Rapid Human Immunodeficiency Virus Decay in Highly Active Antiretroviral Therapy (HAART)-Experienced Children after Starting Mega-HAART

Increasing numbers of patients are treated with mega-highlyactive antiretroviral therapy (HAART), or multiple-combinationantiretroviral therapy, in an attempt to overcome the viralresistance that has contributed to treatment failure. Studiesof human immunodeficiency virus (HIV) viral dynamics are usedto quantify the potency of a given regimen. While mega-HAARTis expected to provide potent therapy, its potency among heavilyexperienced HIV-infected children who have failed previous treatmentis untested. HIV dynamics studies performed in children haveprovided minimal information on viral dynamics during mega-HAART.The present study estimates first- and second-phase viral dynamicsin six children on mega-HAART, following failure of combinationtherapy. The first phase of viral decay was rapid, relativeto rates reported in previous pediatric studies (median ${delta}$ = 0.778d^–¹,range = 0.583 to 1.088, half-life 1 [t1_1/2] = 0.894d), whilethe second phase revealed results similar to those of previousstudies (median µ = 0.026d^–¹, range = –0.005to 0.206, t2_1/2 = 9.316d). This indicates that mega-HAART canprovide potent therapy among heavily experienced pediatric patients.

INTRODUCTION

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Introduction

With the success of highly active antiretroviral therapy (HAART),increasing numbers of human immunodeficiency virus (HIV)-infectedchildren have been treated with antiretroviral therapy for extendedperiods of time. Children who fail HAART regimens are initiallyswitched to alternative regimens consisting of drugs to whichthey have not been exposed. However, those failing numerousregimens run out of new drugs and are in need of salvage therapy.Increasing numbers of such patients are treated with mega-HAART,or multiple-combination antiretroviral therapy, in an attemptto overcome the viral resistance that has contributed to treatmentfailure (10).

The potency of therapeutic regimens is reflected in the rateat which viral load decreases during the first few days afterinitiation of treatment (phase 1 viral decay) and during subsequenttreatment (phase 2 viral decay) (4).

However, studies of viral dynamics in children provide littleinformation concerning the potency of mega-HAART among childrenwho have failed HAART regimens (8, 9). Extrapolation from adultviral dynamics studies to pediatric populations can be problematicbecause of various factors including the following: (i) greaterviral burden in children, (ii) higher CD4 counts and the decreaseof these target cells with age in children, (iii) differencesin pharmacokinetics, and (iv) differences in viral replication.

The primary objective of the present study was to estimate theHIV decay rates among HAART-experienced children who have failedtherapy and who have been changed to mega-HAART regimens. Wehypothesized that these regimens would be relatively potentin the initial phase of viral reduction and would continue toreduce viral load during the second phase, despite the viralresistance which would be expected in such a group of subjects.

MATERIALS AND METHODS

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Materials and Methods

Subjects. Six HIV-infected children who had failed previous HAART dueto increased viral load or rapid decrease in CD4⁺ count wereenrolled. Patient 1 was in Centers for Disease Control and Preventionclass B2, patients 2 and 5 in class B3, and patients 3, 4, and6 in class C3 (1). The median age was 11 years (range, 6 to14 years). Durations of previous antiretroviral regimens were12 to 32 months (median, 16 months). Previous regimens werestopped at least 14 days prior to starting the new regimen.Table 1 presents previous and current treatment regimens. Allpatients had genotype resistance testing done before changingto the current therapy. We used the consensus statement of theInternational AIDS Society-USA Panel to guide our choices (7).In addition, we took into consideration all previously takendrugs and tried to avoid a drug that was previously used, evenif the genotype testing suggested that the isolated strain wassensitive to it. These methods resulted in four of the six patientsreceiving at least two new drugs. In two patients (2 and 5),we had to use seven drugs because we could not identify at leastthree drugs to which they were sensitive and had not been exposed.

