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Medical Rounds

New Heights in Achondroplasia Treatment

While short stature is a hallmark of achondroplasia, the most common form of dwarfism worldwide, those with the condition are also prone to develop sleep apnea, chronic ear infections, neurological problems, spinal stenosis and bowed legs. Frequently, surgical treatments are required to relieve pain and other symptoms.

Now researchers at Johns Hopkins Medicine, the Murdoch Children’s Research Institute in Australia and seven other medical institutions report in a study of 35 children and teenagers with achondroplasia that an experimental drug allowed the average annual growth rate to increase.

The patients’ average boost in height to about 6 centimeters (2.4 inches) per year is close to growth rates among children of average stature, and the side effects of the drug, called vosoritide, were mostly mild, according to the researchers, whose study appeared recently in the New England Journal of Medicine.

“An increase in the annual growth rate alone may have a positive effect on some patients’ quality of life. For other patients, now and in the future, our hope is that the altered bone growth throughout the body could ease such problems as sleep apnea and neurological, leg and back problems, and improve their quality of life,” says Julie Hoover-Fong, director of the Greenberg Center for Skeletal Dysplasia in the Johns Hopkins McKusick-Nathans Institute of Genetic Medicine.

“Right now, the results of the study show an impact on growth, and this effect is sustained, at least over nearly four years in this trial. The potential long-term benefit will take more time to observe,” she adds.

Currently there are no treatments able to reverse achondroplasia, which is caused by mutations in a gene — called FGFR3 — that result in the excess production of proteins that slow bone growth, nor are there ways to treat the genetic culprit itself. Growth hormone has been approved to treat the condition in Japan and occasionally is used off-label elsewhere but is not considered very effective in achondroplasia.

Vosoritide is a synthetic version of a protein present in humans called C-type natriuretic peptide. It is designed to bind to a specific receptor on the surface of chondrocytes, a type of cartilage cell found in the growth plates of bones. Once joined, the vosoritide-receptor connection sends a signal inside the fibroblast to stanch the flow of negative growth factors that were triggered by the mutation in the FGFR3 gene.

“This is the first therapeutic option that targets the molecular cause of the condition,” says Hoover-Fong, who notes that at least four other experimental drugs that target different molecular bone growth receptors or pathways are currently being tested in children and teenagers with achondroplasia at Johns Hopkins and elsewhere.