Shares of biotech company Genta ( GNTA) took a pounding Friday on news that the Food and Drug Administration is questioning the effectiveness of its main cancer therapy. The share price of the Berkeley Heights, N.J., company dropped to $8.88 in late-afternoon trading, a 38% decline from its opening price of $13.15 a share. The slide happened after the FDA posted documents on its Web site that said a clinical trial on Genasense, intended for treatment of melanoma, a form of skin cancer, might yield falsely positive results and said "progression-free survival and antitumor response rate are questionable." The FDA documents precede a critical review of the drug that is required if Genta hopes to get it cleared for sale. In a prepared statement, the company said it would not discuss the agency's comments before the advisory meeting, scheduled for Monday. Genta also said it lost $12.5 million, or 16 cents a share, for the three months ended March 31, up from its loss of $9.6 million, or 12 cents a share, a year ago. Shares in Aventis ( AVE), the French biotech company that paired with Genta in the drug's development, were up slightly in late afternoon trading at $75.90, gaining $2.40 from their Thursday close of $73.50. Genasense did not shrink tumors at a demonstrable rate when combined with another chemotherapy drug, according to the FDA Web site. Patients in the trial who received only the other drug, called DTIC, showed similar effects on their tumors.