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Antiretroviral effect and safety of abacavir alone and in combination with zidovudine in HIV-infected adults

Saag, Michael S.1,9; Sonnerborg, Anders2; Torres, Ramon A.3; Lancaster, Danny4; Gazzard, Brian G.5; Schooley, Robert T.6; Romero, Carmen7; Kelleher, Dennis8; Spreen, William8; LaFon, Stephen8Abacavir Phase 2 Clinical Team

Fast Track

Objectives: To evaluate, over 12 weeks, the antiretroviral activity and safety of abacavir, used alone and in combination with zidovudine (ZDV), as treatment for HIV-1-infected subjects who had limited or no antiretroviral treatment.

Design: Seventy-nine HIV-1-infected subjects, with CD4 cell counts 200–500 × 106/l and <12 weeks of previous treatment with ZDV were enrolled in a multicenter study. Subjects were randomly assigned to one of four cohorts receiving abacavir monotherapy for the first 4 weeks (200, 400, or 600 mg every 8 h daily, or 300 mg every 12 h daily) and, thereafter, combination therapy of abacavir with 600 mg ZDV or ZDV placebo, administered in a double-blind manner for an additional 8 weeks.

Methods: Antiretroviral activity was assessed by measuring changes in plasma HIV-1 RNA levels and CD4+ cell counts. Safety was assessed by monitoring clinical adverse events and laboratory abnormalities during the 12-week period and for 4 weeks post-treatment.

Results: Treatment with abacavir, alone or in combination with ZDV, produced marked decreases in plasma HIV-1 RNA loads and increases in CD4+ cell counts in all groups. At week 4, median plasma HIV-1 RNA loads decreased by 1.11–1.77 log10 copies/ml and median CD4+ cell counts increased by 63–111 × 106/l in all groups. At week 12, median HIV-1 RNA loads decreased by 1.02–2.24 log10 copies/ml (abacavir monotherapy) and by 1.81–2.01 log10 copies/ml (abacavir-ZDV); median CD4+ cell counts increased by 79–195 × 106/l (abacavir monotherapy) and by 93–142 × 106/l (abacavir-ZDV). At week 12, the percentage of subjects who had plasma HIV-1 RNA levels below 400 and 40 copies/ml were 28 and 11%, respectively (abacavir monotherapy) and 69 and 22%, respectively (abacavir-ZDV). Eight subjects (10%) discontinued the study prematurely because of adverse events; nausea (n = 4) and hypersensitivity (n = 3) were the most common reasons for withdrawal. There were no deaths among the study subjects.

Conclusions: In HIV-infected subjects who have received little or no prior antiretroviral therapy, treatment with abacavir alone or in combination with ZDV is well tolerated and resulted in sustained improvements in key immunologic and virologic efficacy parameters through 12 weeks.

1Department of Medicine, University of Alabama at Birmingham, Birmingham, Alabama, USA

2Division of Infectious Diseases, Karolinska Institute, Huddinge University Hospital, Huddinge, Sweden

3Department of Medicine, St Vincent's Hospital and Bentley-Salick Medical Practice, P.C., New York

4Department of Medicine, Methodist Hospital, Memphis, Tennessee, USA

5Department of Medicine, Chelsea and Westminister Hospital, London, UK

6Department of Medicine, University of Colorado Health Sciences Center, Denver, Colorado, USA

7Glaxo Wellcome, Greenford, Middlesex, UK

8Glaxo Wellcome, Inc., Research Triangle Park, North Carolina, USA.

9Requests for reprints to: Dr Michael Saag, The University of Alabama at Birmingham, 908 20th Street South, CCB # 174, Birmingham, AL 35294-2050, USA.

Sponsorship: Supported by a grant from Glaxo Wellcome, Inc., USA.

Date of receipt: 17 July 1998; revised: 24 August 1998; accepted: 1 September 1998.

