Casimersen

Duchenne muscular dystrophy (DMD) is an X-linked recessive allelic disorder characterized by a lack of functional dystrophin protein, which leads to progressive impairment of ambulatory, pulmonary, and cardiac function and is invariably fatal. A related, albeit a less severe, form of muscular dystrophy known as Becker muscular dystrophy (BMD) is characterized by shortened and partially functional dystrophin protein production. Although corticosteroids effectively slow disease progression in both DMD and BMD patients, they do not address the underlying molecular pathogenesis. The application of antisense oligonucleotides in DMD patients with specific mutations allows for exon skipping to produce truncated BMD-like dystrophin proteins, which restore partial muscle function and slow disease progression. Casimersen is a phosphorodiamidate morpholino oligonucleotide (PMO); PMOs are oligonucleotides in which the five-membered ribofuranosyl ring is replaced with a six-membered morpholino ring, and the phosphodiester links between nucleotides are replaced with a phosphorodiamidate linkage. In this manner, PMOs are much less susceptible to endo- and exonucleases and exhibit drastically reduced metabolic degradation compared to traditional synthetic oligonucleotides. Casimersen is the most recent in a line of approved PMOs for treating DMD, including eteplirsen and viltolarsen. However, the specific mutations, and hence the precise exon skipping, targeted by each is different. Casimersen was granted accelerated FDA approval on February 25, 2021, based on data showing an increase in dystrophin levels in skeletal muscle of patients treated with casimersen; this approval is contingent on further verification in confirmatory trials. Casimersen is currently marketed under the tradename AMONDYS 45™ by Sarepta Therapeutics, Inc.