Ex-Hub prosecutor shifts allegiance to drug makers

But a year and a half ago, Loucks, a Republican, left the United States attorney’s office in Boston after he was passed over for the top post and President Obama appointed a Democrat. Instead, Loucks joined Skadden, Arps last July, and has startled former allies by emerging in recent months as zealous a corporate defender as he was a prosecutor, complete with proposals seeking more lenient treatment for companies he once vilified.

In a six-page memo last month to clients, which may include some of the very same corporations he prosecuted repeatedly, Loucks bemoaned strategies he had embraced.

“The government and the whistle-blower have an advantage,’’ he wrote, complaining that federal investigators were now using the law unfairly. “While prosecutors often assert the company has engaged in ‘serious’ misconduct, they keep the company in the dark, often for years, as to the specific allegations.’’

Those who have known him are quick to recall that his crowning achievement was a $2.3 billion settlement against Pfizer after a four-year secret inquiry.

“We’re all disappointed that he’s gone over to the dark side because it seemed that he was a good prosecutor,’’ said Shelley R. Slade, a whistle-blowers’ lawyer in Washington and a former senior counsel for health care fraud at the Justice Department.

“I looked upon it with sadness,’’ said Patrick Burns, spokesman for the whistle-blower advocacy group Taxpayers Against Fraud. “He did great work in the Boston office. He’s a good lawyer. It’s just too bad.’’

Federal ethics rules prohibited Loucks from any dealings with the US attorney’s office in Boston for a year after his resignation, and he can never be involved in cases he investigated directly. But he is not barred from representing clients he once prosecuted on other matters, and his law firm’s roster includes some of the biggest companies he once investigated, including Pfizer, Merck, and Schering-Plough.

In interviews and a lengthy e-mail exchange, Loucks said his views on the whistle-blower law had evolved.