Treatment of HIV/AIDS in Patients Failing Their First HAART Regimen

By Peter J. Piliero, MD
Dr. Piliero is Associate Professor of Medicine, Director of Research of the Division of HIV Medicine and Medical Director of the Clinical Pharmacology Study Unit at Albany Medical College.

ABSTRACT
INTRODUCTION
FACTORS LIMITING THE SUCCESS OF PROTEASE INHIBITOR-BASED HAART
    - Poor adherence
    - Drug-resistant HIV variants
    - Viral reservoirs
    - Suboptimal pharmacokinetic potency of PI-based HAART
CURRENT STRATEGIES FOR RESCUE THERAPY
    - Genotype- and phenotype-guided treatment
    - Pharmacokinetic boosting by combining PIs
ADVANCES IN ANTIRETROVIRAL THERAPY
    - Atazanavir (Reyataz™)
    - Fosamprenavir (Lexiva™)
    - Tipranavir
    - TMC114
CONCLUSIONS
TABLE 1 - Ritonavir-Boosted Combination PI HAART Regimens as Rescue Therapy: Findings from Clinical Trials
TABLE 2 - Hyperlipidemia With Combination-PI Regimens: Findings From Clinical Trials

References

ABSTRACT

A substantial number of HIV-infected patients experience treatment failure while on their initial highly active antiretroviral therapy (HAART) regimen. The development of an effective secondary, or rescue, antiretroviral drug regimen is a complex process that involves consideration of several factors.

These factors include the ability of the patient to diligently adhere to potentially complex, restrictive, and toxic rescue drug regimens; HIV resistance to the current drugs and any associated cross-resistance to agents in the same class; and the presence of lingering viral reservoirs undergoing active replication. Such reservoirs may harbor HIV variants with resistance patterns differing from those currently predominating in the bloodstream.

Sub-optimal adherence in combination with variability in drug pharmacokinetics within HIV-infected individuals can alter drug efficacy, leading to the emergence of resistant HIV variants. These HIV variants persist during all subsequent regimens, potentially compromising rescue therapy. Individually or in combination, such factors can lead to failure of the first-line HAART regimen as well as lead to sub-optimal potency of the rescue therapy.

Strategies aimed at optimizing rescue antiretroviral drug regimens include assessment of HIV genotype and/or phenotype, and the utilization of either protease inhibitor regimens that incorporate ritonavir to improve pharmacokinetics or agents to which the patient has not been previously exposed.

Currently, several new drugs are being made available that produce rapid and sustained virologic and immunologic responses, especially against HIV variants resistant to more established drug regimens.

The combination of simplified dosing schedules, fewer pills per dose, and improved metabolic side effect profiles of rescue regimen agents may facilitate adherence by increasing tolerability.

INTRODUCTION

Highly active antiretroviral therapy (HAART) can suppress plasma HIV RNA levels to below the limits of detection by the most sensitive assay technologies currently available as well as increase the CD4+ cell count. Both of these effects delay progression to AIDS and decrease morbidity and mortality.^1-3 Unfortunately, a substantial number of HIV-infected patients experience treatment failure while on HAART, with their levels of circulating HIV rising to detectable or even to pretreatment levels.⁴ In a recent study done in a primary care facility, only 47% of patients achieved an HIV RNA level of 500 copies/mL or less 7 to 14 months after HAART initiation.⁵

Various characteristics of HAART can hinder its long-term efficacy. Poor patient adherence is the primary factor leading to virologic failure. This frequently results from complex dosing regimens involving many pills per dose and from emergent pharmacotoxicity that can reduce the patient’s willingness to remain on the regimen. As a result, treatment regimens may also fail to fully suppress HIV in all viral reservoirs.^6-9

Patients who experience failure of a primary regimen usually require a second, or rescue, regimen. Since further lapse in patient adherence can reduce the likelihood of a response to the rescue regimen, thus limiting its efficacy, clinicians must address the non-adherence that led to the initial therapy failure prior to initiating the rescue regimen. The rescue regimen must address the antiretroviral resistance selected by the failing drug regimen, and thus resistance testing is an essential step in constructing the new regimen.

This review describes major factors contributing to HAART failure and outlines both current strategies and novel drugs for secondary, or rescue, HAART regimens.

FACTORS LIMITING THE SUCCESS OF PROTEASE INHIBITOR-BASED HAART

Poor adherence

Poor patient adherence to the HAART regimen is the key obstacle to successful antiretroviral therapy.¹⁰ Poor adherence can facilitate the emergence of resistant variants and impede virologic responses to treatment.^9,11 The permissible margin of error for HAART regimen success is very small.¹² In a 6-month study of 99 HIV-infected patients,¹¹ poor adherence to antiretroviral therapy regimen was significantly associated with treatment failure (p<0.001; Figure 1).

Adherence was measured using an electronic monitoring system (MEMS^® TrackCap; APREX; Union City, California) and was quantified as a percentage (number of doses taken vs the number prescribed). The best virologic outcomes require at least 95% adherence to therapy, but several reports suggest that few patients achieve this.^11,13,14

Patient-related reasons for poor adherence can range from simply forgetting to take their medication to more complex and diverse psychosocial issues.^15,16 The patient-provider relationship also plays an important role. Treatment-related reasons for poor adherence include high pill burden, complicated administration requirements, and toxicity contributing to poor tolerability.

