Mechanisms of Action of Photodynamic Therapy with Verteporfin for the Treatment of Age-Related Macular Degeneration

SURVEY OF OPHTHALMOLOGY VOLUME 45 • NUMBER 3 • NOVEMBER–DECEMBER 2000

CURRENT RESEARCH ROBERT WEINREB AND EDWARD COTLIER, EDITORS

Mechanisms of Action of Photodynamic Therapy with Verteporfin for the Treatment of Age-Related Macular Degeneration Ursula Schmidt-Erfurth, MD,1 and Tayyaba Hasan, PhD2 1

University Eye Hospital, Lübeck, Germany, and 2Wellman Laboratory of Photomedicine, Massachusetts General Hospital, Boston, Massachusetts,USA Abstract. Age-related macular degeneration, especially the neovascular form of the disease, is the leading cause of blindness in elderly people in developed countries. Thermal photocoagulation is still the preferred treatment for choroidal neovascularization that does not involve the fovea, but it is suitable for only a small number of patients and it can lead to immediate loss of visual acuity. Photodynamic therapy with use of photochemical light activation of verteporfin as a photosensitizer (verteporfin therapy) has been shown to be effective in treating vascularized tumors, and its potential to treat other conditions involving neovascularization has also been suggested. Preclinical and clinical studies have indicated that verteporfin therapy can be used to treat choroidal neovascularization secondary to age-related macular degeneration effectively and safely. Selective occlusion of choroidal neovasculature by this therapy causes minimal damage to the neurosensory retina and, therefore, does not induce loss of visual acuity. This benefit allows verteporfin therapy to be used in the large proportion of patients who are not eligible for treatment by laser photocoagulation. The mechanistic aspects of the mode of action of light-activated verteporfin are described in this review. (Surv Ophthalmol 45: 195–214, 2000. © 2000 by Elsevier Science Inc. All rights reserved.) Key words. age-related macular degeneration • benzoporphyrin derivative • mechanisms of action • photodynamic therapy • verteporfin

Photodynamic therapy (PDT) is a treatment modality in which a nontoxic light-sensitive compound called a photosensitizer is administered and subsequently activated by light exposure to produce photochemical effects in the target area.16,26 PDT has the potential advantage of dual selectivity: firstly, there is a preferential concentration of the photosensitizer in the target tissue, and secondly, the light irradiation is directed toward and confined to the specific target area. PDT is currently used to treat various

types of solid tumors. However, recent improvements in our understanding of the mechanisms of action, light sources, and light delivery systems, and the development of specific photosensitizing agents with improved selectivity and activity,12,27 such as verteporfin (benzoporphyrin derivative monoacid A, BPD-MA), have expanded the possible therapeutic uses of PDT to nononcologic applications.11,35,43,71,75,88 In the field of ophthalmology, PDT with verteporfin (Visudyne; Wellman Laboratories of Photomedi195

© 2000 by Elsevier Science Inc. All rights reserved.

0039-6257/00/$–see front matter PII S0039-6257(00)00158-2

196

Surv Ophthalmol 45 (3) November–December 2000

SCHMIDT-ERFURTH AND HASAN

Glossary of Terms Relevant to PDT Absorption spectrum Fluorescence Intersystem crossing Irradiance Light dose Pharmacokinetic properties Phosphorescence Photocoagulation Photodynamic therapy (PDT) Photosensitivity Photosensitizer Photosensitizing potency Quantum yield Selectivity Singlet state Triplet state Type I reaction Type II reaction

Defines the optimal wavelength of activating light. Transformation of chemical energy into physical energy at the level of the singlet state with emission of light. Fast luminescence with rapid onset and almost immediate decline after the stop of light excitation. Uptake of light energy causing a switch in the electronic spin of a photosensitizer molecule Light intensity originating from a light source as light energy over time per area (mW/cm2) Total amount of light energy applied to a target structure as light energy per area (J/cm2) Determines the timing of light application, duration of photosensitivity and the site of damage Transformation of chemical energy into physical energy at the level of the triplet state with emission of light. Slow luminescence which may continue to emit light longer than the duration of excitation. A thermal modality to induce structural damage by absorption of high levels of light energy within biological chromophores such as melanin or hemoglobin A non-thermal modality using light, an activable chromophore and oxygen to induce a localized cytotoxic reaction involving chemical radicals and oxidative processes Phototoxic reactivity of tissue following light exposure due to prolonged retention of sensitizer, e.g., within skin or inner organs A light-activable compound which produces highly toxic singlet oxygen radicals upon irradiation with light at its specific absorption peak Determines the light dose required for optimum specificity Ability of a sensitizer to generate excited triplet states Determines degree of iatrogenic damage and cutaneous photosensitivity First level of chemical activation of a sensitizer molecule following light absorption Second level of photochemical activation of a sensitizer molecule with the ability to generate free radicals or excited oxygen 3O2 Photodynamic process with formation of cytotoxic free radicals Interaction of excited sensitizer molecules with oxygen leading to the generation of singlet oxygen radicals, major mechanism of photochemical tissue damage

cine), is being developed for the management of choroidal neovascularization (CNV) secondary to age-related macular degeneration (AMD). This technique also has potential applications in other conditions involving neovascularization.11,16 In this review, we provide a brief overview of the relevance of PDT application to AMD. We then discuss the mechanistic aspects of this treatment with a focus on the use of light-activated verteporfin as a potential new treatment for CNV secondary to AMD.

I. Age-Related Macular Degeneration AMD is a degenerative eye disease that can cause severe irreversible loss of central vision. It occurs most frequently in adults over 60 years of age.13 In the Western world, AMD is the leading cause of blindness in the age group from 65 to 74 years, and the second leading cause of blindness in the population aged from 45 to 69 years. Advanced AMD can be classified into two types: non-neovascular (also called nonexudative or dry) AMD or neovascular (also called exudative or wet) AMD.13 Although less than 20% of patients with AMD have the neovascular form, severe vision loss occurs predominantly in patients with this form of the disease.17 Neovascular AMD is characterized by CNV, which develops most commonly in the area underlying the

center of the foveal avascular zone, and is identified angiographically as subfoveal CNV. The exact etiology of CNV is unknown. Histology reveals that abnormal new blood vessels from the choriocapillaris grow and proliferate through breaks in Bruch’s membrane under the retinal pigment epithelium (RPE; see Fig. 1) and further into the subretinal space between RPE and retina. These new blood vessels are leaky and can lead to subretinal hemorrhage and detachment of the RPE and neurosensory retina, followed by the formation of a fibrovascular scar. These events lead to progressive and irreversible loss of central vision, usually symmetrically in both eyes, over a period of 5 years. Individuals with CNV secondary to AMD can rapidly lose their visual acuity and may be legally blind in the affected area within 2 years of diagnosis in the fellow eye.14 At this time, laser photocoagulation is the only treatment proven to have some therapeutic effect in patients with neovascular AMD. However, it is suitable for only a small proportion of patients—in whom the CNV lesion has a fluorescein angiographic pattern of classic CNV and is of small size with well-defined boundaries.4,20 In addition, laser photocoagulation causes nonselective necrotic damage to the area where the laser is applied. In the subfoveal area, the thermal damage can cause immedi-

197

MECHANISMS OF ACTION OF PDT WITH VERTEPORFIN

juxta and extrafoveal lesions. Extensive preclinical and recent clinical studies suggest that PDT with verteporfin could be promising in this respect.1,49,82

II. Photodynamic Therapy

Fig. 1. Schematic drawings of the damage induced by the CNV complex (top) and the effect of photodynamic therapy (bottom). Top: Blood and fluid are leaked by new blood vessels and cause a build-up of fibroblasts and neovascular endothelial cells between and within the RPE and photoreceptor layers. A detachment of RPE and retina results during the early stage, and later, a persistent fibrovascular scar will form, accompanied by a progressive and irreversible loss of viable photoreceptors. Bottom: PDT directly or indirectly causes an occlusion of the neovascular membrane without affecting overlying neurosensory retina. Following the involution of the CNV and the restoration of the vascular barrier function, extravascular fluid and hemorrhage resolve with reattachment of RPE and photoreceptors.

ate and irreversible loss of visual acuity, particularly in patients with relatively good visual acuity prior to treatment.2 Moreover, in patients eligible for this treatment modality, there is a recurrence rate of at least 50% after laser photocoagulation, which can be accompanied by further visual loss.5 Thus, there is an urgent need for new treatments for AMD that are applicable, safe and effective in a wide range of patients with subfoveal CNV, but potentially also for

Light may be used to cause tissue damage in various ways, e.g., thermal, mechanical, and chemical. Therapeutic applications in ophthalmology include photocoagulation, photodisruption, and photochemistry. Photocoagulation is a thermal procedure in which high levels of light energy are directed to structures containing light-absorbing chromophores, e.g., melanin or hemoglobin. Light energy is transformed into heat, inducing a nonselective coagulative necrosis of the target tissue. Because of heat conduction, other components in the vicinity of the absorber are also affected. Conventional photocoagulation, e.g., panretinal laser treatment in diabetic retinopathy, is based on this principle. Photodisruption implies the use of light, which is applied at high energy levels and short pulse durations to a small focus. Disruption from short light pulses of the duration of milliseconds to tenths of a second produce a vapor bubble, while nanosecond or picosecond pulses lead to the formation of a plasma and shock wave. A disruptive process is used for the dissection of iris tissue in iridotomies or posterior lens capsule in capsulotomies using a YAG laser. Photochemistry differs substantially from these techniques, as it does not involve any thermal or mechanical damage. Chemical photodamage may occur naturally by light exposure alone, inducing oxidative mechanisms, e.g., retinal light damage occurs by oxidative DNA fragmentation.61 The basis of PDT is the initiation of photochemistry at target sites. The two main steps in the process of PDT are the intravenous injection of a photosensitizer and subsequent light irradiation at a specific wavelength that is appropriate for absorption by the sensitizer.29 Although the exact biological mechanisms underlying PDT may vary with the nature of the photosensitizer, the site of localization, and other parameters,29 the primary photochemistry, at least in solution, appears to be similar for all photosensitizers.60 A schematic representation of the photochemical/photophysical steps involved in PDT is presented in Fig. 2. Briefly, light energy is absorbed by the photosensitizer, which is transformed from its ground singlet state S0 to the excited singlet state S1. By intersystem crossing on the level of electrons, S1 gives rise to the excited triplet state T1 (S and T are magnetically different excited states, which result from a quantum mechanical consequence of electron spin). Fluorescence may be directly generated from S1, whereas phosphorescence may derive from T1 after intersystem crossing. Alternatively, T1 may

