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Former Allergan CEO continues battle with Pershing

After his bitter, 8½-month battle with Valeant and activist hedge fund Pershing Square last year, former Allergan chief David Pyott is not going quietly.

"I've visited many members of Congress, doing my best to ensure whatever oversight is possible," Pyott said at a hearing Thursday by the Senate Permanent Subcommittee on Investigations on the U.S. tax code.

Allergan said its U.S. domicile and higher tax code gave it $9 billion more in value to a foreign purchaser; Canada-based Valeant has a tax rate of less than 10 percent.

In an unusual arrangement, Valeant teamed with Bill Ackman's Pershing Square last year in a pursuit of Allergan. Pershing acquired about 10 percent of Allergan and tried to push the Botox maker into a deal.

Pyott, whose company was ultimately sold to Ireland-based Actavis for $66 billion, testified that he hoped the Securities and Exchange Commission will investigate the Ackman-Valeant pursuit, looking into "this novel structure with possible breach of insider trading laws and other securities regulations."

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Pyott said the U.S. tax code needs to be changed because it puts U.S. companies at a disadvantage. But he also took issue with what he called "slow reporting periods" for acquirers of stock, calling the rules "antiquated."

Despite Allergan ultimately going to a different acquirer, Pyott said Pershing recorded a profit of almost $1 billion on the deal.

"You can tell I'm a person of principle, and a lot of people in my shoes just move on," Pyott told the subcommittee. "I'm afraid I feel pretty strongly about a lot of things that happened last year."

Pershing declined CNBC's request for comment. Valeant did not immediately respond to a request.