Lamivudine Plus Zidovudine Compared with Zalcitabine Plus Zidovudine in Patients with HIV Infection

Methods

Study Sample

To be eligible for the study, patients had to meet all of the following criteria: age at least 18 years; documented HIV infection; absolute CD4+ cell count between 100 and 300 cells/mm³; at least 6 months of previous zidovudine therapy, including current zidovudine use at time of study entry; either no experience with didanosine, zalcitabine, or investigational antiretroviral drugs or a maximum of 4 weeks of previous treatment with didanosine; Karnofsky score of at least 60; hemoglobin concentration of at least 92 g/L for men and 88 g/L for women; absolute neutrophil count of at least 1000 cells/mm³; platelet count of at least 5 × 10⁷/L; hepatic aminotransferase and alkaline phosphatase levels no greater than five times the upper limit of normal; serum bilirubin level less than 25.65 mg/L; serum creatinine concentration less than 132.6 mg/L; and total serum amylase level no greater than 1.5 times the upper limit of normal. Patients receiving chemoprophylaxis for Pneumocystis carinii pneumonia, candidiasis, or herpes simplex infections and those receiving erythropoietin or granulocyte colony-stimulating factor were eligible for inclusion.

Exclusion criteria were evidence of active acquired immunodeficiency syndrome (AIDS)-defining opportunistic infections for which therapy had not been completed; current peripheral neuropathy of mild to moderate or greater severity; significant cardiac, hepatic, renal, or neurologic disease; active cancer; or intractable diarrhea or severe malabsorption. Patients were also ineligible if they were pregnant, were breastfeeding, or had childbearing potential and were not using adequate contraception.

Study Design

Our study was a multicenter, randomized, double-blind (the use of zidovudine was open-label) clinical trial designed to compare the safety and activity of two different doses of lamivudine plus zidovudine with the safety and activity of zalcitabine plus zidovudine. The study was originally intended to last 24 weeks, but the duration was amended during the course of the trial so that all patients who completed 24 weeks of study therapy could continue their assigned treatment in a double-blind manner through at least 52 weeks. The study was done at 21 sites in the United States, Canada, and Puerto Rico. The institutional review board at each institution approved the study, and all patients gave written consent. Glaxo Wellcome, Inc., collected and analyzed the primary study data, and the study investigators reviewed and interpreted the results.

Treatment Regimens

Patients were randomly assigned to receive one of three treatment regimens: 150 mg of lamivudine every 12 hours, 200 mg of zidovudine every 8 hours, and zalcitabine placebo every 8 hours (low-dose lamivudine group); 300 mg of lamivudine every 12 hours, 200 mg of zidovudine every 8 hours, and zalcitabine placebo every 8 hours (high-dose lamivudine group); or 0.75 mg of zalcitabine and 200 mg of zidovudine every 8 hours and lamivudine placebo every 12 hours (zalcitabine group).

Each dose of zidovudine (Retrovir; Glaxo Wellcome, Inc., Research Triangle Park, North Carolina) was given as two 100-mg capsules; each dose of zalcitabine (Hivid; Hoffman-La Roche, Inc., Nutley, New Jersey) was given as two 0.375-mg tablets; and each dose of lamivudine (Epivir; Glaxo Wellcome, Inc.) was given as two 75-mg tablets or as one 300-mg tablet with a matching placebo. All adjustments of medications for management of adverse events were made by each study center in a standardized and blinded manner. Adverse events and abnormal laboratory results were graded according to a toxicity rating scale developed by the Division of AIDS, National Institutes of Health.

Study Assessments

The primary outcome measure was the change from baseline in absolute CD4+ cell counts during the first 24 weeks of the study. The secondary outcome measures were the change in log₁₀ plasma HIV RNA levels, clinical progressions of HIV disease, percentages of cells that were CD4+ cells, serum β₂-microglobulin and neopterin levels, and serum immune complex-dissociated p24 antigen levels. Each patient was evaluated within 14 days of randomization, at least 72 hours before randomization, at the time of randomization (day 1), at study weeks 2 and 4, and every 4 weeks thereafter. Safety of the study regimens was evaluated through medical histories and physical examinations, clinical laboratory tests, and the reporting of adverse events. All laboratory studies were done by the Laboratory Corporation of America (formerly Roche Biomedical Laboratories, Research Triangle Park, North Carolina, and Raritan, New Jersey). Virologic activity was evaluated through the measurement of plasma HIV RNA levels by reverse transcriptase quantitative polymerase chain reaction (PCR) (Roche Quantiplex PCR, Laboratory Corporation of America) and the measurement of serum immune complex-dissociated p24 antigen levels (Coulter Corp., Hialeah, Florida) in all patients. The lower limit of detection for the reverse transcriptase PCR assay was 200 copies of HIV RNA per mL.

