Hybrid laser-dye procedure to be tested in multi-center clinical trial in Brazil

Investigators believe that i-MP could be a safe and effective adjunct to existing anti-angiogenic therapies for CNV.

Issue: October 15, 2006

ByRogério A. Costa, MD, PhD

A novel hybrid laser-dye procedure, called indocyanine green-mediated photothrombosis, holds great potential for treating various forms of choroidal neovascularization. This procedure, known as i-MP, seems out of place in the recent era of angiogenic pharmaco-modulation, but it actually could represent a fascinating adjunct to anti-angiogenic agents that are currently available.

After 3 years seeking the means to perform a controlled, multicenter study to assess the clinical value of the treatment and to better understand the major mechanism of action, we have secured funding and are moving forward with such a study.

The study involves the joint effort of physicians – who will not be financially compensated – from 10 ophthalmic centers in Brazil and two Brazilian companies that are providing laser devices, dyes and contract research organization (CRO) monitoring. The companies that are financing the trial are Opto Eletrônica S.A. and Ophthalmos Industria e Comercio de Produtos Farmacêuticos Ltda.

The investigators will evaluate the safety and efficacy of i-MP for the management of neovascular AMD (further information at www.ClinicalTrials.gov).

If our preliminary results are reproducible, we believe i-MP could at some point work synergistically with antiangiogenic agents, such as Macugen (pegaptanib sodium injection, Pfizer/OSI), or Lucentis (ranibizumab, Genentech). If this is possible, the combination could set patient free from the monthly and, to date, eternal eye injection schedule needed to sustain the beneficial effects of pegaptanib/ranibizumab therapy in the setting of neovascular AMD.

Currently, only one photosensitizing agent, Visudyne (verteporfin for injection, QLT/Novartis Ophthalmics) has proven efficacy for use in ophthalmology.

The concept of i-MP is based on the use of low irradiance continuous ~810-nm laser application to photoactivate ICG at one targeted abnormal (pathologic) tissue with recognized increased uptake of the dye, with therapeutic effects essentially arising from photooxidative reactions (types 1 and 2) of the photoactivated dye within the “lesion.” For such a purpose, we have adopted a dedicated two-fold scheme for light application as well as for dye infusion.

Sixty-nine-year-old AMD patient with an ill-defined neovascular lesion with subfoveal involvement who was submitted to ICG-mediated photothrombosis. (A) Fluorescein blockage from sub-retinal and sub-RPE hemorrhage as well as some areas of stippled hyperfluorescence was seen at baseline (BCVA=20/800+1). (B) Six months after treatment, fluorescein staining of the quiescent neovascular lesion and no leakage from previous CNV was observed (BCVA=20/160+2). Note that no clear angiographic damage was evidenced in the presumably normal choroidal and retinal tissues involved in the large (4.5 mm spot) treatment area (dashed circular line).

OCT evaluation of the same patient shown in Figure above. Rapid resolution of the exudative manifestations was disclosed by horizontal 6 mm scans as early as one week after ICG-mediated photothrombosis. Considerable restoration of the macular architecture was seen in the subsequent follow-up visits 12 and 24 weeks after treatment.

Images: Costa RA

Two-fold scheme for ICG infusion

In our initial report on the use of i-MP for CNV management in AMD, a dose of 1.5 mg/kg was used, in keeping with the recommended maximum ICG dose in humans (2.0 mg/kg). A 0.5 mg/kg ICG dose was utilized at that time for an ICG angiography study several minutes prior to the treatment session on those patients.

Based on the induced response over time utilizing quite a few combinations of ICG doses and dosing schedules, we noted that a better therapeutic effect was achieved in those patients in which ICG angiography studies had been performed at the same visit of the i-MP treatment session, even if lower (1.0 mg/kg to 1.5 mg/kg) ICG doses were used for treatment.

In the pilot studies, the positive influence of having some residual ICG uptake in the targeted tissue prior to the treatment dose became more evident. As a result, we adopted the twin ICG infusion scheme for i-MP.

In theory, higher photochemical effects occurs rather in those tissues involved in the treatment laser spot that have been formerly “ICG-loaded” than in the normal tissues that presents minimal or no ICG residual up-take from the first ICG injection.

Two-fold light delivery technique

In addition to a two-step ICG infusion proposal, light delivery for i-MP is also performed in a two-fold scheme.

ICG-mediated photodynamic effects have been experimentally demonstrated in vitro and in vivo after a single 810-nm laser application. However, an increase in the laser intensity (measured in W/cm²) was needed to compensate for the lower maximum human tolerated ICG dose in order to induce a satisfactory effect in the clinical scenario.

Experimental observations and theoretical models have suggested that intensity is the main factor contributing to temperature changes in laser-treated tissues. This theory is borne out by the relative effects of various lasers that are used clinically.

