​London, Dec 1 (IANS) Patients whose human immunodeficiency virus (HIV) strains developed a resistance to older generation drugs are also resistant to modern drugs, a new study led by an Indian-origin scientist has found.

Resistance to a drug typically occurs when patients doesn't take their medication regularly enough, and for first-line treatments to work, patients generally need to take their medication 85-90 per cent of the time. 

The findings revealed that HIV can be resistant to many different drugs simultaneously.

In the study, the researchers found that 16 per cent of people who stopped responding to modern first-line treatments had HIV mutations associated with resistance to an older generation of drugs called thymidine analogues.

Among patients with a thymidine analogue mutation, 80 per cent were also resistant to tenofovir -- the main drug in most modern HIV treatment and prevention strategies.

"We were very surprised to see that so many people were resistant to both drugs, as we didn't think this was possible," said lead author Ravi Gupta, Professor at University College London.

Mutations for thymidine analogue resistance were previously thought to be incompatible with mutations for tenofovir resistance, but now HIV can be resistant to both at once, the researchers said, adding "this emphasises the need to check the genetic profile of patient's virus before prescribing first-line treatments, as they may have already developed resistance to other treatments that they did not mention having taken." 

Further, in order to prevent these multi-resistant strains from developing, the researchers asserted that there is a need for cheap, reliable resistance testing kits to help screen for drug resistance before giving treatment.

"However, until such kits are widely available, we could test the amount of virus in the bloodstream before and after giving treatment that could help detect treatment failure earlier and switch patients to second line drugs," Gupta said.

If a patient's virus becomes resistant to first-line drugs, the next stage is the expensive second-line treatment with greater side effects.

For the study, published in The Lancet Infectious Diseases, the team studied 712 HIV patients across the world whose HIV was not controlled by antiretrovirals.