ABSTRACT
Spinal muscular atrophy (SMA), a rare neuromuscular disorder, is the leading genetic cause of death in infants and toddlers. SMA is caused by the deletion or a loss of function mutation of the survival motor neuron 1 (SMN1) gene. In humans, a second closely related gene SMN2 exists; however it codes for a less stable SMN protein. In recent years, significant progress has been made toward disease modifying treatments for SMA by modulating SMN2 pre-mRNA splicing. Herein, we describe the discovery of LMI070/branaplam, a small molecule that stabilizes the interaction between the spliceosome and SMN2 pre-mRNA. Branaplam (1) originated from a high-throughput phenotypic screening hit, pyridazine 2, and evolved via multiparameter lead optimization. In a severe mouse SMA model, branaplam treatment increased full-length SMN RNA and protein levels, and extended survival. Currently, branaplam is in clinical studies for SMA.
Subject(s)
Brain/drug effects , ERG1 Potassium Channel/metabolism , Muscular Atrophy, Spinal/drug therapy , Pyridazines/chemistry , Administration, Oral , Animals , Brain/metabolism , Cell Line , Crystallography, X-Ray , Dose-Response Relationship, Drug , Drug Evaluation, Preclinical/methods , ERG1 Potassium Channel/antagonists & inhibitors , Humans , Mice, Inbred C57BL , Motor Neurons/drug effects , Muscular Atrophy, Spinal/genetics , Pyridazines/pharmacology , Quantitative Structure-Activity Relationship , RNA Splicing , Rats, Sprague-Dawley , Survival of Motor Neuron 1 Protein/genetics , Survival of Motor Neuron 1 Protein/metabolism , Survival of Motor Neuron 2 Protein/geneticsABSTRACT
Spinal muscular atrophy (SMA), which results from the loss of expression of the survival of motor neuron-1 (SMN1) gene, represents the most common genetic cause of pediatric mortality. A duplicate copy (SMN2) is inefficiently spliced, producing a truncated and unstable protein. We describe herein a potent, orally active, small-molecule enhancer of SMN2 splicing that elevates full-length SMN protein and extends survival in a severe SMA mouse model. We demonstrate that the molecular mechanism of action is via stabilization of the transient double-strand RNA structure formed by the SMN2 pre-mRNA and U1 small nuclear ribonucleic protein (snRNP) complex. The binding affinity of U1 snRNP to the 5' splice site is increased in a sequence-selective manner, discrete from constitutive recognition. This new mechanism demonstrates the feasibility of small molecule-mediated, sequence-selective splice modulation and the potential for leveraging this strategy in other splicing diseases.