Biotech company Amgen said Thursday it is buying ChemoCentryx, a drugmaker focused on treating autoimmune disorders, for roughly $3.7 billion, including debt.

The all-cash deal works out at $52 a share and is a 115% premium to ChemoCentryx's (ticker: CCXI) closing price on Thursday.

Last year, ChemoCentryx got approval for a drug called Tavneos that treats ANCA-associated vasculitis, a collection of relatively rare autoimmune diseases with an unknown cause. "The acquisition of ChemoCentryx represents a compelling opportunity for Amgen to add to our decades-long leadership in inflammation and nephrology," said Amgen (AMGN) CEO Robert Bradway.

Amgen stock was up 0.1% to $247.42 on the news, whereas ChemoCentryx was up 108% to $50.24.

The jump in ChemoCentryx's stock shouldn't be too surprising. In most cases, the company that gets bought sees a jump in its stock price after the announcement because the acquirer is paying a premium for the transaction.

That makes sense because the greater the premium, the higher the chances that shareholders will approve the deal. Take EVO Payments (EVOP) stock, for example. It closed 23% higher on Monday, after Global Payments (GPN) said it was acquiring the payments-software business.

Amgen's transaction is expected to close in the fourth quarter of 2022, subject to approval by ChemoCentryx's shareholders.

Amogen is scheduled to report its second-quarter financial results after market close on Thursday. Analysts tracked by FactSet expect adjusted earnings of $4.39 a share from sales of $6.53 billion.