A novel AIDS pill designed to exploit the discovery that some people appear naturally resistant to HIV has produced good results in early-stage clinical trials, researchers said Tuesday.

Pfizer’s experimental compound UK-427,857 belongs to a new class of medicines that block the virus before it can enter human cells, rather than fighting it once inside.

Such drugs may keep patients healthy for longer with fewer side effects than conventional antiretroviral therapies and provide a new option for people who have developed resistance to conventional AIDS drugs.

Data presented at this week’s 15th International AIDS Conference in Bangkok showed a once-daily dose of the compound produced a 10 to 100-fold fall in the amount of human immunodeficiency virus (HIV) in patients’ blood after 10 days. This is similar to results achieved with potent protease inhibitors.

There was no significant difference in the result whether or not patients had just eaten. Many other AIDS drugs have strict dietary requirements.

Steve Felstead, head of the team developing the product, said the U.S. firm was conducting larger scale investigations following the good results from placebo-controlled monotherapy trials involving 80 patients.

“It has enabled us to test once and twice-daily doses in later trials and remove restrictions on how patients take the medication,” he told Reuters. “It’s very good news for the drug and for patients.”

Pfizer hopes to start a final phase III study, which will involve hundreds of patients, before the end of the year.

CELLULAR DOORWAY

Switzerland’s Roche and U.S. biotech group Trimeris launched the first drug to block HIV’s entry into healthy immune cells last year. But Fuzeon is expensive, must be injected twice daily and sales - $24 million in the first quarter of 2004 - have been disappointing.

Unlike Fuzeon, which is a so-called fusion inhibitor, Pfizer’s drug is a different kind of entry blocker that locks a cellular doorway called CCR5 and which can be given as a tablet.

The idea for blocking it followed the observation that people with a CCR5 mutation can resist HIV infection.

Researchers noticed long ago that some prostitutes in Nairobi, Kenya, and a small number of American men never became infected despite multiple high-risk exposures.

But CCR5 inhibition may provide only a partial solution to blocking HIV since the virus can switch and use alternative routes to enter cells, such as another receptor called CXCR4.

Any successful CCR5 pill is therefore likely to be used in combination with other medicines.

Pfizer is racing with Schering-Plow to be first to develop a CCR5 drug, which, if successful, could generate sales of $500-700 million a year, according to industry analysts.

Schering-Plough reported promising early-stage results with a compound called SCH-D in February, but does not expect to start Phase III tests until next year.

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