Retinitis pigmentosa (RP), a hereditary disease of the retina affecting 100,000 Americans, is the major cause of blindness in adults aged 25-60. There is no approved therapy for the disease and gene therapy, while promising, is limited because of the diversity of underlying genetic mutations. The abnormal localization of rod opsin, the highly-expressed major protein constituent of the photoreceptor outer segment, has long been recognized as a major event preceding photoreceptor cell death in animal models of RP. Recently, in two mouse models of ciliopathy, a reduction in rod opsin expression by genetic methods was shown to markedly delay what is otherwise a rapid loss of photoreceptors in the outer nuclear layer of the retina. Orphagen has discovered specific small molecule ligands to a previously unexplored orphan receptor that is expressed in the photoreceptor layer and is essential for early retinal development. In primary neonatal rat retinal cultures, antagonists (IC50 <250 nM) to this receptor partially suppress rod opsin expression in a receptor-specific manner. After intravitreal injection in rats, our current probe compound also significantly suppresses rod opsin mRNA. The findings suggest that a clinical antagonist to the same receptor will reduce the rate of photoreceptor degeneration for the estimated 40% of RP patients where mislocalization of rod opsin is a factor in rod photoreceptor cell death. In response to PAR-09-260, Optimization of Small Molecule Probes for the Nervous System, we propose to identify a substantially more potent antagonist necessary for in vivo proof-of-principle studies. Two consecutive daily 5 mg intravitreal injections of the current probe compound cause a modest 25% suppression of rod opsin mRNA. However, the potency of the current probe compound is too low for development of an experimental slow release formulation that could deliver an effective intraocular dose over four weeks, a duration required to measurably inhibit photoreceptor degeneration in many rodent RP models.
In Aim 1, we identify new receptor antagonist scaffolds, from commercial compound libraries, based on a 3-D computational model of the proprietary antagonists developed at Orphagen, in order to expand starting points for subsequent medicinal chemistry;
in Aim 2, we carry out focused medicinal chemistry to increase potency with the goal of achieving an IC50 <40 nM for rod opsin suppression in primary retinal cultures;and in Aim 3, we select a lead compound with improved potency and efficacy (rod opsin mRNA inhibition >30% at ? 1 mg/ intravitreal injection) and evaluate retinal function (determined by ERG) to exclude retinal toxicity. If Phase 1 SBIR aims are successfully completed, we plan in Phase 2 to formulate the lead receptor antagonist for sustained intravitreal release and to carry out efficacy and safety studies in rodent models of RP that would justify initiating development of this novel drug class for clinical use.
Retinitis pigmentosa (RP), is a diverse group of inherited degenerative retinopathies characterized by progressive photoreceptor cell death and is the major cause of blindness in working age adults. Gene therapy approaches are very limited and no small molecule therapies are currently available. Aberrant trafficking of rod opsin, which constitutes ~90% of the rod photoreceptor outer segment protein, is thought to trigger photoreceptor cell death in up to half of all RP patients. Orphagen has developed the first small molecule drug- like ligands to a key transcription factor that regulates rod opsin expression. The overall objective of this project is to develop a more potent lead compound that would allow efficacy testing in animal models of retinal degeneration.