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Biomedical Optics & Medical Imaging

Photodynamic therapy: a maturing medical technology

From OE Reports Number 194 - February 2000
31 February 2000, SPIE Newsroom. DOI: 10.1117/2.6200002.0004

"I had what they call subcutaneous nodules that appeared at the operative site following a mastectomy for breast cancer," said Flora VanSant, a university professor in North Carolina. She said the nodules weren't painful, having not yet turned into lesions, but VanSant decided to undergo photodynamic therapy (PDT), a procedure that uses light-activated drugs to target and destroy diseased cells. She received treatment in New York at Buffalo General Hospital's Photodynamic Therapy Center under the care of Ron Allison.

Figure 1. (a) Extensive chest wall recurrence of breast cancer. The lesions had progressed despite salvage surgery, full-dose radiation, and chemo-hormonal therapy. Patient was on round-the-clock narcotics due to pain. (b) This photo shows the lesions one month after using PDT. (c) This photo was taken 3 months after using PDT. The patient exhibited excellent healing, and she no longer required narcotics for pain control. (Photos courtesy of Ron Allison, Buffalo General Hospital, Buffalo, NY)

VanSant was fortunate to have received early PDT. "The nodules above the scar have totally disappeared," she said. "There are two or three nodules below the scar that are now reduced to the size of a head of a pin and they're still decreasing in size. My oncologist here in North Carolina was amazed."

VanSant is fine today, but not everyone with chest wall lesions is so lucky. Many patients have never heard of photodynamic therapy, and many others use it as a last resort behind surgery, radiation, and chemotherapy. "By the time they're referred to PDT," Allison said, "they've gone through every type of salvage known to medicine. It would be nicer if we could see them earlier."

Some of Allison's patients have painful lesions that grow, ulcerate, become infected, and smell (Figure 1). "The patient wakes up every morning and sees these things growing," Allison said. "Psychologically, it's a devastating aspect of cancer." More radiation and chemotherapy leads to no change, often incurring more toxicity, both locally on the chest wall and systemically as infection.

The PDT process begins with the patient receiving a dose of Photofrin (porfimer sodium, a hematoporphyrin derivative; Figure 2), 0.8 mg/kg (which is a lower dose than that for other FDA-approved PDT cases). Forty-eight hours later, the lesions are irradiated with a pumped 630-nm dye laser at 150 joules/cm2. Depending on the size of the lesion, the laser irradiation can take from two to 20 minutes. Most patients arrive with multiple lesions and the whole process generally takes three to six hours. The irradiation is done on the outside of the chest wall and is performed on an outpatient basis.

Figure 2. Molecular structures for several photosensitizers. (a) hematoporphyrin, (b) benzoporphyrin derivative (BPD), and (c) lutetium texaphyrin. The macromolecules of (a) and (b) are composed of four pyrrole rings, a pentagonal structure composed of four carbons and a nitrogen or nitrogen molecule. (a) and (b) exhibit four "nitrogen coordinates" while (c) has five "nitrogen coordinates." The dashed lines in (c) mean that lutetium is fused to the texaphyrin molecule through the five coordinates. The conjugation system (how many ring structures or double bonds) determines the electronic state and far-red absorption properties. In terms of number of double bonds, the texaphyrin is first, then benzoporphyrin, and hematoporphyrin, corresponding to absorption at 730, 689, and 630 nm, respectively. (Diagram courtesy of Kathryn Woodburn, Pharmacyclics Inc., Sunnyvale, CA)

PDT works because tumor cells ingest a photosensitizer such as Photofrin more readily than normal cells. "You have a double situation where the drug gets into the tumor cells but it also lodges in very high concentrations in the abnormal blood vessels that feed the tumor," said Julia Levy, CEO of QLT PhotoTherapeutics (Vancouver, BC, Canada). Because the blood vessels feeding a tumor proliferate rapidly, they don't develop an extracellular matrix formed of smooth muscle cells, extra protein, and polysaccharides that preserve the integrity of the blood vessels. These immature blood vessels have high levels of low-density lipoprotein (LDL -- the so-called "bad" cholesterol) receptors on them. These immature blood vessels need the cholesterol for building the cell membranes and suck the LDL out of the plasma as the plasma moves through. The photosensitizer is formulated in a liposome (a mixture of lipids that form a stable emulsion for dissolving water-insoluble drugs). But as soon as it enters human blood, the affinity that the drug has for LDL is higher. "The drug actually pops off the liposome and onto low-density lipoproteins in the blood. And that happens almost immediately," Levy said. The drug-laden LDL is then absorbed by the LDL receptors on the cells of the newly formed blood vessels.

