Glaxosmithkline will not step in as a rival bidder for U.S. biotech Genzyme and take on Sanofi-Aventis, the British drugmaker's head of research and development told a French newspaper. "An offer by GlaxoSmithKline for Genzyme does not make sense. It is too expensive," Moncef Slaoui told Les Echos on the sidelines of the inauguration of a GSK research centre in France.

Analysts have said it looked unlikely a white knight would emerge and start a bidding war that could force French drugmaker Sanofi to significantly raise its $18.5 billion non-binding offer to buy Genzyme.

The U.S. specialist in drugs against rare diseases has rejected Sanofi's takeover proposal as being too low. Sanofi now has to see by how much it will need to raise its price to even get Genzyme to the negotiating table.

Genzyme would provide Sanofi a new growth platform and expand its U.S. foothold as it seeks to deal with sales and profit losses from patent expiries and challenges on several drugs that each rake in annual sales of at least $1 billion.

GSK, which already has a presence in rare diseases through its partnership with JCR Pharmaceuticals, is looking to balance its R&D portfolio evenly with drugs developed through research in-house and through partnerships. "If the best idea is to be found outside the company, it is better to conclude a partnership agreement," Slaoui said.