Evaluation of verteporfin pharmakokinetics--redefining the need of photosensitizers in ophthalmology

INTRODUCTION

The benzoporphyrine derivative verteporfin has lost its importance to the treatment of the most frequent neovascular eye diseases. Nevertheless, it is still mandatory to define the remaining applications, role, and potential of verteporfin in ocular photodynamic therapy (PDT), including the dosages of administration, effectiveness, and safety profile.

AREAS COVERED

Although verteporfin PDT has forfeited the first-line status and value of treating subfoveal choroidal neovascularization (CNV) due to age-related macular degeneration or pathologic myopia, the treatment remains the standard of care for choroidal haemangioma and polypoidal choroidal vasculopathy. PDT is effective in less pigmented choroidal melanoma as well as in retinal vascular proliferations and retinal angioma. Verteporfin was granted the orphan drug designation for the treatment of chronic or recurrent central serous chorioretinopathy (CSC).

EXPERT OPINION

Evidence-based data regarding optimized parameters (low fluence, reduced dose, fractionated irradiation) adapted to the treated diseases (target structure, dosimetry, blood supply) are scarce. Prospective and large clinical trials are missing, although the scientific community agrees on the fact that the standard treatment protocol does not necessarily provide the optimal efficacy to the specific disease or individual patient. Within the reviewed indications, the adverse effect profile is favorable compared with other therapies.

NEOVASCULAR AGE-RELATED MACULAR DEGENERATION: ROUNDTABLE

Several recent developments may provide an opportunity to improve outcome in individuals who develop neovascular age-related maculopathy (age-related macular degeneration [ARMD]). Concurrent with progress in isolating clinically relevant subtypes of neovascular ARMD, several therapies have been introduced that show promise for halting progression of this disorder. However, data from controlled clinical trials to test the relative efficacy of different management strategies across these subtypes of disease presentation remain limited. In addition, strategies to control ARMD may evolve quickly as more is learned about how specific molecular events, such as cell-mediated inflammation and angiogenesis, contribute to disease expression. A roundtable of investigators was convened to discuss and summarize recent progress in the treatment of ARMD. Case studies were then presented to provide an opportunity for experts to reveal their specific thought processes in the approach to neovascular ARMD based on their own interpretation of current clinical data and empirical experience.

Future pharmacological treatment options for nonexudative and exudative age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of irreversible vision loss in the industrialised world. Within the past decade, researchers have introduced many promising prevention and treatment options in an attempt to minimise the central vision loss imparted from AMD. Based on large-scale, randomised, prospective, placebo-controlled trials, a specially formulated combination of the antioxidants vitamin C, vitamin E, beta-carotene, copper and zinc is the only proven means of AMD prophylaxis. Thermal laser photocoagulation and photodynamic therapy with verteporfin are the only standard treatment options. However, efficacy is limited and treatment is only applicable to a minority of AMD patients. Thus, alternative pharmacological interventions are in all phases of clinical development. Researchers are guardedly optimistic that these advances may change the entire approach to AMD management in the near future. This review article will detail the currently accepted treatment options, as well as describe several of the more promising investigational pharmacological approaches to AMD.

EVALUATION OF PHOTOPOINT PHOTOSENSITIZER MV6401, INDIUM CHLORIDE METHYL PYROPHEOPHORBIDE, AS A PHOTODYNAMIC THERAPY AGENT IN PRIMATE CHORIOCAPILLARIS AND LASER-INDUCED CHOROIDAL NEOVASCULARIZATION

PURPOSE

To assess the potential of a new photosensitizer, indium chloride methyl pyropheophorbide (PhotoPoint MV6401), for ocular photodynamic therapy (PDT) in normal choriocapillaris vessels and experimentally induced choroidal neovascularization in New-World monkeys (Saimiri sciureus).

METHODS

PhotoPoint MV6401 (Miravant Pharmaceuticals, Inc., Santa Barbara, CA) was activated at 664 nm using a DD3-0665 (Miravant Systems, Inc., Santa Barbara, CA) 0.5 W diode laser. The efficacy of MV6401 was evaluated by indirect ophthalmoscopy, fundus photography, fluorescein angiography, and histology. The drug and light doses were 0.10 micromoles/kg to 0.3 micromoles/kg and 10 J/cm to 40 J/cm, respectively, and post-injection activation times ranged from +10 minutes to +120 minutes.