198

Surv Ophthalmol 45 (3) November–December 2000

SCHMIDT-ERFURTH AND HASAN

Fig. 2. Simplified energy-level diagram for the photoexcitation of a molecule. S0, S1, and S2 represent singlet electronic states of the molecule; and T0, T1, and T2, its triplet electronic states. With conventional light sources, only S1 and T1 are reached by the molecule. Higher excited states, such as S2 and T2, may be reached with high-intensity, pulsed irradiation and excitation at two wavelengths.

initiate photochemical reactions directly by generating reactive, cytotoxic free radicals, or indirectly by transferring its energy to ground state oxygen (3O2). This step leads to the formation of excited state oxygen, singlet oxygen (1O2), which causes photo-oxidative damage to biological—cellular and subcellular—targets. The former reaction pathway is referred to as a type I reaction, while the latter (mediated by singlet oxygen) pathway reaction is referred to as a type II reaction. Type I, as well as type II, reactions necessarily require the availability of oxygen. In type I reactions, an electron or hydrogen atom transfer is initiated, which leads to the production of superoxide anions – – O 2 . Hydroxyl radicals and O 2 have been detected during PDT reactions.15 Type II reactions are based on a transfer of energy to molecular oxygen with generation of singlet oxygen 1O2. The highly reactive oxygen products of both reaction types produce similar damage to their biological environment.38 Both reaction types can occur either simultaneously or exclusively, depending on the chemical structure of the photosensitizer. However, it is generally believed that the formation of 1O2 is the primary mechanism of PDT-induced damage for most sensitizers currently being studied, although it has never been detected in any biological system. The possible biological targets of singlet oxygen and free radicals include nucleic acids, enzymes, and cellular membranes, leading to cell damage and cell death.16,23 Sites of photodynamic damage are described on the molecular, subcellular, cellular, and tissue level. Biochemical analysis has shown that PDT inactivates membrane-bound mitochondrial enzymes, such as

cytochrome C oxidase and acyl coenzyme A in endoplasmic reticulum membranes.30 In addition to the alteration of cellular organelles, such as mitochondria and endoplasmic reticula, inactivation of transmembrane pumps like Na/K ATPase has also been described.86 Benzoporphyrin was found to cause damage to lysosomes, resulting in hydrolytic enzyme leakage and intracellular lysis.36 Lysosomal targeting might be a cause for the increased sensitivity of RPE cells to PDT, a cell type with a high lysosomal activity. Sensitizers that fuse with plasma low density lipoproteins (LDL) bind to specific receptors on the surface of the cell membrane of vascular and reticulo-endothelial cells and induce breaks in the cytoplasmic membrane. Transcellular transport of sensitizer/ LDL-complexes leads to accumulation within lysosomes with subsequent cellular lysis. On the tissue level, destruction of the vascular compartment with consecutive ischemia is the predominant cause of damage. Photothrombosis will be described in detail. The mechanisms of PDT-induced cell and tissue destruction are complex and not yet fully understood. However, there appear to be three primary mechanisms: cellular, vascular, and immunological.26,29 Moreover, a mixture of these three mechanisms may be involved in tissue destruction and each mechanism can influence the others.16 The relative contribution of each mechanism is influenced by the characteristics of the photosensitizer, the tissue being treated, and the treatment parameters used. In the case of tumors, the response to PDT is thought to be caused by a combination of direct cytotoxicity and vascular occlusion, which leads to tumor ischemia.

MECHANISMS OF ACTION OF PDT WITH VERTEPORFIN

Direct cellular destruction is mediated largely by singlet oxygen, which has a very short lifetime—on the order of nano- to microseconds.70 This short lifetime restricts the damage to the immediate vicinity of the photosensitizer and singlet oxygen generation; further damage outside the site of light application and photosensitizer activation does not occur. Thus, the distribution and localization of the photosensitizer within the cell is important.16 Singlet oxygen has been invoked as the primary mediator of cytotoxic damage following PDT, largely because PDT effects have been shown to be limited by the availability of oxygen. It is not inconceivable that other oxygen species may be critical for efficient PDT. On the biological side, two main mechanisms of cell destruction have been described: apoptosis and necrosis. Apoptosis is the process whereby a cell undergoes programmed self-destruction that involves the activation of a series of cellular enzymes, resulting finally in the fragmentation of nuclear DNA and disruption of the cell into particles that are engulfed by nearby cells. It has been suggested that sensitizers such as Photofrin (porfimer sodium) that localize in the mitochondria are likely to induce apoptosis, whereas sensitizers that localize in the plasma membrane probably cause necrosis.16 With photosensitizers such as Photofrin, verteporfin, and the purpurins, vascular damage and blood flow stasis—followed by vascular occlusion—appear to be the predominant mechanisms of damage induced by PDT in vivo.18,29 The extent of vascular damage and blood flow stasis has been suggested to be directly related to the level of circulating photosensitizer at the time of irradiation. Serum levels of photosensitizer have been suggested to vary according to the solubilities of different photosensitizers.29 Endothelial cell damage may be the first cellular event induced by PDT that leads to blood flow stasis.18 Ultrastructurally, intensive damage of vascular endothelial cells, which consists mainly of breaks in intraluminal endothelial membranes, is seen.73 It results from the photochemical action of singlet oxygen and other species reactive with components of cell membranes and other sensitive sites of endothelial cells.18 In one scenario, endothelial cell damage causes a rearrangement of the cytoskeletal structure, leading to the shrinkage of endothelial cells away from each other.73 This results in exposure of the vascular basement membrane, which triggers platelet binding and aggregation at the sites of damage. The activated platelets release vasoactive mediators, such as thromboxane, histamine, or tumor necrosis factor-␣ (TNF-␣).18 These can trigger a cascade of events, including amplification of platelet activation, thrombosis, vasoconstriction, and increased vascular permeability, leading to blood flow stasis

199

and resultant tissue hypoxia, and eventual shutdown of the vasculature.18 PDT can also initiate immunological effects.31 The release of inflammatory mediators and the damage to inflammatory cells play a role in PDT-induced tumor destruction. Increased levels of various cytokines, such as interleukin-1␤, interleukin-2 and TNF7␣, have been detected in PDT-treated patients and may contribute to the clinical response. In the case of AMD, the induction of an inflammatory response might stimulate wound-healing and initiate the growth of recurrent CNV. On the other hand, the release of cytokines such as TNF-␣ might facilitate vessel closure by the mechanisms discussed above. Other immunological effects seen with PDT have shown that at low levels of photosensitizer and light (sublethal dosimetry), antigen-presenting cells, such as dentritic or Langerhans’ cells, become less active. The expression of class II histocompatibility antigens and the co-stimulatory molecule B7 are significantly down-regulated, so that their immune function is suppressed. This effect is thought to account for the observation that low levels of verteporfin therapy on skin sections prior to allotransplantation significantly prolong the survival of the grafts in comparison with controls.59 This latter aspect may have relevance to ophthalmologic applications of PDT with verteporfin in enhancing corneal graft “takes” without causing systemic immunosuppression. One of the main aims of PDT is to induce transient local maximal photosensitivity in the target area. Yet, for therapeutic practicability and patient tolerability, it is important to limit the degree and duration of general—mainly skin—photosensitivity caused by PDT. Otherwise, treated patients would need to be confined indoors for unacceptably extended periods of time. On the other hand, there is a direct correlation between the phototoxic effect and the drug and light dose. With lowering of the drug dose, more light has to be applied to achieve identical effects, and vice versa. To avoid drug-related side effects in treated patients, the administered amount of photosensitizer has to be as low as possible, however, without requiring excessively long light-administration times. Lower doses of photosensitizer require higher activating light doses, and higher light doses require a longer duration of light application. For example, a light dose of 150 J/cm2 at 600 mW/cm2 requires an application time of 4.5 minutes, whereas a 50 J/ cm2 light dose (used in phase III clinical trials for AMD1,51) requires an application time of only 83 seconds, during which the light is delivered to the patient in a slit-lamp setting via a hand-held contact lens. It is, therefore, essential to use sensitizers with high efficacy and low skin toxicity.

200

Surv Ophthalmol 45 (3) November–December 2000

Efficacy and selectivity of PDT depend upon numerous factors. These include the amount of photosensitizer injected, the formulation in which it is administered, and the duration of infusion; the absorption wavelength, extinction coefficient, and target area of the photosensitizer; the light source, timing of light delivery, and the amount of light delivered; and the treatment procedure. Based on a study of a number of photosensitizers with different chemical structures, Haimovici et al reported that the localization of photosensitizer in the structures of the rabbit eye is strongly structure- and time-dependent.25 Hence, for a given sensitizing agent, the time interval between drug administration and light treatment plays an important role. When low doses of photosensitizer are used, significant selective localization of the photosensitizer in the neovasculature is obtained after only short time intervals after administration. Furthermore, the selectivity of PDT in general depends upon where the light is directed, how deep the light penetrates through tissues, and where the photosensitizer is concentrated. Since photosensitizers are cleared at different rates from different tissues, careful timing of light exposure within the period when the photosensitizer is at a maximal concentration in the target tissue can increase the selectivity of PDT.60 An angiographic technique that can determine the quality of a photosensitizer (i.e., selective localization in the target tissue) and quantify its concentration in the target tissue over time might substantially help select the optimal parameters in the treatment of CNV by PDT. A. LIGHT SOURCES AND DELIVERY

Although various light sources can be used to activate the photosensitizer, lasers have become the standard light source for PDT.94,95 Lasers have a number of advantages—they are easy to control, produce a monochromatic light beam with reasonably high intensities, can be focused to a small spot, and can be transmitted via a number of light delivery systems, including optical fibers—thus provide precise light delivery to the treatment area.11 For PDT, unlike other laser applications, it is crucial that the light distribution be fairly uniform over the selected spot size, which can be as large as 8000 ␮m, to obtain homogenous phototoxicity. A wide range of lasers is available, but diode lasers are becoming the light sources of choice, because they are small, portable, reliable, and relatively inexpensive. Moreover, they can now produce sufficiently high levels of power to be clinically useful. For example, in the studies of verteporfin therapy in patients with CNV that are outlined in this paper,1,49,82 a specific diode laser was developed and the light was delivered as a single uniformly-illuminated circular spot, using a suitable contact lens via a fiber optic coupled to a slit-lamp.

