Selective photodynamic therapy by targeted verteporfin delivery to experimental choroidal neovascularization mediated by a homing peptide to vascular endothelial growth factor receptor-2.

Abstract

To evaluate the feasibility, efficacy, and selectivity of photodynamic therapy (PDT) using targeted delivery of verteporfin to choroidal neovascularization (CNV) in the rat laser-injury model of CNV. We performed PDT in rat eyes on experimental CNV and normal retina and choroid using verteporfin conjugates. A targeted verteporfin conjugate was made by conjugating verteporfin (after isolation from its liposomal formulation) to a modified polyvinyl alcohol (PVA) polymer (verteporfin-PVA) followed by linkage to the peptide ATWLPPR known to bind the receptor for vascular endothelial growth factor, VEGFR2. The verteporfin-PVA conjugate served as a control. We performed fluorescent fundus angiography to determine the optimal timing of light application for PDT using the conjugates. Closure of CNV was assessed angiographically and graded in a masked standardized fashion. We used standardized histological grading to compare the effects on normal retina and choroid. The verteporfin-PVA conjugation ratio was on average 28:1. The conjugate retained typical emission/excitation spectra and photosensitizing activity and was as efficient as an equivalent amount of verteporfin. Peak intensity of targeted verteporfin in CNV was detected angiographically at 1 hour after intravenous injection. Photodynamic therapy using targeted verteporfin (3 or 4.5 mg/m2) with light application 1 hour after drug injection showed angiographic closure of all treated CNV (17/17) 1 day after treatment. Photodynamic therapy using verteporfin-PVA at the same drug dose achieved closure in 18 of 20 CNV. Histological examination after PDT of normal retina and choroid using targeted verteporfin and irradiation at 1 hour showed minimal effect on retinal pigment epithelium and no injury to photoreceptors, whereas PDT using verteporfin-PVA resulted in retinal pigment epithelium necrosis and mild damage to photoreceptors. Verteporfin bound to the targeting peptide, ATWLPPR, retained its spectral and photosensitizing properties. Angiography demonstrated localization of the targeted verteporfin 1 hour after injection. Photodynamic therapy using targeted verteporfin and the control conjugate were more effective in causing CNV closure than standard liposomal verteporfin. The targeted verteporfin resulted in more selective treatment than the control conjugate or standard verteporfin. These results suggest that targeted PDT strategies based on selective expression of receptors on CNV vasculature may improve current therapy. Targeted PDT for CNV is feasible and may offer a qualitative improvement in current treatments for patients with age-related macular degeneration. This study provides the basis for further preclinical studies of targeted PDT strategies and subsequent clinical trials.