For example, protease inhibitor (PI) regimens such as amprenavir require 8 pills to be taken 2 times daily, for a total of 16 pills. Indinavir must be taken 1 hour before or 2 hours after meals three times a day and requires that approximately 48 ounces of fluid be consumed each day. All of these factors reduce patient willingness to remain on the regimen.^17-19 Strategies intended to improve adherence include modification of existing antiretroviral regimens and pharmacokinetic enhancement with ritonavir. The development of agents that permit once-daily dosing may also improve adherence.

Drug-resistant HIV variants

The error-prone nature of reverse transcription permits emergence of HIV variants that are resistant to current drugs.^20,21 Studies of HIV resistance to antiretroviral monotherapy have demonstrated characteristic genotypic resistance patterns for each drug class.^22-24

HIV resistance to PIs tends to develop more slowly than does resistance to nucleoside reverse transcriptase inhibitors (NRTIs) or non-NRTIs (NNRTIs).⁷ Fortunately, high-level cross-resistance to PIs appears to require 3 or more mutations in the protease gene. Replication of HIV variants that emerge under drug pressure may be attenuated (i.e., replication capacity or viral fitness impaired), compared with replication of wild-type HIV.²⁵

Subsequent removal of drug-selective pressure may actually increase HIV replication by permitting the re-emergence of the wild-type HIV associated with improved replication capacity.

A pivotal study by Deems and coworkers²⁶ confirmed that continuing antiretroviral therapy even in the presence of drug-resistant HIV variants can be beneficial. Although this does not suggest that modification of a failing HAART regimen is unnecessary, complete treatment cessation is probably unwise.

Variants with resistance mutations that have emerged during first-line treatment failure persist when second- and third-line regimens are initiated, although they may represent only a minor population of the entire viral pool. In one study, this was observed for both NRTI and PI treatments, compromising the efficacy of subsequent rescue regimens, but the same phenomenon has been seen with NNRTI-based treatments.²⁷

Viral reservoirs

Several reports have documented CD4+ cell and other anatomic viral reservoirs in HIV‑infected patients even in the presence of undetectable plasma HIV RNA levels and long‑term HAART.^8,28 Such reservoirs are thought to maintain a “library” of archived viral quasispecies that replicate slowly and emerge under selective pressure from antiretroviral therapy.²⁹

Thus, despite HAART, each patient remains a potential source of drug-resistant HIV variants. Recent evidence suggests that distinct populations of HIV with varying drug sensitivities can be identified in blood and semen of patients receiving therapies re-initiated after interruption.

This observation supports the hypothesis that distinct HIV reservoirs exist³⁰ (e.g., in the genital tract) and underscores the difficulty in preventing emergence of drug-resistant variants. Ideally, antiretroviral therapies should penetrate these viral reservoirs to optimize viral suppression during initial treatment.

Suboptimal pharmacokinetic potency of PI-based HAART

Some unboosted PI-based HAART regimens have suboptimal potency.^25,31 Due to their pharmacokinetic limitations, such regimens can yield low and inconsistent drug exposure, facilitating the emergence of drug-resistant virus. For example, saquinavir (hard-gel capsules) and indinavir are extensively metabolized in the intestines and liver by cytochrome P450 (CYP) enzyme system, which limits their bioavailability.

Potency of PI-based HAART can also be impaired by drug-drug interactions whereby either concomitant antiretroviral agents or other medications the patient is taking reduce the PI drug level. Thus, although reverse transcriptase resistance mutation burden is predictive of therapy failure, inadequate PI plasma concentrations exacerbate loss of response to HAART.³²

CURRENT STRATEGIES FOR RESCUE THERAPY

Genotype- and phenotype-guided treatment

The selection of a rescue HAART regimen is most effective when both the patient’s treatment history and data pertaining to protease and reverse transcriptase resistance are factored into therapeutic design.^33-37 However, while resistance testing is now considered the standard of care for the management of patients experiencing virologic failure in North America and Western Europe, failure to detect resistance by genotypic or phenotypic assays does not confirm the absence of drug-resistant HIV.^35,38,39

Although testing can identify the predominant viral variants within an HIV-infected patient, resistant variants may go undetected because their concentrations are below current detection limits.

Genotypic assays can detect the presence of resistance-related mutations using nucleotide sequence analysis.^6,40 Genotypic testing is rapid and provides indirect evidence of resistance relatively quickly, usually within 7 to 12 days. Although genotypic testing is inexpensive, complex mutational patterns resulting in resistance can require expert interpretation to improve clinical outcomes.

Phenotypic testing measures the drug susceptibility of patient-derived viruses in culture assays and provides direct evidence of resistance that is easier to interpret. Unfortunately, the test is labor intensive, requiring 2 to 5 weeks, and it is more expensive than genotypic testing. Another shortcoming of phenotypic testing is variance in definitions of susceptibility for each specific drug, some of which have yet to be established. Clearly, virologic response parameters for each drug tested requires standardization.

However, the development of resistance determined by longitudinal phenotypic testing may be an earlier prognostic marker of regimen failure than viral load.⁴¹ Although it has not yet been determined whether the best tests are genotypic or phenotypic, independent prospective studies show that both types of tests are useful.^33,34,42 Additional studies are needed to assess the relative utility of these assays in guiding clinical therapy decisions.