SCHMIDT-ERFURTH AND HASAN

PDT requires precise dosimetry.87 The exact wavelength of the activating light determines the depth of light penetration into tissues, which is in turn determined by the absorption spectrum of the photosensitizer used. The most appropriate wavelength is that which corresponds to the peak absorption of the individual photosensitizer. In general, light within a “phototherapeutic window” of 600–900 nm is used. Penetration of light through tissues is determined by two mechanisms: light scattering and light absorption.87 Light scatters from inhomogeneous tissue structures at wavelengths below 600 nm; this scattering increases the probability of light absorption by endogenous chromophores, such as melanin and hemoglobin.87 At wavelengths higher than 900 nm, water absorption predominates and reduces the depth of light penetration.87 Additionally, if singlet oxygen generation is critical for phototoxicity to occur, wavelengths above 900 nm are energetically too low to provide the energy for the excitation of triplet oxygen to the singlet state. Light penetration and, consequently, light dosimetry are also influenced by light absorption by the photosensitizer itself (termed self-shielding) and inactivation of the photosensitizer during light exposure (termed photobleaching).95 Typically, red light (630 nm) can penetrate to a depth of 2–3 mm, whereas longer wavelengths (700–800 nm) can penetrate further, to 5–-6 mm.29 At the level of the retina, RPE and choroid, the increase in the absorption of light by the pigments melanin, macular lutein, and hemoglobin in the shorter wavelength range might theoretically lead to a decrease in light penetration. However, since these absorbing layers are relatively thin, the practical loss of light intensity at an irradiance of 600 mW/cm2 is negligible. The hypothesis that decreased penetration of light through pigment and proteinaceous fluid may play a role in the poor response in occult CNV remains unsupported as PDT in occult CNV demonstrates efficacy angiographically, which does not translate into functional benefit. The anatomy of the eye makes it particularly amenable to light treatment. Light of longer wavelengths shows improved transmission even in less transparent lenses; this is crucial when treating an older population, as most of its members have different degrees of cataract. B. CHARACTERISTICS OF PHOTOSENSITIZERS

Studies performed throughout the past decade have indicated that many features of photosensitizers are important for PDT, including selectivity, absorption spectrum, photosensitizing potency and pharmacokinetic properties (Table 1). The success of PDT depends on the photophysical and photochemical properties of the selected photosensitizer—especially the absorption wavelength and

201

MECHANISMS OF ACTION OF PDT WITH VERTEPORFIN

corresponding extinction coefficient—and the efficiency of intersystem crossing to the triplet state, and perhaps the quantum yield of singlet oxygen. Another important factor is the tendency of the photosensitizer to undergo aggregation; this can reduce the extinction coefficient and shorten the lifetime and quantum yield of the triplet excited state.9 The aggregation state of the photosensitizer can also affect its pharmacokinetics and biodistribution.35 The localization of the photosensitizer both at the tissue and subcellular level at the time of light activation is an important determinant of the efficacy of PDT. The structural features of the photosensitizer can affect its affinity for target tissues.12,27 In general, there is some preferential uptake and retention of photosensitizers in tumor tissue, but they are also distributed and retained by normal tissues, particularly those with reticulo-endothelial components (liver, kidney, spleen).28 Various intratumoral sites of photosensitizer localization have been identified, including the mitochondria, lysosomes, plasma membrane, nuclei, and tumor vasculature.16,35 Experimental and clinical studies have demonstrated the importance of the formulation of the photoactive agent in aqueous or lipid solutions, the mode of application with bolus or infusion, and the timing of the light application after dye administration. While hydrophilic compounds diffuse easily into the interstitial tissue, lipophilic compounds are more likely to be confined to vessel walls. Administration in form of a bolus appears to increase the selectivity for neovasculature.41 Irradiation early after intravenous application of sensitizer leads to an enhanced vascular effect, and, in the setting of CNV treatments, to a loss of the retino-choroidal selectivity with dramatic occlusive effects within retinal vasculature.49 The mechanism underlying the selective tumor retention of photosensitizers is complex, but one factor may be their selective affinity for proliferating endothelium.69 It has been suggested that the uptake of photosensitizers is via lipoproteins and, perhaps, the neovasculature, which is a hallmark of growing tumors. The photosensitizer binds to endogenous lipoproteins, in particular LDL, which have an increased expression in malignant cells and neovascular endothelial cells. The LDL-bound photosensitizer is then internalized by endocytosis35,75 and causes direct endothelial cell death following irradiation. A high selectivity of the photosensitizer for neovascular structures, instead of physiological, resting vessels (e.g., retinal or normal choroidal vessels), is essential to maintain the function of photoreceptors in the sensitive macular area. The selective concentration of photosensitizers in neovascularization has been exploited in the treatment of CNV in patients with AMD by PDT. The ra-

tionale for this therapeutic approach is that PDT may selectively destroy the abnormal new choroidal vessels while sparing normal vessels, particularly physiological retinal vasculature. Effective destruction or closure of subfoveal CNV in patients with AMD with concomitant preservation of retinal structures may preserve or even improve vision. Depending on the anatomical site of treatment, selectivity of the photosensitizer for the target tissue may be less important than the need to have a sufficient concentration of the photosensitizer in the target tissue at the time of irradiation. This is because despite the fact that photosensitizers distribute to normal tissues, such as the liver and spleen, PDT does not damage these tissues because the light is not applied there. Delivery of the photosensitizer to the target tissue and tissue retention of the photosensitizer can be enhanced by molecular delivery systems that have a high affinity for the target tissue, such as LDL, liposomes, and monoclonal antibodies.26,35,75 More importantly, the pharmacokinetic profile of a photosensitizer, in particular, its clearance, is an essential determinant of unwanted effects caused by circulating photosensitizers, such as skin and systemic photosensitivity. A problem encountered with photosensitizers that have a long retention time in internal organs is that if a patient treated with this compound requires emergency surgery (e.g., for acute abdominal diseases), the light required for surgery might cause burns of the liver and other internal organs. Thus, for photosensitizers with a short half-life, the time for sensitive structures to be photosensitive is shorter, and, therefore, the risk of systemic photosensitivity is reduced. C. TYPES OF PHOTOSENSITIZERS UNDER OPHTHALMOLOGICAL INVESTIGATION

In early studies of PDT a hematoporphyrin derivative (HPD), synthetic derivative of hemoglobin, was used.23 More recently, porfimer sodium (Photofrin), a purified hematoporphyrin derivative, has been studied extensively and is now licensed for use in several countries for the treatment of various cancers. Photofrin is the only approved PDT product worldwide. Although it has been adopted for several uses, Photofrin, a first-generation photosensitizer, has some limitations including skin photosensitivity lasting 3–6 weeks and limited tissue penetration by activating light at 630 nm.22 The limitations of first-generation photosensitizers prompted the development of alternative photosensitizers. In general, these have been based on the structure of porphyrin, chlorin, purpurin, phthalocyanine, or naphthalocyanine. Several second-generation photosensitizers, such as rose bengal, chloro-aluminum sulfonated phthalocyanine, and bacteriochlo-

202

Surv Ophthalmol 45 (3) November–December 2000

rin A, have been demonstrated to be effective in the photodynamic occlusion of experimental models of ocular neovascularization.39,48,83 However, none of these photosensitizers has been pursued for clinical use in ophthalmology mostly due to systemic side effects such as neurologic complications in animal experiments. Other second-generation photosensitizers, such as verteporfin (Visudyne), tin ethyl etiopurpurin, lutetium texaphyrin, mono-L-aspartyl chlorin e6, aluminum phthalocyanine tetrasulfonate, optrin93 and ATX-S10, have been reported to be effective in animal models of ocular neovascularization and are currently being investigated for their potential use in the treatment of AMD.52,55,57,91 Tin ethyl etiopurpurin, SnET2 (Purlitin), is a lipophilic sensitizer that is administered as a lipid emulsion. Photoactivation is performed in the red wavelength range at 664 nm providing satisfactory transmission and tissue penetration. SnET2 offers a vaso-occlusive potential and was shown to thrombose retinal vessels experimentally.54 In preclinical studies SnET2-combined PDT was successfully used to occlude choriocapillaris in pigmented rabbits when irradiation was started 15–45 minutes after dye injection at a nonthermal irradiance of 300 mW/cm2 and relatively low light doses of 5 to 20 J/cm2. Selective damage to choriocapillary endothelium was identified as the primary thrombogenic stimulus; however, RPE and outer retinal alterations were also documented.62 Based on these preclinical findings prospective open-label phase I/II clinical trials were initiated to evaluate the therapeutic benefit in CNV secondary to AMD, ocular histoplasmosis, and idiopathic causes, and to define appropriate parameters for clinical use. Preliminary reports indicated that 40 patients treated at a drug dose of 0.5–0.75 mg/kg and a light dose of 35 J/cm2 demonstrated a mean increase in visual acuity of ⫹2.7 lines at 3 months following treatment. The initial improvement was followed by an average decline of ⫺1.1 lines at 6 months follow-up with repeated treatment sessions indicated in a subset of patients due to recurrent CNV.90 A prospective, randomized, placebo-controlled phase III trial is currently underway to elucidate effects of retreatments and the potential long-term benefit of PDT using the newly developed sensitizer. As a consequence of the prolonged retention of the sensitizer within the skin, treated patients have to avoid bright light for several weeks following SnET2PDT (study protocol of Phase III clinical trial using SnET2-PDT in subfoveal CNV). The third sensitizer that entered clinical trials in ophthalmology is lutetium texaphyrin (Lutex). This synthetic porphyrin analogue contains a central lanthanidine that causes a red-shift in the absorbance

SCHMIDT-ERFURTH AND HASAN

spectrum, improving the transmission of the sensitizing light and the penetration through melanin and blood.97 In contrast to the lipophilic compounds already mentioned, lutetium texaphyrin is water soluble and therefore easier to administer intravenously. The specific characteristic of this compound is its preferential accumulation in the vascular compartment and lutex-induced PDT has shown activity against atherosclerotic plaques in rabbits.96 In the experimental model of laser-induced CNV in the monkey, absence of fluorescein leakage from CNV was obtained with treatment using 2 mg/kg sensitizer and 50 or 100 J/cm2 light at a wavelength of 732 nm and an irradiance of 600 mW/cm2.7 Damage to neovascular endothelial cells caused thrombosis of CNV; minimal damage to neurosensory retina with rare pyknosis within the outer nuclear layer together with necrosis of RPE was described. Occlusion of the choriocapillary layer was found with all parameters tested, while closure of retinal vessels was observed only at the high light dose when photoactivation was performed within 5 minutes following dye injection.7 Since, in addition to its photosensitizing properties, the compound also exhibits a significant fluorescence peak at 740–752 nm, angiography may be performed to study the biodistribution of the drug over time within retinal and choroidal vessels and the CNV membrane. In the same model of experimental CNV, uptake of sensitizer by neovascularization was documented from 10–45 minutes in a plateau with subsequent lower retention for as long as 2–5 hours, while rapid clearance from retinal vessels was seen early on.24 Hence, lutetium texaphyrin appears to provide a selective accumulation within the target CNV, and combined with angiographic monitoring an individual dosimetry of treatment parameters might be adjustable. Phase I and II clinical trials evaluating safety and efficacy were recently started in Europe with unpublished results so far. All other sensitizers are still exclusively under experimental evaluation, among them a hydrophilic compound, mono-l-aspartyl chlorin e6, which was also used to obtain occlusion of normal choroidal vessels with minimal injury to overlying neurosensory retina in the rabbit eye53 and a water-soluble photosensitizer, ATX-S10, which could selectively occlude experimental CNV in the rat model.58 Further evaluation of all these various sensitizing compounds will show which substances might be more advantageous for the treatment of CNV in AMD due to their chemical characteristics and resulting biodistribution, the selectivity of the obtained tissue effect, and the systemic safety including generalized photosensitivity of the skin and inner organs. Of the second-generation photosensitizers, verteporfin is the furthest advanced in development and

203

MECHANISMS OF ACTION OF PDT WITH VERTEPORFIN

is evaluated in phase III clinical trials in patients with CNV secondary to AMD and is the only sensitizer currently approved by the Food and Drug Administration (FDA).

