Photodynamic therapy (PDT) is a clinically approved, minimally invasive therapeutic procedure that can exert a selective cytotoxic activity toward malignant cells. The procedure involves administration of a photosensitizing agent followed by irradiation at a wavelength corresponding to an absorbance band of the sensitizer. In the presence of oxygen, a series of events lead to direct tumor cell death, damage to the microvasculature and induction of a local inflammatory reaction. Clinical studies revealed that PDT can be curative particularly in early-stage tumors. It can prolong survival in inoperable cancers and significantly improve quality of life. Minimal normal tissue toxicity, negligible systemic effects, greatly reduced long-term morbidity, lack of intrinsic or acquired resistance mechanisms, and excellent cosmetic as well as organ function-sparing effects of this treatment make it a valuable therapeutic option for combination treatments. With a number of recent technological improvements, PDT has the potential to become integrated into the mainstream of cancer treatment.

Photodynamic therapy involves administration of a tumor-localizing photosensitizing agent, which may require metabolic synthesis (i.e., a prodrug), followed by activation of the agent by light of a specific wavelength. This therapy results in a sequence of photochemical and photobiologic processes that cause irreversible photodamage to tumor tissues. Results from preclinical and clinical studies conducted worldwide over a 25-year period have established photodynamic therapy as a useful treatment approach for some cancers. Since 1993, regulatory approval for photodynamic therapy involving use of a partially purified, commercially available hematoporphyrin derivative compound (Photofrin) in patients with early and advanced stage cancer of the lung, digestive tract, and genitourinary tract has been obtained in Canada, The Netherlands, France, Germany, Japan, and the United States. We have attempted to conduct and present a comprehensive review of this rapidly expanding field. Mechanisms of subcellular and tumor localization of photosensitizing agents, as well as of molecular, cellular, and tumor responses associated with photodynamic therapy, are discussed. Technical issues regarding light dosimetry are also considered.

NHIK 3025 cells were incubated with Photofrin II (PII) and/or tetra (3-hydroxyphenyl)porphyrin (3THPP) and exposed to light at either 400 or 420 nm, i.e. at the wavelengths of the maxima of the fluorescence excitation spectra of the two dyes. The kinetics of the photodegradation of the dyes were studied. When present separately in the cells the two dyes are photodegraded with a similar quantum yield. 3THPP is degraded 3-6 times more efficiently by light quanta absorbed by the fluorescent fraction of 3THPP than by light quanta absorbed by the fluorescent fraction of PII present in the same cells. The distance diffused by the reactive intermediate, supposedly mainly 1O2, causing the photodegradation was estimated to be on the order of 0.01-0.02 micron, which corresponds to a lifetime of 0.01-0.04 microsecond of the intermediate in the cells. PII has binding sites at proteins in the cells as shown by an energy transfer band in the fluorescence excitation spectrum at 290 nm. During light exposure this band decays faster than the Soret band of PII under the present conditions. Photoproducts (1O2 etc.) generated at one binding site contribute significantly in the destruction of remote binding sites.

BACKGROUND Photodynamic therapy (PDT) for cancer patients has developed into an important new clinical treatment modality in the past 25 years. PDT involves administration of a tumor‐localizing photosensitizer or photosensitizer prodrug (5‐aminolevulinic acid [ALA], a precursor in the heme biosynthetic pathway) and the subsequent activation of the photosensitizer by light. Although several photosensitizers other than ALA‐derived protoporphyrin IX (PpIX) have been used in clinical PDT, ALA‐based PDT has been the most active area of clinical PDT research during the past 5 years. Studies have shown that a higher accumulation of ALA‐derived PpIX in rapidly proliferating cells may provide a biologic rationale for clinical use of ALA‐based PDT and diagnosis. However, no review updating the clinical data has appeared so far. METHODS A review of recently published data on clinical ALA‐based PDT and diagnosis was conducted. RESULTS Several individual studies in which patients with primary nonmelanoma cutaneous tumors received topical ALA‐based PDT have reported promising results, including outstanding cosmetic results. However, the modality with present protocols does not, in general, appear to be superior to conventional therapies with respect to initial complete response rates and long term recurrence rates, particularly in the treatment of nodular skin tumors. Topical ALA‐PDT does have the following advantages over conventional treatments: it is noninvasive; it produces excellent cosmetic results; it is well tolerated by patients; it can be used to treat multiple superficial lesions in short treatment sessions; it can be applied to patients who refuse surgery or have pacemakers and bleeding tendency; it can be used to treat lesions in specific locations, such as the oral mucosa or the genital area; it can be used as a palliative treatment; and it can be applied repeatedly without cumulative toxicity. Topical ALA‐PDT also has potential as a treatment for nonneoplastic skin diseases. Systemic administration of ALA does not seem to be severely toxic, but the advantage of using this approach for PDT of superficial lesions of internal hollow organs is still uncertain. The ALA‐derived porphyrin fluorescence technique would be useful in the diagnosis of superficial lesions of internal hollow organs. CONCLUSIONS Promising results of ALA‐based clinical PDT and diagnosis have been obtained. The modality has advantages over conventional treatments. However, some improvements need to be made, such as optimization of parameters of ALA‐based PDT and diagnosis; increased tumor selectivity of ALA‐derived PpIX; better understanding of light distribution in tissue; improvement of light dosimetry procedure; and development of simpler, cheaper, and more efficient light delivery systems. Cancer 1997; 79:2282‐308. © 1997 American Cancer Society.