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Walgreens Boots Alliance (WBA - Get Report)  posted stronger-than-expected quarterly earnings and unveiled a $10 billion share buyback Thursday, but a move by Amazon Inc. (AMZN - Get Report) to buy privately owned PillPack sent shares in the newest Dow component tumbling and underscored its sensitivity to new market rivals.

Walgreens Boots, which joined the Dow Jones Industrial Average earlier this week when General Electric Co. (GE - Get Report) was turfed out, said earnings for the three months ending in May, its fiscal third quarter, came in at $1.53 per share, well ahead of the Street forecast of $1.48 and an 15.5% advance from the same period last year. Group sales rose 14% to $34.3 billion, the company said, the bulk ($25.9 billion) coming from its American retail pharmacy division. 

However, weaker comparable store sales, which fell 1.4% at its international pharmacy division and 1.2% domestically, as well as Amazon's deal to buy medication delivery company PillPack for an undisclosed sum, sent the stock into a tailspin.

"PillPack's visionary team has a combination of deep pharmacy experience and a focus on technology," said Amazon's consumer CEO TJ Parker. "PillPack is meaningfully improving its customers' lives, and we want to help them continue making it easy for people to save time, simplify their lives, and feel healthier. We're excited to see what we can do together on behalf of customers over time."

Walgreens Boots shares plunged 10% at the opening bell to change hands at $59.59 each, a move that wipes out all of its gains for the month and put it down 18% for the year. Amazon shares rose 1.1% to $1,678.90 each.

The company said it sees 2018 earnings of between $5.90 and $6.05 per share, a figure that sits largely in-line with the consensus forecast of $5.95 per share.

"I am pleased that, in what has been a challenging environment, we have again delivered solid earnings per share growth combined with healthy cash flow," said CEO Stefano Pessina. "We expect to continue to drive growth, bringing more patients to our U.S. pharmacies through the recent acquisition of Rite Aid stores and through strategic partnerships."

"The $10 billion share repurchase program announced this morning demonstrates our confidence in future business performance and, as ever, our focus on driving long-term stockholder value," he added.