Chief Executive Matthias Wiedenfels, who was appointed just over a year ago, will be replaced by Engelbert Tjeenk Willink, a former board member of privately held German drugmaker Boehringer Ingelheim, Stada said in a statement.

Bernhard Duettmann, previously finance chief at Nivea owner Beiersdorf and then chemicals group Lanxess, will succeed Stada's Chief Financial Officer Helmut Kraft. Willink and Duettmann have been appointed until the end of 2017, Stada said.

The move came only hours after Stada said that private equity firms Bain and Cinven were considering seeking regulatory clearance to make a new takeover offer for the group after their 5.3 billion euro (£4.6 billion) bid fell through, driving Stada's shares higher.

Investors representing 65.52 percent of Stada's equity had signed up for their last bid, missing a 67.5 percent threshold, despite the 49 percent premium offered by the buyout groups to trump a rival offer from private equity duo Advent and Permira.

A renewed approach would need to be approved by both German financial watchdog Bafin and Stada itself because a suitor is normally not allowed to make another offer for at least a year.

"Should further takeover offers be made, we will examine and evaluate them impartially," Stada supervisory board Chairman Ferdinand Oetker said.

Sources told Reuters last week that a new bid could be launched as early as this week. The private equity firms are expected to bid at the same price but lower the acceptance threshold to 65 percent, the sources said at the time.

SUPPORT

Several people close to the matter had said Bain and Cinven were talking to investors to drum up support for a potential new offer.

The bidders wanted to secure irrevocable commitments to tender from shareholders representing more than 15 percent of Stada's voting rights.

Some shareholders - mainly hedge funds, who currently own more than 30 percent of the stock - held back some of their shares, speculating on securing a higher price for any remaining stock after a successful initial tender offer.

Others meanwhile have signed commitments to try and secure the renewed offer's success as it promises a more certain deal than a potential third-party bid from funds such as Advent and Permira or Shanghai Pharma, which had shown interest in Stada.

Kepler Cheuvreux analyst Oliver Reinberg raised his recommendation on Stada shares to "hold" from "reduce" on Tuesday, saying he assumed a new bid by Bain and Cinven would reach its mark.

"Given the low margin of failure in the first bidding process - note they only fell 2 percent short of the outstanding shares tendered - we assume that the deal will ultimately succeed," he said in a note.

Shares in Stada rose 2.1 percent to 63.87 euros by 1530 GMT on Tuesday, making them the biggest gainers on Germany's MDAX index of mid-sized companies.

Stada said that Wiedenfels and Kraft had told its supervisory board they were stepping down for "personal reasons".

People close to the matter said, however, that relations between the executives and the supervisory board had become increasingly strained due to their handling of the takeover process.

"It was clear from the beginning that Wiedenfels would be replaced. But the timing of the move was not ideal as it creates insecurity around the deal," one of the sources said.

(Reporting by Maria Sheahan; Editing by Susan Thomas and Keith Weir)

By Maria Sheahan and Arno Schuetze