Nivalis Therapeutics Inc. said a test of its cavosonstat treatment for cystic fibrosis "failed ... to demonstrate benefit" in a recent trial.

Shares in the Boulder biotech (Nasdaq: NVLS) plummeted more than 50 percent in early Tuesday trading after analysts downgraded the company's shares.

RW Baird downgraded Nivalis to "neutral" from "outperform," and Stifel Nicolaus cut its rating to "hold" from "buy."

"We are disappointed in the outcome of this trial," said Nivalis CEO Jon Congleton, in a statement, who added the company will "continue to investigate the therapeutic potential of cavosonstat."

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