A new HIV drug shows promising results in people for whom other treatments no longer work, a study suggests.
The drug stops HIV getting into cells
Pfizer's Maraviroc - which blocks HIV's entry to immune system cells - is one of a number of drugs being developed for this group of patients.
A study found twice as many people on the drug had undetectable virus levels compared to people on standard drugs.
A UK expert said the development of Maraviroc, detailed at a US conference, was promising.
The drug acts as a block to the CCR5 receptor which is the "doorway" which HIV uses to get into CD4-T-cells.
The 24-week safety study reported to the Conference on Retroviruses and Opportunistic Infections in Los Angeles looked at 1,000 patients.
All were taking their standard medications, but half were also given Maraviroc while the rest were given the dummy - or placebo - version.
A quarter of those in the placebo arm of the study were found to have undetectable viral loads at the end of the study, compared with almost double that number in the group taking Maraviroc - between 41-48% depending on the dosage they were given.
Those taking the drug also had a higher CD4 count.
Michael Carter, an HIV expert from the UK's National Aids Manual (Aidsmap) said the drug was the first in a new class to reach this stage of research.
He said there was another group of drugs, led by Merck's Raltegravir which are also targeted at this group of patients, but which stop HIV integrating into cells.
"HIV therapies have, for so long, focused on people needing drugs for their initial treatment," said Mr Carter.
"But there is a huge and growing number of people who've failed on first and second-line therapies, who need other drugs later on."