Shares of biotechnology company CV Therapeutics (NASDAQ:CVTX) plummeted Monday as investors reacted to a letter from the Food and Drug Administration that outlined hurdles to approval of the company's angina drug, Ranexa. The letter, posted on the FDA's website, prompted the stock's 27% drop, as well as a rash of analyst downgrades.

Although the FDA letter acknowledges signs that the drug is effective, it says CV Therapeutics needs to address a serious side effect, QT prolongation, as well as questions regarding gender differences in effectiveness. The letter also seems to imply that more clinical trials will be required.

The stock has lost about half of its value since October, when hopeful investors were buoyed by word that the FDA had set the review date. There were many reasons for the excitement. If approved, Ranexa could have the distinction of being the first angina drug introduced in two decades, as well as the first drug in a new class.

Angina (chest pain associated with decreased blood flow to the heart) is a growing problem as America's population ages. CV Therapeutics has cited U.S. Census Bureau statistics in its regulatory filings, which estimate that the number of people over the age of 55 will increase by 80% during the next 30 years. Some analysts estimate the market for Ranexa somewhere between $300 million and $500 million.

However, CV Therapeutics has a long way to go before turning that potential into profit. The company is still operating at a loss, with no drugs on the market. And Ranexa is its star candidate. While it has others in various stages of research and clinical trials, this was the one closest to regulatory approval.

Despite Ranexa's promise when and if the FDA gives the green light, potential rewards still seem more than a heartbeat away.

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