Patients with Duchenne muscular dystrophy will be disappointed with news that US regulators have rejected BioMarin’s experimental therapy Kyndrisa, dashing hopes of access to the first treatment specifically approved for the condition.

The US Food and Drug Administration has issued with the firm with a Complete Response letter after concluding that the standard of substantial evidence of effectiveness has not been met.

Kyndrisa (drisapersen) is an investigational antisense oligonucleotide drug being developed for the treatment of the largest subset of Duchenne muscular dystrophy amenable to single exon skipping, acquired by BioMarin via its $840-million purchase of Prosensa in November last year.

The drug has somewhat of a mixed history, having had a stint at GlaxoSmithKline until a failed Phase III trial spurred the drug giant to hand back rights. In the trial, Kyndrisa missed the primary endpoint of a statistically significant improvement of distance walked in six minutes compared to placebo. 

However, subsequent analyses of data from the entire clinical programme showed a 49-metre difference between those on continual treatment (n=52) and those who had been on placebo for 48 weeks followed by active drug (n=31), re-igniting confidence in the drug.

BioMarin said it is now reviewing the FDA’s response and will work with the agency to determine the appropriate next steps for the application.

The drug remains under regulatory review in Europe.