Citigroup ( C), ravaged by its exposure to subprime mortgages, wrote down a pretax sum of $18.1 billion and took a fourth-quarter loss of $9.83 billion, or $1.99 a share. That reverses last year's profit of $5.13 billion, or $1.03 a share, and it compares with Wall Street targets for a loss of $1.03 a share, per Thomson Financial. Revenue plummeted 69.7% to $7.22 billion, also thanks to the massive subprime writedowns. Citi shares were recently sliding $2.26, or 7.8%, to $26.80, in sharp retreat from the upward float they had enjoyed over the past week. On the uptick, Arena Therapeutics ( ARNA) picked up $1.14, or 15%, to $8.55, after an upgrade from Bank of America analyst William Ho. He raised his rating on the stock to buy from neutral, predicting that a data safety monitoring committee reviewing echocardiograms won't find a link between Arena's obesity drug Lorcaserin and heart valve damage. Elsewhere, Lifecore Biomedical ( LCBM) said Tuesday that it will be acquired by Warburg Pincus for $17 a share in cash, a 30% premium over the average price of the shares for the last 30 days, valuing the company at about $239 million. The company expects the transaction will close before the end of the first quarter. Shares climbed $3.89, or 30.3%, to $16.73. Back in earnings, Boston bank State Street ( STT) said fourth-quarter income sank 27.8% year over year to $223 million, 57 cents a share. As previously disclosed , that's primarily because the Boston bank set aside 71 cents a share for covering potential legal costs related to losses from its mortgage-backed investments. Even excluding that one-off expense, however, the bank still came in 7 cents shy of the $1.35 per-share analyst consensus. Shares surrendered 6.1% to $79.70 in recent trading. Shares of Minneapolis-based money-transfer and payment-processing servicer MoneyGram International ( MGI) were cut in half, down 50.7% at $6. The company announced late Monday that, from Sept. 30 to Nov. 30, its investment portfolio lost an unrealized $571 million, bringing total losses to $860 million. MoneyGram is negotiating with Thomas H. Lee Partners for a cash injection of about $800 million. Under the proposed deal, Thomas H. Lee Partners would control between 60% and 65% of MoneyGram. Also losing ground was Memsic ( MEMS), shares of which ceded 19.4% to $6.78. The Andover, Mass., provider of semiconductor and systems solutions exercised its overallotment option to buy 823,000 shares of its stock at $10 a share, the price at the company's IPO, less underwriting discounts and commissions.