Casimersen

Duchenne muscular dystrophy (DMD) is an X-linked recessive allelic disorder that results in the absence of functional dystrophin, a large protein comprising an N-terminal actin-binding domain, C-terminal β-dystroglycan-binding domain, and 24 internal spectrin-like repeats. Dystrophin is vital for normal muscle function; the absence of dystrophin leads to muscle membrane damage, extracellular leakage of creatinine kinase, calcium influx, and gradual replacement of normal muscle tissue with fibrous and adipose tissue over time. DMD shows a characteristic disease progression with early functional complaints related to abnormal gait, locomotion, and falls that remain relatively stable until around seven years of age. The disease then progresses rapidly to loss of independent ambulatory function, ventilatory insufficiency, and cardiomyopathy, with death typically occurring in the second or third decade of life.

The human DMD gene contains 79 exons spread over approximately 2.4 million nucleotides on the X chromosome. DMD is associated with a variety of underlying mutations, including exon duplications or deletions, as well as point mutations leading to nonsense translation through direct production of an in-frame stop codon, frameshift production of an in-frame stop codon, or aberrant inclusion of an intronic pseudo-exon with the concomitant production of an in-frame stop codon. In all cases, no functional dystrophin protein is produced. Becker muscular dystrophy (BMD) is a related condition with in-frame mutations that result in the production of a truncated but partially functional dystrophin protein. BMD patients, therefore, have milder symptoms, delayed disease progression, and longer life expectancy compared to DMD patients.

Casimersen is an antisense phosphorodiamidate morpholino oligonucleotide designed to bind to exon 45 of the DMD pre-mRNA and prevent its inclusion within the mature mRNA before translation. It is estimated that around 8% of DMD patients may benefit from exon 45 skipping, in which the exclusion of this exon results in the production of an internally truncated and at least partly functional dystrophin protein. Although fibrotic or fatty muscle tissue developed previously cannot be improved, this therapy aims to slow further disease progression through the production of partially functional dystrophin and alleviation of the pathogenic mechanism of muscle tissue necrosis.