The U.S. Food and Drug Administration Wednesday approved Mepsevii (vestronidase alfa-vjbk) to treat pediatric and adult patients with an inherited metabolic condition called mucopolysaccharidosis type VII (MPS VII).

MPS VII, traditionally known as Sly syndrome, is an uncommon, progressive condition that affects most tissues and organs and impacts less than 150 patients worldwide. The symptoms of MPS VII vary from patient to patient, but it causes various skeletal abnormalities that become more pronounced with age, including short stature and joint deformities that affect mobility.

The features of MPS VII include a large head, a buildup of fluid in the brain, distinctive-looking facial features that are described as "coarse," and a large tongue. Affected individuals also frequently develop an enlarged liver and spleen hepatosplenomegaly, heart valve abnormalities, and a soft out-pouching around the belly-button or lower abdomen.

The airway may become narrow in some people with MPS VII, leading to frequent upper respiratory infections and short pauses in breathing during sleep. Affected individuals may have developmental delay and progressive intellectual disability, although intelligence is unaffected in some people with this condition.

The life expectancy of individuals with MPS VII depends on the severity of symptoms. Some affected individuals do not survive infancy, while others may live into adolescence or adulthood.

MPS VII is a lysosomal storage disorder caused by the deficiency of an enzyme called beta-glucuronidase, which creates an abnormal buildup of toxic materials in the body’s cells. Mepsevii is an enzyme replacement therapy that works by replacing the missing enzyme.

The safety and efficacy of Mepsevii were analyzed in clinical trials to 23 patients ranging from 5 months to 25 years of age. Patients received treatment with Mepsevii at doses up to 4 mg/kg once every two weeks for up to 164 weeks. A six-minute walk test assessed efficacy in 10 patients who could perform the test. After 24 weeks of treatment, the mean difference in distance walked relative to placebo was 18 meters. Additional follow-up for up to 120 weeks showed continued improvement in three patients and stabilization in the others. Two patients in the Mepsevii development program experienced an increase in pulmonary function.

The most common side effects after treatment with Mepsevii include infusion site reactions, diarrhea, rash, and anaphylaxis.

“This approval underscores the agency’s commitment to making treatments available to patients with rare diseases,” said Julie Beitz, director of the Office of Drug Evaluation III in the FDA’s Center for Drug Evaluation and Research (CDER). “Before today’s approval, patients with this rare, inherited condition had no approved treatment options.”

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