Polypharmacological approach to treatment of Stargardt disease

Stargardt disease (STGD1) is the most common form of inherited macular dystrophy. The primary biochemical defect in STGD1 is excessive formation of cytotoxic lipofuscin bisretinoids in the retinal pigment epithelium (RPE) due to recessive mutations in the ABCA4 gene. There is no treatment for Stargardt disease. The major cytotoxic components of RPE lipofuscin are pyridinium bisretinoids, which are formed as by-products of the visual retinoid cycle. It was suggested that partial inhibition of the visual cycle may reduce the formation of lipofuscin bisretinoids and prolong the RPE and photoreceptor survival in Stargardt disease. A critical step in the visual cycle is the conversion of all-trans-retinyl ester to 11-cis-retinol by the enzyme called isomerohydrolase (IMH). It has been shown that RPE65 represents IMH, which produces 11-cis-retinol from all-trans-retinyl ester in the RPE. The IMH reaction is rate-limiting in the visual cycle function thus making RPE65 an important drug target for the visual cycle inhibition. Pharmacological blockage of the visual cycle accomplished by RPE65 inhibition was shown to effectively reduce bisretinoid production in the Abca4-/- mouse model of Stargardt disease. However, direct RPE65 inhibition as a therapeutic strategy is complicated by the severity of mechanism-based adverse effects (AEs), and it is unlikely that selective RPE65 inhibitors can be used clinically at efficacious doses due to their excessive retinal toxicity. In order to reduce the levels of RPE65 inhibition below the threshold associated with AEs while maintaining a bisretinoid-lowering efficacy, we will develop a class of novel bispecific compounds which in addition to RPE65 inhibition will act as retinaldehyde traps. Retinaldehydes are direct bisretinoid precursors, and their neutralization through the formation of reversible Schiff base or 1,3-oxazinane adducts using primary amine- or aminocarbinol-containing compounds, respectively, would reduce bisretinoid synthesis. The use of bispecific compounds will allow to reduce the level of RPE65 inhibition below the threshold associated with AEs while maintaining a combined bisretinoid-lowering efficacy. Two novel non-retinoid compounds, RPE65-61 and RPE65-71, will serve as the starting points in medicinal chemistry optimization of the new class of bispecific agents in studies related to Specific Aim 1. Traditional ocular pharmacodynamic markers related to the inhibition of the visual cycle and suppression of the rod ERG signal along with a new serum biomarker related to aldehyde trapping will be used to prove the in vivo activity of bispecific compounds and to select the range of safe yet efficacious doses (Specific Aim 2). Evaluation of in vivo efficacy along with the assessment of retinal toxicity after chronic dosing will be conducted in the mouse genetic models of Stargardt disease (Specific Aim 3).