Shares of Spirit Airlines Inc. /zigman2/quotes/205782179/composite SAVE +19.46% tumbled 10% in premarket trading Monday, after the discount air carrier said it adopted a shareholder rights plan, also known as a "poison pill," to help block any person or group of investors from taking advantage of the sharp drop in equity prices to take control of the discount airline. The air carrier said given the coronavirus pandemic's impact on the airline industry, the stock has fallen 63% below the peak prior to the recent decline. The rights plan would become exercisable if a person or investor group acquired 10% or more of the shares outstanding. "The adoption of the Rights Agreement is intended to enable all Spirit stockholders to realize the full potential value of their investment in the company and to protect the interests of the company and its stockholders by reducing the likelihood that any person or group gains control of Spirit through open market accumulation or other tactics (especially in current volatile markets) without paying an appropriate control premium," the company said in a statement. The stock has lost 48% over the past month through Friday, while the S&P 500 /zigman2/quotes/210599714/realtime SPX +1.36% has lost 14%.