The firms made the announcement overnight, sending Trimeris shares down more than 10 percent in after-hours trading.

Roche certificates were indicated around one percent lower in pre-market trading by Bank Leu . Roche closed at 125 Swiss francs on Monday.

"Roche already had a nice run so maybe some people take this as an excuse to sell the stock," a dealer said.

Roche and Trimeris said they had stopped trials of T-1249 because of shortcomings found in its formulation, which the partners did not describe.

"It is a risk we have to face," the Roche spokesman said. "But we remain committed to the development of the next generation of fusion inhibitors," he added.

Roche and Trimeris have already bought HIV/AIDS drug Fuzeon to the market. The spokesman reiterated Roche's confidence in the potential of Fuzeon and expected annual sales eventually of between $500 million and $1 billion.

(Additional reporting by Josiane Kremer)

By Alison McCook

NEW YORK (Reuters Health) - For people with HIV infection, having a steady partner appears to slow the progression to AIDS and postpone death, new research reports.

Among people taking medication for HIV, those who said they were in a stable relationship were more than 20 percent less likely to develop AIDS, and more than 40 percent less likely to die over a multiyear period than were people without a steady partner.

Study author Dr. Heiner C. Bucher of the University Hospital of Basel in Switzerland told Reuters Health these findings show that simply receiving medication for HIV is often "not enough" to keep people healthy.

Bucher noted that having a steady partner may help HIV patients keep track of complicated treatment regimens and help them ward off depression, which research shows can affect the prognosis of other conditions.

Bucher added that HIV-infected patients who are single can likely reap the same benefits of having a steady partner if they surround themselves with people they can trust, confide in, and inform about how they need to manage their disease.

"Being alone with a chronic disease is very hard," Bucher said. "People with HIV need the respect, solidarity and support from the community and society at large." Every six months, Bucher and colleagues asked 3736 people who were being treated for HIV if they had been having sex with a stable partner, and then noted who developed AIDS or died. Half of study participants were followed for more than 3.5 years.

The researchers defined AIDS as the development of an AIDS-related infection, such as tuberculosis or pneumonia.

Half of study participants were at least 36 years old, and 29 percent were women. All began treatment for HIV before the year 2002.

Approximately 80 percent of participants reported being in a steady relationship at least once during the study. People who were sicker at the beginning of the study appeared no more likely to end their relationships than others, suggesting that a more severe illness is not more likely to cause a split.

Those with stable partners were less likely to progress to AIDS or die and were more likely to show an increase in CD4 cells, an indication of the health of their immune systems, the authors report in the British Medical Journal.

Based on these findings, Bucher recommends that doctors caring for HIV-positive patients ask them if they are in a relationship and, if not, if they are depressed and can handle their complicated treatment regimen.

SOURCE: British Medical Journal, January 3 2004.