Additionally, a virtual phenotype test (VPT) (Virtual Phenotype^™; Tibotec-Virco; Yardley, Pennsylvania) has been developed. VPT determines resistance by matching the patient’s HIV genotype with genotypes in a large database of samples for which there is paired phenotypic data. An estimate of the phenotype of the patient's virus is then calculated from the inhibitory concentration (IC₅₀) of viruses in the database with similar genotypes.

Studies comparing measured phenotype and virtual phenotype have shown a very good correlation between the 2 test results (r²=.72).^43-45 Viral phenotypes, estimated by VPT, appear to be useful predictors of treatment response.⁴⁴ Although VPT is simpler and cheaper than phenotypic testing, its reliability depends on several factors, including the specific mutations that are put into the search for a match, the number of matches found, and the distribution of drug susceptibility among the matches. Randomized clinical trials are currently under way to further assess the clinical utility of VPT.

Pharmacokinetic boosting by combining PIs

Current PIs are metabolized predominantly by the 3A4 isoenzyme of the CYP450 system.^40,46 Ritonavir is a potent inhibitor of CYP3A4 and is thought to increase drug bioavailability through its inhibition in the gut wall. It may also reduce drug elimination by inhibiting CYP3A4 in the liver.^40,46-48 Ritonavir’s inhibition of CYP3A4 has been exploited to increase drug levels of concomitantly administered PIs, a so-called “boosting” effect, which can be of particular value in rescue regimens.^46,49

For example, the combination of ritonavir and amprenavir can potentially reduce the required dosage (and therefore pill burden) of amprenavir from 1200 mg twice daily to 600 mg twice daily while improving tolerability. Table 1 summarizes the results of some studies with dual PI–based regimens.^50-55

A critical issue associated with the use of boosted-PI therapy, however, is the increased potential for adverse events, including hyperlipidemia, which can be marked and sustained.^56,57 This has been observed in several studies of ritonavir-based PI combinations (Table 2).^52,58-62 Moreover, treatment of ritonavir-associated hyperlipidemia is complicated because many statins are metabolized by CYP3A4. Ritonavir-induced inhibition of CYP3A4 can result in elevated statin blood levels and associated side effects. Pravastatin, which is not metabolized by CYP3A4, is the statin of choice for treating PI-associated hyperlipidemia.

ADVANCES IN ANTIRETROVIRAL THERAPY

Arguably, the best hope for rescue therapy lies in the availability and development of novel antiretroviral agents. Current strategies in PI research focus on improving the potency and pharmacokinetic and resistance profiles of these agents. Newly available PIs are atazanavir (Reyataz™, ATV) and fosamprenavir (Lexiva™, FPV). 

PIs under clinical investigation include tipranavir and TMC114. The addition of new members to the existing antiretroviral drug classes, along with discovery of drugs having novel mechanisms of action, including immunomodulators and entry inhibitors, is critical to advancing treatment of HIV infection and AIDS.

Atazanavir (Reyataz™)

Atazanavir is a PI with a low pill burden (2 capsules once per day) and a distinct resistance profile in the PI-naïve patient.⁶³ A single daily dose of atazanavir 400 mg achieves adequate serum levels over a 24-hour time period, and, in combination with NRTIs, rapidly and durably suppresses HIV RNA and increases CD4+ cell count in both treatment-naïve and treatment-experienced patients.

Atazanavir as a sole PI has not been observed to be as efficacious as 400 mg of lopinavir boosted with 100 mg of ritonavir twice daily in moderately treatment-experienced patients when each PI regimen was co-administered with 2 NRTIs for 24 weeks.⁶⁴ However, when boosted with ritonavir, minimum serum levels of atazanavir can be increased 5 to 8-fold.⁶⁵

In a study of highly treatment-experienced patients failing at least 2 prior HAART regimens that contained at least 1 PI (BMS-045),⁵⁵ the combination of 300 mg of atazanavir boosted with 100 mg ritonavir once daily resulted in HIV RNA suppression to <400 copies/mL in 64% of patients and to <50 copies/mL in 39% of patients at 24 weeks of treatment (Table 1).

The proportions of patients responding were comparable to those responding to 400 mg of lopinavir boosted with 100 mg of ritonavir twice daily in the same trial (<400 copies/mL, 62%; <50 copies/mL, 42% at 24 weeks). Furthermore, treatment with atazanavir/ritonavir resulted in a reduction or stabilization of lipid levels in these patients (Table 2). The lipid effects of treatment with atazanavir/ritonavir may be associated with a reduction in the risk of cardiovascular events associated with use of PIs.⁶⁶

Fosamprenavir (Lexiva™)

Fosamprenavir is a prodrug of amprenavir with improved pharmacokinetic characteristics that permit reduced pill size and pill count.⁶⁷ The CONTEXT study compared fosamprenavir/ritonavir once daily or twice daily with lopinavir/ritonavir twice daily in 320 patients who had failed 1 or 2 previous PI-containing regimens.

