By: Caitlin Wetzel, Elise Siegel, and Emily Greer
_______________________________________________________________________________________________________________________________
Overview:
Photodynamic therapy is a new method for the treatment of cancer as well as other diseases. The concept of photodynamic therapy was discovered in the 1940s, but its use as cancer treatment was not approved in the United States until 1993. Currently, photodynamic therapy is gaining popularity, but still is not as common as other methods of cancer treatment. It is approved in the U.S. for the treatment of cervical cancer, endobronchial cancer, esophageal cancer, gastric cancer, age related macular degeneration, and actinic keratosis, a pre-cancerous skin condition. Photodynamic therapy has also been shown to be effective in the treatment of basal cell carcinoma, bladder cancer, Bowen's disease, and Darier's disease, which are both genetic skin conditions. Other possible uses currently being researched are the treatment of HIV and other viruses.
Theory:
Photodynamic therapy involves the injection of a drug which is preferentially retained by the targeted tissue. Red light is then applied to the area, and the drug, which is called a photosensitizer, absorbs the energy of this light and becomes excited. The photosensitizer then undergoes a vibrational relaxation, releasing energy, until it reaches the level of its first excited singlet state. The photosensitizer then undergoes an intersystem crossing to its triplet state, and another vibrational relaxation occurs, releasing more energy. The energy from the photosensitizer is then transferred to a molecule of oxygen, exciting the oxygen to its singlet state. The oxygen molecule undergoes one last vibrational relaxation occurs to release more excess energy, and the oxygen molecule stays in its singlet state. This is a long lived intermediary state, and the excited singlet oxygen acts as a destructive agent in the cell.
Photosensitzers:
There are many different types of compounds that can be used as photosensitizers, which as mentioned previously, are the drugs used in photodynamic therapy. Most photosensitzers currently used are derivatives of porphyrins, which are large aromatic rings containing nitrogen. Chlorins and bacteriochlorins are also used which are the same in structure as porphyrins, but lack certain double bonds, making them not fully aromatic. Other effective photosensitizers include cationic dyes and aromatic hydrocarbons.
The two drugs currently approved for photodynamic therapy are sold under the trade names Photofrin and Levulan. Photofrin is a heterogeneous mixture of hematoporphyrin monomers, dimers, and oligomers. The oligomers are formed by an ester linkage between the hydroxyl and carboxylic acid functional groups on the molecule
Levulan is composed of aminolevulinic acid, which is a naturally occurring amino acid. When aminolevulinic acid enters cancer cells which triggers the production of protoporphyrin IX, which is an active photosensitizer. Protoporphyrin IX accumulates only in cancer cells, as it is converted into heme in normal cells by the enzyme ferrochelatase, which cancer cells lack.
Aminolevulinic Acid
Properties of Good Photosensitizers:
When evaluating a drug to be used in photodynamic therapy, it is important to consider several things. First, the drug should have a known composition and chemical purity to make it pharmaceutically viable. It is important that the photosensitizer is not harmful to the cells itself, as it will be present in healthy as well as targeted cells. To ensure this, the dark toxicity of the photosensitizer must be very low, so that it is known that all of the destructive ability of the drug comes from the production of singlet oxygen by irradiation. Selective penetration into the cancer tissue is also important in ensuring that healthy cells will be minimally affected by the drug. A high quantum yield of singlet oxygen is important, so that the drugs can be administered in lower doses. Desirable quantum yield of singlet oxygen are above .6. Lastly, the maximum absorbance wavelength of the photosensitizer must be above 600 nm, because wavelengths smaller than this are absorbed by hemoglobin.
Mechanism of Cancer Treatment:
Treatment with Aminolevulinic acid or Photofrin results in the death of cancer cells by both apoptosis and necrosis. Apoptosis is planned cell death which involves several different processes in the cell. Necrosis occurs when a cell is attacked and killed due to direct physical action. The major target in the cell of all current photosensitizers is the mitochondria of the cell. The photosensitizer decreases the mitochondrial membrane potential, and causes it to release cytochrome c, which causes apoptosis. Cytochrome c oxidase is also inhibited by the presence of photosensitizers. Capsases, which trigger the cellular cascade of apoptosis are produced by photosensitizers as well. Enzymes that bind Calcium cations to the endoplasmic reticulum of the cell are inactivated after the administration of the photosensitizer, which also leads to apoptosis. DNA fragmentation is caused by some drugs used in photodynamic therapy and can lead to necrosis and apoptosis. The main way necrosis occurs in the cells is by inhibiting the tumor vasculature from transporting oxygen-rich blood into the cells. The concentration of singlet oxygen in cells has been shown to be directly proportional to the rate of cell death. Thus, singlet oxygen has been shown to be the compound responsible for these mechanisms of cell destruction. Unfortunately, it is not currently known exactly how singlet oxygen performs the mechanisms previously mentioned.