RESULTS

Best closure of normal choriocapillaris was achieved at a dosage level of 0.15 micromoles/kg in primates. Histology demonstrated that increased post-injection activation times (+60 minutes to +90 minutes) and low laser light doses (10 J/cm to 20 J/cm) in the primate model resulted in selective closure of the choriocapillaris and medium sized choroidal vessels with minimal effect to the retina. Histology from neovascular lesions PDT-treated with MV6401 revealed significant diminution of vascularity, correlating with diminution of leakage observed on angiography.

CONCLUSION

PhotoPoint MV6401, indium chloride methyl pyropheophorbide, is a potent photosensitizer that demonstrates both efficacy and selectivity in primate choriocapillaris and laser-induced choroidal neovascularization occlusion. Maximum selectivity was achieved using a post infusion interval of +60 to +90 minutes.

Current and Future Treatment Options for Nonexudative and Exudative Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is the leading cause of irreversible visual loss in the industrialised world. Although relatively simple to diagnose through direct visualisation augmented with rapid sequence fluorescein angiography, treatment has presented a far greater challenge because the true aetiology of AMD is largely unknown. Within the past decade, researchers have introduced many new, potentially promising treatment and prevention options in an attempt to minimise the damage imparted from AMD. They capitalise on many of the theoretical and known factors contributing to AMD progression. A high-dose of an orally administered combination of the antioxidants ascorbic acid (vitamin C), tocopherol (vitamin E) and beta-carotene, in addition to copper and zinc, is the only widely accepted preventive therapy. Thermal laser photocoagulation and verteporfin photodynamic therapy are the only standard treatment options available based on large scale, randomised, prospective, placebo-controlled trials; however, efficacy is limited and only a minority of patients who present with AMD are eligible for these treatments. Many other preventive and treatment options are in all phases of clinical studies and expected to change the entire approach to AMD management in the near future. For example, alternative antioxidants, drusen ablation, apheresis and HMG-CoA reductase inhibitors have shown promise in some studies by preventing or slowing the progression of certain forms of AMD. In addition, alternative photodynamic therapies, low-intensity laser, antiangiogenic medications, radiation treatment and surgery have demonstrated the ability, albeit to differing degrees, to inhibit or possibly even reverse the severe vision loss often associated with AMD characterised by choroidal neovascularisation.

Clinicopathologic studies of age-related macular degeneration with classic subfoveal choroidal neovascularization treated with photodynamic therapy

BACKGROUND

Photodynamic therapy (PDT) is a relatively new modality that is currently under clinical and experimental evaluation for treatment of subfoveal choroidal neovascularization (CNV). The authors report the case of an 82-year-old woman who underwent verteporfin-mediated PDT for classic subfoveal CNV. Fluorescein angiography performed 2 weeks after treatment disclosed reduction of the initial area of neovascularization and leakage by approximately 60%. Three weeks after PDT, however, the area of leakage was almost the same size as that before treatment. The patient underwent submacular membranectomy almost 4 weeks after treatment. The authors describe the ultrastructural vascular changes after PDT and a clinicopathologic study of classic CNV.

METHODS

The submacular membrane was studied by light and electron microscopy and immunohistochemical techniques.

RESULTS

Ultrastructural examination of the peripheral vessels showed evidence of endothelial cell degeneration with platelet aggregation and thrombus formation. Occasional occluded vessels were surrounded by macrophages, a phenomenon previously reported to describe the process of resorption of such blood vessels. The vessels in the center of the membrane were unremarkable.

CONCLUSION

Photodynamic therapy causes endothelial cell damage, thrombus formation, and vascular occlusion of classic CNV in age-related macular degeneration.

Photodynamic therapy update

Photodynamic therapy uses a photoactivating agent to selectively treat choroidal neovascularization. In April 2000, the United States Food and Drug Administration approved verteporfin photodynamic therapy for the treatment of subfoveal, predominately classic, choroidal neovascularization caused by age-related macular degeneration. The treatment of choroidal neovascularization from other causes such as myopia, angioid streaks, and idiopathy, and presumed ocular histoplasmosis syndrome is still under investigation. Other photoactivating agents are being evaluated. Photodynamic therapy has been shown to halt the progression of visual loss in patients with age-related macular degeneration who have subfoveal predominately classic choroidal neovascularization. The socio-economic impact of verteporfin approval has yet to be determined.

Update on photodynamic therapy

Photodynamic therapy is a relatively selective form of treatment for choroidal neovascularization. Unlike standard laser photocoagulation, photodynamic therapy can close choroidal neovascularization with minimal or no detectable damage to surrounding tissues. Therefore, it allows the clinician to treat subfoveal choroidal neovascularization without immediately adversely affecting central visual function. Several dyes for photodynamic therapy have been under various stages of formal clinical investigation. There is now mounting evidence that shows that photodynamic therapy with at least one of these dyes can significantly reduce the risk of visual loss in eyes with subfoveal choroidal neovascularization from age-related macular degeneration. This form of treatment promises to have a major, favorable impact on public health in areas of the world in which age-related macular degeneration is common.

