Shares of Altaba Inc. formerly known as Yahoo, climbed 1.5% in premarket trade Thursday, toward an 18-year high, after the company said it would buy up to 195 million shares of its common stock at about a 3.8% premium to Wednesday's closing price of $80.28. Altaba was Yahoo until the company completed the sale of its operating business to Verizon Communications Inc. last year, and is now a closed-end management investment company that owns a "substantial" stake in Alibaba Group Holding Ltd. Altaba said the purchase price of its shares will be equal to 35% of Wednesday's stock closing price of Alibaba shares of $208.30, plus 5% of the Alibaba volume-weighed average price of $208.8445. That equals $83.35 a share. Altaba's stock, which is on track to open at the highest level seen during regular session hours since April 2000, has run up 15% year to date through Wednesday, while Alibaba's stock has climbed 21% and the S&P 500 has gained 3.7%.