SRK-015 is an investigational treatment being developed by biotech company Scholar Rock to improve muscle strength and motor function in patients with spinal muscular atrophy (SMA). SMA is a neurodegenerative disease caused by mutations in the SMN1 gene that leads to a loss of motor neurons and consequently muscle atrophy.

How SRK-015 works

SRK-015 is a selective and local inhibitor of the latent (not completely active) form of myostatin, a protein expressed primarily in skeletal muscle cells that largely works to inhibit muscle growth. In concert with other growth factors and hormones, myostatin helps to maintain healthy muscle mass.

Because SRK-015 selectively inhibits latent myostatin from becoming active in muscle tissue, it does not affect the activity of other closely related factors (such as GDF-11 or activin A) as is the case in traditional treatment approaches that block already activated (mature) myostatin or the myostatin receptor. In this way, it is hoped that SRK-015 will have fewer side effects than other myostatin inhibitors.

SRK-015 in clinical trials

Research has shown that animals deficient in myostatin have increased muscle mass and strength. SRK-015-mediated myostatin inhibition has shown similar results in preclinical studies.

Studies in primates and mice showed that SRK-015 helped to improve muscle capacity and strength. It increased primates’ lean body mass, particularly in fast-twitch muscle fibers, which are affected in SMA.

SRK-015 also effectively increased muscle mass and function in a mouse model of SMA in which SMN production was restored one or 24 days after birth to reflect early or later therapeutic intervention. It was also found that SRK-015 improved the bone structure of the animals. 

Scholar Rock announced in May 2018 the completion of a first dose of treatment for a placebo-controlled, double-blind Phase 1 clinical trial testing SRK-015 in healthy volunteers. Positive final results were announced in June 2019 and showed that SRK-015 was well-tolerated across all tested doses.

Scholar Rock started a Phase 2 trial (NCT03921528) in patients with SMA type 2 and type 3 in May 2019. The trial called TOPAZ aims to recruit 55 patients to evaluate the safety and efficacy of SRK-015. It is estimated to be completed in April 2021.

Other details

SRK-015 received orphan drug designation from the U.S. Food and Drug Administration in April 2018.

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