Patients were randomized into 3 treatment groups: fosamprenavir/ritonavir 1400 mg/200 mg once daily; fosamprenavir/ritonavir 700 mg/100 mg twice daily; or lopinavir/ritonavir 400 mg/100 mg twice daily. At 48 weeks, the mean change in viral RNA from baseline was –1.49 log₁₀ copies/mL for once-daily fosamprenavir/ritonavir, –1.53 log₁₀ copies/mL for twice-daily fosamprenavir/ritonavir, and –1.76 log₁₀ copies/mL for twice-daily lopinavir/ritonavir.⁶⁸

Fewer patients on once-daily fosamprenavir/ritonavir achieved an undetectable viral load, so when using fosamprenavir/ritonavir in PI-experienced patients, twice daily dosing should be used.

Tipranavir

Tipranavir is an investigational PI in Phase 3 clinical trials that has potent in vitro activity against HIV variants that are resistant to multiple PIs.⁶⁹ Ritonavir boosting significantly increases the bioavailability and antiretroviral activity of tipranavir. In a recent Phase 2 clinical study, tipranavir/ritonavir combination therapy demonstrated activity in 261 highly treatment-experienced patients.^70,71

These patients were randomly assigned to receive 1 of 3 doses of tipranavir/ritonavir twice daily (Group A, tipranavir/ritonavir 500 mg/100 mg; Group B, tipranavir/ritonavir 500 mg/200 mg; Group C, tipranavir/ritonavir 750 mg/200 mg). Median decreases in HIV RNA levels from baseline were –0.9 log₁₀ copies/mL in Group A, –1.0 log₁₀ copies/mL in Group B, and –1.2 log₁₀ copies/mL in Group C after 2 weeks.

Modest side effects including some diarrhea and nausea were observed. Due to better tolerability, the 500 mg/200 mg tipranavir/ritonavir is the dose being used in the Phase 3 studies.

TMC114

Designed to exhibit high potency with low specificity, TMC114 appears not to permit emergence of drug-resistant variants as readily as current PIs. For this reason, TMC114 has been termed a “resistant-repellent PI.”⁷² In a recent Phase 2 open-label study, TMC114 showed significant antiviral activity in multiple PI-experienced HIV patients failing PI therapy.⁷³ In this study, 50 patients were randomized into 3 treatment cohorts and 1 control cohort. In the 3 treatment cohorts, failing PIs were replaced by 1 of 3 TMC114/ritonavir regimens: 300 mg/100 mg twice daily; 600 mg/100 mg twice daily; 900 mg/100 mg once daily. All other antiretroviral regimens remained unchanged. Patients in the control cohort continued with their failing regimen. The median change in plasma HIV RNA from baseline to end point in the 3 treatment cohorts and the 1 control cohort was –1.24, –1.50, –1.13, and +0.02 copies/mL, respectively. The median reduction in plasma viral load was –1.35 log₁₀ copies/mL HIV RNA after 14 days of treatment with TMC114/ritonavir.⁷³ Phase 2 studies are now underway to evaluate long‑term safety and efficacy and to define the optimal dose of TMC114.

CONCLUSIONS

Factors leading to the failure of an initial HAART regimen include poor patient adherence that may be due to regimen complexity or drug toxicity, the emergence of resistant HIV variants, viral reservoirs, and sub-optimal pharmacokinetic characteristics of PIs.

Although current antiretroviral rescue therapies have limitations, novel antiretroviral agents hold promise for greater control, rapid and durable virologic suppression, and sustained immunologic recovery.

Characteristics of new PIs such as atazanavir, fosamprenavir, and tipranavir (all given in combination with ritonavir), which may improve the response to rescue therapy include: 1) efficacy against resistant HIV variants, 2) improved toxicity profile, and/or 3) simplified dosing and administration to improve patient adherence.

Specifically, atazanavir plus ritonavir requires once-daily administration of only 3 capsules, and uncommonly causes hyperlipidemia associated with PI therapy.

Fosamprenavir allows for a reduced pill burden and shows efficacy when combined with ritonavir in highly treatment-experienced patients.

Tipranavir plus ritonavir also has shown clinical efficacy in patients who have developed significant PI cross-resistance after failing prior regimens. Further development of HAART regimens for rescue therapy will likely include the use of novel PIs in conjunction with other investigational therapies such as entry inhibitors.

[ TABLE 1 ]   

Ritonavir-Boosted Combination PI HAART Regimens as Rescue Therapy*: Findings from Clinical Trials
	Dose and Schedule (PI mg/ritonavir mg)	Findings	Sources
Saquinavir/ ritonavir	600 mg/400 mg bid 400 mg/400 mg bid	20% had HIV RNA <200 c/mL at week 12 65% had HIV RNA <500 c/mL at week 24 38% had HIV RNA <50 c/mL at week 24	Fatkenheur50 Tebas51
Indinavir/ ritonavir	400 mg/400 mg bid 800 mg/100 mg bid	50% had HIV RNA <50 c/mL at week 36 62.5% had HIV RNA <400 c/mL at week 36 37.2% had HIV RNA <500 c/mL at week 12 21.6% had HIV RNA <500 c/mL at week 24	Hsu52 Barreiro53
Atazanavir/ ritonavir	300 mg/100 mg qd	64% had HIV RNA <400 c/mL at week 24 39% had HIV RNA <50 c/mL at week 24	FDA briefing doc.55
Lopinavir/ ritonavir	400 mg/100 mg bid	65% had HIV RNA <400 c/mL at week 48 56% had HIV RNA <50 c/mL at week 48	Clumeck54
Fosamprenavir/ ritonavir	1400 mg/200 mg qd 700 mg/100 mg bid	50% had HIV RNA <400 c/mL at week 48 58% had HIV RNA <400 c/mL at week 48 46% had HIV RNA <50 c/mL at week 48	Vertex Pharmaceuticals74
*Regimens may have also included NRTIs or NNRTIs