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Abacavir [(-)-(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclopentene-1-methanol; 1592U89; ZiagenTM] is a novel carbocyclic nucleoside reverse transcriptase (RT) inhibitor with in vitro activity against a range of HIV-1 and HIV-2 strains, including zidovudine (ZDV)-resistant HIV-1 isolates, and has minimal cytotoxicity in vitro and in vivo [1]. In vitro studies have shown that abacavir and ZDV act synergistically against HIV-1IIIB [1]. Abacavir is phosphorylated by a unique cellular pathway to carbocyclic guanosine triphosphate (intracellular half-life of 3.3 h), which acts as a chain terminator of viral DNA replication [2]. Abacavir-resistant HIV-1 variants have been selected in vitro, some of which are cross-resistant to didanosine, zalcitabine, and lamivudine [3], but none were resistant to ZDV or stavudine. In vitro studies have also shown that, at clinically relevant concentrations, abacavir is not a substrate or inhibitor of cytochrome P450 subunit 3A4 [4].

Abacavir is in clinical development as an antiretroviral agent. A phase I study of abacavir in HIV-infected subjects showed that the drug was rapidly absorbed after oral administration and was well tolerated [5]. On the basis of early studies, a phase I/II clinical trial was conducted to evaluate the safety, tolerance and anti-retroviral activity of abacavir alone and in combination with ZDV in a patient population with limited or no prior antiretroviral treatment. The doses of abacavir were 200, 400 and 600 mg every 8 h (three times daily) and 300 mg every 12 h (twice daily).

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Materials and methods

Study population and design

Confirmed HIV-1-seropositive subjects were either antiretroviral-naive or had received less than 12 weeks of ZDV monotherapy. Subjects (male or female) were aged ≥ 18 years with CD4+ cell counts of 200–500 × 106/l. This study was conducted at 13 study sites (eight sites in the United States and five sites in Europe). All study sites had investigational review board approval to perform the study and all patients gave written informed consent prior to their participation.

Subjects on ZDV monotherapy discontinued treatment 2 weeks before entry. Subjects were sequentially enrolled into four cohorts receiving oral abacavir at 200 mg three times daily (cohort I), 400 mg three times daily (cohort II), 300 mg twice daily (cohort III), and 600 mg three times daily (cohort IV) for 4 weeks. Thereafter, subjects in each cohort were randomly assigned to receive a daily dose of either ZDV or matching ZDV placebo in a double-blind manner in combination with the previously assigned abacavir dose for an additional 8 weeks. Subjects received ZDV as 200 mg three times daily (cohorts I, II, and IV) or 300 mg twice daily (cohort III). Due to limited in vivo animal safety data at the time this study was initiated, extended therapy for greater than 12 weeks was not possible. Therefore, at the end of the 12-week period, abacavir was discontinued and subjects were treated with other antiretroviral agents at the discretion of the investigator.

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Blood samples were collected for analysis of plasma HIV-1 RNA levels (at baseline and at weeks 1, 2, 4, 6, 8 and 12) and for analysis of absolute CD4+ lymphocyte counts (at baseline and weeks 2, 4, 6, 8 and 12). Standard clinical and laboratory evaluations were performed at baseline and at weeks 1, 2, 4, 6, 8 and 12. Adverse events were managed by investigators unaware of the treatment assignments using predetermined guidelines, and the severity of the events was graded according to criteria developed by the US National Institute of Allergy and Infectious Diseases AIDS Clinical Trials Group.

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Laboratory methods

Plasma samples were analyzed for HIV-1 RNA using a commercially available RT-PCR assay with a detection limit of 400 copies/ml by the Clinical Virology Department, Glaxo Wellcome Research Centre (Stevenage, Hertfordshire, UK), according to the manufacturer's instructions (Amplicor HIV-1 Monitor, Roche Molecular Systems, Branchburg, New Jersey, USA). For subjects who had plasma HIV-1 RNA levels below 400 copies/ml, samples were analyzed using an investigational version of an ultrasensitive PCR assay with a detection limit of 40 copies/ml. T-lymphocyte subgroups were quantified by flow cytometry (SmithKline Beecham Clinical Laboratories, Van Nuys, California, USA and Heston, Middlesex, UK).

