Vertex Pharmaceuticals Incorporated (NASDAQ: VRTX) shares sank Monday on results from a trial of one of its cystic fibrosis treatments, but an analyst said further testing may still prove the drug's worth.

Deutsche Bank's Robyn Karnauskas maintained a Buy rating and a $155 price target, and said results of the Phase II study bolster the company's larger ongoing Phase III trial.

The study, of Vertex's VX-661 drug in combination with ivacaftor in 39 adults, met its primary safety end point.

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But lung function improvement of 4.4 percent after four weeks of treatment and 3 percent after 12 weeks was lower than some investors had reportedly hoped.

Karnauskas called the data important for the company but said Vertex may also benefit from future acquisitions to augment its portfolio of cystic fibrosis drugs.

Regulators are expected to rule by July on Vertex's application for its lumacaftor drug in combination with its ivacaftor treatment for cystic fibrosis.

"The pricing of the combo remains the key event to watch for," Karnauskas said.

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