The physics of photodynamic therapy:
Atomic Physics:
The excitation of molecular oxygen is based on the principles of atomic physics. Oxygen is an atom that has 8 electrons in the following configuration: 1s22s22p4. Molecular oxygen, O2 is the active compound in the mechanism. Each of these electrons have a spin of either +1/2 or -1/2 and that is known as the quantum number ms. Each orbital can hold two electrons of opposite spins. According to Hund's rule, the electrons fill each degenerate (equal in energy) orbital halfway before going back to fill them completely. The ground state electron configuration of O2 is 1σ21σ*22σ22π22π*2. This indicates a single sigma bond between the two oxygen atoms. When the ground, or triplet, state of oxygen absorbs energy, it is converted to an excited, or singlet state. The first singlet oxygen state produced by a photosensitizer and light results in one of the unpaired 2π* electron reversing its spin. This causes an increase in energy of 1.63 eV. The energy increase is caused by the two adjacent orbitals containing electrons with antiparallel spins. This electron will then pair with the electron in the other 2π* orbital, releasing energy and forming a double bond between the two oxygen atoms. This singlet state of oxygen has a lifetime of 15-45 minutes, and is the destructive agent causing apoptosis in photodynamic therapy.
This creates the excited, singlet state of oxygen which allows it to react and cause apoptosis.
The singlet and the triplet state of oxygen are separated by 92 kJ/ mol, thus the light that is needed to excite the molecule must provide this much energy per mol. The energy needed to excite one molecule of oxygen is calculated below.
92000 J/mol * 1 mole/ 6.02 * 1023 = 1.528 * 10-19 J = 0.955 eV.
Light Properties
Once the amount of energy required to excite a molecule of oxygen is known the wavelength of light useful for photodynamic therapy can be determined.
E = (hc)/wavelength = (6.63 * 10-34 * 3.0 * 108 ) / 1.528 * 10-19 J = 13.0 µm
Wavelengths typically used in photodynamic therapy are between 600 - 800 nm. Wavelengths below 600 nm are not useful because they are absorbed by tissues in the body. The discrepancy between the calculated wavelength and the actual wavelength required to excite the molecule can be accounted for by the fact that the oxygen is not being directly excited by the light. A much more energetic photon is required to excite the photosensitizer and energy is lost in the transfer to oxygen. The actual maximum wavelength for most photosensitizers is 1270 nm.
Advantages and Disadvantages:
Photodynamic therapy is a great new cancer treatment because it is highly effective, often resulting in more than 80% cell kill. It is also much less invasive than any other form of cancer therapy, such as chemotherapy or radiation treatment. The only invasive part of the procedure is the insertion of an extremely small fiber optic laser to perform the irradiation. There are no dose limitations with photodynamic therapy, so that treatment can be applied until the cancer is completely eliminated. Photodynamic therapy is more selective for cancer cells than other current methods of cancer treatment. There are several disadvantages to the current methods of photodynamic therapy, but they are currently being minimized through research. First, following treatment, the patients will remain photosensitive, and must protect themselves from light. For treatment with Photofrin, this light sensitivity can last up to 2 months. Light sensitivity from aminolevulinic acid treatment lasts only two days, but this drug has conflicting reports of dark toxicity. Another minor disadvantage of photodynamic therapy is that the time between drug administration and irradiation is frequently inconvenient, between 12 and 14 hours for aminolevulinic acid. Unfortunately photodynamic therapy can not treat metastasized cancers. The main reason that photodynamic therapy is not popular in this country does not have to do with the disadvantages of current therapy; it is due to a lack of facilities, support, and skepticism from the general population.
Aside from the ability of photosensitizers to treat cancer, they are also effective at treating many other diseases. For example, many skin diseases such as Bowen's disease (pictured below) can be treated using photodynamic therapy. Studies are currently being conducted using photosensitizers to treat viruses, such as HIV. In fact, Photofrin has been found to decrease intracellular and extracellular concentrations of HIV virus in vitro. Photofrin is also one of the leading current treatment for the retardation of age-related macular degeneration. There is ongoing research to improve the process and photosensitizers available and expand its applications to other cancers and diseases.
