Structural and biochemical basis for retinol-binding protein 4 antagonism by tinlarebant.

Abstract

Background and purpose: Retinol-binding protein 4 (RBP4) is a member of the lipocalin superfamily that is connected to disease states such as insulin resistance, fatty liver disease and ocular disorders including Stargardt disease. Several retinoid and non-retinoid antagonists of this protein have been developed for potential clinical use, but none have yet received FDA approval. Tinlarebant, a derivative of the early lead RBP4 antagonist, A1120, was reported to slow macular lesion growth in Stargardt disease patients. However, its precise mechanism of RBP4 antagonism remains undisclosed.

Experimental approach: The binding of tinlarebant to RBP4 was studied with fluorescence spectroscopy and crystallography. The impact of tinlarebant occupancy in the RBP4 retinol-binding pocket on RBP4 complexation with transthyretin (TTR) was studied by size-exclusion chromatography. The impact of tinlarebant on plasma RBP4 levels was studied in mice.

Key results: Fluorescence quenching assays demonstrate that tinlarebant competitively displaces retinol from holo-RBP4 with an IC₅₀ of 8.75 ± 0.38 nM. The RBP4-tinlarebant complex fails to associate with transthyretin in vitro. Administration of single doses of tinlarebant to mice reduced plasma RBP4 levels by up to 93%. The crystal structure shows that tinlarebant impedes RBP4 association with transthyretin through both conformational and steric effects.

Conclusion and implications: Tinlarebant is a tight-binding RBP4 antagonist that lowers RBP4 plasma levels due to disruption of the RBP4- transthyretin interaction. The crystal structure shows that tinlarebant integrates multiple aspects of RBP4 interaction and antagonism found in its predecessors. Together, these data illuminate the pharmacology of a clinical inhibitor of the RBP4-retinol transport system.