Wyeth (WYE) has filed a lawsuit against the U.S. Food and Drug Administration seeking to reverse the agency's recent approval of a generic version of the company's blockbuster antibiotic Zosyn.

Wyeth claims the generic product can't be safely used in the same manner as the branded product, due to interactivity with an intravenous solution sometimes used in conjunction with Zosyn, and would seriously endanger patients' health. The suit was filed in U.S. District Court for the District of Columbia.

The FDA last week approved a generic version of Zosyn from Orchid Chemicals & Pharmaceuticals Ltd. (524372.BY) of India. Orchid is to have a 180-day period of generic market exclusivity, and has joined with Apotex Inc. of Canada to sell the drug.

Wyeth, Madison, N.J., had previously asked FDA not to approve proposed generic versions of Zosyn because they were based on an earlier formulation of the drug, and Wyeth changed the formulation in 2005. Orchid's generic product is based on the old formulation.

The effects of the old formulation of Zosyn can be diminished when it's given along with an I.V. solution known as Ringer's lactate, Joseph Camardo, senior vice president of Wyeth's global medical affairs, said in an interview Wednesday. Wyeth's newer formulation of Zosyn, however, doesn't have this issue, he said.

Wyeth fears that medical errors will result if health-care professionals assume that generic Zosyn is identical to branded Zosyn and give it with Ringer's lactate, Camardo said.

"We believe [the FDA] came to the wrong conclusion based on the scientific evidence," Camardo said. "That's why we're asking the court to force them to reconsider. We think the law is very clear - if a formulation is different and the ingredients in the formulation have an impact on the safety and efficacy of the drug, then you can't consider them identical."

But the FDA said in a letter last week thatthe prescribing label for Orchid's generic Zosyn will note its incompatibility with Ringer's lactate. The FDA also stated that the generic is as safe and effective as Wyeth's reformulated Zosyn when used in accordance with prescribing labels.

Zosyn had $1.3 billion in sales last year. Wyeth has agreed to be acquired by Pfizer Inc. (PFE); the deal is expected to close by the end of the year.

An FDA spokeswoman said the agency doesn't comment on lawsuits. Zosyn is one of several big-selling branded drugs that have enjoyed continued market exclusivity even after key patents expired, largely because their manufacturers had filed citizen petitions with the FDA asking them not to approve generics. Critics have accused branded drug makers of abusing the citizen-petition process to maintain their monopolies, but branded drug makers have said the petitions were filed for valid safety or efficacy reasons.

Johnson & Johnson's (JNJ) Concerta drug for attention deficit disorder and Sanofi-Aventis SA's (SNY) Lovenox anti-clotting drug also have avoided generic competition partly due to ongoing citizen-petition reviews by the FDA.

Wyeth shares fell 21 cents to $48.33 Wednesday.

-By Peter Loftus, Dow Jones Newswires; 215-656-8289; peter.loftus@dowjones.com