Mutation Takes No Vacation: Can Structured Treatment Interruptions Increase the Risk of Drug-Resistant HIV-1?:

Quantifying the treatment efficacy of reverse transcriptase inhibitors: new analyses of clinical data based on within-host modeling

Background

Current measures of the clinical efficacy of antiretroviral therapy (ART) in the treatment of HIV include the change in HIV RNA in the plasma and the gain in CD4 cells.

Methods

We propose new measures for evaluating the efficacy of treatment that is based upon combinations of non-nucleoside and nucleoside reverse transcriptase inhibitors. Our efficacy measures are: the CD4 gain per virion eliminated, the potential of CD4 count restoration and the viral reproduction number (R₀). These efficacy measures are based upon a theoretical understanding of the impact of treatment on both viral dynamics and the immune reconstitution. Patient data were obtained from longitudinal HIV clinical cohorts.

Results

We found that the CD4 cell gain per virion eliminated ranged from 10^-2 to 600 CD4 cells/virion, the potential of CD4 count restoration ranged from 60 to 1520 CD4 cells/μl, and the basic reproduction number was reduced from an average of 5.1 before therapy to an average of 1.2 after one year of therapy. There was substantial heterogeneity in these efficacy measures among patients with detectable viral replication. We found that many patients who achieved viral suppression did not have high CD4 cell recovery profiles. Our efficacy measures also enabled us to identify a subgroup of patients who were not virally suppressed but had the potential to reach a high CD4 count and/or achieve viral suppression if they had been switched to a more potent regimen.

Conclusion

We show that our new efficacy measures are useful for analyzing the long-term treatment efficacy of combination reverse transcriptase inhibitors and argue that achieving a low R₀ does not imply achieving viral suppression.

HIV-1-Resistant Strains during 8-Week on 8-Week off Intermittent Therapy and their Effect on CD4+ T-cell Counts and Antiviral Response

Background

We aimed here to study drug-associated HIV resistance mutations in peripheral blood mononuclear cells (PBMC) and plasma during intermittent therapy.

Methods

A substudy of 86 patients randomized to the intermittent treatment arm (8-week on/8-week off) of the ANRS 106 Window trial. HIV reverse transcriptase and protease genes were sequenced and resistance mutations identified according to the International AIDS Society list.

Results

Resistance mutations were detected in PBMC of 27/86 (31%) patients at baseline and 25/72 (35%) patients at week 96. Resistance mutations were detected in plasma of 28/86 (33%), 24/83 (29%) and 33/80 (41%) patients at weeks 8, 40 and 88, respectively. The detection of nucleoside reverse transcriptase inhibitor and protease inhibitor associated resistance mutations in plasma remained stable over time, but there was an increase in non-nucleoside reverse transcriptase inhibitor (NNRTI)-associated resistance mutations in patients on an NNRTI-based regimen: 1/33 (3%) versus 7/26 patients (27%) at weeks 8 and 88, respectively ( P=0.02). The proportions of patients with plasma HIV RNA levels ≤400 copies/ml after 8 weeks of treatment at weeks 16, 48 and 96 in patients with drug-resistant and wild-type viruses were 93% versus 74% ( P=0.04), 96% versus 88% ( P=0.43) and 70% versus 84% ( P=0.13), respectively. Patients with drug-resistant virus had a lower CD4+ T-cell decrease from baseline at weeks 40 and 88 as compared to patients with wild-type virus ( P=0.05 and 0.002, respectively).

Conclusions

NNRTI-associated resistance mutations increased over time in plasma of patients who were given NNRTIs. Drug-associated HIV resistance mutations did not seem to impair short-term antiviral response and might be associated with reduced CD4+ T-cell loss during interruptions.

Drug-Sparing Regimens for HIV Combination Therapy: Benefits Predicted for “Drug Coasting”

Structured Treatment Interruptions (STI) during HIV drug therapy were thought to potentially reduce side effects and toxicity, boost immune involvement, and possibly lower the viral set-point. Clinical trials of STI regimens, however, have had mixed results. We use an established mathematical model of HAART to estimate possible therapeutic outcomes for STI and for other, similar patterns in HIV combination therapy. We perform an exhaustive search of patterns of up to 60 days, for triple-drug combinations involving accurate pharmacokinetics for 12 specific antiviral drugs. The results of this analysis are consistent with recent clinical trials which have demonstrated that STI-type patterns, involving therapy interruption of weeks or months, are rarely optimal. Our analysis predicts, however, that the benefit of treatment can often be improved by including very short drug-free periods, during which the patient effectively "coasts" for a day or two on adequate drug concentrations due to the long half-life of some pharmaceuticals. Our analysis predicts many cases in which this may be achieved without increasing the risk of drug-resistance. This suggests that "drug coasting" patterns, significantly shorter than STI patterns, may merit further clinical investigation in efforts to find drug-sparing regimens for HIV.