A faster and more intense increase in tissue temperature occurs when conventional photocoagulation is used than when much higher energy (measured in J/cm²) is delivered in a continuous pulse with lower intensity, for example, as used for TTT in neovascular AMD.

Accordingly, during preliminary i-MP studies, it was also noted that increase of laser intensity to some extent was not related to a better response; to the contrary, increased intensity eventually caused retinal damage without producing any additional therapeutic effects on the targeted tissues.

By adopting a two-fold scheme of low intensity light delivery, we were able to optimize ICG photoactivation in the targeted tissue while avoiding the undesirable thermal effects arising from random absorption of the laser energy by the endogenous pigment in the choroid and retinal pigment epithelium, which are commonly observed in higher-intensity laser treatments.

Persistent central serous chorioretinopathy in a 38-year-old patient submitted to ICG-mediated photothrombosis. (A) Typical focal leak at the level of the RPE (arrowhead) and a large area of serous retinal detachment (arrows) were still seen on fluorescein angiography five months after initial presentation. (B) Three months after treatment subretinal fluid was completely resolved. Minimal RPE alteration with retinal vessels perfused and with no angiographic signs of damage was seen at the site of laser application on fluorescein angiography.

Indocyanine green angiography, red free fundus photography, and OCT evaluation of the same patient shown in the figure above. (A) Note that in spite of the one single large spot utilized for treatment (top), no damage was evidenced on the presumed normal choroidal tissues involved in the laser spot 3 months after treatment (middle). At that time, only minor changes at the level of the RPE were noted by clinical examination (bottom). (B) Sequential 7.00 mm OCT scans performed 72 hours before treatment (baseline), and 1 and 2 weeks, and 3 months after treatment demonstrated rapid and gradual restoration of the macular architecture. No obvious alteration at the level of the neurosensory retina and RPE/choriocapillaris hyperreflective band was seen at the site of laser application.

Mechanisms of action unclear

With respect to the mechanisms of action of i-MP, it is actually impossible to determine exactly how photodynamic or photothermal is the generated response after ICG photoactivation in the clinical scenario.

We hypothesized over a mixed type 1 (heat generation) and 2 (singlet oxygen formation) photooxidation therapy to explain the observed effects in i-MP. For this reason, we changed the initial terminology proposed for the procedure from i-PDT to i-MP.

Although its mechanisms of action remain unclear, we know that selective treatment response has been achieved by combining the aforementioned factors related to ICG-infusion and light-delivery schemes used in i-MP.

In our series, the spot size used for light delivery has been larger than the targeted lesions, and no obvious deleterious effects could be observed in presumably normal tissues within the treatment area by clinical examination, fluorescein and ICG angiography or optical coherence tomography.

Historical context of photosensitizing agents

Interest in cytoxic responses to photosensitizing agents has been recorded by Raab as early as 1900. By using agents (dyes) with known and favorable biodistribution (i.e., tropism for certain pathological tissues), it is expected that a selective effect at such tissue may occur after photoactivation of the agent with laser light at specific wavelength (to provide optimal absorption of laser energy by the agent). One of the first experiments utilizing photosensitizing agents in ophthalmology was reported by Thomas and Langhofer in 1987, in which dihematoporphyrin ether and argon green laser were used for closure of experimental subretinal neovascular vessels. For the past 20 years several agents have been investigated for the management of chorioretinal diseases, but only Visudyne has been shown to be safe and efficacious for ophthalmic use.

diagram
The Jablonski diagram shows the relaxation mechanism for excited state molecules.

Regarding photosensitizer-induced cytotoxicity, the photosensitizer molecule that has absorbed laser light energy can reach the ground state by either radiative or non-radiative decay. In non-radiative decay, the molecule’s absorbed energy can be converted to heat (internal conversion) and transferred to other molecules (photooxidation type 1), thereby damaging cells by raising their intracellular temperature.

Alternatively, the photosensitizer absorbed energy can be transferred to molecular oxygen (photooxidation type II) via a triplet state, which then interacts with oxygen and other compounds to form reactive intermediates, such as singlet oxygen, which can cause irreversible destruction of biologic substrates (Figure 1).

Singlet oxygen possesses a reactive distance of only 0.1 µm, so cytotoxicity is restricted to the immediate vicinity of the photoactivated drug. Therefore, when using a dye-assisted laser approach, different tissue responses can be achieved depending on individual characteristics, techniques, dye and laser properties, or by simply adjusting parameters.

Source: Costa RA

For more information:

Rogério A. Costa, MD, PhD, is director of the Advanced Diagnostic and Treatment Unit in the Hospital de Olhos Araraquara, SP, Brazil. He can be reached at Rua Padre Duarte 989, Apart. 172, Araraquara, São Paulo, Brazil 14801-310; tel/fax: 55-16-3331-1001. Dr. Costa has no financial interest in any of the products mentioned. He is an unpaid consultant for Opto Elêtronica.