[ TABLE 2 ]   

Hyperlipidemia with Combination-PI Regimens: Findings from Clinical Trials
Combination	Lipid Parameter Results	Source
Lopinavir/ritonavir vs nelfinavir	Fasting TG >750 mg/dL 11% for lopinavir/ritonavir vs 2% for nelfinavir TC >300 mg/dL 10% for lopinavir/ritonavir vs 6% for nelfinavir	Ruane62
Indinavir/ritonavir vs indinavir	TC >300 mg/dL 22% for indinavir/ritonavir vs 13% for indinavir Fasting TG >750 mg/dL 12% for indinavir/ritonavir vs 3% for indinavir Fasting TG levels higher in combination groups than with indinavir alone 22/37 patients had grade 3/4 cholesterol or      TG elevations	Harley61 Visnegarwala60 Hsu52
Atazanavir/ritonavir ARV failure; Switched from ³1 PI	Mean changes from baseline: TC –8%, fasting LDL-C –10%, fasting TG –2%	Lichtenstein59
Atazanavir/saquinavir ARV failure; Switched from ³1 PI	Mean changes from baseline: TC –9%, fasting LDL-C –11%, fasting TG –14%	Lichtenstein59
Switch from nelfinavir to atazanavir	Mean change at 24 weeks, mg/dL: TC=202 to 169; fasting LDL-C= 132 to 99; fasting TG = 127 to 102	Murphy58
Lopinavir/ritonavir ARV failure; Switched from ³1 PI	Mean changes from baseline: TC +3%, fasting LDL-C –4%, TG +31%	Lichtenstein59

REFERENCES

1.    Gulick RM, Mellors JW, Havlir D, et al. Treatment with indinavir, zidovudine, and lamivudine in adults with human immunodeficiency virus infection and prior antiretroviral therapy. N Engl J Med. 1997;337:734-739.

         2. Hammer SM, Squires KE, Hughes MD, et al, for the AIDS Clinical Trials Group 320 Study Team. A controlled trial of two nucleoside analogues plus indinavir in persons with human immunodeficiency virus infection and CD4 cell counts of 200 per cubic millimeter or less. N Engl J Med. 1997;337:725-733.

         3. Ledergerber B, Egger M, Opravil M, et al, for the Swiss HIV Cohort Study. Clinical progression and virological failure on highly active antiretroviral therapy in HIV-1 patients: a prospective cohort study. Lancet. 1999;353:863-868.

         4.    Deeks SG, Hecht FM, Swanson M, et al. HIV RNA and CD4 cell count response to protease inhibitor therapy in an urban AIDS clinic: response to both initial and salvage therapy. AIDS. 1999;13:F35-F43.

         5. Rastegar DA, Fingerhood MI, Jasinski DR. Highly active antiretroviral therapy outcomes in a primary care clinic. AIDS Care. 2003;15:231-237.

         6. Angarano G, Monno L. Genotype and phenotype resistance: an overview. J Biol Regul Homeost Agents. 2000;14:11-14.

         7.    Dietrich MA, Butts JD, Raasch RH. HIV-1 protease inhibitors: A review. Infections in Medicine. 1999;16:716-738.

         8.    Finzi D, Hermankova M, Pierson T, et al. Identification of a reservoir for HIV-1 in patients on highly active antiretroviral therapy. Science. 1997;278:1295-1300.

         9.    Cinti SK. Adherence to antiretrovirals in HIV disease. AIDS Read. 2000;10:709-717.

      10.    Tsasis P. Adherence assessment to highly active antiretroviral therapy. AIDS Patient Care STDS. 2001;15:109-115.

      11.   Paterson DL, Swindells S, Mohr J, et al. Adherence to protease inhibitor therapy and outcomes in patients with HIV infection. Ann Intern Med. 2000;133:21-30.

      12.    Vanhove GF, Schapiro JM, Winters MA, Merigan TC, Blaschke TF. Patient compliance and drug failure in protease inhibitor monotherapy [letter]. JAMA. 1996;276:1955-1956.

      13. Kastrissios H, Suárez JR, Hammer S, Katzenstein D, Blaschke TF. The extent of non-adherence in a large AIDS clinical trial using plasma dideoxynucleoside concentrations as a marker. AIDS. 1998;12:2305-2311.

      14.    Singh BN. Effects of food on clinical pharmacokinetics. Clin Pharmacokinet. 1999;37:213-255.

      15.    Bartlett JA. Addressing the challenges of adherence. J Acquir Immune Defic Syndr. 2002;29(suppl 1):S2-S10.

      16. Chesney M. Adherence to HAART regimens. AIDS Patient Care STDS. 2003;17:169-177.

      17.    Bartlett J, DeMasi R, Quinn J, Moxham C, Rousseau F. Correlation between antiretroviral pill burden and durability of virologic response: a systematic overview [abstract ThPeB4998]. Presented at: XIII International AIDS Conference; Durban, South Africa; July 9–14, 2000. Whitehouse, NJ: Merck Sharp & Dohme;

      18.    Mocroft A, Youle M, Moore A, et al. Reasons for modification and discontinuation of antiretrovirals: results from a single treatment centre. AIDS. 2001;15:185-194.

