Intracellular Retinoid-Binding Proteins (CRBP, CRALBP, CRABP) as Emerging Drug Targets in Retinal Disease

Intracellular retinoid-binding proteins, including cellular retinol-binding proteins (CRBPs) and cellular retinoic acid-binding proteins (CRABPs), belong to the intracellular lipid-binding protein (iLBP) family, whereas cellular retinaldehyde-binding protein (CRALBP) is a structurally distinct retinoid-binding protein belonging to the CRAL-TRIO protein family and functions as an active regulator of retinoid trafficking, metabolism, and signaling. Although these proteins are directly implicated in inherited retinal dystrophies, retinoid-dependent cancers, and neurodegenerative diseases, they have received comparatively little attention as pharmacological targets relative to the extracellular carrier Retinol-Binding Protein 4 (RBP4). High-resolution structural studies, including atomic-resolution X-ray co-crystal structures of protein-ligand complexes, have defined the binding pocket architecture of each protein and established a basis for structure-guided drug discovery. This review critically evaluates the druggability of CRBP, CRALBP, and CRABP by integrating structural, biochemical, and pharmacological evidence. We discuss known small-molecule modulators, including the first-in-class CRBP1 inhibitor abn-CBD and next-generation non-retinoid scaffolds, alongside gene therapy strategies targeting CRALBP deficiency. We further address the selectivity challenges inherent to the conserved iLBP fold and identify future directions for the development of isoform-selective therapeutics for retinal degeneration, oncology, and neurodegeneration.