III. Application of Photodynamic Therapy in Macular Degeneration Because the pathogenesis of age-related macular degeneration is still unclear, a prophylactic or causative treatment is not available. However, several studies documenting the course of the disease in the various subtypes of AMD clearly identified the choroidal neovascular complex as the histological substrate responsible for progressive and irreversible visuale loss.14,21 CNV is anatomically localized extremely close to structures with relevance for visual function, such as photoreceptor outer segments, RPE, and physiological choriocapillary. Therefore, any nonselective approach to eliminate subretinal neovasculature that is unable to preserve adjacent extravascular structures offers only a limited therapeutic benefit. When subfoveal membranes were treated during the MPS trial, coagulated eyes lost approximately three lines of vision immediately following treatment.2 No benefit was demonstrated for indirect scatter photocoagulation of the macula with laser burns applied beyond the area of angiographically defined CNV.8 Longer wavelengths with deeper transmission through the retina and increased absorption within the hemoglobin of the choriocapillary layer were suggested, but dye and krypton lasers failed to improve functional results.3 Thomas et al were the first investigators to use a photoactivatable dye, dihematoporphyrin ether, to chemically augment effects obtained by argon green laser coagulation and lower the level of laser energy required in an attempt to reduce the thermal effect.89 The improvement was limited because the laser energy was still within the range of thermal mechanisms and because the hematoporphyrin dye did not localize selectively enough to the pathologic proliferation. At the same time, the clinical evaluation of the photodynamic modality using hematoporphyrin for the treatment of intraocular tumors in patients was discontinued in response to severe nonselective damage to other vascularized structures in the eye with retinal ischemia and neovascular glaucoma.84 Full advantage of the nonthermal potential of photodynamic therapy was taken with the administration of phthalocyanin dye and low-intensity light activation to induce photodynamic occlusion of experimental choroidal neovascularization by Kliman et al.40 Identical results with occlusion of subretinal neovascularization with low-intensity light application were later reproduced with rose bengal, another phototoxic substance, in the same model of la-

ser-induced CNV in the monkey choroid.47 Both sensitizers, however, require high drug concentrations, are not safe for human use, due to systemic side effects, and exhibit a limited selectivity for the CNV membrane as a consequence of their hydrophilic character with diffuse leakage from the neovascularization into neighboring tissue and the risk to damage adjacent retinal structures. With the introduction of a second generation photosensitizer without any relevant systemic side effects into clinical evaluation for the treatment of vascularized skin tumors, the concept of a safe and selective photodynamic treatment in clinical CNV disease finally became realistic.46 Benzoporphyrin derivative monoacid A or verteporfin was designed as a lipophilic agent and administered in a liposomal formulation providing the hypothetical advantage of reducing leakage of the sensitizer into extravascular tissue and of targeting receptors for lipoproteins abundantly expressed by proliferating vascular endothelium, further increasing the selectivity of the treatment. Proof of principle for uptake of the sensitizer in neovasculature, successful photothrombosis of neovascularization, and a tight spatial confinement of the photodynamic damage to vascular structures as crucial conditions for maintainance of neural retina in CNV therapy was demonstrated in animal models by Schmidt-Erfurth and Hasan73,76,78,79 and opened the field for further preclinical and clinical evaluation of verteporfin therapy in CNV of AMD patients.

IV. Verteporfin (Visudyne) as a Photosensitizer A. CHARACTERISTICS OF VERTEPORFIN

Verteporfin is a potent second-generation photosensitizing agent derived from porphyrin. It is a chlorin-type molecule which is a chemically stable compound and has been shown to be an efficient generator of singlet oxygen.9,10 It has all the features theoretically necessary for effective PDT in ocular neovascularization. In addition to an absorption maximum in the UVA range, verteporfin offers an absorption peak between 680–695 nm with the theoretical advantages of dye illumination at that range already mentioned (Fig. 3).9,10 Consequently, it can be activated with light from a low-power, nonthermal laser at wavelengths that can penetrate blood, melanin, and fibrotic tissue.65,73,78,85 Richter et al have shown that compared with earlier photosensitizers, such as hematoporphyrin, verteporfin is about four times more efficient in absorbing light at wavelengths that penetrate tissues best (i.e., around 700 nm) and thus provides a much higher cytotoxic effect than hematoporphyrin (10 times more in human ad-

204

Surv Ophthalmol 45 (3) November–December 2000

SCHMIDT-ERFURTH AND HASAN

Thus, for clinical use, verteporfin is administered intravenously as a liposomal formulation. It is provided as a lyophilized powder that is reconstituted in water to provide a stable liquid form, allowing accurate and reliable concentrations to be administered. During preparation and administration, adequate care must be taken to protect verteporfin from intensive light sources. B. LOCALIZATION, BIODISTRIBUTION AND PHARMACOKINETICS WITHIN OCULAR STRUCTURES

Fig. 3. Absorption spectrum and chemical structure of verteporfin.

herent cell lines).65 Other in vitro and in vivo studies have confirmed that verteporfin is a potent photosensitizer.64–68,73,85 Verteporfin is lipophilic and is more readily taken up by malignant or activated cells, compared with normal or resting cells.6,34 Additionally, in vivo studies have shown that verteporfin is rapidly and selectively taken up by neovascular endothelium.69,79 A proposed mechanism for the preferential accumulation of verteporfin in CNV is as follows: In the circulation, liposomally delivered verteporfin binds with LDL to form a complex, which is then taken up into proliferating cells (e.g., neovascular endothelial cells) probably via LDL receptors and endocytosis.6,75 Schmidt-Erfurth et al showed that liposomal verteporfin accumulated to the same degree within neovascularization as did verteporfin directly complexed with LDL.74 The increased expression of LDL receptors and increased uptake of LDL in rapidly proliferating endothelial cells (such as the new choroidal vessels in CNV) may enhance the preferential binding and uptake of verteporfin into neovascular tissues.19,21 However, other mechanisms of cellular uptake may also be involved. Once inside the cell, verteporfin may become bound to intracellular or membrane components. The selectivity of verteporfin for neovasculature has been enhanced by preassociation with liposomes.79 Richter et al reported that the liposomal delivery system enables verteporfin to 1) partition more rapidly into plasma lipoproteins, 2) reach higher levels in tumor tissues and neovasculature, and 3) be a more potent photosensitizer in vivo.67

Several techniques have been used to confirm the localization and selectivity of verteporfin for neovasculature in vivo. Light-induced fluorescence (LIF) of verteporfin has provided information on the location and time course of accumulation of verteporfin. For example, in rabbit eyes with experimentally induced corneal neovascularization, LIF showed peak accumulation of liposomal verteporfin in the neovasculature 60–90 minutes after injection.79 Verteporfin can also be used for angiographic visualization of choroidal vessels and CNV, which demonstrates that the photosensitizer accumulates rapidly in experimental CNV in monkeys.41 The biodistribution over time of verteporfin following intravenous injection influences the timing of irradiation for optimal occlusion of CNV and determines the extent of iatrogenic damage. Haimovici et al showed that verteporfin accumulates rapidly in the established vasculature of the choroid, RPE, and photoreceptors of rabbit eyes.25 However, there was negligible accumulation of verteporfin in structures such as the cornea, lens, and vitreous body. The rapid accumulation in the RPE and photoreceptor outer segments may be due to the high lipid content of these structures or their high expression of LDL receptors.56 Similar to other photosensitizers, verteporfin distributes to other tissues, especially the liver and spleen.64,66 One of the main advantages of verteporfin is that it is cleared rapidly from the body. In mice, verteporfin reaches maximal tissue levels within 3 hours of intravenous injection, followed by a rapid decline within 24 hours.64,66 Similarly, verteporfin was completely eliminated from the corneal neovascular tissue and normal tissue in the rabbit eye within 48 hours.79 Studies in various animal models have found that verteporfin has a serum half-life of 2–5 hours following intravenous injection.64 Moreover, verteporfin is metabolized to a less active form in vivo and is cleared very rapidly, predominantly in the feces and a very small proportion excreted in urine.64,66 Verteporfin appears to be cleared more rapidly than other photosensitizers. For example, verteporfin was no longer detectable in the outer retina of

MECHANISMS OF ACTION OF PDT WITH VERTEPORFIN

rabbit eyes 2 hours after injection, whereas Photofrin was still detectable 48 hours after injection.25 Furthermore, in contrast to other photosensitizers which are cleared in their active forms, verteporfin is cleared in its less active form, thereby reducing the risk of prolonged skin photosensitivity.29 Whether other agents will be useful in the treatment of AMD depends not only on their ability to occlude or alter CNV, but also on their systemic effects, such as an extended photosensitivity of the skin requiring several weeks of protection from light or a long retention of the drug within inner organs, complicating emergency surgery, as reported with SnET2. Because verteporfin undergoes rapid clearance, the risk of systemic photosensitivity is reduced and sensitive structures, such as the eyes and skin, are only photosensitive for a short period.64,68 Irradiation of mice 24 hours after administration of verteporfin caused only a minimal skin reaction. Pharmacokinetic studies in humans show that the plasma half-life of verteporfin is 5–6 hours. Skin photosensitivity, assessed by a UVB-filtered solar simulator, showed short-term return to baseline in a dose-dependent manner. At 6 mg/m2, which is the clinically relevant dose being used in neovascular AMD, no skin photosensitivity was detected at 24 hours, whereas at 18 mg/m2, baseline was reached by 5 days.44 This is in contrast to the prolonged skin photosensitivity lasting up to 6 weeks with HPD or Photofrin.68 Furthermore, in patients with cancer, NPe6 has been shown to persist in the plasma for as long as 6 weeks.37 Preliminary clinical studies of verteporfin therapy for the treatment of skin cancer have shown that verteporfin is well tolerated and safe for human use,37 and it causes minimal side-effects, both locally and systemically.16

