Amazon.com Inc. said Thursday that it has agreed to acquire PillPack, an online pharmacy that pre-sorts medications into different doses and handles both refills and renewals. PillPack delivers medications to customers and specializes in customers who take multiple prescriptions per day. The deal is expected to close in the second half of the year, and terms weren't disclosed. "PillPack's visionary team has a combination of deep pharmacy experience and a focus on technology," said Jeff Wilke, the chief executive officer of Amazon's Worldwide Consumer segment, in a release. Shares of pharmacy stocks CVS Health Corp. Walgreens Boots Alliance Inc. and Rite-Aid Corp. are down in premarket trading. Rite-Aid shares turned lower on the news, after they were initially up following earnings results. Insurers UnitedHealth Group Inc. and Aetna Inc. have seen their stocks fall as well. Shares of Walmart Inc. McKesson Corp. and Express Scripts Holding Co. are also down. Amazon shares are up 68% over the past 12 months, while the S&P 500 has gained 11%.