The U.S. Food and Drug Administration has rejected a GlaxoSmithKline proposal to offer a new, stronger dosage of its top-selling asthma inhaler drug, and GSK executives said they were “very surprised and disappointed” by the decision.

The FDA said no to a supplemental drug application for Advair Diskus, which is used to treat emphysema, or chronic obstructive pulmonary disease (COPD). GSK had sought approval for a 500/50 strength of the drug which includes fluticasone propionate and salmeterol inhalation powder. A 250/50 mixture has FDA approval.

The drug is delivered through an inhaler.

Forty-six percent of Advair’s $6 billion in revenues in 2006 came from the U.S.

The setback with the FDA is the second in less than two weeks for GSK, which has been battling to support its diabetes drug Avandia. An FDA panel recommended stronger labels for the drug warning of potential heart attack risks. Sales of Avandia have plummeted since a May report in the New England Journal of Medicine raised safety concerns.

GSK said it will meet with the FDA to discuss the so-called “not approvable letter” which raises questions about appropriate dosing recommendations.

"We are very surprised and disappointed by this FDA decision particularly given the outcome of the FDA advisory committee meeting earlier this year," said Katharine Knobil, the vice president of respiratory clinical development for chronic obstructive pulmonary disease.

An advisory panel voted to support the new dosage in May.

"The advisory committee voted unanimously that Advair 500/50 demonstrated a significant reduction in the risk of exacerbations,” Knobil said. “We believe in the strength of the data; this application is based on the results of the largest COPD study conducted in more than 6,000 patients over three years. We are committed to working withthe FDA to address any questions they have and to pursue a way forward."

Advair 250/50 is used to treat airflow obstruction in patients with COPD associated with chronic bronchitis.