V. Mechanisms of Action of Verteporfin Therapy: Experimental Studies Several animal models of ocular neovascularization have been used to investigate the potential of verteporfin therapy for closing abnormal neovascular vessels and to elucidate the mechanisms of action.32,42,50,73,74,79 The effects of verteporfin therapy were assessed using fundus photography and angiography (using fluorescein, indocyanine green [ICG], and verteporfin), and were confirmed by histopathology (light and electron microscopy). Overall, the results of these studies suggest that the main mechanism of action of verteporfin therapy is vascular occlusion due to thrombus formation induced by photodynamic damage to endothelial cells and subsequent platelet activation. The first application of PDT with verteporfin to induce vascular occlusion in ocular pathologies was re-

205

ported in 1994.73,77,78 To study the vaso-occlusive potential of verteporfin, neovascularization was induced in the rabbit cornea to provide an easily accessible neovascular net in a transparent matrix. The uptake of verteporfin, coupled with LDL and the liposomal formulation of verteporfin, was directly measured by monitoring the fluorescence of the photosensitizer accumulated in the neovascular tissue over time. Subsequently, PDT using verteporfin was performed at various doses and time intervals. Angiography of the treated neovascular nets clearly showed complete and selective occlusion at low doses of drug and light without alteration of the adjacent stroma, as shown by light microscopy or electron microscopy.79 In a subsequent study, subretinal implantation of the fragments of a nonpigmented, highly vascularized tumor was used to obtain a dense choroidal neovascular net.73 Verteporfin therapy achieved complete angiographic occlusion of the neovascular compartment by thrombosis of vascular channels, following selective endothelial damage (Fig. 4). No nonspecific damage was found in adjacent structures within the eye, indicating that the treatment appeared to be efficient in eliminating subretinal neovascular proliferation and safe in maintaining physiological tissues (e.g., retinal structures). Occluded vessels remained closed and did not recanalize during follow-up for 12 weeks.73,76 To demonstrate sparing of sensitive overlying retinal structures while occluding subretinal capillary layers, verteporfin PDT was used to occlude the choriocapillary layer in normal rabbit eyes.78 With the use of appropriate parameters, verteporfin therapy selectively induced reproducible and isolated choriocapillary occlusion without alteration of overlying photoreceptors or ganglion cells, as shown by light and electron microscopy. These results gave proof of principle that light-activated verteporfin could be a useful tool to treat selectively subretinal neovascularization, such as CNV in AMD. In continuation of this basic work, verteporfin therapy was subsequently applied to primate eyes with laser-induced neovascularization (Ryan’s model). Verteporfin therapy effectively and selectively prevented fluorescein dye leakage from experimentally induced CNV in monkeys.32,42,50 Fluorescein angiography at 24 hours after PDT showed early hypofluorescence in the treated area of experimental CNV in monkeys.42,50 Moreover, there was persistent hypofluorescence suggesting closure of CNV up to at least 4 weeks.33 Light microscopy showed occluded vessels in the area of CNV in monkeys. Furthermore, electron microscopy showed that the neovasculature was occluded with red blood cells, white blood cells, platelets, and fibrin, and that endothelial cells were

206

Surv Ophthalmol 45 (3) November–December 2000

SCHMIDT-ERFURTH AND HASAN

Fig. 4. Photodynamic therapy (PDT) with verteporfin of a tumor-induced model of choroidal neovascularization. Top left: Ophthalmoscopy before PDT with verteporfin shows a highly vascularized tumor implanted into the suprachoroidal space; rapid growth and surface hemorrhage can be seen. Top right: Two weeks after PDT with verteporfin, the tumor is avascular and has shrunk markedly. Treatment in the tumor model was performed at a high drug dose resulting in occlusion of the entire light-exposed choroid. Center left: Angiography before PDT with verteporfin demonstrates the dense and well-perfused tumor vasculature which makes the entire lesion appear hyperfluorescent. Center right: Six weeks after PDT with verteporfin, the treated area of the tumor is devoid of any neovascular network, and no recanalization is observed during the entire follow-up. Bottom left: Light microscopy of a tumor immediately after PDT with verteporfin shows that the neovasculature is completely thrombosed, while the structures outside the lumen such as the outer vascular wall and tumor cells appear intact. Bottom right: Electron micrograph of a PDT-treated vessel reveals the endothelial membranes with multiple defects and wall blebbing. (Top and center figures are reprinted from Schmidt-Erfurth U et al,77 and bottom right figure is reprinted from Schmidt-Erfurth U et al,76 with permission from Ophthalmology; bottom left figure is reprinted from Schmidt-Erfurth U et al73 with permission from Experimental Eye Research.)

MECHANISMS OF ACTION OF PDT WITH VERTEPORFIN

damaged.32,42,50 It should be noted, however, that Ryan’s model has limitations with respect to CNV in AMD. It is basically a wound-healing response and its pathogenic mechanisms differ significantly from those of the neovascular form of AMD. Also, the angiogenic stimulus is a transient, and not chronic, process, which might explain the complete regression of CNV as the experimentally induced stimulus phase subsides. In both the rabbit and monkey models, the level of damage to the retina and choroid was dependent on the verteporfin dose, the light dose, and the time between administration of verteporfin and light. Damage was minimal when irradiation was performed 20 minutes after verteporfin injection.42,78 Furthermore, the RPE cells damaged by light-activated verteporfin showed subsequent recovery within 3–4 weeks, although with a loss of pigmentation and an arrangement in multiple layers.33,45 Reinke et al have recently reported that verteporfin therapy in monkeys could be repeated (three consecutive treatments over the center of the macula, i.e., the fovea) with only minimal damage to choriocapillaris and mild damage to the RPE and outer photoreceptors.63 Although hypofluorescence following PDT was consistent with choriocapillary occlusion, choroidal perfusion changes were only transient and choroidal vessels reopened during follow-up. Overall, from preclinical studies, it can be concluded that verteporfin therapy causes minimal damage to normal intraocular tissues, such as retinal vessels, the underlying choroid, and the overlying neurosensory retina, but may cause reversible damage to the RPE and outer retina.32,42,45,50,78 The damage to the RPE and retina is probably a direct photochemical effect due to early localization of verteporfin in these tissues.25 Nevertheless, the eventual RPE damage induced by verteporfin therapy is significantly less than that caused by thermal lasers. The light of these lasers is primarily absorbed by melanin directly, and leads to heat generation and thermal nonselective necrosis of adjacent CNV. Histopathologic studies have shown that laser photocoagulation of human eyes causes full-thickness damage to the retina and choroid, leading to a scar and visual scotoma.92 In comparison, verteporfin therapy causes minimal damage to the retina and choroid in animals,42,78 although comparable histopathological data from human eyes are not available at this point. Whether damage to these structures is limited with a single application or a low number of treatments, ongoing clinical trials will show if multiple PDT applications adversely affect retinal function. Before clinical studies in patients with AMD could be initiated, the optimal dose of verteporfin, light dose, and timing of irradiation were determined in

207

the primate model of experimentally induced CNV.32,42,50 In these studies the optimal dose of verteporfin was 0.375 mg/kg (approximately 6 mg/m2), and the optimal time for irradiation using light at a wavelength of 692 nm was 20–50 minutes after commencing intravenous injection of verteporfin. Irradiation performed too early (i.e., within 5 minutes of verteporfin injection), caused some damage to retinal and larger choroidal vessels.42,50 The light parameters used were a spot size of 1,250 ␮m and a light intensity of 600 mw/cm2, delivered over 4 minutes to give a light dose of 150 J/cm2. The high light dose used possibly explained why the optimal irradiation time was ⱖ 20 minutes. At 50 J/cm2, light can be applied earlier and, therefore, may prevent the uptake of verteporfin into the RPE and outer retina. These dosimetry studies indicated that one clinical advantage of verteporfin over some other photosensitizers is that the photosensitizer administration and light irradiation can be performed in a single treatment session, whereas an interval of several hours is required with ATX. The PDT effect was less intensive if irradiation was performed at 10 minutes instead of 15 minutes after verteporfin infusion, although verteporfin plasma levels were higher at the earlier time point (i.e., 10 minutes). This result suggests that uptake by the endothelial cells was not optimal at 10 minutes after verteporfin infusion. The optimal time for irradiation therefore depends on endothelial cell uptake and, indeed, may vary between species.82

VI. Clinical Experience With Verteporfin Therapy in Neovascular AMD The aim of verteporfin therapy in patients with CNV secondary to AMD is to occlude or destroy the CNV selectively while maintaining perfusion in the deeper, larger choroidal vessels and overlying retinal tissue and retinal vessels. This would ensure that the health of the choroid and the overlying retina in the treated area is maintained. A multicenter phase I and II nonrandomized clinical trial was initiated in patients with subfoveal CNV who were not eligible for laser photocoagulation.49,81,82 This study evaluated the short-term safety and maximum tolerated dose of verteporfin, established the optimal dosimetry, and determined if fluorescein leakage from subfoveal CNV could be stopped by verteporfin therapy without immediate permanent visual loss, as it often occurs with subfoveal laser photocoagulation. The ability to do so would indicate that the treatment could at least control further growth of the lesions and the accompanying destruction of the retina by the exudation and the formation of fibrovascular tissue. Patients were assessed