Human immunodeficiency virus isolates were banked for future drug-resistance studies. Immunologic activity was evaluated through the measurement of T-lymphocyte subsets, β₂-microglobulin levels, and neopterin levels in all patients. T-lymphocyte subsets were determined three times before therapy with study medications was initiated, and the baseline absolute CD4+ cell count was defined as the mean of the final two values obtained at the last visit made before and at the time of randomization. Clinical progressions of HIV disease were classified according to the criteria from the Centers for Disease Control and Prevention (CDC); these criteria consist of commonly recognized AIDS-related events (class B conditions) and AIDS-defining events (class C conditions) [28]. All diagnoses were reviewed by clinical research personnel who were blinded to treatment assignments. The criteria for the discontinuation of assigned treatment included serious adverse events; a 50% decline from baseline in absolute CD4+ cell counts on two consecutive occasions at least 28 days apart or a new AIDS-indicator illness (at the discretion of the center); pregnancy; unreliable follow-up (at the discretion of the center); or noncompliance. Patients who discontinued treatment were asked to return for monitoring of CD4+ cell counts every 4 weeks until study week 52.

Statistical Analysis

An equal number of patients was randomly assigned to the three treatment regimens through the use of permuted blocks at each study center. A sample size of 75 participants per treatment group was chosen to allow 80% power to detect a difference of 25 CD4+ cells/mm³ between the mean CD4+ cell counts of the three groups at the 0.05 level of significance. A summary metric [29] was used to characterize the HIV disease marker profile during the first 24 weeks of the study for the analyses of primary study outcomes. The time-weighted area under the curve (trapezoidal rule) minus the baseline value (DAVGT) was used to compare the profile of data collected longitudinally over the first 24 weeks of study. This method permits the inclusion of maximal available data in the analyses. In patients who completed 52 weeks of study treatment, the average of the last three evaluations (at weeks 44, 48, and 52) minus the baseline value was used as a “durability measure.”

Treatment groups were compared using the van Elteren test [30], which is an analogue of the Cochran-Mantel-Haenszel test used to compare ordinal or continuous outcomes across treatment groups stratified by study center [31]. Planned primary pairwise comparisons (the low-dose lamivudine group compared with the zalcitabine group and the high-dose lamivudine group compared with the zalcitabine group) were adjusted using the Bonferroni method for multiple comparisons. Planned secondary comparisons of the low-dose lamivudine group with the high-dose lamivudine group were not adjusted for multiple comparisons. All P values are unadjusted two-sided values. Exact permutation tests were used to compare the incidence of adverse events occurring up to 30 days after the end of study treatment. Baseline patient characteristics were compared using either the Cochran-Mantel-Haenszel general association test stratified by center (discrete variables) or the van Elteren test stratified by center (continuous variables). Time to permanent discontinuation of study therapy was plotted using Kaplan-Meier curves and tested using the log-rank test. The relative risk for progression to a new clinical end point (with 95% CIs) was calculated using the Cochran-Mantel-Haenszel statistic. All analyses of efficacy were done on an intention-to-treat basis and included the available information on all evaluable participants. Four scheduled interim evaluations of safety were reviewed by an independent data and safety monitoring board. The data from the trial were analyzed as planned after the last patient randomly assigned to treatment completed the 24-week double-blind period of the study.

Results

Patient Characteristics

A total of 483 patients were screened and 254 patients were enrolled in our study between June 1993 and May 1994. Two hundred fifteen patients did not meet the entry criteria, and 14 declined to participate before randomization. Fourteen patients who entered the study had at least one eligibility violation (6 had had less than 24 weeks of previous zidovudine use), and these 14 patients were included in all analyses.

