Potent and specific small molecule stabilizers of amyloidogenic antibody light chains

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Light chain amyloidosis (AL) is a systemic degenerative disease caused by the misfolding and aggregation of free antibody light chain (LC) proteins that are secreted from a monoclonal plasma cell expansion. This process results in buildup of LC aggregates, including amyloid fibrils, ultimately leading to organ failure. Existing treatments for AL focus on eradicating the plasma cell expansion using cytotoxic chemotherapy. However, many patients, especially those with cardiac involvement, cannot tolerate these treatment regimens. Despite advances in AL treatment, each year, a thousand patients in the US die within a year of diagnosis, and 12,000 Americans currently live with the disease. Therefore, new treatments for AL represent a pressing unmet medical need. Aggregation-prone LCs have low kinetic stability, i.e. high rates of transient unfolding from the natively folded, nontoxic LC into aggregation-competent conformations. Small molecules which stabilize the native state of aggregation-prone proteins have shown clinical efficacy, e.g., the kinetic stabilizer drug tafamidis that targets the protein transthyretin is a frontline treatment for the transthyretin amyloidoses. I envision that an analogous kinetic stabilizer for LCs would be similarly efficacious. This treatment strategy should be well-tolerated and is complementary to existing AL therapies. Using high-throughput screening, we identified small molecules that kinetically stabilize LCs, slowing aberrant proteolysis enabled by conformational excursions. These small molecules will likely stabilize most LC sequences, as the binding site we discovered is highly conserved in AL LC sequences–critical because each AL patient has a unique LC sequence. However, the hits from the screen exhibit low M dissociation constants, which is insufficient for drug candidates. I hypothesize that highly potent (low nanomolar KD’s) and selective small molecules can be generated to kinetically stabilize aggregation-prone LCs. To test this hypothesis, I will employ hit-to-lead medicinal chemistry, X-ray crystallography, and computer-aided structure-based design to improve the potency and selectivity of the LC kinetic stabilizers. I will use one screening hit, 7-diethylamino-4-methylcoumarin (1), as a template for optimization. In Specific Aim 1, I will improve the potency of this hit by appending a substructure that extends into an unoccupied pocket revealed by the co-crystal structure of the LC•1 complex. In Specific Aim 2, I will use a “scaffold hopping” approach to improve potency and remove the metabolic liabilities of the coumarin core and anchor substructure. In Specific Aim 3, I will assess the potency and selectivity of kinetic stabilizers coming from Aims 1 & 2 using a fluorescently-labeled LC protease sensitivity assay conducted in human plasma, using sequence diverse disease-associated LCs. To inform further optimization in Aims 1 & 2, I will use X-ray crystallography to obtain structural insight into kinetic stabilizer•LC complexes and use modern computational approaches to facilitate structure-based design. The primary outcome of my proposal will be the identification of lead LC kinetic stabilizers that will be carried forward to lead optimization by others.