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Statistical analysis

Analyses included all subjects who had a baseline determination and had received at least one dose of abacavir. Baseline virologic and immunologic marker values were calculated as the mean values of two assessments, 2 weeks prior to and on the first day of dosing. Summary statistics and quartiles were calculated for the absolute values and for changes from baseline at each scheduled evaluation. The Wilcoxon rank-sum test was used to detect overall treatment differences between the abacavir monotherapy and abacavir-ZDV groups in the change in HIV-1 plasma viral load and CD4+ cell count from baseline to Week 12 for all cohorts combined. Differences were considered statistically significant if the P value was less than 0.05.

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Seventy-nine HIV-1-infected subjects were enrolled into four cohorts at sites in the United States and Europe. Treatment groups were comparable with regard to demographics and baseline characteristics (Table 1), except that the baseline median plasma HIV-1 RNA load in cohort I tended to be higher than in other cohorts. Most subjects (91%) had not received prior ZDV therapy; no subject had received other anti-retroviral therapy.

Table 1

Table 1

Of the 79 subjects, 69 completed the study (17, 15, 20, and 17 subjects in cohorts I, II, III, and IV, respectively). Of those who discontinued the study prematurely, eight withdrew because of adverse events and two withdrew their consent.

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Plasma HIV-1 RNA

Treatment with abacavir alone or in combination with ZDV resulted in a marked decrease in plasma HIV-1

RNA levels in all groups (Fig. 1a). At week 4, abacavir monotherapy elicited a median plasma HIV-1 RNA load reduction of 1.11–1.77 log10 copies/ml in the cohorts combined (Table 2). At week 12, median plasma HIV-1 RNA load reductions after treatment with abacavir monotherapy or abacavir-ZDV were 1.02–2.24 log10 copies/ml and 1.81–2.01 log10 copies/ml, respectively. For the combined cohorts, the change in median plasma HIV-1 RNA viral load from baseline to week 12 was significantly larger for the abacavir-ZDV group compared with the abacavir monotherapy group (P < 0.05).

Table 2

Table 2

Fig. 1

Fig. 1

At week 4, 28 and 45% of subjects on abacavir monotherapy who were subsequently assigned to receive placebo or ZDV, respectively, had plasma HIV-1 RNA levels of below 400 copies/ml (Fig. 1b). At week 12, 28 and 69% of subjects on abacavir or abacavir-ZDV, respectively, had plasma HIV-1 RNA levels below 400 copies/ml. The percentage of subjects who had viral loads below 40 copies/ml were 5 and 11% at week 4, and 11 and 22% at week 12, on abacavir monotherapy and abacavir-ZDV therapy, respectively.

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CD4+ cell counts

Treatment with abacavir alone or in combination with ZDV resulted in a sustained increase in CD4+ cell counts in all groups (Fig. 2). At week 4, abacavir elicited a median CD4+ cell count increase of 63–111 × 106/l (Table 2). At week 12, median CD4+ cell count increased by 79–195 × 106/l and 93–142 × 106/l, respectively, after treatment with abacavir or abacavir-ZDV. For the combined cohorts, the change in median CD4+ cell count from baseline to week 12 was not significantly different for the abacavir-ZDV group compared with the abacavir monotherapy group (P > 0.05).

Fig. 2

Fig. 2

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Adverse events

Abacavir monotherapy and abacavir-ZDV combination therapy were generally well tolerated. Adverse events were usually mild and reversible, with the most common adverse events being nausea (57%), headache (41%), and asthenia (29%; Table 3). Severe adverse events (attributable to abacavir) were infrequent and tended to be observed primarily in the higher dose cohorts (cohorts II and IV).