No significant differences were seen in the demographic or other characteristics of the three groups before treatment (Table 1). The 254 study patients included 42 women (17%), 90 members of racial or ethnic minority groups (35%), and 33 patients who previously or currently used injection drugs (13%). One hundred six patients (42%) had symptomatic HIV disease. The median duration of previous zidovudine use was 20 months. Thirteen patients had previously received antiretroviral therapy with agents other than zidovudine; 9 of the 13 had previously received didanosine for less than 1 month. The mean absolute CD4+ cell count at baseline was 214.5 cells/mm³, and the median plasma HIV RNA level was 4.7 log₁₀ copies/mL.

Patient Disposition

Of the 254 patients originally assigned to treatment, 197 (78%) received study therapy for 24 weeks (Table 1). Of the 57 patients who did not complete the first 24 weeks of the study, 30 (12% of the patients originally assigned to treatment) discontinued treatment because of an adverse event; this did not differ according to treatment group (P > 0.2). Twenty-six patients (10% of the patients originally randomly assigned to treatment) were lost to follow-up, withdrew from the study at the discretion of the investigator, or withdrew from the study because of a decrease in absolute CD4+ cell count over 24 weeks. The patients who received study therapy for 24 weeks and those who did not complete 24 weeks of therapy did not differ with regard to baseline demographic characteristics, absolute CD4+ cell counts, or log₁₀ plasma HIV RNA levels (data not shown). One hundred sixty-one patients (63%) received their originally assigned medications for 52 weeks in a double-blind extension phase of the study. Of the 93 patients who did not complete the first 52 weeks of the study, 40 (16% of the patients originally randomly assigned to treatment) discontinued treatment because of an adverse event (12% of the low-dose lamivudine group, 17% of the high-dose lamivudine group, and 19% of the zalcitabine group). Kaplan-Meier plots of the time to permanent discontinuation of study therapy showed no significant difference in discontinuation rates among the treatment groups over 52 weeks (P > 0.2; data not shown). The patients who received study medication for 52 weeks and those who did not receive study medication for 52 weeks had similar baseline demographic characteristics and absolute CD4+ cell counts (data not shown). However, patients who completed 52 weeks of treatment had lower baseline plasma HIV RNA levels than did patients who did not complete 52 weeks of treatment (medians of 4.62 log₁₀ copies/mL and 4.83 log₁₀ copies/mL, respectively; P = 0.031).

Immunologic and Virologic Effects

Immunologic Activity

We assessed immunologic responses to each drug combination primarily by measuring changes in absolute CD4+ cell counts in the intention-to-treat sample (Figure 1). The median changes in the DAVGT of absolute CD4+ cells during the first 24 weeks of the study were plus 23.05 cells/mm³ for the low-dose lamivudine group, plus 36.47 cells/mm³ for the high-dose lamivudine group, and − 0.38 cells/mm³ for the zalcitabine group (Table 2). The changes in absolute CD4+ cell count during the 24-week period differed significantly (P < 0.001), favoring both the low-dose and the high-dose lamivudine regimens over the zalcitabine regimen (Table 2). No significant difference was seen between the low-dose lamivudine group and the high-dose lamivudine group (P > 0.2; data not shown). Changes in absolute CD4+ cell counts did not differ according to age, sex, race or ethnicity, or log₁₀ plasma HIV RNA levels or absolute CD4+ cell count at baseline (data not shown). For patients evaluated at week 52, the median changes from baseline in the durability measure were plus 42.50 cells/mm³ for the low-dose lamivudine group, plus 23.33 cells/mm³ for the high-dose lamivudine group, and − 29.58 cells/mm³ for the zalcitabine group (Table 3). The pairwise comparisons between the 52-week durability measures showed significant treatment differences favoring both the low-dose and the high-dose lamivudine regimens over the zalcitabine regimen (P = 0.012 for the low-dose lamivudine group compared with the zalcitabine group; P = 0.014 for the high-dose lamivudine group compared with the zalcitabine group). No significant difference was seen in the durability measure between the low-dose lamivudine group and the high-dose lamivudine group at week 52 (P > 0.2; data not shown). The secondary markers of immunologic activity included percentages of cells that were CD4+ cells, β₂-microglobulin levels, and neopterin levels (Table 2 and Table 3). At 52 weeks, the durability measure of the percentage of cells that were CD4+ cells and neopterin and β₂-microglobulin levels (after adjustment for multiple comparisons) did not differ significantly according to treatment group.

