The Sensex, which opened 72 points up in the opening trade was down by nearly 150 points at 18,052 points at 1100 hours with initial gains erased by the emergence of heavy profit-booking in capital goods and realty stocks. Similarly, the broad-based National Stock Exchange Nifty index was down by 46 points at 5,409 points after a gain of 18 points in the morning trade. Brokers said the emergence of profit-taking after a rally during the previous two sessions mainly dragged the Sensex down.