There is no cure for HIV

Early trials of two new classes of drugs show they are safe and that they work as well as existing treatments.

While further tests are needed, the drugs could boost the weaponry available to doctors to fight HIV.

A growing number of people are becoming resistant to anti-HIV medicines. New drugs are needed to enable doctors to continue to treat these patients.

Combination therapy

There are currently around 20 anti-HIV drugs. They are generally used in various combinations called highly active antiretroviral therapy (HAART).

When it works well, HAART can keep the virus at very low levels in patients, keeping them healthy and able to live virtually normal lives.

I think that it clearly will have clinical implications Dr Mark Laughlin, Schering-Plough

However, the virus eventually finds ways of resisting the drugs, forcing doctors to use different combinations to try to control it. But over time, they can run out of combinations.

New classes of anti-HIV drugs offer patients the best hope. This is because they work differently to current drugs and patients are less likely to become resistant to them.

Last year, pharmaceutical company Roche launched a drug called Fuzeon - the first new class of anti-HIV drug for seven years.

Most of the big pharmaceutical companies are working on potentially new classes of anti-HIV drugs.

Two companies, Schering-Plough and Bristol-Myers Squibb, presented results from early trials at the annual Conference on Retroviruses and Opportunistic Infections in San Francisco.

Dr Mark Laughlin, a research executive at Schering-Plough, said experimental tests on patients showed its drug, called SCH-D, can help to reduce HIV levels.

The drug blocks a cellular doorway called CCR5, which HIV uses to enter and infect healthy cells.

While further trials are needed, Dr Laughlin said he was confident the drug will benefit patients.

"I think that it clearly will have clinical implications," he said.

Scientists from Bristol-Myers Squibb said they tested their latest drug, called BMS-488043, on a group of patients over two weeks. They said the drug reduced HIV levels safely.

Their drug stops HIV from attaching itself to healthy cells, a so-called attachment inhibitor. It is also a potentially new class of drug.

Hepatitis study

The conference also heard how a combination of drugs can help HIV positive patients who are also infected with hepatitis C.

A trial of over 800 patients in 19 countries found that using a drug called Pegasys in combination with ribavirin offers the greatest chance of curing hepatitis C.

The APRICOT study found that 40% of patients treated with both drugs cleared the virus compared to 12% of those treated with conventional treatments.