On a molecular level, when light of the appropriate wavelength is shone onto these cells, the photosensitizing molecule is excited and can decay via spin-allowed transitions -- internal conversion or fluorescence -- or by spin-inhibited transitions -- phosphorescence or intersystem crossing (Figure 3). The intersystem crossing is key for PDT. The photosensitizer decays without radiation from an excited state to a slightly lower state of different spin; i.e, from singlet to triplet or triplet to singlet. Intersystem crossing is a relatively long-life excited state, microseconds rather than the nanoseconds of fluorescence, but shorter than the milliseconds of phosphorescence. This is key because oxygen diffusion occurs next. An oxygen molecule collides with the sensitizer molecule, which excites the O2 molecule from the ground state to the singlet oxygen state. Singlet oxygen is highly toxic to cells. Prostaglandins are produced that close blood vessels down. Mitochondria, which control cell metabolism, shut down. Cells die; tissues die.

Figure 3. Once an electron is excited to a higher energy level by light, the molecule can decay directly by spin-allowed transitions or indirectly by spin-prohibited transitions. Spin-allowed transitions (singlet to singlet) include internal conversion, ic -- the conversion of electronic energy to molecular vibrational and/or rotational energy -- and fluorescence, f, after vibrational relaxation, vr. Spin-prohibited transitions are singlet to triplet or vice versa via phosphorescence, p, or intersystem crossing, isc. Only fluorescence and phosphorescence are radiation processes.

The photosensitizing agents are porphyrins or porphyrin-like derivatives. Porphyrins are ring-like molecules (Figure 2) found in nearly all living organisms. They are responsible for energy production, metabolism, and transport functions. They readily absorb light and can transfer that light energy into chemical and physical energy. Their use as a photosensitizer was probably first done in 1911. Porphyrins and related compounds are ideal not only for the above characteristics, but because they also have low systemic toxicity. In the late 1940s, they were known to localize in certain organs such as the liver and also in cancerous tissues.

PDT applications include lung cancer, ovarian cancer, brain tumors (see interview with Paul Muller, OE Reports, Feb. 1998), as well as its more studied applications in cutaneous, subcutaneous, and bladder cancers.

The data so far do not suggest PDT will increase a patient's chances for survival. "There's not much literature to suggest that offering local control will alter your survival rate that much, simply because it's the systemic disease that is going to kill you," Allison said. These patients have other areas of metastasis, and chest wall lesions are just a painful aspect of tumor progression. Still, some patients are still alive two to three years later and they were regressing until the PDT treatment.

"I do believe that offering local control to these individuals improves their survival, but that's not necessarily the aim of this treatment," Allison said. "The aim is to treat these terrible local tumors that are causing everybody a lot of grief and pain and try to give them a high quality of palliation." In other words, give patients relief from pain and a better quality of life.

Treatment for noncancers

In addition to treating cancer, a new application for PDT is treating noncancerous medical afflictions such as heart disease and age-related macular degeneration.

Kathryn Woodburn, a scientist at Pharmacyclics (Sunnyvale, CA) is using a compound developed by Pharmacyclics called Antrin (motexafin lutetium, a texaphyrin; Figure 2) to decrease the plaque presence in arteries. A photosensitizer accumulates in plaque cells in the same way that it does in tumor cells (Figure 4). Phase 1 studies have been done on peripheral arteries such as the femoral and iliac arteries. Researchers have moved into Phase 2 studies to help prevent restenosis, the recurrent buildup of plaque after stenting and balloon angioplasty. Of course, the goal is to see if this is a viable procedure for heart arteries. Antrin is injected 24 hours before the procedure and then a fiber optic is inserted to illuminate the plaque-filled area using a 732-nm diode laser.

Figure 4. Antrin localization in atheromatous plaque. The left picture shows a rabbit aorta, sliced longitudinally, ladened with atheromatous plaque. After receiving an Antrin injection, fluorescence analysis revealed select localization of the photosensitizer, at 750 nm, within the plaque, with little detected in the normal aortic wall. (Courtesy Kathryn Woodburn, Pharmacyclics Inc., Sunnyvale, CA.)

