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WASHINGTON, D.C. -- Results of a 21-center study show that a combination of the antiviral drugs AZT and 3TC is more effective in retarding the progression of the AIDS virus than a combination of AZT with another antiviral agent called ddC, researchers at Duke University Medical Center reported Tuesday.

All three drugs interrupt a step in the virus's life cycle when its genetic material is inserted into a host cell's genes. Known as nucleoside analogues, the drugs are reverse transcriptase inhibitors that keep viral RNA from being copied into DNA in a host cell, an essential step in viral replication.

The trial, which involved 254 patients, was the first large test of the AZT-3TC combination in the United States involving patients who have been treated with AZT. Recent trials in Europe have observed potential advantages of the therapy.

According to Dr. John Bartlett, principal investigator of the study and research director of the Duke University Center for AIDS Research, the AZT-3TC combination led to an absolute increase in the number of CD4 lymphocytes, which are key immune system cells whose numbers drop as the disease compromises the immune system. Bartlett prepared the results of the study for presentation Tuesday at the Third National Conference on Human Retroviruses and Related Infections. The trial results extend and support the preliminary trial results Bartlett presented at last year's meeting.

"The AZT-3TC combination has been receiving much attention in the medical community, and it is currently being used by many physicians," Bartlett said in an interview. "This study provides important scientific data to support what we have been seeing in the clinics. "AZT-3TC is an excellent combination of drugs that provides enduring improvement in CD4 counts and is well-tolerated by patients," he said. "That the increase in the CD4 counts was maintained after 52 weeks supports the use of this combination for AZT-experienced patients." All patients enrolled in the trial had moderately advanced disease and all had been taking AZT -- the first federally approved drug to treat AIDS -- for an average of 2 years before enrolling in the trial. The randomized double-blind trial followed patient progress for 52 weeks. For the study, which was conducted at 21 medical centers in the United States, Canada and Puerto Rico, patients were randomized into three groups: high-dose 3TC plus AZT, low-dose 3TC and AZT, and ddC and AZT.

After one year, the AZT-ddC patients saw an average decrease of 29.58 CD4 cells per cubic millimeter of blood, while the low-dose 3TC plus AZT increased an average of 42.5 cells and the high-dose 3TC plus AZT increased an average of 23.33 cells. The difference between the high-dose and low-dose groups was deemed statistically insignificant, Bartlett said.

"We also found that patients in all three groups achieved a high degree -- 75 percent -- of suppression in the levels of HIV RNA in the plasma," Bartlett said. "On average, patients held this suppression for one year."

The amount of viral RNA in the bloodstream, as well as the level of CD4 cells, are the primary measurements physicians make in predicting the clinical outcomes of AIDS patients, Bartlett said. The primary goal of the trial was to gauge the effects of the drug combinations on these two important measurements. He said the trial also revealed another interesting trend.

"We saw that the AZT-3TC patients developed fewer new complications of their disease than did the other patients," Bartlett said. These complications, arising primarily as a result of patients' compromised immune systems, include such disorders as Kaposi's sarcoma or pneumocystis carinii pneumonia (PCP). "Since we saw this occur in only a few patients, this observation must be assessed in future large-scale trials."

AZT, 3TC and ddC are three of the five nucleoside analogues currently approved by the U.S. Food and Drug Administration (FDA) for use in treating AIDS patients. In November, the FDA gave marketing approval for the AZT-3TC combination.