Molecular pharmacodynamics of emixustat in protection against retinal degeneration
Jianye Zhang, … , Gregory P. Tochtrop, Krzysztof Palczewski
Jianye Zhang, … , Gregory P. Tochtrop, Krzysztof Palczewski
Published June 15, 2015
Citation Information: J Clin Invest. 2015;125(7):2781-2794. https://doi.org/10.1172/JCI80950.
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Research Article Ophthalmology

Molecular pharmacodynamics of emixustat in protection against retinal degeneration

Abstract

Emixustat is a visual cycle modulator that has entered clinical trials as a treatment for age-related macular degeneration (AMD). This molecule has been proposed to inhibit the visual cycle isomerase RPE65, thereby slowing regeneration of 11-cis-retinal and reducing production of retinaldehyde condensation byproducts that may be involved in AMD pathology. Previously, we reported that all-trans-retinal (atRAL) is directly cytotoxic and that certain primary amine compounds that transiently sequester atRAL via Schiff base formation ameliorate retinal degeneration. Here, we have shown that emixustat stereoselectively inhibits RPE65 by direct active site binding. However, we detected the presence of emixustat-atRAL Schiff base conjugates, indicating that emixustat also acts as a retinal scavenger, which may contribute to its therapeutic effects. Using agents that lack either RPE65 inhibitory activity or the capacity to sequester atRAL, we assessed the relative importance of these 2 modes of action in protection against retinal phototoxicity in mice. The atRAL sequestrant QEA-B-001-NH2 conferred protection against phototoxicity without inhibiting RPE65, whereas an emixustat derivative incapable of atRAL sequestration was minimally protective, despite direct inhibition of RPE65. These data indicate that atRAL sequestration is an essential mechanism underlying the protective effects of emixustat and related compounds against retinal phototoxicity. Moreover, atRAL sequestration should be considered in the design of next-generation visual cycle modulators.

Authors

Jianye Zhang, Philip D. Kiser, Mohsen Badiee, Grazyna Palczewska, Zhiqian Dong, Marcin Golczak, Gregory P. Tochtrop, Krzysztof Palczewski

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Figure 7

Effects of the emixustat derivative MB-002 on RPE65 activity and protection from light-induced retinopathy.

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Effects of the emixustat derivative MB-002 on RPE65 activity and protect...
(A) Chemical structures of emixustat and MB-002. (B) Inhibitory effect of MB-002 on retinoid isomerase activity in vitro. (C) Inhibitory effects of visual cycle modulators on 11-cis-retinal regeneration in C57BL6J mice. Post-bleaching recovery of ocular 11-cis-retinal was measured after a a 6-hour dark incubation period. (D) OCT images of retinae from pigmented Abca4–/– Rdh8–/– mice treated with MB-002 or retinylamine. The images, obtained 3 days after light exposure, show a lack of protective effects with MB-002. Scale bars: 100 μm. (E) Representative 3D TPM image of the retinal photoreceptor and RPE cell layers from an albino Abca4–/– Rdh8–/– mouse treated with MB-002 and exposed to intense light showing numerous swollen photoreceptor outer segments (red arrowheads). RPE cells are at the top of the section, and the photoreceptor cell layer section is shown at 10 μm below the RPE. Mice were exposed to bright light 1 day after they were gavaged with either oil (control), Ret-NH2 (2 mg/mouse), or MB-002 (2 mg/mouse). TPM images of intact mouse eyes were obtained on day 2 after light exposure. Enlarged photoreceptors were counted in each group at a distance of 8 to 10 μm from the RPE. The mean numbers (± SD) of enlarged photoreceptors for each treatment group are as follows: 252 ± 33 for the oil-treated group; none in the no-light group; 5 ± 5 in Ret-NH2–treated animals; and 233 ± 45 in MB-002–treated animals. n = 4.