Here are some of the biggest premarket U.S. stock movers today:Supernus Pharmaceuticals surged 20.7% after agreeing to an all-stock merger of equals with Indivior Pharmaceuticals. Under the deal, Supernus shareholders will receive 1.5401 Indivior shares for each share held, while Indivior shareholders will receive a $1 billion special cash dividend before closing. The combined company will retain the Supernus name and continue trading under the SUPN ticker. Ferguson climbed 7.1% as investors positioned ahead of the building products distributor's inclusion in the S&P 500 before trading begins on Aug. 5. GameStop slipped 6% after announcing agreements with existing noteholders to exchange approximately $1.4 billion of convertible debt for newly issued Class A common stock.