      19.    Max B, Sherer R. Management of the adverse effects of antiretroviral therapy and medication adherence. Clin Infect Dis. 2000;30(suppl 2):S96-S116.

      20.    Hu WS, Temin HM. Genetic consequences of packaging two RNA genomes in one retroviral particle: pseudodiploidy and high rate of genetic recombination. Proc Natl Acad Sci U S A. 1990;87:1556-1560.

      21. Domingo E, Menendez-Arias L, Holland JJ. RNA virus fitness. Rev Med Virol. 1997;7:87-96.

      22.    Molla A, Korneyeva M, Gao Q, et al. Ordered accumulation of mutations in HIV protease confers resistance to ritonavir. Nat Med. 1996;2:760-766.

      23.    Condra JH, Holder DJ, Schleif WA, et al. Genetic correlates of in vivo viral resistance to indinavir, a human immunodeficiency virus type 1 protease inhibitor. J Virol. 1996;70:8270-8276.

      24.    Patick AK, Duran M, Cao Y, et al. Genotypic and phenotypic characterization of human immunodeficiency virus type 1 variants isolated from patients treated with the protease inhibitor nelfinavir. Antimicrob Agents Chemother. 1998;42:2637-2644.

      25.    Picchio GR, Valdez H, Sabbe R, et al. Altered viral fitness of HIV-1 following failure of protease inhibitor-based therapy. J Aquir Immune Defic Syndr. 2000;25:289-295.

      26.    Deeks SG, Wrin T, Liegler T, et al. Virologic and immunologic consequences of discontinuing combination antiretroviral-drug therapy in HIV-infected patients with detectable viremia. N Engl J Med. 2001;344:472-480.

      27.   Svedhem V, Lindkvist A, Lidman K, Sonnerborg A. Persistence of earlier HIV-1 drug resistance mutations at new treatment failure. J Med Virol. 2002;68:473-478.

      28.    De Pasquale MP, Allega J, Sutton L, et al. Drug-selected HIV-1 mutations can differ in cervico-vaginal and blood plasma RNA [abstract 446]. Available at: http://retroconference.org/2001/abstracts/abstracts/abstracts/446.htm. Accessed April 25, 2001.

      29.    Ellis R. Dynamics of HIV replication and persistence in the CNS after antiretroviral therapy [abstract S19]. Available at: http://retroconference.org/2001/abstracts/abstracts/abstracts/S19.htm. Accessed April 25, 2001.

      30. Choudhury B, Pillay D, Taylor S, Cane PA. Analysis of HIV-1 variation in blood and semen during treatment and treatment interruption. J Med Virol. 2002;68:467-472.

      31.    Havlir DV, Hellmann NS, Petropoulos CJ, et al. Drug susceptibility in HIV infection after viral rebound in patients receiving indinavir-containing regimens. JAMA. 2000;283:229-234.

      32.    Masuhr A, Mueller M, Simon V, et al. Predictors of treatment failure during highly active antiretroviral therapy (racing trial). Eur J Med Res. 2002;7:341-346.

      33.    Durant J, Clevenbergh P, Halfon P, et al. Drug-resistance genotyping in HIV-1 therapy: the VIRADAPT randomised controlled trial [published erratum appears in Lancet 1999;354:1128]. Lancet. 1999;353:2195-2199.

      34. Clevenbergh P, Durant J, Halfon P, et al. Persisting long-term benefit of genotype-guided treatment for HIV-infected patients failing HAART. The Viradapt study: week 48 follow-up. Antivir Ther. 2000;5:65-70.

      35.    Hirsch MS, Brun-Vézinet F, D'Aquila RT, et al. Antiretroviral drug resistance testing in adult HIV-1 infection: recommendations of an International AIDS Society–USA Panel. JAMA. 2000;283:2417-2426.

      36.    Baxter JD, Mayers DL, Wentworth DN, et al, and the CPCRA 046 Study Team for the Terry Beirn Community Programs for Clinical Research on AIDS. A randomized study of antiretroviral management based on plasma genotypic antiretroviral resistance testing in patients failing therapy. AIDS. 2000;14:F83-F93.

      37.   Haubrich R, Demeter L. Clinical utility of resistance testing: retrospective and prospective data supporting use and current recommendations. J Aquir Immune Defic Syndr. 2001;26(suppl 1):S51-S59.

      38. Carpenter CCJ, Cooper DA, Fischl MA, et al. Antiretroviral therapy in adults. Updated recommendations of the International AIDS Society—USA Panel. JAMA. 2000;283:381-390.

      39.    Pozniak A, Gazzard B, Babiker D, et al. British HIV association (BHIVA) guidelines for the treatment of HIV-infected adults with antiretroviral therapy. HIV Med. 2001;2:276-313.