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TABLE 1. Baseline characteristics and phase 1/2 viral kinetics^a

Quantification of plasma HIV-RNA copy number by RT-PCR. HIV-1 RNA was quantified in 200 µl of plasma (stored at–70C within 6 to 18 h after phlebotomy) by reverse transcription-PCR(RT-PCR) (Amplicor; Roche), according to the manufacturer'sspecifications. The lower detection limit of the assay is 400copies of HIV-1 RNA per ml of plasma. All assays were performedin the Children's Memorial Special Infectious Diseases Laboratory,which participates in an ongoing quality assurance program ofthe Centers for Disease Control and Prevention.

Statistical analysis. Preliminary data analysis consisted of graphic displays of thedata, based upon the basis spline smoothing method (5). Theprimary analysis of plasma RNA data utilized the biphasic viraldynamics model proposed by Wu and Ding (13).

The basis spline smoothing method is a nonparametric techniqueused to examine data, with respect to pattern and distribution,without prior distribution assumptions. The graphic displayof patterns in the data is informative for subsequent modelfitting and parameter estimation (5).

The statistical model used to estimate viral decay rates canbe written as follows: V(t) = P1 e^–^d¹^t + P2 e^–^d^{2^t}+ e(t), where d1 and d2 are the first- and second-phase decayrates, P1 and P2 are macroparameters, representing baselineviral levels, such that P1 plus P2 is equal to the model estimateof the baseline viral load, and e(t) is measurement error.

This model does not include parameter estimates for the so-called"shoulder effect, " representing the delay between the onsetof therapy and the time at which viral load begins to drop (6,12, 13). Moreover, this method does not require the assumptionof pretreatment steady state.

The nonlinear mixed-effects (NLME) model approach was used tofit this model (14). Comparisons of this method with other methodscan be found in two recent papers by Ding and Wu (2, 3). Thenonlinear mixed-effects model approach uses all data pointstogether to get population estimates of the decay rates (fixedeffects); it then uses individual data to estimate parametersaccounting for patient variability in decay rates (random effects);finally, it combines them to get empirical Bayes estimates ofeach patient's first- and second-phase viral decay, denotedas d1 and d2.

The half-life of the two phase decays can be calculated as follows:H1 = ln(2)/d1 and H2 = ln(2)/d2.

Written informed consent for the Children's Memorial HospitalInstitutional Review Board was obtained. The children were admittedto the Children's Memorial Hospital Clinical Research Centerbefore the new mega-HAART therapy was started. Blood was drawn(for baseline determination) just before the first dose of thenew regimen and every 6 h for the first 3 days, every 12 h ondays 4 and 5, and every 24 h on days 6 and 7. Blood sampleswere also taken on days 14, 28, and 84. At each time point,HIV RNA copies per milliliter was measured by the RT-PCR RocheAmplicor assay.

RESULTS

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Results

Data used for the NLME analysis. Since the primary purpose of the study was to assess viral decayrates, rather than overall patient success on the treatmentregimens, data reflecting periods in which the drugs were nottaken were eliminated. Patient 2 had Pseudomonas aeruginosasepsis, which resulted in a drug interruption during the timeof the 84-day RNA measure. Treatment with mega-HAART was reinitiated10 days later. Patients 4 and 6 developed toxicity rash andintolerance to the mega-HAART therapy after days 9 and 6, respectively.Thus, the last data point for patient 2 is at day 28 and thelast data point for patients 4 and 6 is at day 6. For the remainingpatients, all data available through day 84 were used.

Results from the NLME analysis. The results of modeling RNA change among the six study subjectsare shown in Fig. 1. Note that the model fits each patient'sdata quite well. The first- and second-phase estimates of viraldecay rates are presented in Table 1. The first phase estimatesare relatively steep (median ${delta}$ = 0.778d^–¹, range = 0.583to 1.088, t1_1/2 = 0.894d) in comparison with those found ina previous study which included experienced children (3) (median ${delta}$ = 0.43d^–¹, range 0.18 to 0.77, t1_1/2 = 1.6d) and comparefavorably with those reported from adult studies which haveused standard HAART therapy (10-12). The second-phase viraldecay estimates (median µ = 0.026d^–¹, range = –0.005to 0.206, t2_1/2 = 9.316d), while similar to those observed inprevious pediatric studies, should be interpreted with caution,since only three patients had complete data for the phase 2time periods.