Costs versus benefits: best possible and best practical treatment regimens for HIV

Current HIV therapy, although highly effective, may cause very serious side effects, making adherence to the prescribed regimen difficult. Mathematical modeling may be used to evaluate alternative treatment regimens by weighing the positive results of treatment, such as higher levels of helper T cells, against the negative consequences, such as side effects and the possibility of resistance mutations. Although estimating the weights assigned to these factors is difficult, current clinical practice offers insight by defining situations in which therapy is considered "worthwhile". We therefore use clinical practice, along with the probability that a drug-resistant mutation is present at the start of therapy, to suggest methods of rationally estimating these weights. In our underlying model, we use ordinary differential equations to describe the time course of in-host HIV infection, and include populations of both activated CD4(+) T cells and CD8(+) T cells. We then determine the best possible treatment regimen, assuming that the effectiveness of the drug can be continually adjusted, and the best practical treatment regimen, evaluating all patterns of a block of days "on" therapy followed by a block of days "off" therapy. We find that when the tolerance for drug-resistant mutations is low, high drug concentrations which maintain low infected cell populations are optimal. In contrast, if the tolerance for drug-resistant mutations is fairly high, the optimal treatment involves periods of reduced drug exposure which consequently boost the immune response through increased antigen exposure. We elucidate the dependence of the optimal treatment regimen on the pharmacokinetic parameters of specific antiviral agents.

Structured treatment interruptions (STI) in chronic unsuppressed HIV infection in adults

BACKGROUND

Structured treatment interruptions (STI) of antiretroviral therapy (ART) have been investigated as part of novel treatment strategies, with different aims and objectives depending on the populations involved. These populations include: 1) patients who initiate ART during acute HIV infection; 2) patients with chronic HIV infection, on ART, with successfully suppressed viremia; and 3) patients with chronic HIV infection and treatment failure, with persistent viremia due to multi-drug resistant HIV (Hirschel 2001; Deeks 2002; Miller 2003). In an earlier Cochrane review (Pai 2005), we had summarized the evidence about the effects of STI in chronic suppressed HIV infection. In this review, we summarize the evidence on STI in patients with chronic unsuppressed HIV infection due to drug-resistant HIV. Unsuppressed HIV infection describes those patients who cannot suppress viremia, due to the presence of multi-drug-resistant virus. It is also referred to as treatment failure. Drug resistance is identified by the presence of resistant mutations at baseline.STI as a treatment strategy in HIV-infected patients with chronic unsuppressed viremia involves interrupting ART in controlled clinical settings, for a pre-specified duration of time. These interruptions have various aims, including the following: 1) to allow wild virus to re-emerge and replace the resistant mutant virus, with the hope of improving the efficacy of a subsequent ART regimen; 2) to halt development of drug resistance and to preserve subsequent treatment options; 3) to alleviate treatment fatigue and reduce drug-related adverse effects; and 4) to improve quality of life (Miller 2003; Montaner 2001; Vella 2000;).

OBJECTIVES

The objective of our systematic review was to synthesize the evidence on the effect of structured treatment interruptions in adult patients with chronic unsuppressed HIV infection.

SEARCH STRATEGY

We included all available intervention studies (randomized controlled trials and non-randomized trials) conducted in HIV-infected patients worldwide. We searched nine databases, covering the period from January 1996 to February 2006. We also scanned bibliographies of relevant studies and contacted experts in the field to identify unpublished research, abstracts and ongoing trials. In the first screen, a total of 3186 potentially eligible citations from nine databases and sources were identified, of which 2047 duplicate citations were excluded. The remaining 1139 citations were examined in detail, and we further excluded 951 citations that were modeling studies, animal studies, case reports, and opinion pieces. As shown in Figure 01, 188 citations were identified in the second screen as relevant for full-text screening. Of these, 60 basic science studies, editorials and abstracts were excluded and 128 full-text articles were retrieved. In the third screen, all full-text articles were examined for eligibility in our review. These were subclassified into three categories: 1) chronic suppressed HIV infection; 2) chronic unsuppressed HIV infection; and 3) acute HIV infection. Studies were further excluded if their abstracts did not contain enough information for inclusion in our reviews. A total of 62 studies were finally classified into chronic suppressed, acute, and chronic unsuppressed categories. Of these, 17 trials met the eligibility criteria for this review.