Antiretroviral Activity

We assessed the antiretroviral activity of each drug combination by measuring changes in log₁₀ plasma HIV RNA levels (using reverse transcriptase PCR) and immune complex-dissociated p24 antigen levels in the intention-to-treat sample. The DAVGT of log₁₀ plasma HIV RNA levels and immune complex-dissociated p24 antigen levels through week 24 are presented in Table 2 by treatment group with pairwise comparisons. The median change from baseline in log₁₀ plasma HIV RNA levels through week 52 is presented in Figure 2 by week of study. At study week 2, maximal effects on plasma HIV RNA levels were achieved in both the low-dose lamivudine group and the high-dose lamivudine group, and this suppression was much greater than that seen in the zalcitabine group (Figure 2). At study week 2, the peak median reduction in log₁₀ plasma HIV RNA levels was − 1.5 log₁₀ copies/mL for the low-dose lamivudine group, − 1.54 log₁₀ copies/mL for the high-dose lamivudine group, and − 0.70 log₁₀ copies/mL for the zalcitabine group. Over the first 24 weeks of the study, the suppression of log₁₀ plasma HIV RNA levels was significantly greater in the low-dose lamivudine group and the high-dose lamivudine group than in the zalcitabine group in the DAVGT measure (P = 0.024 for the low-dose lamivudine group compared with the zalcitabine group; P = 0.006 for the high-dose lamivudine group compared with the zalcitabine group) (Table 2), although the median changes at week 24 were similar (− 0.83 log₁₀ copies/mL for the low-dose lamivudine group, − 0.91 log₁₀ copies/mL for the high-dose lamivudine group, and − 0.64 log₁₀ copies/mL for the zalcitabine group). The difference in the DAVGT measure over the 24-week period was driven by the transient, early potent antiretroviral effect of lamivudine plus zidovudine. The difference seen between the low-dose lamivudine group and the high-dose lamivudine group through the first 24 weeks was not significant (P > 0.2; data not shown). Suppression of immune complex-dissociated p24 antigen through the first 24 weeks of the study did not differ significantly between the groups in the pairwise comparisons after correction for multiple comparisons. Antiretroviral activity was seen in all subgroups regardless of age, sex, race or ethnicity, or log₁₀ plasma HIV RNA levels or absolute CD4+ cell counts at baseline (data not shown). The durability measure of log₁₀ plasma HIV RNA levels at 52 weeks showed a median change of − 0.48 log₁₀ copies/mL for the low-dose lamivudine group, − 0.51 log₁₀ copies/mL for the high-dose lamivudine group, and − 0.39 log₁₀ copies/mL for the zalcitabine group (Table 3). In pairwise comparisons, the durability measure of log₁₀ plasma HIV RNA levels at 52 weeks did not differ significantly among the three treatment groups (Table 3).

Clinical Progression of HIV Disease

We assessed clinical progression of HIV disease by using the CDC classification system in the intention-to-treat sample with all available follow-up data until the study was unblinded. Overall, 44 clinical progressions to new class B or class C conditions occurred. Eleven were in the low-dose lamivudine group (13.1%), 14 were in the high-dose lamivudine group (16.7%), and 19 were in the zalcitabine group (22.1%). Six patients progressed to new CDC class C conditions (0 in the low-dose lamivudine group, 2 in the high-dose lamivudine group, and 4 in the zalcitabine group). No patients died during study follow-up. Because the overall rate of progression to new class C conditions was low (2.3%), the 168 patients in the high-dose and low-dose lamivudine groups were combined in an exploratory analysis because of their similar immunologic and virologic profiles through 52 weeks. These patients were then compared with the 86 patients in the zalcitabine group. The results favored lamivudine plus zidovudine over zalcitabine plus zidovudine, but the difference was not significant (relative risk for a new class C condition, 0.256 [95% CI, 0.054 to 1.215]; P = 0.086).

Safety Data

No significant differences were seen between treatment groups in the number of grade 3 or grade 4 toxicity-related clinical events reported, but more toxicity-related events occurred in the high-dose lamivudine group (P = 0.071) (Table 4). The high-dose lamivudine group reported more toxicity-related gastrointestinal symptoms, neurologic symptoms (headaches), and hepatobiliary tract or pancreatic symptoms. Grade 3 or grade 4 neuropathy did not commonly occur in any of the treatment groups. In addition, no significant differences were seen in the numbers of patients that reported at least one grade 3 or grade 4 toxicity-related laboratory event (P > 0.2) (Table 4). Of the 13 patients who developed grade 3 or grade 4 elevations in hepatic aminotransferase levels, 8 had had abnormal aminotransferase values at baseline. More patients in the high-dose lamivudine group developed grade 3 or grade 4 elevations in alanine aminotransferase or aspartate aminotransferase levels, but the difference was not statistically significant (P = 0.187). No differences were seen when the data were examined according to subgroups based on age, sex, and race or ethnicity (data not shown).

