A clinical study into an experimental AIDS pill made by Pfizer Inc, the world’s biggest drugmaker, should be halted because it offers patients no safety net, activists said Tuesday.

The European AIDS Treatment Group (EATG) said the design of the trial for Pfizer’s so-called CCR5 inhibitor drug maraviroc, previously known as UK-427,857, was putting people with HIV at unnecessary risk of developing full-blown AIDS.

“The trial design should be changed or otherwise stopped,” EATG Chairman Mauro Guarinieri said in a statement.

“We demand all concerned regulatory authorities to assume their responsibility and act accordingly.”

Concerns about the study design have already forced Pfizer to suspend plans to test the drug in France, Germany and Spain. But a company spokesman said its research had won the support of the majority of stakeholders and regulators around the world.

EATG criticism centers on the fact that previously untreated patients can enroll in the phase IIb/III study, regardless of their immune system health or the amount of virus they are carrying.

The group, an influential voice in European AIDS policy, said this was unethical, since it could mean people with severely compromised immune systems who were in vital need of a proven treatment would instead get an untested investigational drug.

Pfizer disagreed and said the study contained appropriate checks and balances to protect patient safety, while at the same time allowing researchers to work out the best future use of the medicine.

RACE TO MARKET

The U.S.-based company is in a race with other drugmakers to be first to market with a CCR5 inhibitor - a new kind of pill that can block the AIDS virus before it enters human cells.

If successful, CCR5 inhibitors should have fewer toxic side effects than conventional treatments and offer hope to patients whose virus has developed resistance to existing antiretroviral medicines.

Industry analysts say Pfizer is a few months ahead of rivals with maraviroc, which was discovered at the group’s research laboratories in Sandwich, southern England. But both GlaxoSmithKline Plc and Schering-Plow Corp are on its tail.

The first CCR5 drugs could reach the market in 2007 or 2008, Dr. Graeme Moyle, an HIV expert at London’s Chelsea and Westminster hospital, said earlier this year.

Industry analysts estimate that a successful CCR5 drug should generate sales of $500-700 million a year.

Pfizer is currently enrolling patients, or has approval to start trials, in Australia, Belgium, Canada, Italy, Mexico, Netherlands, Switzerland, the UK and the United States - but not France, Germany or Spain.

“Pfizer believes a thorough evaluation of maraviroc is required in a population representative of individuals initiating HIV therapy today,” the company said.

Maraviroc and the rival products from GSK and Schering-Plough are the second class of anti-AIDS drugs designed to stop HIV entering healthy immune cells.

Switzerland’s Roche Holding AG and U.S. biotech group Trimeris Inc launched the first such drug, Fuzeon, in 2003. But Fuzeon is expensive, must be injected twice daily and sales, which totaled $135 million in 2004, have been disappointing.

The new drugs - which lock a cellular doorway, or co-receptor, called CCR5 - work in a different way and can be given as a tablet, which should make them more attractive to doctors and patients.