SELECTION CRITERIA

Inclusion criteriaAll available randomized or non-randomized controlled trials investigating planned treatment interruptions among patients with chronic unsuppressed HIV infection. Early pilot non-randomized prospective studies on treatment interruptions of fixed and variable durations were also included. Relevant abstracts on randomized controlled trials were also included if they contained sufficient information. Exclusion criteriaEditorials, reviews, modeling studies, and basic science studies were excluded. Studies on STI among patients with chronic suppressed HIV infection were summarized in a separate review. Studies on STI in primary HIV infection were beyond the scope of this review.

DATA COLLECTION AND ANALYSIS

Two reviewers independently extracted data, evaluated study eligibility and quality. Disagreements were resolved in consultation with a third reviewer.A total of seventeen studies on STI were included in our review. However, due to significant heterogeneity across studies (i.e. in study design, populations, baseline characteristics, and reported outcomes; and in reporting of measures of effect, hazard ratios, and risk ratios), we considered it inappropriate to perform a meta-analysis.

MAIN RESULTS

In early pilot non-randomized trials, a pattern was evident across studies. During treatment interruption, a decline in CD4 cell counts, increase in viral load, and a shift in the level of genotypic drug resistance towards more of a wild-type HIV virus was reported. This suggests that STI may be used to increase drug susceptibility to an optimized salvage regimen upon treatment re-initiation. These studies generated useful data and hypotheses that were later tested in randomized controlled trials. Randomized controlled trials rated high on quality. Of the eight randomized controlled trials reviewed, seven had been completed while one was ongoing and remains blinded. Of the seven completed randomized controlled trials, six have reported consistent virologic and immunologic patterns, and found no significant benefit in virologic response to subsequent ART in the STI arm, compared to the control arm. In addition, the largest completed randomized trial reported greater numbers of clinical disease progression events and evidence of prolonged negative impact on CD4 cell counts in the STI arm (Beatty 2005; Benson 2004; Deeks 2001; Lawrence 2003; Walmsley 2005; Ruiz 2003). The single RCT with divergent findings from the others (GigHAART), reporting a significant virologic and immunologic benefit due to STI, was different in prescribing a shorter STI duration and a salvage ART regimen of 8-9 drugs. There were also differences in the patient population characteristics with this study, targeting those with very advanced HIV disease (Katlama 2004). Although we await the unblinded results of the eighth RCT (OPTIMA), the evidence so far does not support STI in the setting of chronic unsuppressed HIV infection with antiretroviral treatment failure (Brown 2004; Holodniy 2004; Kyriakides 2002; Singer 2006).

AUTHORS' CONCLUSIONS

The current available evidence primarily supports a lack of benefit of STI before switching therapy in patients with unsuppressed HIV viremia despite ART. There is evidence of harm in attempting STI in patients with relatively advanced HIV disease, due to the associated CD4 cell decline and the increased risk of clinical disease progression. At this time, there is no evidence to recommend the use of STI in this clinical category of patients with treatment failure.

Structured treatment interruptions (STI) in chronic suppressed HIV infection in adults

BACKGROUND

Although antiretroviral treatment (ART) has led to a decline in morbidity and mortality of HIV-infected patients in developed countries, it has also presented challenges. These challenges include increases in pill burden; adherence to treatment; development of resistance and treatment failure; development of drug toxicities; and increase in cost of HIV treatment and care. These issues stimulated interest in investigating the short-term and long-term consequences of discontinuing ART, thus providing support for research in structured treatment interruptions (STI). Structured treatment interruptions of antiretroviral treatment involve taking supervised breaks from ART. STI are defined as one or more planned, timing pre-specified, cyclical interruptions in ART. STI are attempted in monitored clinical settings in eligible participants. STI have generated hopes of reducing drug toxicities, decreasing costs and total time on treatment in HIV-positive patients. The first STI was attempted in the case of a patient in Germany, who later permanently discontinued treatment. This successful anecdotal case report led to several trials on STI worldwide.

