DRUG-resistant strains of HIV appear to have 鈥渓ife immunity鈥� to particular
drugs, researchers in Britain have found. The findings dash hopes that people
could combat resistant HIV strains by taking a break from a drug and returning
to it once viral resistance has worn off.
Researchers at the PHLS Antiviral Susceptibility Reference Unit in Birmingham
looked at HIV mutations that make the virus resistant to drugs in 550 patients
who had already received drug therapy. Those whose treatment failed had a high
proportion of drug-resistant virus in their blood. But resistant viral strains
seemed to disappear in patients who then took a different set of drugs, they
found.
The researchers wanted to know what would happen if the patients took the
first set of drugs again. 鈥淐an you give the drug back?鈥� says unit head Deenan
Pillay. They discovered that the resistant viruses not only returned, but came
back more quickly and at a higher level than before.
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鈥淥nce you have evolved drug resistance mutations, they are with you for
life,鈥� Pillay says. 鈥淔or certain classes of drugs, it is entirely possible that
recycling of drugs is not going to be effective.鈥� He says that the resistant
virus stays on at low levels in the body鈥檚 lymph nodes, ready to return if the
patient ever takes that drug again.
Mike Youle of the HIV Research Unit at the Royal Free Hospital in London says
the research is worrying, but points out that in some situations, recycling
certain drugs might still work. 鈥淚f you take people off and later put them back
on the same drug, you can still get a reduction in virus,鈥� he notes.
Previous work on HIV has shown that in the lab, drug-resistant strains don鈥檛
grow as well as the original virus. The next priority for the researchers is to
work out whether the disease progresses as quickly in patients who are
harbouring drug-resistant strains. 鈥淲e don鈥檛 know the course of the illness.
Maybe the drug-resistant strains are more benign than the wild-type ones,鈥� says
PHLS researcher Pat Cane.
Pillay also told the conference that HIV-resistant strains are increasing in
Britain at an alarming rate. His team estimates that nearly 30 per cent of new
infections are with viruses already resistant to at least one drug.
The researchers sequenced the DNA of viruses from people who had only just
become infected, to look for mutations which make the viruses resistant to
different drugs. Eight out of 28 patients tested in the past year were infected
with virus that was already resistant to at least one drug, compared with two
out of 36 patients between 1994 and 1998 (see Table).
Some of the strains were even resistant to all the main classes of
anti-retroviral drugs available, the researchers found.