208

Surv Ophthalmol 45 (3) November–December 2000

using ophthalmoscopic examination, fluorescein angiography, and best-corrected visual acuity. Verteporfin angiography was performed in some patients and showed accumulation of the photosensitizer in the CNV. The dose-ranging study showed that the minimally effective light dose was greater than 25 J/cm2 and that the maximally tolerated light dose was less than 150 J/cm2.49,81 At a light dose of 150 J/cm2, nonperfusion of neurosensory retinal vessels was observed.49 In these studies, the effective single treatment providing the best visual outcome consisted of a verteporfin dose of 6 mg/m2 infused intravenously over 10 minutes, and a light dose of 50 J/cm2 delivered over a period of approximately 83 seconds. The optimal time for irradiation (at 690 nm, light intensity 600 mW/cm2) was 15 minutes after the start of verteporfin infusion. This treatment procedure is painless, does not require general anesthesia and is being used in several ongoing phase III randomized clinical trials.1,51 A. EFFECTS ON FLUORESCEIN LEAKAGE FROM CNV AND VISUAL ACUITY

The same clinical study showed that a single treatment with light-activated verteporfin was well tolerated and caused an immediate treatment effect. There was a complete absence of fluorescein leakage from CNV in most lesions 1 week after treatment (Fig. 5).49,81 The effect was temporary, however, with some leakage reappearing by 4 weeks and, by 12 weeks, leakage from CNV had reappeared in most treated eyes (70%–80%). Nevertheless, the area of leakage was less than that seen before treatment. These studies demonstrated, therefore, that retreatment of the area showing recurrent leakage was necessary for long-term beneficial effects on CNV (Fig. 5).49,82 In addition to stopping fluorescein leakage, verteporfin therapy did not damage mean visual acuity in the short term. Unlike laser photocoagulation, there was no immediate loss of vision at the site of verteporfin activation in most cases. The overlying retina was preserved and there was no clinical damage to the RPE visible on post-treatment photography (Fig. 5).81 Furthermore, there was no significant loss of vision during the 12 weeks after treatment. At this time, 20% of patients had gained 2 or more lines of vision, 56% had gained 1 line of vision, and 24% had lost 2 or more lines of vision.49 In addition, in patients receiving the dose of verteporfin judged optimal for phase III trials, 36% had improvements of 2 or more lines of vision at the 12-week follow-up examination. Functional testing of the central visual field, as determined with microperimetry, showed that the pre-existing central defects became smaller

SCHMIDT-ERFURTH AND HASAN

in 80% and remained stable in 20% of the treated patients if treatment parameters were adequate.72 This suggested that verteporfin therapy can lead to visual function improvement when optimal treatment parameters are used. Schmidt-Erfurth et al also found that retreatment at intervals of up to 8 weeks was well tolerated, and resulted in a reproducible stabilization of CNV leakage that had recurred.82 In addition, there was maintenance or improvement of visual acuity in many cases. At follow-up examinations 16 and 20 weeks after an initial treatment with verteporfin, the area of classic CNV was no greater than that seen before treatment in 40% of patients, and there was an increase of 2 or more lines of visual acuity in 30% of patients.82 Unlike experimental studies in animal models, histopathologic evidence confirming the thrombotic occlusion of CNV after verteporfin therapy in patients with AMD is not available. Furthermore, the cause of the characteristic choroidal hypofluorescence following PDT is still unclear. It is conceivable that primarily no immediate occlusion of the CNV is induced by PDT; however, endothelial damage leads to a breakdown of the vascular barrier. Consecutively enhanced leakage occurs, which would explain the increase in metamorphopsia noted by the patient and the increase in edema seen by the ophthalmologist within the first days after treatment. If PDT first enhances extravasation of fluid, the interstitial pressure and edema within the neighboring choriocapillary layer might increase the flow resistance in choriocapillaris and the CNV complex and decrease the perfusion of both vascular beds. Massive extravasation is seen in angiograms performed within 24 hours after PDT application (our unpublished data). Occlusion of the CNV might then be a secondary consequence of transient changes in perfusion dynamics. Early hypofluorescence could as well result from masking by a protein-rich exudate originating from vascular or RPE barrier breakdown within the treated area. Several weeks after PDT, hypofluorescence disappears consistent with resolution of exudate, an improvement of choriocapillary perfusion, and the restoration of vascular barrier functions. Accordingly, the neovascular channels never disappear completely from the angiogram, but show involution and absence of leakage. ICG angiography (ICG-A) of the choroidal vasculature has been performed in a subgroup of patients in the phase I/II trial to give further insight into the mechanism of action of verteporfin therapy.72,80 Using ICG-A, disappearance of large parts of the CNV was demonstrated. However, persistence of feeder vessels within the treated area was found in more than 50% of eyes.72,80 This may explain the recur-

209

MECHANISMS OF ACTION OF PDT WITH VERTEPORFIN

Fig. 5. Photodynamic therapy with verteporfin of choroidal neovascularization (CNV) in AMD. Top left: Ophthalmoscopy before PDT with verteporfin shows neovascular AMD with typical characteristics, such as retinal thickening, subretinal hemorrhage and fluid, and lipid exudates. Top right: Four weeks after PDT with verteporfin, fundus appearance shows normal transparency of the treated macular retina, with less subretinal blood and fluid than before treatment and decreased lipid exudates. Bottom left: Angiography before PDT with verteporfin shows classic CNV located in the center of the macula during the early phase. Bottom right: Intensive leakage from the CNV lesion is seen in the late phase. (Fig. 5 is continued on next page.)

rence of CNV seen by fluorescein angiography at 4 weeks after treatment. By ICG-A, CNV remnants did not exhibit any significant progression after verteporfin therapy, but vessels became progressively atrophic and leakage subsided. The role of light-activated verteporfin is now to prevent regrowth of a neovascular net with leakage from the persistent new vessels or ideally to induce progressive involution of the neovascular complex by using repeated applications for as long as the vessels or angiogenic stimuli persist. Furthermore, ICG-A substantiated a complete recovery of the surrounding normal choriocapillaries in the majority of treated eyes at follow-up examinations 12 weeks after PDT.72,80 In con-

cordance with the findings of the phase I/II trial with maintenance of visual acuity in most patients 12 weeks after therapy,49,81,82 current trial regimens are based on a 3-month retreatment interval, as long as leakage is still delineated angiographically (i.e., during the growth phase of the CNV complex). B. ONGOING STUDIES

Three multicenter, randomized, placebo-controlled, masked studies are ongoing to determine the long-term safety and efficacy of verteporfin therapy in patients with CNV.1,51 The treatment of AMD with PDT (TAP) investigation in patients with CNV secondary to AMD should clarify whether this treat-

210

Surv Ophthalmol 45 (3) November–December 2000

SCHMIDT-ERFURTH AND HASAN

Fig. 5. (continued).Top left: One week following PDT with verteporfin, classic CNV is absent in the early phase. Top right: Faint leakage originating from the upper portion of the treatment site is seen in late phase. Bottom left: Four weeks after PDT with verteporfin, early-phase angiography shows a small recurrence of classic CNV, which can be delineated in the temporal portion of the treated lesion. Bottom right: Late-phase angiography shows markedly reduced CNV leakage compared with pretreatment. (Reprinted from Schmidt-Erfurth U et al,81 with permission from Graefes Archiv fur klinische und experimentelle Ophthalmologie).

ment is safe and effectively reduces long-term vision loss compared with the natural course of the disease.1 The Verteporfin In Photodynamic Therapy (VIP) trial should establish whether verteporfin therapy is effective in CNV at an early stage of the disease process and in a wider range of patients, including those with occult-only CNV or CNV secondary to pathologic myopia.51 The first-year results of the TAP investigation showed that verteporfin therapy can preserve vision in a significant number of patients. In the primary efficacy analysis at 12 months, a total of 61.4% of patients receiving verteporfin therapy lost less than 3 lines of vision, compared with 45.9% of patients receiving placebo treat-

ment.1 Patients receiving verteporfin and light were also more than twice as likely to have improved vision (ⱖ1 line gained). In subgroup analyses, the visual acuity benefit (less than 15 letters lost) was clearly demonstrated when the area of classic CNV occupied 50% or more of the area of the entire lesion, defined as predominantly classic lesions. Sixtyseven percent of patients with predominantly classic CNV demonstrated stabilization of visual acuity following verteporfin treatment, compared to only 39% of placebo-treated patients after 1 year. This functional benefit was even more pronounced when occult CNV components were completely absent— identifying the photodynamic effect on the classic

211

MECHANISMS OF ACTION OF PDT WITH VERTEPORFIN

component as the most important factor in PDTrelated benefit. In contrast, no significant difference in visual acuity was noted when the area of classic CNV was less than 50% of the area of the entire lesion. Investigators therefore recommended to restrict the use of verteporfin therapy to eyes with predominantly classic neovascular lesions in AMD.

VII. Conclusion PDT is an established therapy for certain solid tumors and is evolving rapidly as a treatment modality for a variety of other conditions that are characterized by neovascularization. PDT with verteporfin is an exciting and promising new treatment option for patients with CNV secondary to AMD in whom a safe and selective treatment may be critical to prevent blindness. The main advantages of verteporfin therapy are its photosensitizing potency, absorption peak around 690 nm enabling better tissue penetration, rapid and selective accumulation by endothelial cells in neovasculature, and rapid clearance from the body. Experimental studies suggest that the main mechanism of action of verteporfin therapy is intravascular damage leading to thrombus formation and selective vascular occlusion. Direct endothelial cell damage probably results from the production of singlet oxygen and other free radicals with alteration of cell membranes. The clinical data on the mechanism of action of verteporfin therapy so far appear to be consistent with the data from experimental studies with animal models.73,78,79 Importantly, verteporfin therapy results in selective damage of ocular neovasculature and causes minimal damage to the surrounding physiological structures such as the neurosensory retina. Therefore, verteporfin therapy is a potentially selective treatment to reduce the risk of vision loss from predominantly classic CNV in patients with AMD.

Method of Literature Search MEDLINE was used with the search words photochemotherapy, choroidal neovascularization, age-related macular degeneration, photocoagulation, ocular, choroidal, neovascularization, photosensitizer, benzoporphyrin derivative, tin ethyl etiopurpurin, lutetium texaphyrin, mono-l-aspartyl chlorin e6, and ATX-S10 covering the years 1975 to 1999. In addition, articles related to the topic collected during our own work in the field were used. For recent data presented at meetings only abstract books of the specific meetings were used. Only information from peer-reviewed journals was included. Articles in English and German were used by abstract and text, other languages were omitted.

