* Net present value of royalties support $51/share price - Piper Jaffray
* Theravance shares jump 18 percent (Adds analyst, GSK comments, background; updates share movement)
March 4 (Reuters) - Shares of Theravance Inc rose as much as 18 percent after a brokerage said a positive outcome on the approval of its respiratory drugs could lead to a takeover of the drugmaker, which is one-quarter owned by GlaxoSmithKline Plc.
Theravance is developing Relovair - expected to be sold as Breo in the United States and Relvar in the European Union - and Anoro to treat chronic obstructive pulmonary disease in partnership with GSK, Britain's biggest drugmaker.
An advisory panel to the U.S. Food and Drug Administration is slated to review Breo on Thursday, following which, the FDA will issue a final decision on its approval by May 12. A final decision for Anoro is expected by Dec. 18.
"The economics on the respiratory business is only shared with Glaxo, that's why I think Glaxo would be the front runner," Piper Jaffray analyst Ian Somaiya told Reuters.
Theravance is also developing potential treatments for hypertension and heart failure with Merck & Co.
The potential royalty payments that Theravance would receive if Breo, Relvar and Anoro are approved, support a purchase price of about $51 per share, Somaiya said.
"Given the cost of capital for Glaxo is so much lower, I could assume that they could pay a higher price for it."
Breo is pitched as a more convenient alternative to GSK's $8 billion-a-year blockbuster Advair.
Analysts expect total Breo sales of about $314 million in 2014 and growing to about $1.25 billion by 2017 if the drug is approved this year, according to Thomson Pharma.
Anoro is estimated to generate sales of about $186 million next year, with about $1.25 billion by 2017.
Theravance, which has a market capitalization of about $2.06 billion, could not immediately be reached for comment.
"We don't comment on rumor or speculation" Kevin Colgan, a North Carolina-based spokesperson for GSK said in a phone interview responding to the Piper Jaffray research.
Shares of Theravance jumped up as much as 18 percent to a four-month high of $24.63, before easing back a little to $24.12 on the Nasdaq. (Reporting by Pallavi Ail and Saumyadeb Chakrabarty in Bangalore; Editing by Roshni Menon)