Discussion

The well-tolerated combination of lamivudine and zidovudine showed immunologic activity superior to that of the currently approved combination of zalcitabine and zidovudine in HIV-infected patients who had previously received zidovudine therapy for at least 6 months and had CD4+ cell counts of 100 to 300 cells/mm³. In addition, although the patient sample had an advanced stage of HIV disease and extensive previous nucleoside exposure (approximately 2 years), sustained suppression of plasma HIV RNA levels to less than baseline values was achieved by all three treatment groups. The maximum effect on plasma HIV RNA levels in both the low-dose and the high-dose lamivudine groups was achieved at study week 2, and this peak viral suppression was much greater than that seen in the zalcitabine group. In both the low-dose and the high-dose lamivudine groups, this early effect contributed to the significantly greater reductions in log₁₀ plasma HIV RNA levels seen over the 24-week period as measured by the time-weighted difference from baseline; however, the median reductions in these levels at 24 and 52 weeks were similar among the three groups.

A significantly greater and more sustained increase in absolute CD4+ cell count was seen for both the low-dose lamivudine group and the high-dose lamivudine group over 24 and 52 weeks of study. In the zalcitabine group, however, the mean CD4+ cell count initially increased and then returned to baseline by week 16; this was followed by a median decrease of 30 cells/mm³ at 52 weeks. There was also an apparent dissociation through 52 weeks between plasma HIV RNA levels (similar sustained reductions were seen in all three groups) and changes in absolute CD4+ cell count (the high-dose and low-dose lamivudine groups had sustained increases over the study period, whereas the zalcitabine group had a continued decrease from baseline). This has been seen in other trials of zalcitabine plus zidovudine in patients who had previously received zidovudine [3, 14, 32].

In patients followed through 52 weeks, the immunologic and antiretroviral effects of lamivudine plus zidovudine appear to endure. However, definite conclusions about the durability of responses cannot be made because of the high rate of discontinuation of study treatment (37% of patients discontinued therapy over 52 weeks, a rate similar to that seen in other trials of antiretroviral therapy [1, 3, 14, 33]). Although the clinical characteristics of patients completing and not completing the study were similar at baseline, a potential bias may have been introduced by the greater dropout rate of patients with less favorable immunologic and virologic responses. It is reassuring to note that the numbers of study patients who discontinued treatment did not differ significantly between groups.

These virologic and immunologic observations are also consistent with the results of other clinical studies of lamivudine plus zidovudine. A European trial [34] done in patients who had had at least 6 months of previous zidovudine therapy and had CD4+ cell counts of 100 to 400 cells/mm³ (NUCB 3002) showed superior increases in absolute CD4+ cell counts and decreases in plasma HIV RNA levels during therapy with lamivudine plus zidovudine compared with continued zidovudine monotherapy over the 24-week period. In addition, patients who received zidovudine monotherapy to which lamivudine was added after 24 weeks had similar increases in absolute CD4+ cell counts and decreases in plasma HIV RNA levels. In two other clinical trials done in patients who had not previously received zidovudine (NUCA 3001 and NUCB 3001), lamivudine plus zidovudine showed greater increases in absolute CD4+ cell counts and greater decreases in plasma HIV RNA levels than did zidovudine monotherapy [27, 33, 35]. Our study (NUCA 3002) is the first clinical trial to directly compare lamivudine plus zidovudine with another nucleoside reverse transcriptase inhibitor combination, and it shows the immunologic superiority of lamivudine plus zidovudine over zalcitabine plus zidovudine.

