The growth of abnormal blood vessels in the eye is a leading cause of vision loss in people with the wet form of age-related macular degeneration (wet AMD). In this disease, blood vessels grow into the retina and leak blood and fluid causing a condition called edema, which damages the macula. The macula is the part of the retina responsible for seeing fine detail, reading, and recognizing faces. Proteins, notably vascular endothelial growth factor (VEGF), stimulate the growth of these leaky blood vessels, a process called angiogenesis or neovascularization. New drugs that inhibit VEGF, including ranibizumab (Lucentis) and bevacizumab (Avastin), are injected directly into the eye and have helped many patients with wet AMD and similar eye diseases maintain their sight. Other types of eye disease are marked by blood vessel growth in the cornea, which is normally free of blood vessels.

Now, a new study by eye researchers in China has found that a natural VEGF inhibitor found in the licorice root, called isoliquiritigenin (ISL), can suppress angiogenesis in the retinas and corneas of laboratory mice. The researchers used a number of laboratory tests, called assays, to measure the antiangiogenic potency of ISL in the eye. A topical formulation of ISL applied to the corneas of mice with abnormal blood vessel growth suppressed the formation of these vessels, while intravitreal (injected into the eye) application of ISL had the same effect on mice with retinal angiogenesis. In one test, a formulation of ISL was more potent in suppressing angiogenesis than bevacizumab, the leading treatment for wet AMD. Intravitreal ISL was also effective at suppressing the leaking of blood vessels in the retina.

Licorice extract and its components have been shown to have antioxidative, anti-inflammatory, pro-apoptotic (promoting normal cell death), and antiangiogenic activity in both cancerous and normal tissues. In the current study, ISL derived from licorice downregulated production of VEGF and upregulated production of pigment-epithelium-derived growth factor (PEGF). PEGF has an important role in eye diseases, such as wet AMD. In healthy eyes, PEGF suppresses the growth of blood vessels in parts of the eye that are normally free of vessels, such as the cornea. In people with wet AMD and corneal neovascularization, however, PEGF levels are abnormally low. PEGF also suppresses inflammation, which is a factor in some eye diseases, such as diabetic macular edema. Further research is needed to validate the findings of this study and determine whether ISL could become a new therapy for the treatment of neovascular eye diseases.