Shares of Palantir Technologies Inc. PLTR, -0.25% have seesawed to a loss of 0.6% in midday trading, reversing an earlier intraday gain of as much as 2.3%, as the data integration and software company’s stock is set to suffer its worst week since going public. The stock has plunged 17.8% this week, as it is set to lose ground in every session, and is on track to decline in 11 of the past 12 trading days. That would be the biggest one-week selloff since the stock went public on Sept. 30, surpassing the previous record drop of 15.1% for the week ended Dec. 31. During the stock’s recent weakness, a number of insiders made millions trading the stock. The stock has now lost 18.0% over the past three months, while the S&P 500 SPX, -0.48% has gained 6.0%.