Shares of Digitas plunged nearly 20 percent overnight after it lowered its full-year revenue and earnings estimates because of "specific client challenges."

According to AdWeek, Boston-based Digitas says it now projects 2006 fee revenue of $380-395 million, instead of $405-425 million; net earnings of 41-49 cents per share, rather than 46-56 cents per share; and adjusted cash earnings of 54-62 cents per share, instead of 61-71 cents per share. CEO David Kenny said despite near-term, "specific client challenges, our largest and longest-standing client relationships remain strong."

Apparently American Express is mulling some shifts - about one-tenth of the account - to smaller, less-expensive shops. Digitas has not commented on AmEx, which has reaffirmed its relationship with Digitas. AmEx and General Motors accounted for half of the company's 2005 revenue.

Digitas reported third-quarter net income of $13.5 million, or 14 cents per diluted share, compared with $11.9 million, or 12 cents per share, for the year-ago period - an increase of about 13 percent. Revenue was $191 million, up more than 40 percent.