A top American virologist has developed
what appears to be the most sensitive test to date for
detecting drug-resistant strains of HIV — up to
1,000 times more sensitive than the most widely used
commercial tests.
"With this test, we can determine
if a patient is carrying drug-resistant HIV strains
that make up as little as 0.1 to 0.01% of their total
viral load," says Dr Feng Gao, a leader in HIV research
from Duke University whose findings were published in
the advance online edition of Nature Methods
on January 7. The highly sensitive assay could become
indispensable in informing physicians' therapeutic choices,
since it would provide them with advance warning of
eventual drug resistance, he adds.
SENSITIVITY
TRAINING
The novel testing method — parallel allele-specific
sequencing (PASS) — is able to accurately determine
the presence of different resistance mutations in individual
viral genomes when a large number of such genomes are
analyzed in parallel. At least 15% of newly diagnosed
HIV patients are thought to carry drug-resistant strains.
The assay functions by chemically
labelling individual sections of viral DNA with fluorescent
markers: green for mutated gene locations known to cause
resistance to a particular antiretroviral drug, and
red for the same, non-mutated locations. These markers
are then read by a scanner and the percentages of different
drug-resistant HIV strains are calculated.
To assess the test, the researchers
analyzed plasma samples from treatment-naòve
patients; patients who had received, but weren't currently
undergoing treatment; and patients with treatment failure.
The assay was sensitive enough to detect multiple mutations
on a single virus out of a pool of 10,000 non-mutated
ones.
The results indicate the PASS method
is up to 1,000 times more sensitive than either genotypic
or phenotypic assays — those currently employed
by most laboratories — which only pick up mutant
HIV strains once they exceed 20% of the total viral
population. The next-best assay, real-time polymerase
chain reaction (PCR), can pick up drug-resistant strains
at between 1% and 0.1% concentrations. But like the
PASS test, real-time PCR is not yet commercially available.
"These [other] tests don't tell
you whether a genome has one or multiple drug-resistant
mutations," says Dr Gao. "Our assay shows you precisely
what kind, as well as what combination, of mutations
are present in each viral genome that's tested."
Duke University has filed for a
provisional patent on PASS technology and is seeking
financial partners to make the assay commercially viable.
PRESCRIBE
SMART
Drug resistance is the main cause of HIV treatment failure
and thus a major concern to the estimated 58,000 people
currently living with HIV/AIDS in this country. As Dr
Gao points out, currently used tests just aren't equipped
to deal with the problem. "You really don't need to
reach 20% [the threshold of standard test detection]
for drug resistance to occur. One or 0.1% may be enough
to cause drug-resistant HIV strain dominance," he says.
"By prescribing drugs that a patient
has not developed resistance to, you can significantly
delay drug resistance, and therefore the onset of AIDS
and subsequent death for many more years," he adds.
Dr Gao's research team is now trying
to determine whether using PASS to direct drug treatment
indeed slows down the progression to drug resistance.
He hopes the PASS method will provide future researchers
a means of tracking the evolution of drug-resistant
HIV — and potentially, that of other viruses, such
as hepatitis B and C. Ultimately, he envisions this
leading to the development of new drugs that evade resistance.
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