There has been little chatter about GlaxoSmithKline’s mepolizumab, an experimental treatment for severe eosinophilic asthma that is expected to go before an FDA advisory committee panel on Thursday.

However, the drug is of note because the respiratory category accounted for 33% of the company’s pharmaceuticals and vaccines businesses last year and it is flagging. Respiratory product sales fell 22% in the US in the first quarter of 2015, and the decline was strong enough that executives said the falloff was largely responsible for the 23% decline in total US pharmaceutical sales during this period.

GlaxoSmithKline submitted mepolizumab to the FDA in November. It is meant to tamp down on the production of eosinphils, which are white blood cells, the overproduction of which can be triggered by allergens as well as bacteria. When they accumulate in the lungs they can damage them.

GSK’s struggle is tied to its blockbuster asthma drug Advair. The drugmaker not only had to lower the price of the branded medication before generic competition surfaced, but it has also failed to build a market for its successor line of Ellipta-based inhalers, including Breo Ellipta and Anoro Ellipta, as quickly as it anticipated. Advair also has the disadvantage of being a medication patients and physicians like, making it difficult to have them switch from older, less expensive Advair to newer, more expensive Ellipta. Further, the company said this year that a US generic version of Advair will probably surface earlier than it expected, which could depress sales even further.

Sanford Bernstein analyst Dr. Tim Anderson wrote in a May research note that GSK chose a tough category to depend on because the respiratory franchise is one in which new drugs offer incremental improvements over older medications and is subject to price reductions to remain competitive.

Anderson said this category is unlike oncology where there is little pressure to reduce prices and there is a substantial amount of unmet need, yet this is an area GSK walked away from when it sold its cancer business to Novartis last year.

GSK has said it expects respiratory sales to rebound in 2016. But the company’s respiratory sales base will shrink from nine products to four by 2020.