Influence of photodynamic therapy on immunological aspects of disease - an update

Photodynamic therapy (PDT) utilises light-absorbing compounds combined with directed photo-irradiation to produce clinical effects. This review updates advances in the understanding of the biochemical pathways triggered by PDT within cells, its influence upon different immune parameters and progress in the use of PDT against human immune-mediated disease. Several works have further defined the notable capacity of PDT to foster anticancer immunity.

Problems with and pitfalls of photodynamic therapy

OBJECTIVE

To delineate the various factors that may influence the outcome of photodynamic therapy of the retina and choroid.

DESIGN

Experimental animal study.

ANIMALS

Pigmented and nonpigmented rabbits; rhesus monkeys.

INTERVENTION

The hydrophilic photosensitizer, mono-L-aspartyl chlorin e6, which is maximally activated at 664 nm, was studied after intravenous injection into pigmented and nonpigmented rabbits and rhesus monkeys. Laser light was supplied by a red diode laser coupled to a modified slit-lamp biomicroscope and delivered to the ocular fundus after passing through a standard fundus contact lens. Standard photodynamic parameters were used. The effects of fundus pigmentation, intraocular pressure, spot focus and defocus, region of fundus treated, equivalent fluence, and retreatment were observed in the different animal species.

MAIN OUTCOME MEASURES

Slit-lamp biomicroscopy, fluorescein angiography, light and transmission electron microscopy.

RESULTS

Fundus pigmentation appeared to be a factor only at the lowest fluence level tested, where only 4 of 12 lesions attempted in pigmented fundi were noted on fluorescein angiography, compared with 12 of 12 lesions in albino rabbits. At normal intraocular pressures and a given fluence, 10 of 10 lesions were fully manifested on fluorescein angiography, compared with 4 of 10 at 30 mmHg and 0 of 10 at pressures sufficient to blanch the optic nerve (>60 mmHg). For laser spots either focused or defocused, there were 6 of 6 lesions that were fully manifested on fluorescein angiography for each of the parameters. Lesions treated in the fovea resulted in larger spots on fluorescein angiography. The fluence of 5 mW for 10 seconds resulted in a larger lesion on angiography than the equivalent fluence of 10 mW for 5 seconds. Areas of retreatment in rabbits demonstrated more thinning of the neurosensory retina and loss of photoreceptor outer segments and nuclei than corresponding areas receiving one treatment.

CONCLUSIONS

Photodynamic therapy results varied, depending on intraocular pressure, region of fundus treated, ocular pigmentation, and the total time of exposure to the photosensitizer. Retreatment resulted in progressive thinning of the neurosensory retina with loss of photoreceptor outer segments and nuclei in the rabbit eye.

Changing therapeutic paradigms for exudative age-related macular degeneration: antiangiogenic agents and photodynamic therapy

Age related macular degeneration (AMD) is the leading cause of irreversible visual loss in the United States. Overall, approximately 10 - 20% of patients with AMD exhibit the exudative form, which is responsible for most of the estimated 1.2 m cases of severe visual loss from AMD. Visual loss develops in the exudative form of AMD due to abnormal choroidal neovascular membranes (CNVM) that develop under the retina, leak serous fluid and blood, and ultimately cause a blinding disciform scar in, and under, the retina. Currently, the only well-studied and widely accepted method of treatment is laser photocoagulation of the CNVM. However, only a minority of patients with exudative AMD show well-demarcated 'classic' CNVM amenable to laser treatment, and at least half of these patients suffer persistent or recurrent CNVM formation within two years. In addition, since the treatment itself causes a blinding central scotoma when the CNVM is located subfoveally, many clinicians do not treat subfoveal CNVM. With these treatment limitations, there has been a great deal of interest in alternative therapies for AMD, including anti-angiogenic agents and photodynamic therapy. Angiogenesis involves a complex interplay of cellular events involving a cascade of factors that are both inhibitory and stimulatory. Soluble growth factors have been the best-known cell modulating agents in ophthalmology, but there are a multitude of potential sites for inhibition of angiogenesis by pharmacological agents. With regard to photodynamic therapy, a photosensitising dye is injected intravascularly and low power laser light is used to activate the dye within the CNVM to cause vascular occlusion by a photochemical reaction. Closure of the CNVM is achieved without severe collateral damage to the non-vascular tissues as occurs with laser photocoagulation.