The results conflict with those from two earlier studies that found that Truvada is more effective, particularly for people who start treatment with higher viral loads (amount of HIV in the blood). However, another large study had found no difference between the two.

“These results support the use of either NRTI [nucleoside reverse transcriptase inhibitor] backbone in the initial therapy of antiretroviral therapy-naive patients and would support continuing Epzicom as a ‘preferred’ NRTI option,” wrote the study authors.

They also noted that the reason for the discrepancy in results is not clear.

Epzicom (abacavir/lamivudine) and Truvada (emtricitabine/tenofovir) both consist of two NRTIs and are commonly prescribed as the “backbone” of antiretroviral regimens, in combination with a third antiretroviral from a different drug class.

Results of previous studies comparing the two backbones have been conflicting. One large study found that Epzicom was not as effective as Truvada, with participants who had initial viral loads higher than 100,000 copies per milliliter of blood experiencing treatment failure significantly faster on Epzicom than on Truvada. In addition, a second study found that participants taking Epzicom were less likely to successfully achieve an undetectable viral load after 48 weeks.

However, results of another large study found no difference in efficacy between the two regimens, including for participants with high initial viral loads. Two additional studies in which participants switched from Truvada to Epzicom or vice versa also found similar efficacy between the drugs.

Since there are some safety concerns with Epzicom (some people can experience a rare but potentially fatal hypersensitivity reaction), the U.S. Department of Health and Human Services currently considers Truvada the “preferred” NRTI backbone, while Epzicom is considered an “alternative” backbone.

However, according to the study authors, Epzicom is still considered a preferred option in several international and European treatment guidelines.

In this study, the researchers aimed to determine whether there was a difference in efficacy or tolerability between the two regimens. The researchers examined the medical records of 1,764 HIV-positive adults who started antiretroviral therapy for the first time after January 1, 2000.

The median age of participants was 40 years old; most (86 percent) were male. None were injection drug users.

All participants took either Epzicom (588 participants) or Truvada (1,176 participants) in combination with Sustiva (efavirenz), Viramune (nevirapine), Kaletra (lopinavir/ritonavir), or Norvir (ritonavir)-boosted Reyataz (atazanavir).

For each participant, the researchers examined the risk of treatment failure, risk of switching or stopping backbone regimens due to side effects or any other reason, and time to successfully achieve undetectable viral loads.

The median follow-up time was 34 months for participants taking Epzicom and 20 months for participants taking Truvada.

Results showed that there was no difference in the risk of treatment failure between the two regimens regardless of participants’ initial viral loads, although participants taking the protease inhibitors Kaletra or Reyataz were more likely to experience treatment failure with both Epzicom and Truvada.

The researchers also found no difference in the risk of stopping or switching backbone regimens. Instead, female participants, those living in Ontario or Quebec (versus British Columbia), and those taking Kaletra were at higher risk of stopping or switching their initial regimens.

The authors speculated that the higher risks of treatment failure and regimen stopping or switching in participants taking protease inhibitors were due to physicians prescribing these drugs to more challenging patients, rather than actual problems with protease inhibitor-based regimens.

In addition, the median time to successfully achieve undetectable viral loads was four months for both participants taking Epzicom and participants taking Truvada. After 24 weeks, participants taking Epzicom had a 55 percent chance of having undetectable viral loads, compared to a 60 percent chance for participants taking Truvada. The difference was not considered significant.

For more information, please see the study in the Journal of Acquired Immune Deficiency Syndromes (abstract).

Selzentry was originally approved for treatment-experienced patients in 2007. Pfizer, Selzentry’s manufacturer, applied for its expanded use in September after receiving results from study 1026.

Study 1026 compared a 300 mg dose of Selzentry given twice daily in combination with Combivir (zidovudine/lamivudine) to treatment of Combivir and Sustiva (efavirenz) in treatment-naïve patients. Sustiva is an effective antiretroviral already approved as an option for patients undergoing treatment for the first time.

Selzentry specifically targets HIV strains that use a protein, called CCR5, to enter human T cells. Other strains of HIV use the CXCR4 protein and are not susceptible to the effects of Selzentry. It has been suggested that treatment-naïve patients are the most appropriate group to receive Selzentry because most are infected with CCR5-using HIV only.

Pfizer initially applied for Selzentry approval after week 48 results became available from study 1026. The reviewing FDA committee determined that the results showed a significant rate of virologic failure, or reemergence of a high viral load, observed in participants taking Selzentry. As a result, the committee recommended waiting for the completion of the study at week 96 to monitor the long-term changes in viral load. These results are considered to be a more accurate assessment of the efficacy of Selzentry use in treatment-naive patients and are included in Pfizer’s recent application.

A more sensitive test to determine the type of protein, CCR5 or CXCR4, used by a patient’s HIV strain was developed during the course of the study. This test established that 44 percent of the virological failures seen were due to improper administration to individuals infected with CXCR4-using HIV.

The results from week 96 indicated that Selzentry was both safe and effective as a first-line treatment. Participants taking Selzentry showed similar changes in viral load and CD4 cell counts to those taking Sustiva. The Antiviral Drugs Advisory Committee has now voted to recommend approval of Selzentry for use in treatment-naïve patients.

For more information, please read the Antiretroviral Drugs Advisory Committee Briefing Document available on the FDA Web site, as well as the press release on the Pfizer Web site.