Health regulators want more data on Protalix Biotherapeutics Inc and Pfizer Inc's experimental drug for Gaucher disease before deciding whether to approve the therapy for patients with the rare genetic disorder.

News of the delay sent Protalix shares tumbling more than 27 percent.

It also spells some uncertainty for the $2 billion Gaucher market, which has been dogged by shortages of the leading therapy -- Genzyme Corp's Cerezyme.

In a letter to Protalix on Friday, the Food and Drug Administration did not call for any new clinical trials, the Israeli drugmaker said. However, the FDA did ask for additional data from several existing studies.

While gathering extra data is less onerous than launching an entirely new study, the news still scared off some investors as Wall Street analysts had largely expected approval and decent market share, given the muscle of the company's marketing partner, Pfizer.

Protalix said the FDA had sought more information about clinical use, drug chemistry and manufacturing controls. The agency has already inspected the manufacturing site.

Chief Executive Officer David Aviezer said in a statement that Protalix was disappointed, but would work with the agency and ask FDA officials for a meeting as soon as possible to clarify their request.

Pfizer also pledged its commitment to the drug, known chemically as taliglucerase.

Gaucher disease stems from an enzyme deficiency that prevents the breakdown of certain fats in the body. It can cause organ damage or death and affects about one in 50,000 to 100,000 people, but is particularly prevalent among Ashkenazi Jews, according to the National Institutes of Health.

Unlike most biotech drugs, which use cells from mammals, taliglucerase is made with genetically modified carrot cells.

Most analysts had expected the companies' drug to take a bite out of the Gaucher market from Genzyme, which has already seen Cerezyme sales slump amid manufacturing and distribution problems. Genzyme is being acquired by Sanofi-Aventis SA.

Oppenheimer & Co analysts have said sales of Protalix's drug could reach $225 million by 2014, with less of an impact given that British drugmaker Shire Plc's rival product, Vpriv, has already made it to the U.S. market.

Still, Pfizer and Protalix may be able to offer their intravenous therapy at a lower price, making it a more attractive option for health insurers, the analysts said earlier in February.

Pfizer's marketing prowess could also give the drug a boost if it eventually wins approval, other analysts have said.

Although Genzyme's Cerezyme made up 41 percent of the $2 billion Gaucher drug market in 2009, its share could shrink to 27 percent in 2015, when the market is expected to reach $4 billion, according to Thomson Reuters data.

Genzyme is also working on a new, capsule treatment for the disorder.

Shares of Protalix were down 27.5 percent at $6.77 in morning trading, while Pfizer dipped 0.3 percent to $18.85.

(Reporting by Susan Heavey; Additional reporting by Debra Sherman in Chicago; editing by John Wallace and Lisa Von Ahn)