Table 3

Table 3

Eight subjects (10%) discontinued the study prematurely because of adverse events, all of which were considered by the investigator to be attributable to abacavir. None of these subjects were on 300 mg twice daily. Four subjects were withdrawn because of nausea, two receiving abacavir-ZDV therapy [200 mg three times daily (cohort I) and 400 mg three times daily (cohort II)] and two receiving abacavir alone [400 mg three times daily (cohort II) and 600 mg three times daily (cohort IV)]. One subject was withdrawn due to dizziness, photophobia, and palpitations (cohort IV). Three subjects, one on abacavir-ZDV (cohort I) and two on abacavir alone (cohort IV), were withdrawn within 4 weeks of dosing because of hypersensitivity reaction (erythematous generalized rash, low-grade fever, malaise and nausea). After a temporary interruption of study drugs due to hypersensitivity, subjects were rechallenged at least once. Rechallenge with abacavir resulted in rapid onset of hypersensitivity. There were no deaths among the study subjects.

Four additional subjects experienced a serious adverse event but remained in the study through week 12. Amongst abacavir-ZDV subjects, one experienced headache (cohort I) and another reported depression [300 mg twice daily (cohort III)]. Amongst abacavir monotherapy subjects, one experienced leukopenia (cohort II) and another had a syncopal episode (cohort III).

Five subjects (6%) experienced grade 3 or 4 laboratory abnormalities. Three cases of hematological abnormalities were reported: amongst subjects receiving ZDV monotherapy, one had leukopenia (grade 3, but grade 4 at one evaluation) and another had decreased hemoglobin count (grade 4); one subject receiving abacavir-ZDV therapy had decreased platelet count (grade 3). Two cases of clinical chemistry abnormalities were reported: one subject receiving ZDV monotherapy had elevated alanine aminotransferase levels (grade 3), and one subject receiving abacavir-ZDV therapy had hypoglycemia (grade 3). The one report of grade 4 leukopenia was also noted as a serious adverse event. In all cases, laboratory values had returned to within normal limits at the scheduled follow-up visit.

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In this 12-week study, treatment with abacavir both as monotherapy and in combination with ZDV resulted in sustained reductions in plasma HIV-1 RNA and marked increases in median CD4+ lymphocyte counts in HIV-infected subjects with limited or no previous use of ZDV. However, the study did not identify clear differences between different dosing regimens.

Abacavir monotherapy demonstrated a median viral load reduction of 1.11–1.77 log10 copies/ml at week 4 to 1.02–2.24 log10 copies/ml at week 12, and a median increase in CD4+ cell count of at least 79 × 106/l sustained over 12 weeks. In contrast, other RT inhibitor monotherapy regimens have demonstrated smaller viral load reductions during the first 12 weeks of therapy (0.5–0.8 log10 copies/ml at week 4 to 0.3–0.6 log10 copies/ml by week 12), with less pronounced reductions observed thereafter [6–9]. Median CD4+ cell count increases have also been modest (20–30 × 106/l at week 4), and this effect wanes over time [7–11]. These findings suggest that abacavir is more potent than currently available nucleoside RT inhibitors.

Our results also suggest that subjects who received abacavir with ZDV had a greater and more consistent improvement in plasma HIV-1 RNA levels than subjects receiving abacavir alone. However, with the limited sample size in our study, small differences in CD4+ cell count changes from baseline between the abacavir-ZDV and abacavir monotherapy groups could not be detected. In addition, although 69% of subjects receiving abacavir-ZDV therapy achieved reduction in HIV-1 RNA level below the limit of detection of the Roche Amplicor assay, only 22% were able to achieve values below the limit of detection using the ultrasensitive assay. Thus, like other antiretroviral agents, abacavir will most likely exhibit highest potency as part of a three-drug (or more) antiretroviral regimen.

