FILE PHOTO - Leon Black, Chairman, CEO and Director, Apollo Global Management, LLC, speaks at the Milken Institute's 21st Global Conference in Beverly Hills, California, U.S. May 1, 2018. REUTERS/Lucy Nicholson

(Reuters) - Apollo Global Management APO.N and its Chief Executive Leon Black are being sued by Caldera Holdings, a venture founded by Imran Siddiqui, a former managing director at the buyout firm, a court filing on Thursday showed.

Caldera’s complaint, filed in the New York Supreme Court, alleges that Apollo pursued a campaign of “systematic defamation and interference,” including “disparaging the company in the marketplace.”

Caldera is also asking for at least $1.5 billion in damages, according to the filing.

“We strongly believe Caldera’s claims completely lack any merit, and we intend to defend ourselves vigorously,” Apollo spokesman Charles Zehren said in an emailed statement.

Caldera and Apollo's life insurance unit Athene Holding Ltd ATH.N are both bidding for control of American Equity Investment Life Holding Co AEL.N, a U.S. provider of annuities and life insurance products, according to a Financial Times report on Thursday.

In May, Iowa-based American Equity said it would explore a sale, attracting interest from firms such as Athene Holding and FGL Holdings FG.N. Thursday's filing alleges that Apollo hindered Caldera from competing with Athene, causing rifts between the company and its investors.

Caldera could not be reached for comment, outside regular U.S. business hours.