More Information on Atomic Energy Levels
References
For more on the chemistry of PDT...
Overview:
Photodynamic therapy is a new method for the treatment of cancer as well as other diseases. The concept of photodynamic therapy was discovered in the 1940s, but its use as cancer treatment was not approved in the United States until 1993. Currently, photodynamic therapy is gaining popularity, but still is not as common as other methods of cancer treatment. It is approved in the U.S. for the treatment of cervical cancer, endobronchial cancer, esophageal cancer, gastric cancer, age related macular degeneration, and actinic keratosis, a pre-cancerous skin condition. Photodynamic therapy has also been shown to be effective in the treatment of basal cell carcinoma, bladder cancer, Bowen's disease, and Darier's disease, which are both genetic skin conditions. Other possible uses currently being researched are the treatment of HIV and other viruses.
Theory:
Photodynamic therapy involves the injection of a drug which is preferentially retained by the targeted tissue. Red light is then applied to the area, and the drug, which is called a photosensitizer, absorbs the energy of this light and becomes excited. The photosensitizer then undergoes a vibrational relaxation, releasing energy, until it reaches the level of its first excited singlet state. The photosensitizer then undergoes an intersystem crossing to its triplet state, and another vibrational relaxation occurs, releasing more energy. The energy from the photosensitizer is then transferred to a molecule of oxygen, exciting the oxygen to its singlet state. The oxygen molecule undergoes one last vibrational relaxation occurs to release more excess energy, and the oxygen molecule stays in its singlet state. This is a long lived intermediary state, and the excited singlet oxygen acts as a destructive agent in the cell.
Photosensitzers:
There are many different types of compounds that can be used as photosensitizers, which as mentioned previously, are the drugs used in photodynamic therapy. Most photosensitzers currently used are derivatives of porphyrins, which are large aromatic rings containing nitrogen. Chlorins and bacteriochlorins are also used which are the same in structure as porphyrins, but lack certain double bonds, making them not fully aromatic. Other effective photosensitizers include cationic dyes and aromatic hydrocarbons.
The two drugs currently approved for photodynamic therapy are sold under the trade names Photofrin and Levulan. Photofrin is a heterogeneous mixture of hematoporphyrin monomers, dimers, and oligomers. The oligomers are formed by an ester linkage between the hydroxyl and carboxylic acid functional groups on the molecule
Levulan is composed of aminolevulinic acid, which is a naturally occurring amino acid. When aminolevulinic acid enters cancer cells which triggers the production of protoporphyrin IX, which is an active photosensitizer. Protoporphyrin IX accumulates only in cancer cells, as it is converted into heme in normal cells by the enzyme ferrochelatase, which cancer cells lack.
Aminolevulinic Acid
Properties of Good Photosensitizers:
When evaluating a drug to be used in photodynamic therapy, it is important to consider several things. First, the drug should have a known composition and chemical purity to make it pharmaceutically viable. It is important that the photosensitizer is not harmful to the cells itself, as it will be present in healthy as well as targeted cells. To ensure this, the dark toxicity of the photosensitizer must be very low, so that it is known that all of the destructive ability of the drug comes from the production of singlet oxygen by irradiation. Selective penetration into the cancer tissue is also important in ensuring that healthy cells will be minimally affected by the drug. A high quantum yield of singlet oxygen is important, so that the drugs can be administered in lower doses. Desirable quantum yield of singlet oxygen are above .6. Lastly, the maximum absorbance wavelength of the photosensitizer must be above 600 nm, because wavelengths smaller than this are absorbed by hemoglobin.
Mechanism of Cancer Treatment:
Treatment with Aminolevulinic acid or Photofrin results in the death of cancer cells by both apoptosis and necrosis. Apoptosis is planned cell death which involves several different processes in the cell. Necrosis occurs when a cell is attacked and killed due to direct physical action. The major target in the cell of all current photosensitizers is the mitochondria of the cell. The photosensitizer decreases the mitochondrial membrane potential, and causes it to release cytochrome c, which causes apoptosis. Cytochrome c oxidase is also inhibited by the presence of photosensitizers. Capsases, which trigger the cellular cascade of apoptosis are produced by photosensitizers as well. Enzymes that bind Calcium cations to the endoplasmic reticulum of the cell are inactivated after the administration of the photosensitizer, which also leads to apoptosis. DNA fragmentation is caused by some drugs used in photodynamic therapy and can lead to necrosis and apoptosis. The main way necrosis occurs in the cells is by inhibiting the tumor vasculature from transporting oxygen-rich blood into the cells. The concentration of singlet oxygen in cells has been shown to be directly proportional to the rate of cell death. Thus, singlet oxygen has been shown to be the compound responsible for these mechanisms of cell destruction. Unfortunately, it is not currently known exactly how singlet oxygen performs the mechanisms previously mentioned.
