It is the purpose of this conference to look forward over the next five years in the field of photodynamic therapy. In my opinion, unless certain critical goals are achieved, PDT actually may be all but gone in five years. However, before looking forward, it may be useful to look back and assess how we have arrived at where we are today.
This review presents the current clinical status of photodynamic therapy (PDT)in the treatment of human cancer, as well as its potential and the technical needs currently perceived as required for PDT to achieve that potential. Progress in the treatment of endobronchial cancer, superficial bladder cancer, head and neck cancer, gynecological cancer, intracranial tumors, gastrointestinal cancers, ocular cancers, and cutaneous and subcutaneous cancers is rev'ewed. While a nearly universal need is apparent for reliable, user-friendly PDT lasers, other technicAl advances in the mechanism of light delivery will be required to optimize treatment of intrdcranial, head and neck, gynecologic and intraocular tumors. Development of topical formulations of PHOTOFRINÃ‚Â® may allow treatment of gynecologic and cutaneous cancer while obviating the systemic photosensitivity caused by intravenous administration of the drug. Development of new photosensitizers activated at wavelengths above 630 nm may allow effective debulking or obliteration of large tumors. Indeed, in the future, specific photosensitizers may be tailored to different indications. Practical clinical situations should be considered when designing and testing new photosensitizers for future clinical development. Potential applications of POT in the intraoperative adjuvant therapy of tumors with a high risk of recurrence, and in the intraoperative conversions of surgical partial to complete tumor reponses are described.
Photodynamic therapy is a relatively new modality for therapy of neoplastic disease and also shows promise in other therapeutic settings. While PDT research has proceeded without any special programs being put in place, further development depends on support- ing clinical data and perhaps some new approaches with regard to re- search support.
Electrochemical measurements of transcutaneous tumor oxygen tension are combined with compartmentalized modeling techniques in order to develop a more complete picture of the early, oxygen-dependent events during photodynamic therapy (PDT). Irradiation of Photofrin II-treated VX-2 skin carcinomas in rabbit ears results in severe local oxygen depletion which is proportional to the applied light dose. For 50 mW/cm2 irradiations (at 630 nm), energy fluences of approximately 200- 300 kJ/m2 are required to irreversibly deplete tumor transcutaneous oxygen levels. Light-induced structural damage further decreases tumor oxygen tension by disrupting blood flow. Measured and modeled tumor oxygen depletion rates are used to estimate the production of cytotoxic oxygen intermediates as well as provide a qualitative assessment of tumor circulatory status.
Syntheses and chemical characterization of a number of new porphyrin-derived sensitizers, related to Photofrin-II and otherwise, are described. The major thrust of our more recent work centers on preparation of porphyrin oligomers linked by ester, ether, or carbon-carbon bonds, as well as photosensitizers with long-wavelength absorbance. Where appropriate, PDT activity is briefly mentioned.
In recent years, a number of new photosensitizers have been proposed as alternatives to Photofrin II, for use in photodynamic therapy. Most studies in this area have targeted the absorption characteristics of the photosensitizer as the important parameter however there is currently no general consensus on where this absorption should be. In this article, the spectroscopic requirements for photodynamic therapy are examined with respect to generation of phototoxic species, penetration of light through tissues, tumor location and compatibility with available light sources. Finally, additional properties which may dictate the choice of sensitizer, such as stability, purity, hydrophobicity and biological lifetime are discussed.
Metallophthalocyanines and naphthalocyanines are under intensive study as second generation photosensitizers for PDT. They have attractive photophysical and chemical properties including strong absorption maxima at wavelengths where tissues provide optimal light transmissions, good capacity to generate singlet oxygen and facile chemical accessibility. Their photophysical properties are mainly determined by the nature of the central metal ion whereas ring substituents and axial ligands on the metal ion modulate solubility, tendencies to aggregate or associate with biomolecules, cell penetrating properties and the pharmacokinetics of the dyes. The availability of series of differently substituted metallophtha- locyanines and naphthalocyanines provides for the opportunity to assess structure- activity relationships with various parameters of importance in the overall out- come of PDT, an approach which could guide future syntheses of improved photosen- sitizers for the photodynamic therapy of cancer.
Mechanisms of dye sensitized photooxidation are introduced, and techniques of distinguishing and analytically determining mechanisms in these systems by the use of chemical and physical traps, kinetic tests and luminescence are described.
