NEW YORK (AP) — Shares of drug developer Dendreon Inc. surged when they resumed trading after hours Tuesday after Nasdaq halted trading earlier in the day following a mysterious plunge in shares before release of data on an experimental cancer drug. The study of Dendreon's Provenge treatment for prostate cancer was almost universally hailed by stock analysts and doctors as positive, with the study showing it extended survival in patients with advanced prostate cancer by about a month compared with the only approved treatment, Sanofi-Aventis' chemotherapy drug Taxotere. But between 1:25 and 1:27 p.m. Tuesday and before release of the study results, shares plummeted from the mid-$20 range to $11.81, and the exchange halted trading. Nasdaq later said it investigated potentially erroneous trades that may have spurred the selloff, but that the trades would stand. Trading resumed after hours and shares rocketed to $25.75, more than doubling the lows seen during the fall and topping prices they had been before it happened. Merriman Curhan Ford analyst Joe Pantginis said he regarded the plunge as a "blip" and sees the study's results for effectiveness and safety as "nothing but a green light from the FDA" for a drug he forecasts being approved by mid-2010. The drug has come before the Food and Drug Administration before. Two years ago, the agency went against its advisers and delayed a decision on the drug, asking for more proof of safety and effectiveness. The most recent study was intended to provide that proof. Provenge, studied as a treatment for advanced prostate cancer, works like a vaccine, stimulating the body's own immune system to attack the cancer.