OBJECTIVES

The objective of this systematic review was to assess the effects of structured treatment interruptions (STI) of antiretroviral therapy (ART) in the management of chronic suppressed HIV infection, using all available high-quality studies.

SEARCH STRATEGY

Nine databases covering the time period from January 1996 to March 2005 were searched. Bibliographies were scanned and experts contacted in the field to identify unpublished research and ongoing trials. Two reviewers independently extracted data, and evaluated study eligibility and quality. Disagreements were resolved in consultation with a third reviewer. Data from 33 studies were included in the review.

SELECTION CRITERIA

STI is a planned, timing pre-specified experimental intervention. In our review, we decided to include all available intervention trials in HIV-infected patients, with or without control groups. We reviewed evidence from 18 randomized and non-randomized controlled trials, and 15 single arm trials. Single arm trials were included because these pilot studies made significant contribution to the early development and refutation of hypotheses in STI.

DATA COLLECTION AND ANALYSIS

Trials included in this review varied in study participants, methodology and reported inconsistent measures of effect. Due to this heterogeneity, we did not attempt to meta-analyse them. Results were tabulated and a qualitative systematic review was done

MAIN RESULTS

For the purpose of this review, STI strategies were classified either as a timed-cycle STI strategy or a CD4-guided STI strategy. In timed-cycle STI strategy, a predetermined period of fixed duration (e.g. one week, one month) off ART was attempted followed by resumption of ART, while closely monitoring changes in CD4 levels and viral load levels. Predetermined criteria for interruption and resumption were laid out in this strategy. Timed-cycle STI fell out of favor due to reports of development of resistance in many studies. Moreover, there were no significant immunological and virological benefits, and no reduction in toxicities, reported in these studies. In CD4-guided STI strategy, ART was interrupted for variable durations guided by CD4 levels. Participants with high nadir CD4 levels qualified for this approach. A reduction in costs of ART, a reduction in mutation, and a better tolerability of this CD4-guided STI strategy was reported. However, concerns about long-term safety of this strategy on immunological, virological, and clinical outcomes were also raised.

AUTHORS' CONCLUSIONS

Timed-cycle STI have not been proven to be safe in the short term. Although CD4-guided STI strategy has reported favorable outcomes in the short term, the long-term safety, efficacy and tolerability of this strategy has not been fully investigated. Based on the studies we reviewed, the evidence to support the use of timed-cycle STI and CD4-guided STI cycles as a standard of care in the management of chronic suppressed HIV infection is inconclusive.

Strategies to Decrease Viral Load Rebound, and Prevent Loss of Cd4 and Onset of Resistance during Structured Treatment Interruptions

Background

Toxicity and other drug adherence-related factors have contributed to decreased compliance to antiretroviral regimens amongst HIV-infected patients. Irregular therapy disruption causes loss of CD4 T cells, onset of drug resistance and rapid rebound of plasma viral load (VL). However, an appropriate choice of drugs and properly scheduled structured treatment interruptions (STIs) may limit VL rebound, maintain CD4 counts and minimize resistance.

Methods

We conducted a clinical study of STIs, RIGHT 901, involving 60 drug-naive patients with chronic HIV infection (CD4 >300, VL >10000) randomized to receive didanosine-stavudine-indinavir (IDV group) or didanosine-stavudine-hydroxyurea (HU group), for 12 weeks. Subsequently, all patients were randomized again to start STI (short induction) or to continue the therapy for an additional 24 weeks before starting STI (long induction). Both groups underwent four STI cycles and then stopped therapy as long as viraemia remained below 10000 copies/ml before reinitiating another four cycles of STI.

Results

During continuous therapy VLs were suppressed at similar rates in both the HU and IDV groups, while a blunted CD4 count was documented in the HU group. Following the first stop median VL rebounded close to baseline values in both groups, however, during the following STI median VL rebound decreased in the HU group, while in the IDV group VL continued to rebound to values close to baseline, and the difference between the two groups was statistically significant. Moreover, patients treated with HU had a constant and stable CD4 increase during STI, whereas CD4 counts fluctuated in the IDV group, with sharp falls during treatment interruptions and partial CD4 recovery following treatment restart. Even in the presence of IDV resistance predisposing mutations at baseline, no genotypic change in the protease sequence was observed during STI. A relevant mutation in the reverse transcriptase sequence (K70R) emerged in one patient interrupting treatment after 36 weeks of continuous therapy and in one patient after four STI cycles. Side effects (no major events) were similar among groups.