This paper concerns the use of deep-red light absorbing chromophores as potential photosensitizers for photodynamic therapy (PDT). In setting the scene, the role of scattering by tissues and of the availability of convenient laser light sources are considered. In terms of long wavelength extremes for the production of singlet oxygen, it is pointed out that the energy gap between S1 and T1 (EST) is the critical factor. Some of the photophysical and photochemical properties of naphthalocyanines and octabutoxyphthalocyanines are presented. Both species absorb strongly at 740 nm and beyond. Finally, the tissue distribution and phototherapeutic effectiveness in animal tumor models is outlined. Delivered doses as low as 0.5 mg/kg body weight are optimally effective.
The continued examination of injury sites and mechanisms of cytotoxicity associated with photodynamic therapy (PDT) can take advantage of current molecular and/or biochemical techniques. The increased expression of oxidative stress proteins can be studied as a function of photosensitizer type, treatment conditions and cell type. However, while in-vitro studies can address questions regarding subcellular PDT targets there is growing evidence that in-vivo effects of PDT are mediated by both vascular and direct tumor cell injury. Preclinical PDT studies using mono-l-aspartyl chlorin e6 (NPe6) confirm that the efficacy of this photosensitizer is correlated with plasma levels of this compound and not tumor cell levels.
Past research has shown that photodynamic therapy, using Photofrin II as sensitizer, exerts its tumor-destructive effects primarily through assault on the tissue microvasculature. This is accompanied by less than optimal treatment selectivity and prolonged normal tissue photo- sensitivity. The development of new sensitizers should therefore add to its goals of finding improved photochemical and photophysical characteristics the goal of increased tumor selectivity. The latter can only be achieved through detailed comparative studies of different second generation sensitizers, which will serve to unravel structure/function relationships and so lead to the rational design of new and truly superior compounds.
There are currently a large number of photosensitizers under investigation for their possible applications to PDT. Many of them appear to share the property of PhotofrinR of selective accumulation in malignant or other abnormal tissue. In developing the area of photodynamic therapy there are a number of investigative approaches that might be taken to add rigour to the application of photosensitizers for treatment of disease. One is to try to understand the mechanisms whereby selective accumulation takes place, and the other is to try to develop technology to improve selective uptake by targeted tissue. The following report covers, at preliminary level, our attempts to address both these issues using a new group of photosensitizers, the benzoporphyrin derivative (BPD) analogues described previously (1-3). The BPD analogues all have an identical reduced tetrapyrrol ring, and differ by the position of a cyclohexadiene ring (fused at either ring A or ring B of the porphyrin) and the presence of either two acid groups or one acid and one ester group at rings C and D of the porphyrin. The monoacid derivatives (ring A or B designated as BPD-MA or BPD-MB) are considerably more active in in vitro phototoxic killing of cell lines and in PDT using a murine tumour model, than are the diacids (designated BPD-DA or BPD-DB (3) Because of the close similarity in structures between these compounds and the clear differences in photodynamic effect, we considered them to be useful molecular models in which to determine whether there were basic differences between them in serum distribution, and if these differences would affect the efficacy of PDT.
"Second generation" photochemotherapies might employ a multiagent approach using cationic photosensitizers which selectively target tumor cells together with anionic photosensitizers which act on blood vessels. Combination photochemotherapies might also include glycolysis inhibitors and/or hyperthermia to rationally enhance light-induced killing and still maintain tumor selectivity. Some unresolved issues are also discussed: We generally do not understand the biological bases for tumor selectivity and photochemotherapeutic efficacy. We need to determine the desirability of absolute malignant cell specificity. Both in vitro and in vivo test systems should be improved and standardized, and their (ir)relevance to clinical PCT carefully examined. More mechanistic data should be obtained from ongoing and future clinical trials. Finally, we need to better define the critical biochemical and cellular mechanisms of cell injury and death, and better understand why therapy fails.
For nearly 40 years the field of psoralen photobiology has been focused on the effects of photoactivated 8-MOP on nuclear DNA. The results of these extensive studies are reviewed. In addition, new targets for modification are described. 8-MOP and UVA was first used to treat skin afflicted with two common dermatological disorders, vitiligo and psoriasis. More recently, several other disease have been treated using an extracorporeal form of this photochemotherapy in which the patient's blood is irradiated with UVA. Clinical results and possible modes of action are described.
This review discusses various opportunities for applications of photodynamic therapy in the field of bone marrow trans- plantation. These include the elimination of tumor cells from autologous bone marrow grafts (marrow purging), the pro- phylaxis and treatment of graft-versus-host disease, the prevention of graft rejection, and the inactivation of viruses in bone marrow grafts and blood products.
