* Alcantara buys 9.3 percent
* Plans to join shares with Polanco family stake
* Head of radio and news to become new CE0 (Adds details on capital increase and debt)
By Robert Hetz
MADRID, July 22 (Reuters) - The chief executive of Spain's Prisa will leave the media group in October as Mexican businessman Roberto Alcantara takes a 9.3 percent stake to become the largest single shareholder, the company said on Tuesday.
Prisa, the indebted publisher of the country's most widely-read newspaper El Pais, said CEO Fernando Abril-Martorell would step down on Oct.1, to be replaced by current head of radio and news, Jose Luis Sainz Diaz.
Mexico's Alcantara, who runs a transport business, will pay 100 million euros ($135 million) to purchase about 189 million new shares in Prisa at 0.53 euros each, a 40 percent premium to the average share price over the past three months.
Prisa shares closed on Tuesday up 5.8 percent at 0.33 euros.
Prisa, which ran into debt trouble more than five years ago, said it would use the new funds to start paying back at least 600 million euros of debt at a minimum 25 percent discount to face value as part of an agreement reached with creditor banks.
The company had more than 3 billion euros of debt in 2013.
Spain's Polanco family, which founded El Pais after a long dictatorship ended in the 1970s, will group its stake with Alcantara's shares to jointly hold nearly 20 percent of the company, Prisa Chairman Juan Luis Cebrian told Reuters.
Cebrian, who has close ties with the Polanco family, initiated the top management change but also requested that Abril-Martorell remain on the company's board of directors and El Pais's editorial committee.
"The profile of the new CEO will be less financial and more focused on the group's business and strategy," Cebrian told Reuters in a telephone interview. ($1 = 0.7425 Euros) (Reporting by Tracy Rucinski and Robert Hetz; Editing by Sarah Morris and Keiron Henderson)