      40. Romanelli F, Pomeroy C. Human immunodeficiency virus drug resistance testing: state of the art in genotypic and phenotypic testing of antiretrovirals. Pharmacotherapy. 2000;20:151-157.

      41.    Servais J, Plesseria JM, Lambert C, et al. Longitudinal use of phenotypic resistance testing to HIV-1 protease inhibitors in patients developing HAART failure. J Med Virol. 2002;67:312-319.

      42.    Cohen CJ, Hunt S, Sension M, et al, and the VIRA3001 Study Team. A randomized trial assessing the impact of phenotypic resistance testing on antiretroviral therapy. AIDS. 2002;16:579-588.

      43.    Larder BA, Kemp SD, Hertogs K. Quantitative prediction of HIV-1 phenotypic drug resistance from genotypes: the virtual phenotype (VirtualPhenotype) [abstract 63]. Antivir Ther. 2000;5(suppl 3):49.

      44.    Graham N, Peeters M, Verbiest W, Harrigan R, Larder B. The Virtual phenotype is an independent predictor of clinical response [abstract 523]. Presented at: 8th Conference on Retroviruses and Opportunistic Infections; Chicago, Illinois; February 4-8, 2001.

      45.    Mazzotta F, Lo Caputo S, Scudeller L, Torti C, Castelli F, Carosi G. Real-vs-virtual phenotype: 16 week results of a multicenter randomized trial (The Genepherex study) [abstract 574]. Presented at: 1st International AIDS Society Conference on HIV Pathogenesis and Treatment; Buenos Aires, Argentina; July 8, 2001.

      46.    Piscitelli SC, Gallicano KD. Interactions among drugs for HIV and opportunistic infections. N Engl J Med. 2001;344:984-996.

      47.    Kumar GN, Dykstra J, Roberts EM, et al. Potent inhibition of the cytochrome P-450 3A-mediated human liver microsomal metabolism of a novel HIV protease inhibitor by ritonavir: a positive drug-drug interaction. Drug Metab Dispos. 1999;27:902-908.

      48.    Kempf DJ, Marsh KC, Kumar G, et al. Pharmacokinetic enhancement of inhibitors of the human immunodeficiency virus protease by coadministration with ritonavir. Antimicrob Agents Chemother. 1997;41:654-660.

      49.    Deeks SG. Failure of HIV-1 protease inhibitors to fully suppress viral replication. Implications for salvage therapy. Adv Exp Med Biol. 1999;458:175-182.

      50. Fätkenheuer G, Hoetelmans RMW, Hunn N, et al. Salvage therapy with regimens containing ritonavir and saquinavir in extensively pretreated HIV-infected patients. AIDS. 1999;13:1485-1489.

      51.    Tebas P, Patick AK, Kane EM, et al. Virologic responses to a ritonavir¾saquinavir-containing regimen in patients who had previously failed nelfinavir. AIDS. 1999;13:F23-F28.

      52.    Hsu A, Zolopa A, Shulman NS, et al. Final analysis of ritonavir (RTV) intensification in indinavir (IDV) recipients with detectable HIV RNA levels [abstract 337]. Available at: http://retroconference.org/2001/abstracts/abstracts/abstracts/337.htm. Accessed April 26, 2001.

      53.    Barreiro P, Soriano V, Oller V, González-Lahoz J. Potent activity but limited tolerance of ritonavir plus indinavir in salvage interventions [letter]. J Acquir Immune Defic Syndr. 2000;24:488.

      54.   Clumeck N, Brun S, Sylte J, et al. Kaletra (ABT-378/r) and efavirenz: one-year safety/efficacy evaluation and phenotypic breakpoints in multiple-PI-experienced patients [abstract 525]. Available at: http://retroconference.org/2001/abstracts/abstracts/abstracts/525.htm. Accessed September 27, 2001.

      55.    Bristol-Myers Squibb Company. BMS-232632: Atazanavir Briefing Document May-2003. Available at: http://www.fda.gov/ohrms/dockets/ac/03/briefing/3950B1_01_bristolmyerssquibb-atazanavir.pdf. Accessed July 17, 2003.

      56.    Fellay J, Boubaker K, Ledergerber B, et al. Prevalence of adverse events associated with potent antiretroviral treatment: Swiss HIV Cohort Study. Lancet. 2001;358:1322-1327.

      57.   Tsiodras S, Mantzoros C, Hammer S, Samore M. Effects of protease inhibitors on hyperglycemia, hyperlipidemia, and lipodystrophy: a 5-year cohort study. Arch Intern Med. 2000;160:2050-2056.

      58.    Murphy R, Pokrovsky V, Rozenbaum W, et al. Long-term efficacy and safety of atazanavir with stavudine and lamivudine in patients previously treated with nelfinavir or ATV: 108-week results of BMS Study 008/044 [abstract and poster 555]. Available at: http://www.retroconference.org/2003/Abstract/Abstract.aspx?AbstractID=2075. Accessed August 12, 2003.

      59. Lichtenstein K, Clumeck N, Bellos N, et al. Lipid benefits are observed in antiretroviral (ARV)-experienced HIV-infected patients when switched to atazanavir (ATV)-containing regimens [abstract]. Presented at: 2nd International AIDS Society Conference on HIV Pathogenesis and Treatment; Paris, France; July 13-16, 2003.

