Virco first to introduce drug-specific biological cut-offs for its HIV resistance tests

October 22, 2000

New cut-offs will provide more realistic picture of resistance

GLASGOW, SCOTLAND, October 23, 2000 - Data presented today at the 5th International Congress on Drug Therapy in HIV Infection identified and discussed new drug-specific, biological cut-off values developed by Virco for its HIV drug resistance tests. These will tell clinicians instantly whether their patient's virus is within the normal susceptible range found in untreated patients or whether biologically relevant resistance has developed. The new cut-offs are unique to each drug, varying from 2.5 for some of the protease inhibitors up to 6-10 for the non-nucleosides, and will provide clinicians with an indicator of resistance that more realistically mirrors the situation in the clinic.

"These are the first biologically relevant cut-offs to be used in HIV resistance monitoring and represent an important advance in terms of the practical utility of drug resistance information," said Julio Montaner, MD, Professor of Medicine and Chair in AIDS, University of British Columbia, Vancouver, Canada. "Using these new cut-offs will take some of the guess work out of the interpretation of resistance results, which is good news, both for clinicians and their patients."

The new cut-offs are based on the natural biological variation of tens of thousands of virus samples from patients in the USA and Europe and will be integrated into Virco's HIV drug resistance testing services (Antivirogram™ and VirtualPhenotype™) worldwide over the next few weeks.

Until now resistance tests have used somewhat arbitrary cut-offs (the threshold above which HIV is categorised as being resistant to a drug) that were, for example, based on the reproducibility of the tests themselves. Resistance was measured directly in terms of how much drug was required to inhibit the replication of the virus by 50% (the IC50), and then, compared to the IC50 for a laboratory standard reference virus. By repeatedly running a test with the standard reference virus, the reproducibility of the test was measured and a cut-off was set at this level, (e.g., a 2.5 increase in IC50.) The limitation of setting a single cut-off for all available drugs in this way is that it told the clinician very little about the significance of any change in susceptibility reported by a test, apart from it being large enough to be measured.

In order to develop biologically relevant cut-off values for each drug, Virco measured the IC50 values for 1,000 untreated patients as well as many thousands of samples of HIV-1 with no resistance mutations. The average and the range of susceptibility were calculated for each drug. The cut-offs were then set at two standard deviations above the mean. This statistical term means that a test result falling above the cut-off can be said to be above the normal susceptible range with 97.5% confidence.

Since the susceptibility of untreated and unmutated virus varied considerably from drug to drug, the new cut-offs are different for each drug. All the cut-offs are above the reproducibility limit of the Virco assay, which is approximately a 2-fold change in susceptibility. The use of the new cut-offs will change the amount of resistance being reported. For example, the new cut off values for the dideoxynucleoside analogues, including d4T, are lower than before and, in a study of 5,000 random clinical samples, revealed a higher and more realistic incidence of resistance.

"We are very excited about this development. The new cut-offs will tell clinicians at-a-glance whether the susceptibility of their patient's virus to each drug has changed to a biologically significant degree," said Brendan Larder, PhD, Chief Scientific Officer of the Virco Group. "We are hopeful that the new cut-offs will help clinicians select the most effective drugs for their patients and ultimately improve the outcome of therapy."
Virco is a multinational biotechnology company with operating subsidiaries in the United States, Belgium, The United Kingdom and Ireland. It applies the most advanced technologies in molecular biology, automation, ultra high-throughput screening and electronic data processing to the diagnosis and management of HIV, other infectious diseases and cancer. Virco is the only company to provide genotyping, phenotyping and the new, unique VirtualPhenotype™. The company believes it provides the most sophisticated and comprehensive portfolio of phenotyping and genotyping decision support tools for physicians treating HIV infected patients.

Virco sells its testing services via Laboratory Corporation of America Holdings (LabCorp), SRL in Japan, as well as directly to HIV/AIDS centers all over the world. Virco has a two-year agreement with Gilead for the promotion of Virco Resistance Monitoring Services to US HIV prescribers by the Gilead sales force.

For further information, please visit Virco's website:

U.S. Contact: Peter Vigliarolo, Cooney/Waters; Tel: 212/886-2200
Virco: Dr. Andrew Revell; Tel: + 44 7967 126498 or at the Virco exhibition

Cooney Waters Group, Inc.

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