Considering the Promise of Vamorolone for Treating Duchenne Muscular Dystrophy

The BALB/c.mdx62 mouse exhibits a dystrophic muscle pathology and is a model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating monogenic skeletal muscle-wasting disorder. Although many pharmacological and genetic interventions have been reported in preclinical studies, few have progressed to clinical trials with meaningful benefit. Identifying therapeutic potential can be limited by availability of suitable preclinical mouse models. More rigorous testing across models with varied background strains and mutations can identify treatments for clinical success. Here, we report the generation of a DMD mouse model with a CRISPR-induced deletion within exon 62 of the dystrophin gene (Dmd) and the first generated in BALB/c mice. Analysis of mice at 3, 6 and 12 months of age confirmed loss of expression of the dystrophin protein isoform Dp427 and resultant dystrophic pathology in limb muscles and the diaphragm, with evidence of centrally nucleated fibers, increased inflammatory markers and fibrosis, progressive decline in muscle function, and compromised trabecular bone development. The BALB/c.mdx62 mouse is a novel model of DMD with associated variations in the immune response and muscle phenotype, compared with those of existing models. It represents an important addition to the preclinical model toolbox for developing therapeutic strategies.

AMONDYS 45 (Casimersen), a Novel Antisense Phosphorodiamidate Morpholino Oligomer: Clinical Considerations for Treatment in Duchenne Muscular Dystrophy

AMONDYS 45 (casimersen) is an antisense oligonucleotide therapy used to treat Duchenne muscular dystrophy (DMD), a rare genetic disorder characterized by a mutation in the DMD gene. Symptoms include progressive muscle weakness, respiratory and cardiac complications, and premature death. Casimersen targets a specific mutation in the DMD gene that results in the absence of dystrophin protein, a key structural component of muscle fibers. While there is currently no cure for DMD, exon-skipping therapy works by restoring the reading frame of the mutated gene, allowing the production of a partially functional dystrophin protein. Clinical trials of casimersen have shown promising results in increasing dystrophin production, as measured by polymerase chain reaction (PCR) droplets when compared to placebo. In a randomized double-blind trial, patients who received casimersen had significantly higher dystrophin levels when compared to those who received placebo. Casimersen therapy is administered through repeated intravenous infusions, although the optimal dosage and duration of treatment are still under investigation. Based on the completed and ongoing clinical trials, casimersen has been well tolerated, with most adverse events being mild and unrelated to casimersen. In 2021, AMONDYS 45 (casimersen) received approval from the US Food and Drug Administration (FDA) for the treatment of Duchene muscular dystrophy in patients with a mutation of the DMD gene that is amenable to exon 45 skipping. These collective findings indicate that casimersen has the potential to elicit functional changes in individuals with DMD, although further studies are necessary to comprehensively evaluate the specific functional improvements. Regardless, the FDA approval and ongoing clinic trials mark a significant milestone in the development of DMD treatments and offer hope for those affected by this debilitating disease.