      60. Visnegarwala F, Sajja P, Fasano P, Sepcie B, White A. Comparison of efficacy, safety, and tolerability of indinavir/ritonavir (IND/RIT) combination to indinavir (IND) alone in a community clinic [abstract 430]. Available at: http://www.ias.se/print.asp?abstract_id=430. Accessed August 15, 2001.

      61.    Harley W, DeJesus E, Pistole M, et al. A 24-week randomized, controlled, open-label evaluation of adherence and convenience of continuing indinavir versus switching to ritonavir/indinavir 400 mg/400 mg BID (The NICE Study) [abstract 334]. Available at: http://www.retroconference.org/2001/abstracts/abstracts/abstracts/334.htm. Accessed August 22, 2001.

      62.    Ruane P, Mendonca J, Timerman A, et al. Kaletra vs. nelfinavir in antiretroviral-naive subjects: week 60 comparison in a phase III, blinded, randomized clinical trial [abstract 6]. Available at: www.ias.se/abstract/show.asp?abstract_id=6. Accessed September 27, 2001.

      63.    Colonno R, Rose R, Cianci C, Aldrovandi G, Parkin N, Friborg J. Emergence of atazanavir resistance and maintenance of susceptibility to other PIs is associated with an I50L substitution in HIV protease [abstract 597]. Available at: http://www.retroconference.org/2003/Abstract/Abstract.aspx?AbstractID=1925. Accessed July 28, 2003.

      64.    Nieto-Cisneros L, Zala C, Fessel WJ, et al. Antiviral efficacy, metabolic changes and safety of atazanavir (ATV) vs lopinavir/ritronavir (LPV/RTV) in combination with 2 NRTIs in patients who have experienced virologic failure with prior PI-containing regimen(s): 24-week results from BMS A1424-043 [abstract]. Presented at: International AIDS Society Meeting 2003; Paris, France; July 13-16, 2003.

      65.    Badaro R, DeJesus E, Lazzarin A, et al. Efficacy and safety of atazanavir (ATV) with ritonavir (RTV) or saquinavir (SQV) vs lopinavir/ritonavir (LPV/RTV) in combination with tenofovir (TFV) and one NRTI in patients who have experienced virologic failure to multiple HAART regimens: 16-week results from BMS AI424-045 [abstract 118]. Antivir Ther. 2003;8(suppl 1):S212-S213.

      66.    Sanne I, Piliero P, Squires K, Thiry A, Schnittman S, for the AI424-007 Clinical Trial Group. Results of a phase 2 clinical trial at 48 weeks (AI424-007): a dose-ranging, safety, and efficacy comparative trial of atazanavir at three doses in combination with didanosine and stavudine in antiretroviral-naive subjects. J Acquir Immune Defic Syndr. 2003;32:18-29.

      67.    Wood R, Arasteh K, Pollard R, Kaur P, Naderer O, Wire MB. GW433908, a novel prodrug of the HIV protease inhibitor (PI) amprenavir (APV): safety, efficacy, and pharmacokinetics (PK) (APV20001) [abstract 333]. Available at: http://www.retroconference.org/2001/abstracts/abstracts/abstracts/333.htm. Accessed October 19, 2001.

      68.    Vertex Pharmaceuticals. Vertex reports preliminary 48-week data from phase III study of 433908, an investigational HIV protease inhibitor. Press release. Available at: www.vpharm.com/Pressreleases2003/pr072403.html. Accessed October 16, 2003.

      69.    Rusconi S, La Seta CS, Citterio P, et al. Susceptibility to PNU-140690 (tipranavir) of human immunodeficiency virus type 1 isolates derived from patients with multidrug resistance to other protease inhibitors. Antimicrob Agents Chemother. 2000;44:1328-1332.

      70.    Curry R, Markowitz M, Slater L, Neubacher D, Robinson P, Cotton G. Safety and efficacy of tipranavir, a non-peptidic protease inhibitor, in multiple PI-failure patients (BI 1182.2). Available at: http://www.ias.se/abstract/show.asp?abstract_id=3. Accessed January 12, 2004.

      71.    Gathe J, Kohlbrenner VM, Pierone G, et al. Tipranavir/ritonavir demonstrates potent efficacy in multiple protease inhibitor experienced patients: BI 1182.52 [abstract 179]. Available at: http://www.retroconference.org/2003/Abstract/Abstract.aspx?AbstractID=2197. Accessed September 5, 2003.

      72.    Pauwels R. The development of the next generation of antiretrovirals with activity against drug-resistant strains of HIV-1. Available at: http://www.natap.org/2001/8thEccathi/day4.htm. Accessed December 16, 2003.

      73.    Arasteh K, Clumeck N, Pozniak A, et al. First clinical results on antiretroviral activity, pharmacokinetics, and safety of TMC114, an HIV-1 protease inhibitor, in multiple PI-experienced patients [abstract 8]. Available at: http://www.retroconference.org/2003/Abstract/Abstract.aspx?AbstractID=1767. Accessed February 10, 2003.

      74.    Vertex Pharmaceuticals. Vertex reports preliminary 48-week data from phase III study of 433908, an investigational HIV protease inhibitor. Press release. Last update: 2003. Available at: http://www.vpharm.com/Pressreleases2003/pr072403.html. Accessed October 16, 2003.