AIDSWEEKLY Plus, Monday, 14 April 1997
Daniel J. DeNoon, Senior Editor
The "future of antiretroviral therapy" has arrived ahead of schedule.
A new blood test permits physicians to rapidly determine whether a patient's HIV strain is susceptible to particular anti-HIV drugs. The genotypic assay, dubbed HIV-1 GenotypR Plus, is manufactured by Specialty Laboratories Inc., Santa Monica, California.
It has been less than three months since a leading AIDS researcher expressed the hope that such a test would be available by the end of the decade.
"I think the future of antiretroviral therapy will certainly be improved with a phenotypic or genotypic assay," University of California, San Diego researcher Douglas D. Richman said in a state-of-the-art presentation on HIV resistance to the 1997 Retrovirus conference (see AIDS Weekly Plus, February 17, 1997).
"I'm more convinced that the genotypic assays will be more useful for reverse-transcriptase inhibitors than protease inhibitors," Richman added. "But no well validated, reasonably constant assays are available for clinical management today. Hopefully they will be available in next few years."
Specialty Laboratories is calling their new test a "breakthrough." The assay uses polymerase chain reaction (PCR) technology and automated DNA sequencing to determine the genotype of HIV-1 in patient blood samples. Viral genotypes are then evaluated for the presence of mutations associated with resistance to anti-HIV drugs. Genotypic assays previously have been available only to researchers.
Blood samples must be sent to Specialty Laboratories for testing. Results can be expected in two to three weeks, according to materials provided by the manufacturer.
"Genotyping allows us to choose antiviral drugs that are active against a patient's strain of the virus, and to choose them right away, without making mistakes and without having to experiment to see if a certain drug works for the individual," said Neil Flynn of the University of California, Davis.
"We can give patients feedback on how well the drugs are working against their virus within two or three weeks. They can see the result of taking those medications instead of waiting six months or a year to see if they get sick with an AIDS illness. I think this provides a very strong incentive for patients to continue therapy."
The assay is able to identify resistance mutations for all anti-HIV drugs currently approved in the U.S. These include the reverse-transcriptase inhibitors zidovudine (AZT, trade name Retrovir), zalcitabine (ddC, trade name HIVID), stavudine (d4T, trade name Zerit), didanosine (ddI, trade name Videx), and lamivudine (3TC, trade name Epivir); and the protease inhibitors saquinavir (trade name Invirase), ritonavir (trade name Norvir), indinavir (trade name Crixivan), and the newly approved nelfinavir (trade name Viracept).
The cost of the test is $400. The manufacturer claims that most health insurers will cover this cost, and that it will be covered in the same way as other PCR tests by Medicaid and Medicare.
Specialty Laboratories said that the cost of the test is comparable to that of other sophisticated assays, and claims that testing will actually reduce patient costs by avoiding or withdrawing expensive AIDS medications that no longer work for particular patients.
Physicians will most likely call for the assay prior to beginning antiretroviral therapy, in order to determine whether the patient's virus is already resistant to antiretroviral drugs. For example, it is estimated that AZT- resistant strains are responsible for 10 to 15 percent of new HIV infections in the U.S.
According to what has fast become standard practice, the physician would then monitor the patient's viral load during therapy. An increase in viral load not attributable to other factors (e.g., vaccination) would signal the need for new genotypic testing.
Other researchers speaking at the 1997 Retrovirus conference addressed the need to tailor anti-HIV therapy to individual patients.
Ann Collier of the University of Washington, Seattle, suggested that given the great interpatient differences in HIV disease, there may never be a single standard drug regimen (see AIDS Weekly Plus, February 24, 1997).
"We need to get better at individualizing our treatments," she said. "Each patient is an individual and we now have sensitive measures of disease activity to help us. That said, we're all aware that the number of combinations which could be used will only increase as new drugs are added to our armamentarium."
Collier said that if it turns out that HIV must be treated with lifelong suppressive therapy, treatment will have to be altered as needed to deal with drug-resistant HIV strains that might emerge.
In a discussion of the possible simultaneous use of two different protease inhibitors, University of Pittsburgh researcher John Mellors said that one strategy would be to identify, early in the course of treatment, individuals who will have only an incomplete response to dual protease- inhibitor therapy (see AIDS Weekly Plus, February 17, 1997). The new assay would be one means of accomplishing this.
Specialty Laboratories in 1988 introduced the first commercially available PCR test for HIV. It also manufactures the HIV-1 RNA AccuQuant test for measuring HIV viral load.
More information on the test can be obtained from Specialty Laboratories, 2211 Michigan Avenue, Santa Monica, California 90404-3900. Telephone: (800) 421-7110. Website: http://www.specialtylabs.com.
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