The predictive value of treatment-induced changes in mean absolute CD4+ cell counts and median plasma HIV RNA levels for such clinical outcomes as progression to AIDS or death has been recently analyzed in several clinical trials of antiretroviral therapy [36-39]. O'Brien and associates [36] retrospectively analyzed the relative values of both measures in a trial of immediate compared with delayed zidovudine therapy in mildly symptomatic patients who had never received zidovudine and had absolute CD4+ cell counts of 200 to 500 cells/mm³ (Veterans Affairs Cooperative Study 298) [40]. These researchers found that a treatment-induced decrease of 75% (− 0.6 log₁₀) in plasma HIV RNA level at 24 weeks accounted for 59% of the benefit of zidovudine treatment in delaying progression to AIDS. In comparison, the predictive value of a 10% increase in absolute CD4+ cell count at 24 weeks explained only 31% of the benefit of zidovudine on clinical outcomes. The combined use of treatment-induced changes in both plasma HIV RNA levels and absolute CD4+ cells at 24 weeks explained 79% of the benefit of zidovudine on clinical outcomes. In our trial, which studied a different sample of patients who had previously received zidovudine, had CD4+ cell counts of 100 to 300 cells/mm³, and were treated with combination therapies, the median change over 24 weeks in the DAVGT of plasma HIV RNA levels was − 0.83 log₁₀ copies/mL (− 85%). The median change over 24 weeks in the DAVGT of absolute CD4+ cell counts was plus 23.05 cells/mm³ (plus 11%) for the low-dose lamivudine group. Other recent studies [37-39] have also shown that treatment-induced changes in both absolute CD4+ cell counts and plasma HIV RNA levels may be most useful in assessing the effect of antiretroviral therapies.

Several theories may explain the enhanced, sustained immunologic and antiretroviral activity seen in vivo during therapy with lamivudine plus zidovudine. One explanation derives from observations about resistance to lamivudine and viral replicative capacity. In clinical studies of lamivudine or lamivudine plus zidovudine, early potent suppression of plasma HIV RNA is seen at week 2. Thereafter, plasma HIV RNA levels increase but do not return to baseline values. This corresponds to the development of phenotypic resistance to lamivudine and the appearance of the reverse transcriptase codon 184 lamivudine resistance-conferring mutation [26, 27]. Preliminary data from clinical trials in either patients who had not previously received zidovudine (NUCA 3001) or those who had received zidovudine (NUCB 3002) show that although lamivudine resistance develops by 12 weeks in most recipients of lamivudine monotherapy or lamivudine plus zidovudine, plasma HIV RNA levels remain suppressed at 24 weeks [27, 41]. Reverse transcriptase codon 184 is located in a highly conserved region of the HIV reverse transcriptase enzyme that is believed to be a critical domain for catalytic function; in vitro molecular analyses of substitutions at the YMDD motif in this region have altered the infectious potential of the virus [42]. Therefore, the lamivudine resistance-conferring mutation at reverse transcriptase codon 184 may result in the evolution of a “less fit” viral population during the selective pressure of lamivudine therapy; this may contribute to lower plasma HIV RNA levels. Another possible explanation for the durable antiviral activity of lamivudine plus zidovudine is derived from the previously mentioned in vitro molecular interactions between the lamivudine and zidovudine resistance-conferring mutations. In this circumstance, lamivudine plus zidovudine in patients who had previously received zidovudine and have zidovudine-resistant genotypes may result in restored susceptibility to ongoing zidovudine treatment, thus providing net viral suppression from both agents in the lamivudine plus zidovudine regimen. Additional information is forthcoming on a subset of patients from our study who were analyzed for phenotypic and genotypic antiretroviral resistance.

Several limitations should be considered when our clinical outcome results are interpreted. First, HIV disease progression was not our primary efficacy end point, but our protocol did specify the collection of data on HIV disease progression as a secondary end point. Second, there was a differential duration of exposure to study treatment, a differential duration of follow-up, and a complete loss to follow-up of many patients; all of these factors may have biased our clinical findings. Third, the rate of clinical events was insufficient to permit definite conclusions because only 6 of the 254 study patients had clinical disease progression (that is, the development of new CDC class C conditions) during the treatment and follow-up period. Because they had similar plasma HIV RNA and absolute CD4+ cell responses, the two groups that received lamivudine plus zidovudine were pooled so that lamivudine plus zidovudine could be compared with zalcitabine plus zidovudine. Results favored the lamivudine plus zidovudine groups over the zalcitabine plus zidovudine group only after the lamivudine plus zidovudine groups were pooled, but the difference was not significant. Taken together, these limitations reiterate that the confirmatory clinical studies of lamivudine plus zidovudine that are designed to assess clinical outcomes as primary study end points need to be completed.