The physics of photodynamic therapy:
Atomic Physics:
The excitation of molecular oxygen is based on the principles of atomic physics. Oxygen is an atom that has 8 electrons in the following configuration: 1s22s22p4. Molecular oxygen, O2 is the active compound in the mechanism. Each of these electrons have a spin of either +1/2 or -1/2 and that is known as the quantum number ms. Each orbital can hold two electrons of opposite spins. According to Hund's rule, the electrons fill each degenerate (equal in energy) orbital halfway before going back to fill them completely. The ground state electron configuration of O2 is 1σ21σ*22σ22π22π*2. This indicates a single sigma bond between the two oxygen atoms. When the ground, or triplet, state of oxygen absorbs energy, it is converted to an excited, or singlet state. The first singlet oxygen state produced by a photosensitizer and light results in one of the unpaired 2π* electron reversing its spin. This causes an increase in energy of 1.63 eV. The energy increase is caused by the two adjacent orbitals containing electrons with antiparallel spins. This electron will then pair with the electron in the other 2π* orbital, releasing energy and forming a double bond between the two oxygen atoms. This singlet state of oxygen has a lifetime of 15-45 minutes, and is the destructive agent causing apoptosis in photodynamic therapy.
This creates the excited, singlet state of oxygen which allows it to react and cause apoptosis.
The singlet and the triplet state of oxygen are separated by 92 kJ/ mol, thus the light that is needed to excite the molecule must provide this much energy per mol. The energy needed to excite one molecule of oxygen is calculated below.
92000 J/mol * 1 mole/ 6.02 * 1023 = 1.528 * 10-19 J = 0.955 eV.
Light Properties
Once the amount of energy required to excite a molecule of oxygen is known the wavelength of light useful for photodynamic therapy can be determined.
E = (hc)/wavelength = (6.63 * 10-34 * 3.0 * 108 ) / 1.528 * 10-19 J = 13.0 µm
Wavelengths typically used in photodynamic therapy are between 600 - 800 nm. Wavelengths below 600 nm are not useful because they are absorbed by tissues in the body. The discrepancy between the calculated wavelength and the actual wavelength required to excite the molecule can be accounted for by the fact that the oxygen is not being directly excited by the light. A much more energetic photon is required to excite the photosensitizer and energy is lost in the transfer to oxygen. The actual maximum wavelength for most photosensitizers is 1270 nm.
Advantages and Disadvantages:
Photodynamic therapy is a great new cancer treatment because it is highly effective, often resulting in more than 80% cell kill. It is also much less invasive than any other form of cancer therapy, such as chemotherapy or radiation treatment. The only invasive part of the procedure is the insertion of an extremely small fiber optic laser to perform the irradiation. There are no dose limitations with photodynamic therapy, so that treatment can be applied until the cancer is completely eliminated. Photodynamic therapy is more selective for cancer cells than other current methods of cancer treatment. There are several disadvantages to the current methods of photodynamic therapy, but they are currently being minimized through research. First, following treatment, the patients will remain photosensitive, and must protect themselves from light. For treatment with Photofrin, this light sensitivity can last up to 2 months. Light sensitivity from aminolevulinic acid treatment lasts only two days, but this drug has conflicting reports of dark toxicity. Another minor disadvantage of photodynamic therapy is that the time between drug administration and irradiation is frequently inconvenient, between 12 and 14 hours for aminolevulinic acid. Unfortunately photodynamic therapy can not treat metastasized cancers. The main reason that photodynamic therapy is not popular in this country does not have to do with the disadvantages of current therapy; it is due to a lack of facilities, support, and skepticism from the general population.
Aside from the ability of photosensitizers to treat cancer, they are also effective at treating many other diseases. For example, many skin diseases such as Bowen's disease (pictured below) can be treated using photodynamic therapy. Studies are currently being conducted using photosensitizers to treat viruses, such as HIV. In fact, Photofrin has been found to decrease intracellular and extracellular concentrations of HIV virus in vitro. Photofrin is also one of the leading current treatment for the retardation of age-related macular degeneration. There is ongoing research to improve the process and photosensitizers available and expand its applications to other cancers and diseases.
More Information on Atomic Energy Levels
References
For more on the chemistry of PDT...