References 1. _____: Photodynamic therapy of subfoveal choroidal neovascularization in age- related macular degeneration with verteporfin: one-year results of 2 randomized clinical trials—TAP report. Treatment of age-related macular degeneration with photodynamic therapy (TAP Study group). Arch Ophthalmol 117:1329–45, 1999 2. _____: Macular Photocoagulation Study Group: Laser photocoagulation of subfoveal neovascular lesions of age-related macular degeneration. Updated findings from two clinical trials. [see comments]. Arch Ophthalmol 111:1200–9, 1993 3. _____: The Canadian Ophthalmology Study Group: Argon green vs krypton red laser photocoagulation of extrafoveal choroidal neovascular lesions. One-year results in age-related macular degeneration. [see comments]. Arch Ophthalmol 111:181–5, 1993 4. _____: Macular Photocoagulation Study Group: Subfoveal neovascular lesions in age-related macular degeneration. Guidelines for evaluation and treatment in the macular photocoagulation study. [see comments]. Arch Ophthalmol 109:1242–57, 1991 5. _____: Macular Photocoagulation Study Group: Persistent and recurrent neovascularization after krypton laser photocoagulation for neovascular lesions of age-related macular degeneration. Arch Ophthalmol 108:825–31, 1990 6. Allison BA, Pritchard PH, Levy JG: Evidence for low-density lipoprotein receptor-mediated uptake of benzoporphyrin derivative [published erratum appears in Br J Cancer 1995 Jan;71(1):214]. Br J Cancer 69:833–9, 1994 7. Arbour JD, Connolly EJ, Graham K, et al: Photodynamic therapy of experimental choroidal neovascularization in a monkey model using intravenous infusion of lutetium texaphyrin. Invest Ophthalmol Vis Sci 40:S401, 1999 8. Arnold J, Algan M, Soubrane G, et al: Indirect scatter laser photocoagulation to subfoveal choroidal neovascularization in age-related macular degeneration. Graefes Arch Clin Exp Ophthalmol 235:208–16, 1997 9. Aveline B, Hasan T, Redmond RW: Photophysical and photosensitizing properties of benzoporphyrin derivative monoacid ring A (BPD-MA). Photochem Photobiol 59:328– 35, 1994 10. Aveline BM, Hasan T, Redmond RW: The effects of aggregation, protein binding and cellular incorporation on the photophysical properties of benzoporphyrin derivative monoacid ring A (BPDMA). J Photochem Photobiol B 30:161–9, 1995 11. Bown SG: Science, medicine, and the future. New techniques in laser therapy. BMJ 316:754–7, 1998 12. Boyle RW, Dolphin D: Structure and biodistribution relationships of photodynamic sensitizers. Photochem Photobiol 64:469–85, 1996 13. Bressler NM, Bressler SB, Fine SL: Age-related macular degeneration. Surv Ophthalmol 32:375–413, 1988 14. Bressler SB, Bressler NM, Fine SL, et al: Natural course of choroidal neovascular membranes within the foveal avascular zone in senile macular degeneration. Am J Ophthalmol 93:157–63, 1982 15. Buettner GR, Need MJ: Hydrogen peroxide and hydroxyl free radical production by hematoporphyrin derivative, ascorbate and light. Cancer Lett 25:297–304, 1985 16. Dougherty TJ, Gomer CJ, Henderson BW, et al: Photodynamic therapy. J Natl Cancer Inst 90:889–905, 1998 17. Ferris FL 3d, Fine SL, Hyman L: Age-related macular degeneration and blindness due to neovascular maculopathy. Arch Ophthalmol 102:1640–2, 1984 18. Fingar VH: Vascular effects of photodynamic therapy. J Clin Laser Med Surg 14:323–8, 1996 19. Fogelman AM, Berliner JA, Van Lenten BJ, et al: Lipoprotein receptors and endothelial cells. Semin Thromb Hemost 14:206–9, 1988 20. Freund KB, Yannuzzi LA, Sorenson JA: Age-related macular degeneration and choroidal neovascularization. Am J Ophthalmol 115:786–91, 1993

212

Surv Ophthalmol 45 (3) November–December 2000

21. Gaffney J, West D, Arnold F, et al: Differences in the uptake of modified low density lipoproteins by tissue cultured endothelial cells. J Cell Sci 79:317–25, 1985 22. Gomer CJ: Preclinical examination of first and second generation photosensitizers used in photodynamic therapy. Photochem Photobiol 54:1093–107, 1991 23. Gomer CJ: Photodynamic therapy in the treatment of malignancies. Semin Hematol 26:27–34, 1989 24. Graham KB, Arbour JD, Connolly EJ, et al: Digital angiography using lutetium texaphyrin in a monkey model of choroidal neovascularization. Invest Ophthalmol Vis Sci 40:S402, 1999 25. Haimovici R, Kramer M, Miller JW, et al: Localization of lipoprotein-delivered benzoporphyrin derivative in the rabbit eye. Curr Eye Res 16:83–90, 1997 26. Hasan T, Parrish JA: Photodynamic therapy of cancer, in Holland JF (ed): Cancer Medicine. Baltimore, Williams & Wilkins, 1997, pp 739–51 27. He J, Larkin HE, Li YS, et al: The synthesis, photophysical and photobiological properties and in vitro structure-activity relationships of a set of silicon phthalocyanine PDT photosensitizers. Photochem Photobiol 65:581–6, 1997 28. Henderson BW, Bellnier DA: Tissue localization of photosensitizers and the mechanism of photodynamic tissue destruction. Ciba Found Symp 146:112–25, [discussion 125– 30] 1989 29. Henderson BW, Dougherty TJ: How does photodynamic therapy work? Photochem Photobiol 55:145–57, 1992 30. Hilf R, Smail DB, Murant RS, et al: Hematoporphyrin derivative-induced photosensitivity of mitochondrial succinate dehydrogenase and selected cytosolic enzymes of R3230AC mammary adenocarcinomas of rats. Cancer Res 44:1483–8, 1984 31. Hunt DWC, Chan AH, Levy JG: Immunological aspects of photodynamic therapy. Photodynamics News 1:2–4, 1998 32. Husain D, Miller JW, Michaud N, et al: Intravenous infusion of liposomal benzoporphyrin derivative for photodynamic therapy of experimental choroidal neovascularization. Arch Ophthalmol 114:978–85, 1996 33. Husain D, Miller JW, Michaud N, et al: Long-term effect of photodynamic therapy (PDT) using liposomal benzoporphyrin derivative (BPD) on experimental choroidal neovascularization (CNV) and normal retina and choroid. Invest Ophthalmol Vis Sci 37:S223, 1996 34. Jamieson CH, McDonald WN, Levy JG: Preferential uptake of benzoporphyrin derivative by leukemic versus normal cells. Leuk Res 14:209–19, 1990 35. Jori G: Factors controlling the selectivity and efficiency of tumour damage in photodynamic therapy. Lasers Med Sci 5: 115–20, 1990 36. Jori G, Reddi E: The role of lipoproteins in the delivery of tumour-targeting photosensitizers. Int J Biochem 25:1369–75, 1993 37. Kessel D: Pharmacokinetics of N-aspartyl chlorin e6 in cancer patients. J Photochem Photobiol B 39:81–3, 1997 38. Kessel D: Sites of photosensitization by derivatives of hematoporphyrin. Photochem Photobiol 44:489–93, 1986 39. Kliman GH, Puliafito CA, Stern D, et al: Phthalocyanine photodynamic therapy: new strategy for closure of choroidal neovascularization. Lasers Surg Med 15:2–10, 1994 40. Kliman GH, Stern D, Gregory WA, et al: Angiography and photodynamic therapy of experimental choroidal neovascularization using phthalocyanin dye. Invest Ophthalmol Vis Sci 30:S371, 1989 41. Kramer M, Kenney AG, Delori F, et al: Imaging of experimental choroidal neovascularization (CNV) using liposomal benzoporphyrin derivative mono-acid (BPD-MA) angiography. Invest Ophthalmol Vis Sci 36:S236, 1995 42. Kramer M, Miller JW, Michaud N, et al: Liposomal benzoporphyrin derivative verteporfin photodynamic therapy. Selective treatment of choroidal neovascularization in monkeys. Ophthalmology 103:427–38, 1996 43. Levy JG: Photodynamic therapy. Trends Biotechnol 13:14–8, 1995

SCHMIDT-ERFURTH AND HASAN

44. Levy JG, Chan A, Strong HA: The clinical status of benzoporphyrin derivative. SPIE 2625:86–95, 1996 45. Lin SC, Lin CP, Feld JR, et al: The photodynamic occlusion of choroidal vessels using benzoporphyrin derivative. Curr Eye Res 13:513–22, 1994 46. Lui H, Kollias N, Wimberly J, et al: Photodynamic therapy of malignant skin tumors with BPD-MA. Lasers Derm Plast Surg 1876:37, 1993 47. Miller H, Miller B: Photodynamic therapy of subretinal neovascularization in the monkey eye. Arch Ophthalmol 111:855–60, 1993 48. Miller H, Miller B: Photodynamic therapy of subretinal neovascularization in the monkey eye. Arch Ophthalmol 111:855–60, 1993 49. Miller JW, Schmidt-Erfurth U, Sickenberg M, et al: Photodynamic therapy with verteporfin for choroidal neovascularization caused by age-related macular degeneration: results of a single treatment in a phase 1 and 2 study [comment] [see comments] [published erratum appears in Arch Ophthalmol 2000 Apr;118(4)]. Arch Ophthalmol 117: 1161–73, 1999 50. Miller JW, Walsh AW, Kramer M, et al: Photodynamic therapy of experimental choroidal neovascularization using lipoprotein-delivered benzoporphyrin. Arch Ophthalmol 113: 810–8, 1995 51. Mones J, VIP Study Group: Photodynamic therapy (PDT) with verteporfin of subfoveal choroidal neovascularization in age-related macular degeneration: study design and baseline characteristics in the VIP randomized clinical trial. Invest Ophthalmol Vis Sci 40:S321, 1999 52. Mori K, Katagiri T, Sano A, et al: Photodynamic therapy for choroidal neovascularization with a hydrophilic sensitizer: NPe6. Invest Ophthalmol Vis Sci 38:S93, 1997 53. Mori K, Yoneya S, Ohta M, et al: Angiographic and histologic effects of fundus photodynamic therapy with a hydrophilic sensitizer (mono-L-aspartyl chlorin e6). Ophthalmology 106:1384–91, 1999 54. Moshfeghi DM, Peyman GA, Khoobehi B, Crean DH: Photodynamic occlusion of retinal vessels using tin ethyl etiopurpurin (SnET2). an efficacy study. Invest Ophthalmol Vis Sci 36:S115, 1995 55. Nishiwaki H, Danna S, Grebe R, Zeimer R: Laser targeted photodynamic therapy occludes experimental choroidal neovascularization without visible damage to RPE and choriocapillaris. Invest Ophthalmol Vis Sci 39:S276, 1998 56. Noske UM, Schmidt-Erfurth U, Meyer C, Diddens H: [Lipid metabolism in retinal pigment epithelium. Possible significance of lipoprotein receptors]. Ophthalmologe 95:814–9, 1998 57. Obana A, Gohto Y, Kanai M: Photodynamic occlusion of the choroidal neovascularization of primate eyes using a new hydrophilic photosensitizer, ATX-S10. Invest Ophthalmol Vis Sci 39:S389, 1998 58. Obana A, Gohto Y, Kaneda K, et al: Selective occlusion of choroidal neovascularization by photodynamic therapy with a water-soluble photosensitizer, ATX-S10. Lasers Surg Med 24:209–22, 1999 59. Obochi MO, Ratkay LG, Levy JG: Prolonged skin allograft survival after photodynamic therapy associated with modification of donor skin antigenicity. Transplantation 63:810–7, 1997 60. Ochsner M: Photophysical and photobiological processes in the photodynamic therapy of tumours. J Photochem Photobiol B 39:1–18, 1997 61. Organisciak DT, Darrow RM, Barsalou L, et al: Light history and age-related changes in retinal light damage. Invest Ophthalmol Vis Sci 39:1107–16, 1998 62. Peyman GA, Moshfeghi DM, Moshfeghi A, et al: Photodynamic therapy for choriocapillaris using tin ethyl etiopurpurin (SnET2). Ophthalmic Surg Lasers 28:409–17, 1997 63. Reinke MH, Canakis C, Husain D, et al: Verteporfin photodynamic therapy retreatment of normal retina and choroid in the cynomolgus monkey. Ophthalmology 106:1915–23, 1999 64. Richter AM, Cerruti-Sola S, Sternberg ED, et al: Biodistribu-

