Shares of Cougar Biotechnology Inc. fell 22 percent Monday on exceptionally large trading volume despite no important news about the company.

Shares of the Los Angeles-based cancer drug developer lost $5.20 to close at 18.10. Around 600,000 shares changed hands today, compared to a 10-day average of 14,000. The company has 20.5 million shares outstanding.

Broadpoint Capital Inc. analyst Simos Simeonidis said the sudden drop may have come from a large institutional investor unloaded all or part of its stake. Cougar has at least 12 holders of more than 600,000 shares.

The company recently reported positive data on its lead prostate cancer drug candidate, Simeonidis said, and company management told him there was no subsequent news that could have swung shares so violently.

“It’s hard for existing shareholders, but they should be aware that can happen to a thinly traded stock,” said Simeonidis, adding that earlier in the day he had advised clients to see the drop as a buying opportunity. Some investors may have taken his advice; Cougar stock gained $1.71, or 9.5 percent, to hit 19.81 in after-hours market.