Abstract
Purpose:
Production of cytotoxic lipofuscin bisretinoids in the retina depends on the influx of retinol from serum to the RPE. Partial pharmacological reduction of serum retinol represents a treatment strategy for Stargardt disease and dry AMD. We focus on discovery of Retinol Binding Protein 4 (RBP4) antagonists as a means to partially reduce serum retinol and inhibit bisretinoid formation. As part of the NIH Blueprint Neurotherapeutics Network project we conducted the optimization project to identify novel RBP4 antagonists with the exceptional potency and drug-like characteristics. Here we present characterization of the advanced RBP4 antagonist with the desired attributes of a potential preclinical candidate capable of advancing to the IND-enabling studies.
Methods:
In vitro potency was assessed in the RBP4 binding and RBP4-TTR interaction assays. In vitro ADME properties were characterized in a battery of established tests (cytochrome P450 inhibition, metabolic stability in liver microsomes, intrinsic clearance, plasma protein binding). Pharmacokinetic (PK) studies coupled with the analysis of the serum biomarker (levels of serum RBP4) were conducted in rats, dogs, mice and non-human primates. The effect of compound dosing on concentrations of visual cycle retinoids was assessed in dark- and light- adapted eyes. The effect of the test compound on the visual cycle rates was studied using the post-bleach ERG rod response recovery and 11-cis-retinal regeneration tests. Pre-clinical efficacy in inhibiting bisretinoid formation was assessed in the Abca4-/- mouse model.
Results:
We identified a potential treatment candidate with the exceptional in vitro potency and optimal ADME characteristics. Outcomes of PK studies and results of the in vitro intrinsic clearance experiments performed in human hepatocytes predict once a day human dosing. Serum RBP4 reduction induced by the compound was associated with a measurable decrease in visual cycle retinoids. The rate of the visual cycle was similar in compound-treated and control animals. Following compound dosing in Abca4-/- mice we detected 50% reduction in bisretinoid accumulation.
Conclusions:
Optimization efforts yielded a potential drug candidate with optimized potency, improved pharmacokinetic characteristics, proven mechanism of action, and established pre-clinical efficacy that may allow its further characterization as a treatment for Stargardt disease and dry AMD