A prototype instrument is described, based on optical fiber light delivery and detection, for quantitative, non-invasive, in vivo studies of photosensitizers used in photodynamic therapy. The primary purpose of the instrument is the measurement of absolute photosensitizer uptake in tissues. This is achieved by measuring the spectrum of diffusely reflected light and identifying the characteristic absorption spectrum of the photosensitizer. This absorbance measurement is converted to equivalent absolute photosensitizer concentration by a procedure which separately determines the intrinsic optical absorption and transport scatter coefficients of the tissue. The procedure involves measuring the radial dependence of the diffusely reflected light on the tissue surface as a function of wavelength. The principles and operation of the instrument have been evaluated in tissue-simulating phantoms and in normal and tumor tissues in animal models in vivo. The outstanding problems and future development of this method, and of analogous quantitative measurements of photosensitizer fluorescence in vivo are discussed.
The physical principles of dosimetry for photodynamic therapy are discussed. The presentation emphasizes a discussion of prognosticated maximum obtainable depth of tumor necrosis and of expected maximum depth of selective necrosis, i.e. the maximum depth which can be obtained before normal tissue within the light field at the surface is destroyed at the same rate as the neoplastic cells at the given depth into the tumor. These parameters are discussed in terms of bleaching, retention and clearance of the photosensitizer and of optical properties of the tissue.
A complete instrumentation has been developed for photodynamic therapy (PDT) and combined PDT- hyperthermia in the upper aerodigestive tract and the bronchi. These instruments consist of several light distributors which permit optimal light dosage to "superficial" tumors, as well as an injector for laser beams into an optical fiber and a fiberoptic coupler for cw laser beam powers at least 100 Watts. PDT is carried out with HpD and Photofrin II at 630 nm, whereas occasional simultaneous hyperthermia is at 1.06 microns. PDT of 41 cases of "early" squamous cell carcinoma is reported with follow-up between 5 and 62 months. In the oesophagus and bronchi the results are good for cancers staged in situ or microinvasive at endoscopy (2 recurrences for 23 lesions treated). For more advanced cancers (submucosal in the oesopha- gus or invading the bronchial cartilage) the results are less satisfactory with 3 recurrences for 8 lesions treated. In the bronchi (1 case) and the oesophagus (1 case) the largest disease - free survival is now more than 5 years. We encountered 6 complications (3 cicatrical stenosis, 2 fistulae, 1 severe sunburn), most of them resulting from the lack of selectivity of PDT with these porphyrin mixtures at the applied conditions. These experiments show that PDT is efficient at destroying early squamous cell carcinomas in the pharynx, oesophagus and bronchi. Tumour selectivity of HpD and photofrin II is poor in the aerodigestive tract lined with squamous cell epithelium. The future lies in the synthesis of a more selective efficient photosensitizer.
An optical system for digitized imaging (OSDI) consisting of a computer-controlled charge-coupled-device (CCD) array with 576 x 384 detection elements and 1.4 bits of digitization is used for in-vivo measurements of light distributions from diffusing tips of optical fibers in tumors. A macro lens assembly allows imaging with a spatial resolution of approximately 46 pm. Radiation-induced fibrosarcoma were implanted and grown subcutaneously in C3H mice. A fiber with a cylindrical diffuser tip (1.5 x 5 mm) was inserted into tumors. Tumor light intensity distribution images were made with the OSDI at wavelengths varying from 457.9 to 800 nm with argon, argon-dye and titanium-sapphire lasers. These tumor images show that the volume of tumor exposed to actinic light intensities increases for wavelengths between 400 and 800 run and reaches a maximum at about 800 nm. The uniformity of light distribution also increases at longer wavelengths. At shorter wavelengths, blood vessels in the tumor are clearly delineated as dark lines and networks of lines that might shield sensitized tumor cells from adequate light exposure. The light-acti.vated drug, Photofrin II (PF II), 20 mg/kgbw, was administered intravenously to anesthetized mice. With optical excitation at 457.9 nm consecutive (0, 1., 2.5, 4 hours) fluorescence-only images were made of PF II fluorescence distribution in the tumor and surrounding the diffusing optical fiber. Serial images after PF II injection showed drug fluorescence increasing with time in the tumor around the fiber. The OSDI provides a way to measure actual light intensity distributions and could be used in vivo to guide adjustments in light intensity and drug distributions before and during tumor phototherapy.