NEW YORK – A former hedge fund portfolio manager was arrested Tuesday in what prosecutors called perhaps the most lucrative insider trading scheme of all time – an arrangement to obtain secret, advance results of tests on an experimental Alzheimer’s drug that helped his fund and others make more than $276 million.

The case also led authorities to investigate the activities of one of the nation’s wealthiest hedge fund managers, Steven A. Cohen.

Mathew Martoma, the portfolio manger, was accused in U.S. District Court in Manhattan with using the information to advise other investment professionals to buy shares in the companies developing the drug, then later to dump those investments and place financial bets against the companies when the tests returned disappointing results.

“The charges unsealed today describe cheating coming and going,” U.S. Attorney Preet Bharara said. The scheme unfolded “on a scale that has no historical precedent.”

Martoma’s trades helped reap a hefty profit from 2006 through July 2008, while he worked for CR Intrinsic Investors LLC of Stamford, Conn., an affiliate of SAC Capital Advisors, a firm owned by Cohen.

Cohen is not referred to by name in court papers but is frequently alluded to for his dealings with the defendant in the weeks leading up to an announcement about the drug trial.

The government has been scrutinizing SAC since at least November 2010, when the FBI subpoenaed SAC and other influential hedge funds.

SAC spokesman Jonathan Gasthalter said the company and Cohen “are confident they have acted appropriately.”

The FBI said the scheme developed after Martoma met a doctor in Manhattan involved in an Alzheimer’s drug trial in October 2006.

Martoma’s attorney, Charles Stillman, said, “What happened today is only the beginning of a process that we are confident will lead to Mr. Martoma’s full exoneration.”

Martoma was arrested at his home in Boca Raton, Fla., and made an initial appearance in federal court in West Palm Beach, Fla., where he was released on $5 million bail on charges of conspiracy to commit securities fraud and securities fraud.

The Securities and Exchange Commission filed civil papers in the case against CR Intrinsic Investors, Mathew Martoma and Dr. Sidney Gilman. The SEC complaint said Martoma carried out the scheme with Gilman, an 80-year-old professor of neurology at the University of Michigan Medical School who served as chairman of a safety committee overseeing the clinical trial.