Casimersen, sold under the brand name Amondys 45, is an antisense oligonucleotide medication used for the treatment of Duchenne muscular dystrophy (DMD) in people who have a confirmed mutation of the dystrophin gene that is amenable to exon 45 skipping.

[2][5] Casimersen was approved for medical use in the United States in February 2021,[1][2][6] and it is the first FDA-approved targeted treatment for people who have a confirmed mutation of the DMD gene that is amenable to skipping exon 45.

Casimersen is an antisense phosphorodiamidate morpholino oligonucleotide designed to bind to the exon 45 of the DMD pre-MRNA, which prevents its exclusion into the mature RNA before translation.

[2] All participants were male, between 7 and 20 years of age, and had a genetically confirmed mutation of the DMD gene that is amenable to exon 45 skipping.

[2] The benefit was evaluated by measuring the level of dystrophin in muscle biopsies in 43 participants before treatment and at week 48, in an interim analysis.