We show that lamivudine plus zidovudine is generally well tolerated by patients who had previously received zidovudine. More grade 3 or grade 4 clinical and laboratory toxicity-related events were seen in the high-dose lamivudine group than in the low-dose lamivudine group. Given the similar antiretroviral and immunologic activity of the regimens in the low-dose and high-dose lamivudine groups and the higher overall incidence of grade 3 or grade 4 toxicity-related events in the high-dose lamivudine group, 150 mg of lamivudine plus zidovudine may be preferred for patients who have previously received zidovudine.

In summary, we show encouraging evidence of the immunologic and antiretroviral activities of lamivudine plus zidovudine in patients who had previously received zidovudine. These effects of the well-tolerated lamivudine plus zidovudine combination may reflect a previously unwitnessed durability in the combined activity on absolute CD4+ cell counts of two nucleoside reverse transcriptase inhibitors for patients with advanced HIV disease who had previously received zidovudine. Lamivudine plus zidovudine must now be evaluated for its ability to prolong therapeutic benefit in HIV-infected patients in larger-scale studies specifically designed to assess clinical outcomes. The rationale behind selecting a combination regimen for the treatment of HIV-infected patients is that a durable reduction in viral burden in vivo will delay disease progression. The recent introduction of HIV protease inhibitors and non-nucleoside reverse transcriptase inhibitors may offer additional opportunities to potently and durably suppress viral activity in combination with nucleoside reverse transcriptase inhibitors.

Appendix

Members of the North American HIV Working Party are Bisher Akil, MD (ComBAT Group, Los Angeles, California); Nicholas Bellos, MD (Southwest Infectious Disease Associates, PA, Dallas, Texas); K. Wayne Bockmon, MD (Houston Clinical Research Network, Houston, Texas); Charles Brummitt, MD (Wisconsin Community-Based Research Consortium, Milwaukee, Wisconsin); Bill Cameron, MD (Division of Infectious Disease, Ottawa General Hospital, Ottawa, Ontario, Canada); Timothy P. Cooley, MD (Boston City Hospital, Boston, Massachusetts); Joseph G. Jemsek, MD (Nalle Clinic, Charlotte, North Carolina); William Lang, MD (ViRx, Inc., San Francisco, California); George F. McKinley, MD [HIV]/AIDS Clinical Trials Program, New York, New York); Melanie Thompson, MD (AIDS Research Consortium of Atlanta, Atlanta, Georgia); Sharon Walmsley, MD (The Toronto Hospital, Toronto, Ontario, Canada); Earl Matthew, MD (Central Texas Medical Foundation, Austin, Texas); Daniel Pearce, DO (HIV Research Group, San Diego, California); Dolores Peterson, MD (University of Texas Southwestern Medical Center, Dallas, Texas); John C. Pottage Jr., MD (Chicago Center for Clinical Research, Chicago, Illinois); Jim Sampson, MD (The Research & Education Group, Portland, Oregon); and William B. Smith, MD (Louisiana Cardiovascular Research Center, New Orleans, Louisiana).

Sharon L. Benoit and Mary Ann Fallon, two authors of this manuscript, are currently employed in the Division of Virology at L.A.B. Biopharmaceutical International, Inc., Durham, North Carolina.

Ms. Benoit and Ms. Fallon: L.A.B. Biopharmaceuticals, Inc., University Corporate Center, Suite 212, 2634 Chapel Hill Boulevard, Durham, NC 27707.

Mr. Quinn: Medco Research, PO Box 13886, Research Triangle Park, NC 27709.

Ms. Self and Dr. Rubin: Glaxo Wellcome, Inc., Five Moore Drive, Research Triangle Park, NC 27709.

Dr. Johnson: 1900 University Boulevard, THT 229, University of Alabama at Birmingham School of Medicine, Birmingham, AL 35294-0006.

Dr. Sepulveda: Calle 23-0-26, Box 7949, Hospital Regionale De Ponce, Ponce, Puerto Rico 00732.

Dr. Ehmann: Department of Hematology, Hershey Medical Center, Hershey, PA 17036.

Dr. Tsoukas: Montreal General Hospital, 1650 Cedar Avenue, Room A5.140, Montreal, Quebec H3G 1A4, Canada.