymphocytes are known to release cytokines
which can contribute to both direct cell kill and vascular damage. The identity of these
cytokines and their precise role in promoting cell survival or cell death via apoptosis or
necrosis, remains to be defined.
More information about physical and biological mechanisms of PDT you can gain,
if refer to the literature:
Миронов А.Ф.
Фотодинамическая терапия рака – новый эффективный метод диагностики и лечения
злокачественных опухолей Download (in Russian)СОЖ. № 8, 1996. С. 32–40.
Залесский В.Н., Дынник О.Б.
Апоптоз и цитотоксические эффекты фотосенсибилизации Download (in Russian)Сучасні проблеми токсикології. № 4, 2003.
Luksiene Z.
Photodynamic therapy: mechanism of action and ways to improve the efficiency of treatment DownloadMedicina (Kaunas). №12, 2003. Р.1137.
