ProQR Therapeutics (PRQR - Get Report) presented clinical data Thursday showing for the first time that its experimental drug QR-010 improves the function of a key protein that is defective in patients with cystic fibrosis.

The development of QR-010 is still early but Thursday's preliminary study results were encouraging enough to boost ProQR's stock price by 35% to $8.30 after a 38% gain in early trading.

Cystic fibrosis is caused by mutations in the gene responsible for the production of a protein known as CFTR, which stands for Cystic Fibrosis Transmembrane Conductance Regulator. The mutations create defective CFTR protein. The lack of functional CFTR protein causes mucus to build up in the airways of the lungs. As a result, cystic fibrosis patients suffer from severe breathing problems and bacterial infections. Fatal respiratory failure is a common outcome.

The ProQr drug QR-010 is a snippet of messenger RNA designed to correct the error in the gene so that functional CFTR protein is produced. The drug is delivered as a mist inhaled by cystic fibrosis patients into their lungs.

By contrast, Vertex Pharmaceuticals' (VRTX - Get Report) approved cystic fibrosis drugs Kalydeco and Orkambi work by improving the function of defective CFTR protein and bringing more of the protein to the surface of cells.

In the study presented Thursday, ProQR swabbed the nasal passages of cystic fibrosis patients with QR-010 for one month and measured a biomarker, nasal potential difference (NPD), to asses the ability of to restore CFTR function.

In seven cystic fibrosis patients with two copies of the F508 mutation, the most common causes of the disease, four weeks of treatment with QR-010 reduced the NPD value by 4.1mV, or significantly compared to baseline, ProQR said.

In another group of patients with one copy of the F508 mutation plus an additional, different mutation, QR-010 had no effect on NPD values, the company said.

"NPD is a reliable, direct and specific measurement of CFTR activity and is therefore used as an important endpoint to assess CFTR function in clinical trials in CF. As CFTR dysfunction is the key problem in CF, the restoration of CFTR function as measured by NPD is an important early signal for potential future clinical benefit for patients", said Dr. John Clancy of the Cincinnati Children's Hospital and the principal investigator of the QR-010 study, in a statement.

Improving the function of CFTR protein is not sufficient for the approval of a cystic fibrosis drug. To reach the market, ProQR will need to demonstrate treatment with QR-010 can improve lung function in cystic fibrosis patients.

The first such data from a small clinical trial is expected in the middle of 2017.

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