Theravance Inc. (NASDAQ: THRX) got a big boost today from a purchase of 10 million additional shares by GlaxoSmithKline plc (NYSE: GSK) at a 7.5% premium to a volume-weighted average share price for the past five trading days thru last Friday. GSK now owns about 26.8% of Theravance, up from Glaxo’s previous stake of 18.3%.

The two companies are working on a new once-a-day drug, Relovair, to treat asthma and chronic obstructive pulmonary disease (COPD). The drug is important to Glaxo as a replacement for its Advair drug which has lost its patent protection in the US and will lose it next year in Europe.

GSK and Theravance are gearing up to file for approval use as a treatment for COPD in the US and Europe for Relovair later this year. Approval for the drug’s use against asthma is also on track for this year, but a US filing may be done later due to the stringency of US rules on once-daily drugs.

Theravance shares are up nearly 16% today at $22.57 in a 52-week range of $12.91-$28.95.