In the mouse laser-induced model of AMD, REDD14 was shown to inhibit expression of RTP801 in the retinal pigment epithelium/choroid in a sequence-dependent manner, inhibit neovascularization in a sequenceand dose-dependent manner, inhibit vessel leakage in a sequenceand dose-dependent manner, work in an additive fashion in combination with the antiangiogenic molecules (pegaptanib) and an antimouse VEGF antibody to prevent both neovascularization and vessel leakage, and exhibit anti-inflammatory activity.

The efficacy seen in the mouse model was confirmed in a monkey laser-induced model of AMD.

The data suggest that the different mechanism of action of REDD14NP (i.e., blocking the RTP801 hypoxia/stress pathway) is independent of, and complementary to, the antiangiogenic mechanism of current therapeutic approaches for selected retinal vascular diseases such as neovascular AMD and DME. REDD14NP has also been evaluated in a primate model. Low levels of REDD14NP were detectable in plasma of monkeys up to 22 hours after intravitreal injection (the latest sampling time point), and in the plasma of rabbits up to 6 days after intravitreal injection (the latest sampling time point). The persistence of detectable REDD14NP in plasma likely reflects continued slow egress of the test article from the eye. As such, it attests to the likely persistence of REDD14NP in the eye.

No systemic toxicity or other effects outside the eye were observed with REDD14NP, and there was no indication of any potential for genetic toxicity. Findings in the intravitreal studies that were possibly related to REDD14NP include an increased incidence of cellular infiltration and secondary changes in the eye (e.g., increased intraocular pressure) that occurred at the highest dose level tested in monkeys (i.e., at

3.0 mg/eye); and an increased incidence of hemorrhage that occurred at the highest dose level tested in rats (30 g/eye). REDD14NP has not

been evaluated in human diseases prior to studies in ocular models. Based on the preclinical data demonstrating bioactivity and safety, current clinical trials are being conducted by Pfizer and Quark to evaluate the potential therapeutic role of REDD14NP in neovascular AMD and DME.

SUMMARY AND KEY POINTS

The application of siRNA technology for retinal diseases and other ocular diseases is very early, and the full potential has yet to be determined. The bevasiranib studies have demonstrated safety of intravitreal siRNA in approximately 300 eyes at the time this manuscript was written. SIRNA-027 also appears to be safe based on the smaller study numbers in the phase I trial. The early studies of bevasiranib have also emphasized the importance of eliminating existing target protein before siRNA therapy is initiated, since blocking the synthesis of the target protein does not eliminate existing target protein. Target protein may be degraded at variable rates and can continue to produce damage before the siRNA has a chance to have any biologic effect.

siRNA can be targeted against proteins such as VEGF that contribute to retinal diseases such as CNV and DME. siRNA directed against VEGF and VEGF receptor appear to be well tolerated as intravitreal injections and no systemic absorption or toxicity has been identified to date. In preclinical and phase I studies, bevasiranib and SIRNA-027 showed a possible biologic effect on CNV, but the phase III studies have not confirmed such results. REDD14NP is being evaluated without any definitive results at this time. A detailed analysis of the bevasiranib and SIRNA-027 clinical trial data will help to elucidate why these compounds were not as effective in inhibiting CNV as ranibizumab.

The potential use of siRNAs to treat ocular diseases is exciting, as many conditions are caused by protein overproduction in the eye, and the use of siRNA in human trials to treat ocular diseases will help to clarify the role of mRNA and proteins in causing retinal disease. However, the failure of two siRNA molecules, bevasiranib and SIRNA, to demonstrate efficacy in neovascular AMD, in the era of anti-VEGF therapy, suggests that there are challenges such as targeted delivery of the siRNA molecule that must be overcome before one can consider siRNA in the therapeutic arena for retinal vascular diseases. Further

preclinical and clinical studies are needed to define better all the potential uses of siRNA to treat ocular diseases.

ACKNOWLEDGMENT

We wish to thank Sam Reich and his colleagues from OPKO Health, for providing the bevasiranib macular degeneration data, Jonathan Prenner, MD for providing the bevasiranib diabetic macular edema data, and Tuyen Ong, MD and his colleagues at Pfizer for providing information on REDD 14NP, for this chapter prior to the studies’ publications.

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