A key finding of this study is that the reductions in viral load and increases in CD4+ cell count achieved with abacavir monotherapy were of comparable magnitude to those achieved with protease inhibitors. With protease inhibitor monotherapy, studies have reported reductions in HIV-1 RNA levels of 0.5–2.3 log10 copies/ml and increases in CD4+ cell counts of 68–104 × 106/l in antiretroviral-naive subjects [12–15]. In contrast, combination regimens involving two currently available nucleoside RT inhibitors (ZDV with lamivudine, didanosine, or zalcitabine) have yielded viral load reductions of 1.1–1.7 log10 copies/ml and CD4+ cell count increases of 55–90 × 106/l in anti-retroviral-naive subjects [6–11].

The present study investigated three escalating three times daily dosing regimens and one twice daily dosing regimen. The 200 mg three times daily and 300 mg twice daily regimens produced similar activity with fewer adverse events compared with the other dosing regimens. Considering the increased adherence to treatment achieved with lower pill burdens and twice daily dosing, the 300 mg twice daily dosing regimen was therefore selected for evaluation in subsequent clinical trials. The potency of abacavir combined with the convenience of twice daily dosing should facilitate patient adherence to therapy.

Abacavir was generally well tolerated. Five subjects (6%) experienced a grade 3/4 laboratory abnormality (none of which resulted in premature treatment discontinuation), and eight subjects (10%) withdrew because of an adverse event. Three patients treated with abacavir in this study experienced a hypersensitivity reaction. A broader experience with the use of abacavir from the treatment of over 5000 patients in subsequent clinical trials of this drug has revealed a hypersensitivity syndrome in approximately 3% of recipients worldwide. The hallmark of this syndrome is fever, varyingly accompanied by rash, weakness, nausea, and vomiting. The drug must be discontinued in order to achieve resolution of the symptoms, which usually resolve quickly. The symptoms universally recur rapidly upon rechallenge with the drug. Rechallenge following the occurrence of this reaction has resulted in severe manifestations of the original syndrome, in addition to hypotension, respiratory distress, and in one instance, death. No deaths occurred in this study despite rechallenge in each of the three cases of hypersensitivity.

Nonetheless, it is imperative that individuals who discontinue abacavir due to hypersensitivity syndrome should not be rechallenged with the drug.

Abacavir represents an important new agent for use in subjects with HIV infection. The precise role of abacavir in clinical practice remains to be determined. Future studies are needed to assess its activity in nucleoside-experienced subjects and those who are failing existing therapy with other nucleoside agents, protease inhibitors, and non-nucleoside agents. Studies are underway to evaluate the role of triple nucleoside therapeutic regimens that represents a ‘convergent’ approach to antiretroviral treatment via multiple attack of a single target.

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The investigators gratefully acknowledge the contributions of the Abacavir Phase 2 Clinical Team members: Richard D'Aquila (Massachusetts General Hospital, Boston, Massachusetts), Judith Feinberg (University of Cincinnati, Cinicinnati, Ohio), William Lang (ViRx, Inc., San Francisco, California), Melanie Thompson (AIDS Research Consortium of Atlanta, Atlanta, Georgia), Jan W. Mulder (Slotervaartziekenhuis, Amsterdam, The Netherlands), Merce Gurgui Ferrer (Hospital de Santa Creu i Sant Pau, Barcelona, Spain) and Miguel Santin Cerezales (Hospital de Bellvitge, Barcelona, Spain), and the Glaxo Wellcome team members: William Symonds, Stephanie Tortell, Gillian Pearce, Seth Hetherington, and Helen Steel for the clinical conduct of the study, Randall Lanier and Richard Harrigan for virologic support, Amy Cutrell for statistical analysis, and Teri Lorbacher for data entry and editing.

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Abacavir; zidovudine; combination drug therapy; reverse transcriptase inhibitors; HIV infection

© 1998 Lippincott Williams & Wilkins, Inc.