In heart disease, the cholesterol builds up in the arterial wall, which is followed by the deposition of inflammatory cells called macrophages. The macrophages send out signals to the smooth muscle cells, which in turn accumulate and proliferate, causing eventual obstruction of the lumen, the opening of the artery. PDT targets the macrophages, which ingest the photosensitizer and are subsequently killed when the light shines on them. Since the macrophages can no longer signal the smooth muscle cells, the buildup of plaque is contained. "In oncology, you don't want one cell remaining," Woodburn said. "However, with atherosclerosis, you want to stabilize the disease. You don't have to knock out all those cells, just enough so that blood can still flow through the lumen."

In the wet form of age-related macular degeneration (AMD), abnormal blood vessel growth occurs across the macula, the central part of the retina. These blood vessels leak and cause scar tissue to form on the macula, causing the loss of central vision. Wet age-related macular degeneration is the most common form of legal blindness in people over 50 and about 200,000 new cases occur each year in North America. To date, there has been no effective treatment

However, PDT offers some hope. Visudyne (BPD-MA; Figure 2) is injected intravenously into the patient. As in tumors, the leaky blood vessels are hyperproliferative. Because the blood vessels are immature and dividing rapidly, they have the low density lipoprotein receptors, which absorb the Visudyne-laden LDL. Thirty minutes later, 689-nm light from a diode laser is shone onto the macula and the leaky blood vessels are destroyed (Figure 5). The degree of leakage is greatly reduced and there is no significant vision loss. Initial results are encouraging, though blood vessels releak at the four-week and 12-week checkups. Still, researchers believe PDT will become a significant first step in the treatment of AMD.

Figure 5. Age-related macular degeneration. (a) This photo shows neovascular growth (new blood vessels) that cover the macula. (b) One week after photodynamic therapy, the blood vessels obstructing the macula are gone, destroyed by the production of singlet oxygen in the PDT process. Julia Levy of QLT Phototherapeutics (Vancouver, BC, Canada) said each molecule of Visudyne can generate half a million molecules of singlet oxygen per second. Vision is stable at 20/70. (Photos provided by QLT Phototherapeutics and Jason Slakter, MD.)

In another application, PDT is used to treat non-small-cell lung cancer, which makes up more than 75 percent of all lung cancers. These non-small-cell lung cancers -- adenocarcinomas, squamous cell carcinomas, and large cell carcinomas -- are hard to treat because they are often so advanced in development. When found early enough, surgery is the most effective. PDT is used for patients whose cancers have been caught at an early stage. It can be used to treat patients with early, endobronchial microinvasive lesions and who are not candidates for surgery. It can also be used to help alleviate shortness of breath caused by obstructing endobronchial lesions. The patients are injected with Photofrin 24 to 48 hours prior to tumor illumination with a 1-watt 630-nm dye laser using a cylindrical diffusing fiber that delivers 200 to 300 joules/cm2. "We have excellent palliation in patients with advanced disease, and excellent respiratory rate in patients with early lesions who are not candidates for surgery because of other medical problems," said Tracey Weigel of Memorial Sloan Kettering (New York, NY).

Anne Moor, a post-doctoral research fellow at Wellman Laboratories of Photomedicine (Harvard Univ., Cambridge, MA) is using PDT in preclinical trials for ovarian cancer, primarily cell studies but some animal studies as well. "Ovarian cancer is a late-diagnosed cancer in most cases because women don't have a lot of symptoms before the tumor starts to obstruct the bowel," Moor said. "So once people come to the clinic they often have a lot of stage 3 or stage 4 cancer already. The therapy of choice is surgery and chemotherapy. However, we think that PDT can be a useful additional therapy because it is hard to remove everything by only surgical methods."

In her cell studies, Moor uses BPD-MA (Figure 2), illuminating the tumor cells with a 689-nm diode laser. She targets what are called the epidermal growth factor receptors (EGF-R) in the tumor cells. She believes that it is the EGF-R that is partly responsible for tumor cell growth and proliferation and, by targeting them with PDT, one can kill the tumor cells. "That is why we try to get the photosensitizer specifically into the tumor cells by using this receptor -- because tumor cells in some cases have a higher level of these receptors and that is a way to get specificity between your tumor cells and your normal cells."

And the prognosis for the future of PDT? Levy said its applications will broaden to include other treatments, such as age-related macular degeneration. "The enthusiasm with which PDT has been looked upon by the ophthalmic community is the first example of this. We're going to see PDT used in combination, probably with other therapies, particularly in cancer. It is tough to have a stand-alone therapy in a disease like cancer. I think you're going to see it become a mainstream technology."

Frederick Su
Frederick Su is a freelance writer based in Bellingham, WA. www.bytewrite.com.