A study in the Journal of the American Medical Association has found that one in four new cases of HIV infection in San Francisco is resistant to some classes of antiretroviral drugs.

Researchers from the Gladstone Institute of Virology and Immunology said the finding highlighted the need for new classes of drugs to be developed.

About one in four are acquiring a virus with signs of drug resistance

Dr Robert Grant, Gladstone Institute of Virology

They also suggested that newly-infected patients should be tested for drug resistance before they start treatment.

Dr Robert Grant and colleagues at Gladstone Institute of Virology and Immunology studied 225 newly infected and untreated patients between June 1996 and June 2001.

Genetic tests

They tested these patients for anti-retroviral drug resistance by analysing the genetic makeup of the virus.

They found a sharp increase in the number of patients who were resistant to non-nucleoside reverse transcriptase inhibitors (nnRTIs).

The nnRTIs are one of three major classes of anti-HIV drugs and are an important weapon in the fight against the disease.

In 1996, none of the patients studied were resistant to these drugs.

However, by 2001, 13% were found to have been infected with nnRTI-resistant virus.

They also found an increase in the number of patients resistant to protease inhibitors (PI), another key class of anti-HIV drugs.

In 1996, 2.5% of patients had a PI-resistant strain of HIV. By 1997, that had increased to 7.7%.

Success

However, doctors have been successful in reducing resistance among newly-infected patients to a third class of drugs.

This is important news for anyone who thinks that they don't need to worry about contracting HIV

Terrence Higgins Trust spokesman

In 1996, 20% of patients were resistant to nucleoside reverse transcriptase inhibitors (NRTIs), including the well known AZT drug.

But by 2001, just 5.5% of newly-infected patients had a nnRTI-resistant strain of HIV.

Nevertheless, the number of people who were resistant to more than one class of drugs has jumped.

In 1996, 2.5% were what doctors describe as dual-resistant. By 2001, that figure was 12%.

Of the 225 people studied, one was resistant to all three major classes of anti-HIV drugs.

Overall, 27% of newly-infected patients were resistant to at least one class of drug.

Dr Grant said: "About one in four are acquiring a virus with signs of drug resistance."

The study found that over time the majority of patients still benefited from anti-retroviral therapy.

"Viral suppression was eventually possible in the majority of patients because the transmitted resistance affected only some of the drugs in the regimen," Dr Grant said.

Warning

However, he called for patients to be tested for drug resistance before starting treatment.

He added that doctors should continue to treat patients even if they are resistant to some classes of drugs.

He said the benefits far outweighed any disadvantages, not least because they can prolong patients' lives.

Dr Grant said: "Our findings should not dissuade doctors from offering antiretroviral treatments.

"Rather, our study supports combining treatment with prevention interventions."

The Terrence Higgins Trust described the findings as important.

A spokesman said: "This is important news for anyone who thinks that they don't need to worry about contracting HIV on the grounds that it's treatable.

"The study shows high levels of sexually transmitted drug resistant HIV, and means that people could start off with a virus that's already very difficult to treat and even - in a small but growing number of cases - untreatable.

"It also means that people have restricted treatment options if their treatments fail after the first or second combinations and may run out of treatment options much faster."