GlaxoSmithKline and Pfizer, two leading developers of Aids drugs, are merging their HIV operations into a new company that will hold nearly a fifth of the market for treatments against the virus.

The two companies said on Thursday that Glaxo would have an 85 percent stake in the joint venture and Pfizer 15 percent, a split which reflects Glaxo's stronger position in marketed products.

The tie-up underlines a growing trend by big pharmaceutical companies to collaborate in the costly and risky process of developing new medicines.

The partners said the new business would be more sustainable and broader in scope than either company's individually, and would hold a 19 percent share of the growing market for HIV/AIDS treatments.

It will include 11 already marketed products -- including Glaxo's top-sellers, Combivir and Epzicom -- that together generated sales of 1.6 billion pounds ($2.4 billion) last year, plus a pipeline of six new medicines, of which four are in mid-stage Phase II development.

"At the core of this specialist business is a broad portfolio of products and pipeline assets, which can be more effectively leveraged through the new company's strong revenue base and dedicated research capability," said Glaxo Chief Executive Andrew Witty.

One of the goals will be to develop new fixed-dose combination therapies using existing and novel medicines. Fixed-dose tablets are important in providing more convenient treatments for fighting HIV infection.

The new company will contract R&D services directly from Glaxo and Pfizer to develop these medicines. Dominique Limet, currently head of Glaxo's Personalised Medicine Strategy, has been appointed chief executive designate of the new company.

There is no cure for the human immunodeficiency virus (HIV) that causes AIDS, but combinations of drugs can keep the virus from replicating and damaging the immune system.

An estimated 33 million people globally are infected with the AIDS virus, most of them living in Africa and other developing countries. - Reuters