Merger-Arb Bet at Amgen-Horizon Deal’s Lowest Hour Spurs 25% Win

Hedge funds Pentwater Capital Management, HBK Investments and Farallon Capital Management are reaping a windfall from bets that US regulators wouldn’t be able to stop Amgen Inc. from completing its $28 billion takeover of Horizon Therapeutics Plc.

The deal closedFriday, leaving Pentwater with a $1.8 billion stake in Horizon, based on the 15.5 million shares it had amassed, the most among hedge funds and second only to Vanguard Group Inc. overall, data compiled by Bloomberg show.

The Illinois-based money manager stands out for the bold timing of its decision to boost its holdings, with a net purchase ...