Feb. 4 (Bloomberg) -- Eisai Co. fell the most in more than four years in Tokyo trading after the Japanese drugmaker was forced to delay seeking U.S. approval for its first potential new medicine since the Aciphex stomach ulcer treatment was released in 1999.

A third phase of patient studies is required on Eisai's experimental breast cancer treatment before it can apply to the Food and Drug Administration, the Tokyo-based company said after markets closed on Feb. 1. It had requested bypassing that step, enabling a review as early as last quarter. Eisai now expects to seek FDA approval in the year ending March 2010, it said.

The delay is frustrating Eisai President Haruo Naito's attempts to develop successors to Aciphex and Aricept, the company's best-selling treatment for Alzheimer's disease. Aricept sales, which accounted for 40 percent of group revenue last quarter, will slump once the drug loses patent protection in the U.S. in November 2010.

The development delay is a disappointment,'' said Hirohisa Shimura, an equities analyst at UBS Securities Japan Ltd., in a Feb. 1 report. ``Prospects for developing a new drug candidate for Alzheimer's are also getting worse. Eisai may be forced to focus on other backup compounds.''

Eisai, Japan's fourth-largest drugmaker, plunged 6.6 percent to 4,100 yen on the exchange, the biggest one-day slide since July 7, 2003. Today's decline erased almost all the stock's gain this year.

Mizuho Securities Co. health-care analyst Hiroshi Tanaka cut his rating on Eisai shares to ``reduce'' from ``hold,'' and at least three brokerages lowered their price target for the shares after the company's announcement.

Bristol-Myers

The FDA refused to expedite a review of Eisai's drug, known as E7389 or eribulin mesylate, after the agency granted Bristol- Myers Squibb Co. accelerated approval for a similar treatment last year, Eisai said.

New York-based Bristol-Myers won FDA approval in October for Ixempra, an injectable drug to treat the most advanced form of breast cancer and the first new type of chemotherapy for the disease in 14 years.

The FDA granted Ixempra, also known as ixabepilone, priority review last June, reducing the approval process to six months from at least 10, because of its potential to treat unmet medical needs.

Women living in North America have the highest rate of breast cancer. About 180,000 new cases were diagnosed in the U.S. last year, and more than 40,000 women died from the disease, according to the American Cancer Society. Worldwide, about 1.15 million new cases of breast cancer are diagnosed annually, and 475,000 women die.

Eribulin mesylate contains a synthetic form of a substance first isolated from a type of marine sponge in 1992, according to Eisai. The Japanese drugmaker is also investigating whether its medicine can help a broader range of patients, including those with prostate and lung cancer.