May 21 (Reuters) - Shares of Mayne Pharma MYX.AX plunged 30% on Wednesday after U.S.-based Cosette Pharmaceuticals initiated a review of its A$672 million ($432.1 million) acquisition, citing a "material adverse change" in the Australian firm's business and finances.

Shares of Mayne Pharma, a dermatology and women's health company, were down 30.3%, as of 0108 GMT, and on course for their worst day since early-March 2009, if losses hold.

Under the February agreement, Cosette's acquisition of Mayne is subject to a "material adverse change" condition, allowing the deal to collapse if significant negative developments impact Mayne's financial health, business, or outlook.

Cosette did not immediately respond to a request for comment outside of business hours.

The Bridgewater, New Jersey-headquartered pharmaceutical products manufacturer has indicated that Mayne's trading performance, including its nine-month earnings update, legal case with TherapeuticsMD, and its recent talks with the U.S. Food and Drug Administration (FDA) constitute the "adverse change".

Last month, Mayne received an FDA letter concerning promotional claims made about its oral contraceptive, Nextstellis.

Cosette's claim triggers a mandatory 10-business-day consultation period with Mayne to discuss the change in good faith.

Cosette has indicated that it will issue a notice to terminate the deal if the consultation process fails to reach a satisfactory outcome, Mayne said in a statement on Wednesday.

Mayne, which rejected the issues raised in the Cosette notice, said it remains open to talks to resolve the matter.

($1 = 1.5552 Australian dollars)

(Reporting by Himanshi Akhand in Bengaluru; Editing by Sherry Jacob-Phillips)

((Himanshi.Akhand@thomsonreuters.com;))