Risdiplam

Spinal muscular atrophy (SMA) is a severe and progressive congenital neuromuscular disease resulting from mutations in the survival of motor neuron 1 (SMN1) gene responsible for making SMN proteins. Clinical features of SMA include degeneration of motor neurons in the spinal cord which ultimately leads to muscular atrophy and, in some cases, loss of physical strength. SMN proteins are expressed ubiquitously throughout the body and are thought to hold diverse intracellular roles in DNA repair, cell signaling, endocytosis, and autophagy. A secondary SMN gene (SMN2) can also produce SMN proteins, but a small nucleotide substitution in its sequence results in the exclusion of exon 7 during splicing in approximately 85% of the transcripts - this means that only ~15% of the SMN proteins produced by SMN2 are functional, which is insufficient to compensate for the deficits caused by SMN1 mutations. Emerging evidence suggests that many cells and tissues are selectively vulnerable to reduced SMN concentrations, making this protein a desirable target in the treatment of SMA.

Risdiplam is an mRNA splicing modifier for SMN2 that increases the inclusion of exon 7 during splicing, which ultimately increases the amount of functional SMN protein produced by SMN2. It does so by binding to two sites in SMN2 pre-mRNA: the 5' splice site (5'ss) of intron 7 and the exonic splicing enhancer 2 (ESE2) of exon 7.