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FIG. 1. HIV-RNA from the six patients (dots) and the fitted curves based on the basis spline smoothing method. Right panels, HIV-RNA from the six patients (dots) and the corresponding fitted trajectories using NLME modeling.

The statistical model used in the present study did not excludedata from the first few hours after initiation of mega-HAARTtherapy, which could have been influenced by the shoulder effect.This effect consists of a delay in initial viral reduction,due to virions produced before the pharmacologic effect of therapy,which are still infectious and capable of producing viral particlesthat would be detected by the RNA assay. Exclusion of thesedata would have resulted in even steeper phase 1 estimates ofviral decay. This suggests that the estimates from the presentstudy are conservative, relative to those from studies whichhave not used data taken during the first few hours after theintroduction of study treatment (13).

DISCUSSION

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Discussion

The relatively steep phase 1 decay rates found in the childrenstudied here indicate that a large component of their baselinevirus was sensitive to the mega-HAART regimens (e.g., wild-typeand/or partially resistant virus). The fact that their first-phaserates of elimination were more rapid than those of experiencedpediatric patients in previous, non-mega-HAART studies suggeststhat the clearance rate of sensitive HIV strains may increaseas the number of potent antiretroviral drugs increases.

The first-phase elimination of the sensitive component of thevirus leaves a residue of resistant virus and/or virus fromlatently infected cells. Change in this residual component ofthe virus is estimated by the phase 2 decay rates. In the presentstudy, these rates were comparable to those reported in pediatricstudies involving children who were less experienced and weretreated less aggressively (8, 9). Thus, although the heavilyexperienced patients in the present study may have had greaterviral resistance than subjects in previous pediatric studies,the quantity of medication administered in the mega-HAART regimensmight have been sufficient to overcome resistance and/or preventits emergence. However, the findings presented here may alsoreflect a complex effect in which therapeutic activity againstvirus from latently infected cells and/or moderately resistantvirus outweighs the replication of highly resistant virus. Ourinterpretation of the results may be limited by the small samplesize of only six patients and further studies are needed.

The results presented here reflect data gathered in the first84 days following initiation of mega-HAART treatment. Whilenot part of the present study, clinical follow-up informationwas available for four of the study patients. In patient 3 therapywas discontinued within 2 weeks of study completion, due toa potential interaction between the mega-HAART regimen and initiationof therapy for atypical mycobacteria infection. Patient 5 maintainedviral load suppression for 6 months but then broke through dueto major problems with compliance. In two patients (1 and 2),the viral suppression achieved by day 84 was maintained for26 and 25 months, respectively.

These results suggest that mega-HAART, or multiple-combinationantiretroviral therapy, can dramatically increase the first-phaseviral decay rates in experienced children and provide furtherbenefit during the second phase of viral decay. For patientswho tolerated the mega-HAART, it continued to be effective duringprolonged follow-up. Thus, mega-HAART is an attractive formof salvage therapy for experienced patients who have failedprevious HAART regimens.

ACKNOWLEDGMENTS

We thank the patients for participation.

This work was supported in part by grant no. RR-00048 from theNational Center for Research Resources, National Institutesof Health.

FOOTNOTES

* Corresponding author. Mailing address: Division of Infectious Diseases, Section of Pediatric, Adolescent, and Maternal HIV Infection, Children's Memorial Hospital, 2300 Children's Plaza, Box 155, Chicago, IL 60614. Phone: (773) 880-4757. Fax: (773) 880-3208. E-mail: ryogev{at}childrensmemorial.org.

REFERENCES

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