S&P SmallCap 600 in Focus: CENTA REGN

The S&P SmallCap 600 Index -which tracks stocks with a market capitalization between $300 million and $2 billion -rose 1.9 points to 390.96.

Small-cap companies make up about 3 percent of the total U.S. securities market, but are often cited as good investments due to their low valuations and potential to grow into big-cap stocks.

Central Garden shares gained 74 cents, or 11.5 percent, to $7.20. Piper analyst Mitchell Kaiser raised the company to "Buy" from "Neutral," saying its new chief executive has stabilized the business and margins are poised to improve.

Shares of Arris Inc. climbed 57 cents, or 7.1 percent, to $8.59 after a Friedman, Billings, Ramsey analyst upgraded the communications technology provider to "Outperform" from "Market Perform," citing confidence in its cable modem termination system.

Drug developer PharmaNet Development Group Inc. advanced 82 cents, or 5.5 percent, to $15.85.

Among the decliners, Regeneron Pharmaceuticals Inc. plunged $3.05, or 14.2 percent, to $18.42 after an ovarian cancer drug it is developing with Sanofi-Aventis failed to meet its goal in a mid-stage study.

Shares of Guaranty Financial Group Inc. tumbled 63 cents, or 10.6 percent, to $5.34 after reaching a fresh 52-week low of $5.01 earlier in the session. On Wednesday, the Austin, Texas-based regional bank announced a stock offering of up to $350 million.

Shares of Red Robin Gourmet Burgers lost $3.04, or 7.9 percent, to $35.63 after the restaurant chain said its first-quarter profit fell 3 percent to miss Wall Street expectations.