PARIS, Sept 26 (Reuters) - Franco-German drugmaker Aventis SA said on Thursday it would spin off some of its work in infectious diseases as part of an overhaul of research and development operations.

Shares rose as much as 7.3 percent after Aventis said a restructuring would boost spending on R&D in cancer, diabetes, thrombosis, multiple sclerosis, schizophrenia, Alzheimer's, coronary heart disease, asthma and rheumatoid arthritis.

For infectious diseases, the world's fifth largest drugs firm plans a two-pronged approach -- keeping antibiotic Ketek and vaccine work in-house, but looking for external partners for other anti-bacterial and anti-fungal activities.

Aventis wants to set up a joint venture to make optimum use of the expertise built up in its Romainville and Vitry research lab, Gilles Brisson, head of Aventis Pharma France, said.

The idea is still at an early stage, he added.

Aventis has already spun off its osteoporosis research into a new company, ProSkelia, which was set up with financing from private equity group Warburg Pincus earlier this year.

"The focus of our activity involves being selective and the goal is to seek leadership positions in a number of therapeutic areas," Brisson said.

Aventis said the restructuring of its research division would mean a reduction of 300-400 out of its 2,200 jobs in France, although it would try to avoid redundancies.

Aventis's 2002 results will not be affected by the move and any restructuring provisions to be taken on the books in 2003 or 2004 will be offset by cost savings, he told a conference call.

While parts of the reorganisation were expected, the move prompted investors to pull back from recent weakness in its share price, said Jacques Aymon Laplye of KBC Securities.

At 1425 GMT, Aventis was up 6.2 percent at 55.65 euros, leaving an underperformance of 1.9 percent when compared with the European healthcare sector so far this year. Blue-chip French shares were up by an average four percent.

Aventis's "priority of priorities" is the treatment of cancer, the number three cause of deaths in the world. Its Taxotere drug is a leader in cancer treatment.

"Even in this sector we work in networks. From now on our activities will be driven by partnerships and networks of internal and external teams," Brisson said.

Aventis also said it planned to convert its Romainville site outside Paris into a "technology park" offering R&D facilities to other companies.