Two decades into the AIDS pandemic, the world has two dozen drugs to fight the virus and researchers have high hopes for new classes of medicine that block the virus before it can enter human cells.

But an effective vaccine - the best hope for the developing world where drugs remain out of reach for millions - is still only a distant hope.

Pharmaceutical makers will showcase their latest advances at the International AIDS Conference in Bangkok from July 11 to 16.

Joep Lange, president of the International AIDS Society, expects a focus on experimental compounds that stop HIV from entering cells, rather than fighting it once inside. Such drugs may keep patients healthy for longer with fewer side effects.

“The inhibition of HIV entry is the field with the most promise at the moment,” Lange, a professor of internal medicine at the University of Amsterdam, said in a telephone interview.

“One of the main problems with current drugs is their long-term toxicity and particularly their metabolic side effects. It is very unlikely that the entry inhibitors will give those sort of side effects.”

Switzerland’s Roche and U.S. biotech group Trimeris last year launched the first drug of this type, known as a fusion inhibitor. But Fuzeon is expensive, must be injected twice daily and its sales - which totaled $24 million in the first quarter of 2004 - have been disappointing.

PFIZER VS SCHERING-PLOUGH

Now Pfizer and Schering-Plough are racing to develop a different kind of entry inhibitor that blocks a cellular doorway called CCR5 and which can be given as a pill.

Schering-Plough reported promising early-stage results with a compound called SCH-D in February and Pfizer is expected to report in Bangkok that its rival drug, UK-427,857, is at least as effective as existing antiretrovirals in short-term tests.

Pfizer aims to start final phase III tests later this year, putting it ahead in the race to develop a drug which, if successful, could generate sales of $500-700 million a year, according to Cathay Financial analyst Sena Lund in New York.

GlaxoSmithKline is also working on a CCR5 product but is believed to be further behind in development.

David Reddy, head of Roche’s HIV/AIDS business, says the war against the virus is becoming more challenging, since many of the best targets for medical intervention have already been hit.

“We’re moving into a harder era in terms of discovering new drugs, so I think some caution needs to be applied,” he said.

Still, the arrival of new drugs like the CCR5 class and other new approaches, such as an attachment inhibitor from Bristol-Myers Squibb, is expected to keep HIV/AIDS a growth business.

Market analyst Datamonitor predicts the HIV/AIDS drugs market will double in size to $12 billion by 2012, despite the decision by major companies - under pressure from activists - to slash the cost of medicines in poor countries.

UPHILL VACCINE STRUGGLE

Vaccines, however, are the poor relation and little concrete news is expected in Bangkok on a safe and effective shot.

Trials of 30 vaccine candidates are currently underway in 19 countries - but there is only one pivotal phase III trial and hopes for this project, involving 16,000 volunteers in Thailand, are not high following the failure of a related study last year.

“There is more effort now than there was but it is still not enough,” said Seth Berkley, president and chief executive of the non-profit International AIDS Vaccine Initiative.

“An effective AIDS vaccine promises to end the epidemic. A 50 percent effective vaccine, given to two third of the adult population, could reduce infections up to 60 percent.”

To get there, Berkley wants to see 10 percent of global AIDS spending devoted to finding a vaccine, up from 2-3 percent now.