THE head of research and development at GlaxoSmithKline conceded yesterday that the drugs maker may not launch its billion-dollar painkiller for arthritis sufferers in 2006.

But Tachi Yamada, chairman of R&D, insisted that the unnamed medicine, which belongs to the same family of drugs as Merck’s Vioxx, would eventually become a bestseller.

In a robust campaign aimed at nervous City institutions, Mr Yamada pledged that GSK would not follow Merck in dropping the treatment, known only as “406381”.

He said that the company had identified different safety profiles between drugs in the class. GSK could demonstrate that ’381 was more effective than a rival painkiller by Pfizer, called Celebrex, and that both were safer than Vioxx because neither led to an increase in blood pressure, he added.

Concerns have surfaced over the safety of all painkillers in the class, known as Cox-II inhibitors, since Merck withdrew Vioxx from pharmacy shelves at the end of September.

A senior official in the US Food and Drug Administration later alleged that Vioxx might have led to more than 27,000 fatal heart attacks. Merck is facing more than 300 lawsuits involving about 1,000 plaintiff groups.

Mr Yamada said that ’381 would almost certainly face longer safety trials, jeopardising an expected launch in 2006.

However, he reiterated the group’s commitment to the drug, which is expected to generate annual sales of $1 billion (£535 million) or more. “I would agree that perceptions are poor at the moment, but we believe that ’381 has the potential to be a very, very big drug,” he said.

Mr Yamada said that GSK, together with other makers of Cox-II drugs, would meet US drugs regulators in February. The company would have a clearer idea of how to proceed with phase III trials after the meeting. “My guess is that we are going to have to do a lot before we launch the drug,” he said.

Mr Yamada’s support for the experimental drug came as GSK updated analysts on its pipeline for new treatments for disorders of the central nervous system. The unit accounts for more than 20 per cent of all medicines being researched by the giant.

At present GSK, Europe’s largest drugs maker, has one of the biggest pipelines in the sector, with 90 new chemical entities under trial.

The company believes that it has about 22 so-called “blockbuster” medicines under development — drugs that have the potential to generate annual sales of $1 billion or more.

Last night analysts reacted favourably to the presentation, pointing to the company’s ability to find new uses for old drugs. GSK is seeking to turn Radafaxine, which recently failed clinical tests as a treatment for depression, into a new medicine for obesity.

The company is also working on plans to turn Avandia, an established diabetes drug, into a treatment for Alzheimer’s disease, after scientists found that the chemical reduces inflammation of the brain.

GSK shares closed up 2p at £11.33 yesterday.

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