A Closer Look: Virtual Phenotypic Resistance Test

Part of A Guide to HIV Drug Resistance

December 2006

Mark Holodniy, M.D., F.A.C.P., C.I.C., Stanford University; Veteran's Affairs Medical Center, Palo Alto, CA

Patient: Kyle C.
Doctor: Mark Holodniy, M.D.
CD4 count at the time of the test: 287
Viral load at the time of the test: 34,500
Current CD4 count: 361
Current viral load: undetectable
Resistance mutations: K20T, L33F, M36I, I54L, Q58E, L63P, T74S, L90M, K103N, M184V, K219R, L228R, K238N

Kyle C. (not his real name) has been on HIV treatment for a long time. His first regimen consisted of only 3TC and AZT. After two years on that regimen, he switched to his first triple-drug regimen: Combivir (AZT/3TC) and efavirenz. At the time this resistance test was taken, Kyle was taking 3TC, ddI (Videx), ritonavir and saquinavir (Invirase).

Although Kyle's overall health was good, his CD4 count was 287 and his viral load was 34,500, which indicated that his HIV medications weren't suppressing HIV as well as they should. His doctor, Mark Holodniy, M.D., ordered a virtual phenotypic test.

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This test found that Kyle's HIV was highly resistant to many HIV medications: 3TC and FTC (both NRTIs), every NNRTI and many protease inhibitors, especially if given unboosted. To some medications, such as ddI, abacavir, unboosted indinavir (Crixivan) or darunavir (Prezista), Kyle's virus had developed some, but not complete, resistance. Other drugs remain fully active, such as the NRTIs AZT, d4T and tenofovir. The protease inhibitors indinavir, saquinavir, lopinavir and tipranavir retained full activity, but only when boosted with ritonavir. Although Kyle was on a regimen containing boosted saquinavir, predicted on this test to have Maximal Response, his medications did not fully suppress HIV. This could be for several reasons, including the possibility that Kyle was finding it difficult to take all his medications on time.

Based on the results of the test, Dr. Holodniy prescribed a new regimen: Epzicom (abacavir/3TC), tenofovir and Kaletra. Six months later, Kyle is doing extremely well: His CD4 count is up from 287 to 361, and his viral load is now undetectable.