Conclusions

An appropriate choice of STI schedule and regimens containing drugs less prone to resistance and/or able to prevent CD4 fluctuation may contribute to optimizing STI for chronically infected patients with respect to limiting viral rebound, improving CD4 counts and maintaining a resistance profile comparable to continuous highly active antiretroviral therapy.

Lack of persistent drug-resistant mutations evaluated within and between treatment interruptions in chronically HIV-1-infected patients

OBJECTIVE

To determine the effect of treatment interruptions (TI) on the evolution and persistence of drug-resistant viruses in chronically HIV-1-infected suppressed patients.

METHODS

The emergence of viral resistance to combination antiretroviral therapy was monitored in 11 suppressed chronically HIV-1-infected patients undergoing from one up to four sequential TI (a total of 25 TI), by genotyping of the virus for known mutations in the genes for protease and reverse transcriptase. Resistance assays were performed at the first viral rebound > 100 copies/ml.

RESULTS

All subjects achieved resuppression of HIV-1 under the same antiretroviral therapy, regardless of the number of TI. Five of eleven patients showed no development of resistance. In the remaining six patients, the following patterns of mutations associated with viral resistance were found: one mutation (K70R), which was observed in one patient during the 1st TI and persisted during follow-up; two mutations (L90M, M184V), which were observed in four patients during the 1st TI and were intermittently present or lost following extended TI, treatment reinitiation and/or during subsequent TI; and evolution of two mutations (M184V, K219E) observed in two patients. These two mutations were not present during the 1st TI and were subsequently lost following therapy reinitiation or during the next TI.

CONCLUSIONS

Detection of drug resistance during TI by virus genotyping assays does not predict failure to resuppress after antiretroviral therapy reinitiation nor persistence of a resistant viral population during extended interruptions or subsequent TI.

Drug Resistance Mutations during Structured Treatment Interruptions

Background

We assessed whether treatment interruptions induce selection of mutations associated with drug resistance in the Swiss–Spanish Intermittent Treatment Trial (SSITT). Patients had been on HAART without previous failure and had undetectable viraemia for at least 6 months. Their HAART was interrupted for 2 weeks and restarted for 8 weeks. After four of these cycles, treatment was definitively interrupted at week 40.

Methods

Genotypic resistance testing was performed in 87/97 Swiss patients: in those failing treatment before week 40, at the time of first viral rebound >500 copies/ml off treatment and preceding failure to reach RNA <50 copies/ml after 8 weeks of re-treatment; for patients without virological failure, on the first sample with HIV-1 RNA >1000 copies/ml after week 40.

Results

Mutations associated with drug resistance were detected in 9/25 (36%) patients with virological failure during the first 40 weeks and in 6/59 (10%) patients after week 40. Overall, drug resistance mutations were detected in 17% of patients, all but two with the 184V/I mutation. Among the 74 patients receiving lamivudine, the M184V/I mutation was detected in 13/74 (17.6%) patients. A wild-type codon at position 184 was detected in previous samples in all b'ut two. The relative risk for virological failure was 2.55-fold higher in patients with the M184V/I mutation than in patients without detectable mutation ( P=0.007).

Conclusions

The M184V/I mutation is frequently selected during repeated treatment interruptions.

Structured treatment interruption in patients infected with HIV: a new approach to therapy