213

MECHANISMS OF ACTION OF PDT WITH VERTEPORFIN

65. 66.

67.

68.

69. 70. 71. 72. 73.

74. 75.

76. 77. 78. 79.

80.

81.

82.

83. 84.

tion of tritiated benzoporphyrin derivative (3H-BPD-MA), a new potent photosensitizer, in normal and tumor-bearing mice. J Photochem Photobiol B 5:231–44, 1990 Richter AM, Kelly B, Chow J, et al: Preliminary studies on a more effective phototoxic agent than hematoporphyrin. J Natl Cancer Inst 79:1327–32, 1987 Richter AM, Waterfield E, Jain AK, et al: Photosensitising potency of structural analogues of benzoporphyrin derivative (BPD) in a mouse tumour model. Br J Cancer 63:87–93, 1991 Richter AM, Waterfield E, Jain AK, et al: Liposomal delivery of a photosensitizer, benzoporphyrin derivative monoacid ring A (BPD), to tumor tissue in a mouse tumor model. Photochem Photobiol 57:1000–6, 1993 Richter AM, Yip S, Waterfield E, et al: Mouse skin photosensitization with benzoporphyrin derivatives and Photofrin: macroscopic and microscopic evaluation. Photochem Photobiol 53:281–6, 1991 Roberts WG, Hasan T: Role of neovasculature and vascular permeability on the tumor retention of photodynamic agents. Cancer Res 52:924–30, 1992 Rodgers MA: On the problems involved in detecting luminescence from singlet oxygen in biological specimens [news]. J Photochem Photobiol B 1: 371–3, 1988 Rowe PM: Photodynamic therapy begins to shine [news]. Lancet 351:1496, 1998 Schmidt-Erfurth U: Indocyanine green angiography and retinal sensitivity after photodynamic therapy of subfoveal choroidal neovascularization. Semin Ophthalmol 14:35–44, 1999 Schmidt-Erfurth U, Bauman W, Gragoudas E, et al: Photodynamic therapy of experimental choroidal melanoma using lipoprotein-delivered benzoporphyrin. Ophthalmology 101: 89–99, 1994 Schmidt-Erfurth U, Birngruber R, Hasan T: Photodynamic therapy in ocular vascular disease. Laser Physics 8:191–8, 1998 Schmidt-Erfurth U, Diddens H, Birngruber R, Hasan T: Photodynamic targeting of human retinoblastoma cells using covalent low- density lipoprotein conjugates. Br J Cancer 75: 54–61, 1997 Schmidt-Erfurth U, Flotte TJ, Gragoudas ES, et al: Benzoporphyrin-lipoprotein-mediated photodestruction of intraocular tumors. Exp Eye Res 62:1–10, 1996 Schmidt-Erfurth U, Hasan T, Flotte T: Photodynamic therapy of experimental, intraocular tumors with benzoporphyrin-lipoprotein. Ophthalmology 91:348–56, 1994 Schmidt-Erfurth U, Hasan T, Gragoudas E, et al: Vascular targeting in photodynamic occlusion of subretinal vessels. Ophthalmology 101:1953–61, 1994 Schmidt-Erfurth U, Hasan T, Schomacker K, et al: In vivo uptake of liposomal benzoporphyrin derivative and photothrombosis in experimental corneal neovascularization. Lasers Surg Med 17:178–88, 1995 Schmidt-Erfurth U, Michels S, Hager A, et al: ICG-angiographic analysis of the photodynamic mechanism in treatment of choroidal neovascularization. Invest Ophthalmol Vis Sci 39:S242, 1998 Schmidt-Erfurth U, Miller J, Sickenberg M, et al: Photodynamic therapy of subfoveal choroidal neovascularization: clinical and angiographic examples. Graefes Arch Clin Exp Ophthalmol 236:365–74, 1998 Schmidt-Erfurth U, Miller JW, Sickenberg M, et al: Photodynamic therapy with verteporfin for choroidal neovascularization caused by age-related macular degeneration: results of retreatments in a phase 1 and 2 study [comment] [see comments] [published erratum appears in Arch Ophthalmol 2000 Apr;118(4):488]. Arch Ophthalmol 117:1177–87, 1999 Schuitmaker JJ, van Best JA, van Delft JL, et al: Bacteriochlorin a, a new photosensitizer in photodynamic therapy. In vivo results. Invest Ophthalmol Vis Sci 31:1444–50, 1990 Sery TW, Shields JA, Augsburger JJ, Shah HG: Photodynamic therapy of human ocular cancer. Ophthalmic Surg 18:413– 8, 1987

85. Soncin M, Busetti A, Biolo R, et al: Photoinactivation of amelanotic and melanotic melanoma cells sensitized by axially substituted Si-naphthalocyanines. J Photochem Photobiol B 42:202–10, 1998 86. Specht KG, Rodgers MA: Depolarization of mouse myeloma cell membranes during photodynamic action. Photochem Photobiol 51:319–24, 1990 87. Svaasand LO, Gomer CJ, Morinelli E: On the physical rationale of photodynamic therapy. Future Directions and Applications in Pharmacodynamic Therapy. IS6 SPIE Institute Series 233–248, 1990 88. Tao J, Sanghera JS, Pelech SL, et al: Stimulation of stressactivated protein kinase and p38 HOG1 kinase in murine keratinocytes following photodynamic therapy with benzoporphyrin derivative. J Biol Chem 271:27107–15, 1996 89. Thomas EL, Langhofer M: Closure of experimental subretinal neovascular vessels with dihematoporphyrin ether augmented argon green laser photocoagulation. Photochem Photobiol 46:881–6, 1987 90. Thomas EL, Rosen R, Murphy R, et al: Visual acuity stabilizes after a single treatment with Snet2-photodynamic therapy in patients with subfoveal choroidal neovascularization. Invest Ophthalmol Vis Sci 40:S401, 1999 91. Thomas EL, Rosen R, Murphy R, et al: Purlytin (SnET2)photodynamic therapy produces closure of subfoveal choroidal neovascularization in humans. Invest Ophthalmol Vis Sci 39:S242, 1998 92. Tso MO, Wallow IH, Elgin S: Experimental photocoagulation of the human retina. I. Correlation of physical, clinical, and pathologic data. Arch Ophthalmol 95:1035–40, 1977 93. US Securities and Exchange Commission. Form 10-K. Pharmacyclics Inc. 1999 94. Wilson BC: Light sources for photodynamic therapy. Photodynamics News 1:6–8, 1998 95. Wilson BC: Photodynamic therapy: light delivery and dosage for second-generation photosensitizers. Ciba Found Symp 146:60–73; discussion 73-7, 1989 96. Woodburn KW, Fan Q, Kessel D, et al: Phototherapy of cancer and atheromatous plaque with texaphyrins. Clin Laser Med Surg 14:343–8, 1996 97. Young SW, Woodburn KW, Wright M, et al: Lutetium texaphyrin (PCI-0123): a near-infrared, water-soluble photosensitizer. Photochem Photobiol 63:892–7, 1996

Outline I. Age-related macular degeneration II. Photodynamic therapy A. Light sources and delivery B. Characteristics of photosensitizers C. Types of photosensitizers under ophthalmological investigation III. Application of photodynamic therapy in macular degeneration IV. Verteporfin (Visudyne) as a photosensitizer A. Characteristics of verteporfin B. Localization, biodistribution and pharmacokinetics within ocular structures V. Mechanisms of action of verteporfin therapy: experimental studies VI. Clinical experience with verteporfin therapy in neovascular AMD A. Effects on fluorescein leakage from CNV and visual acuity B. Ongoing studies VII. Conclusion

The Wellman Laboratories of Photomedicine, Massachusetts General Hospital, Harvard Medical School, Boston are an owner of a patent covering the use of verteporfin. Should the Massachu-

214

Surv Ophthalmol 45 (3) November–December 2000

setts General Hospital receive royalties or other financial remuneration, Dr. T. Hasan and Dr. U. Schmidt-Erfurth would receive a share or same in accordance with the Wellman Laboratories of Photomedicine’s institutional patent policy and procedures which include royalty-sharing provisions. Support for Dr. Hasan by the National Institutes of Health and the Department of Defense, FEL program, during the writing of this review, is gratefully acknowledged. The authors also thank Dr. Julia Levy, QLT Phototherapeutics Inc., Vancouver, Canada,

SCHMIDT-ERFURTH AND HASAN

and Dr. Neil Bressler, The Wilmer Institute, Baltimore, MD, USA, for their critiques and additions to this review. QLT Phototherapeutics, Vancouver, supplied the photosensitizer for the preclinical and clinical studies of the authors and the TAP study group. Cibavision, Switzerland, made editorial services available in the preparation of the manuscript. Reprint address: Medical Information, CIBA Vision Ophthalmics, 11460 Johns Creek, Parkway, Duluth, Atlanta, GA 30097. Email: [email protected]