Nvidia (NVDA) shares are dropping in premarket trading on "triple witching" day, where various stock contracts expire simultaneously, causing market volatility. Meanwhile, Gilead Sciences (GILD) is climbing after its lenacapavir drug proved effective in preventing HIV in women. Biotech Sarepta Therapeutics (SRPT) is seeing a surge in shares as the FDA expands approval for its gene therapy. Chipotle Mexican Grill (CMG) is preparing for a 50-for-1 stock split next week, leading to renewed focus on the company. Even though CarMax's (KMX) quarterly results missed expectations, the used-car retailer is still seeing gains in premarket trading. With U.S. stock futures falling after Nvidia's decline, investors should keep a close eye on these developments today.