Current antiretroviral regimens are limited by issues of potency, adherence, toxicity, resistance and cost. With these limitations and the realisation that eradication of HIV infection currently is not possible, there is enthusiasm for strategies that allow discontinuation of medications, such as the structured treatment interruption (STI). STI is hypothesised to have benefits in three distinct clinical scenarios: acute treated infection, chronic treated infection with controlled viraemia, and chronic treated infection without controlled viraemia (salvage therapy). In patients with acute treated HIV infection, STI may preserve or enhance cellular immune responses to allow continued virological suppression in the absence of ongoing treatment. The Berlin patient presented with acute HIV infection prior to seroconversion and received antiretroviral therapy. After two treatment interruptions (for intercurrent infections), he permanently discontinued therapy and remained virologically suppressed for 2 years. Investigators from Massachusetts General Hospital described eight patients with acute or early HIV infection who received treatment and then underwent one or two STI. After the STIs, five of eight patients showed enhanced cellular immune responses and continued with virological suppression off treatment for a median of 2.7 years. In patients with chronic treated infection with controlled viraemia, STI may enhance immune responses as in the case of acute infection, or may allow decreased drug exposure and toxicity. Investigators from the National Institutes of Health enrolled 18 patients with chronic HIV infection and virological suppression while taking antiretroviral regimens. With a single STI, all patients rebounded, although one (6%) ultimately continued off therapy with virological suppression. The largest study of STI is the Spanish Swiss Intermittent Treatment Trial in which 128 patients with chronic suppressed HIV infection on antiretroviral therapy underwent four cycles of STI. At 52 weeks, 17% had suppressed viral load levels of <5000 copies/ml in the absence of therapy. In patients with chronic treated infection without controlled viraemia (salvage therapy), STI promotes a shift from resistant to wild-type (i.e. no mutations) virus. In the Hamburg cohort, the shift to wild-type virus was seen in 28 of 45 heavily treatment-experienced patients after an STI. Seventy-two percent of these patients experienced a virological response on a subsequent regimen, although many ultimately experienced virological rebound. In the San Francisco cohort, a shift to wild-type virus was seen in 15 of 17 protease inhibitor-experienced patients and six of these patients achieved virological suppression to <200 copies/ml on a new regimen. Risks associated with STI include increases in viral load levels with the risk of loss of virological control (i.e. failure to resuppress on therapy), repopulation of viral reservoirs and antiretroviral resistance, and decreases in CD4+ cell counts with the risk of loss or dysregulation of immune function and the occurrence of clinical events. Other risks include acute retroviral syndrome and the recurrence of short-term adverse effects. Currently, STI cannot be recommended as part of routine clinical care. Prospective studies are needed to assess the risks and benefits of this strategy in all clinical settings.

HIV drug resistance and implications for the introduction of antiretroviral therapy in resource-poor countries

The development and transmission of HIV drug-resistant viruses is of serious concern and has been shown to significantly diminish the effectiveness of antiretroviral therapy. In addition, cross-resistance between drugs of the same class can seriously limit therapeutic options and may potentially be most problematic in resource-poor settings where new drugs are not widely available. Strategies based on avoidance of virological failure are therefore essential for the long-term success of therapy. In this regard, regionally adapted programs to facilitate proper adherence with therapy need to be urgently implemented, concomitant with expanded access to new antiretroviral drugs. The value of genotypic resistance testing as a prognostic tool to help guide therapeutic decisions has been established. However, the relatively high cost of this novel technology does not warrant its routine utilization at this time in resource-poor countries. Lastly, the genetic barrier of the antiretroviral agents that are prescribed is also an important consideration that needs to be integrated with knowledge of HIV-1 subtypes, drug pharmacology, and medical management of concurrent illnesses. The selection of appropriate first-line antiretroviral combination regimens may be an even more important consideration in developing than developed countries, given that options in the aftermath of treatment failure may be more limited in such settings.

Planned interruptions of anti-HIV treatment

Highly active antiretroviral therapies (HAART) lower morbidity and mortality of HIV infection, but are unable to eradicate HIV and may cause side-effects. Planned interruptions of HAART are studied for three reasons: (1) to stimulate the anti-HIV immune response after viraemia has been suppressed by treatment; (2) to increase time off drug, to improve quality of life and diminish side effects and costs; and (3), among individuals whose virus has become resistant to treatment, to induce reversion of resistance to wild-type and therefore to improve the chances of success of subsequent salvage therapy. Regarding immune stimulation, the most promising results come from rare patients who started HAART during primary HIV infection. Up to 60% controlled viraemia below 5,000 copies/mL for up to 1 year without HAART. Among the majority of patients who start HAART later, during chronic HIV infection, preliminary results suggest that after repeated cycles on and off therapy, about 20% remain with a viraemia below 5,000 copies/mL after 12 weeks' treatment interruption. For the 80% who rebound to higher viraemia, additional immune-stimulatory manoeuvres are being considered such as administration of cytokines, or specific immune treatment by anti-HIV vaccines. When treatment is interrupted in patients who fail HAART and who have drug-resistant HIV, the drug-sensitive wild-type replaces the resistant quasispecies in 30-60% of patients. It remains to be seen whether this phenomenon increases the chances of successful salvage therapy. The risks and benefits of treatment interruption still have to be compared in large clinical trials, which are currently in the planning stage.