* Gilead seeking approval for aztreonam

* U.S. panel to review drug on Thursday

* Gilead shares down 0.77 percent (Adds background, analyst comment)

NEW YORK - U.S. Food and Drug Administration staff cited concerns on Tuesday with the design and analysis of clinical studies intended to support the effectiveness of Gilead Sciences Inc's experimental cystic fibrosis drug.

Gilead, the large U.S. biotechnology company, is seeking approval for its inhaled drug aztreonam for treating respiratory symptoms and pulmonary function in some cystic fibrosis patients.

In looking at Gilead's application ahead of an FDA advisory panel on the medicine on Thursday, FDA staff reviewed two clinical studies designed to support the efficacy of Gilead's aztreonam, a reformulated version of an intravenous antibiotic.

In one study of 211 patients, FDA staff said dropouts due to adverse events and other reasons may have led to biases favoring the Gilead drug. A company analysis of the study also may have been flawed, the staff said.

There are concerns that assumptions used in the analysis could have been determined retrospectively, the staff said in their 48-page briefing document.

In a second study of 164 patients, staff raised several concerns about a questionnaire used to determine how a patient's symptoms may have been affected during the study.

The problems cited included missing data from incomplete questionnaires, and unclear and vague responses.

There are several concerns about these questions and the responses, the staff said.

Cystic fibrosis is an inherited chronic disease that affects the lungs and digestive system of about 70,000 people worldwide, according to the Cystic Fibrosis Foundation.

The drug won marketing approval in September in Europe and Canada under the brand name, Cayston.

The FDA said in September 2008 it would not approve the drug unless the company conducted an additional trial, surprising Wall Street. Gilead's appeal of the decision was denied, but the agency agreed to an advisory panel review of the drug, according to the company.

Aztreonam sales could reach at least $200 million globally by 2015, according to Summer Street Research analyst Carol Werther. The medicine could strengthen Gilead's pulmonary franchise but pales in importance when compared with its stable of HIV drugs, Werther said.

Gilead shares were down 36 cents, or 0.77 percent, at $46.26 in early afternoon trading on Nasdaq. (Reporting by Lewis Krauskopf, editing by Dave Zimmerman)

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