Activist investor William Ackman's Pershing Square Capital Management LP on Monday put forth a slate of six nominees for Botox-maker Allergan Inc.'s (AGN) board of directors.

Pershing, which owns a 9.7% stake in Allergan, has backed Valeant Pharmaceuticals International Inc.'s roughly $53 billion hostile takeover bid for Allergan.

Allergan has repeatedly rebuffed the advances questioning the viability of the deal for its shareholders.

On Monday, Allergan called Pershing's announcement "a further attempt by co-bidders Pershing Square and Valeant to acquire Allergan at a grossly inadequate price that substantially undervalues the Company and creates significant risks and uncertainties for Allergan stockholders.

"Today's announcement also fails to address the serious concerns raised by Allergan and important members of the investment community about Valeant's anemic organic growth driven by unsustainable price increases, among other fundamental business model issues," Allergan said in a statement.

"Under the current Board's leadership, which includes individuals with significantly more industry experience than Pershing Square's nominees, Allergan continues to execute on its plan to drive long-term organic growth, enhance its growth prospects and generate significant value for all of Allergan's stockholders," the California pharmaceutical company said.

Pershing and Allergan agreed in June to a deal that averts a dilutive "poison pill" defense and allows Mr. Ackman and his firm to coordinate with shareholders to call a special meeting, which would be the first step toward replacing a majority of Allergan's nine directors.

Last week, Valeant's pursuit received a vote of confidence from Credit Suisse Group AG as the bank agreed to advise Pershing in the Allergan bid

Allergan has said Valeant's bids undervalue the company, while it has also raised concerns about the sustainability of Valeant's acquisition-favorable business strategy.

Pershing's nominees include Betsy Atkins, 61 years old, chief executive of independent venture-capital firm Baja LLC and a member of the Council on Foreign Relations; Cathleen P. Black, 70 years old, senior adviser to early stage venture-capital firm RRE Ventures LLC and a member of the Council on Foreign Relations; Fredric N. Eshelman, 65 years old, principal at Eshelman Ventures LLC; Steven J. Shulman, 63 years old, managing director of private equity investment firm Shulman Ventures Inc.; David A. Wilson, 73 years old, former president and chief executive of not-for-profit education association Graduate Admission Council; and John J. Zillmer, 58 years old, former executive chairman of Univar Inc., a global distributor of industrial and specialty chemicals and related services.

Pershing said the nominees are independent of Pershing and Valeant.

"Each member of our slate of nominees is an independent, skilled leader with relevant domain, industry and/or executive management experience," Mr. Ackman said in a news release. "By supporting this slate, Allergan shareholders can ensure that the interests of shareholders will be well represented on the board of Allergan."

(Maria Armental contributed to this article.)