Viral resistance patterns selected by antiretroviral drugs and their potential to guide treatment choice

Finding a Role for Zalcitabine in the HAART era

Zalcitabine (ddC) is a nucleoside analogue reverse transcriptase inhibitor with demonstrated clinical benefit in combination use. More widespread use of zalcitabine has been limited by a number of factors including peripheral neuropathy and three times daily dosing. However, screening for the risk factors for peripheral neuropathy may enable a reduction in the incidence of neuropathy to below 10%. Additionally, new data on the use of zalcitabine twice daily suggest, based on the long intracellular half-life of the active triphosphate, that this is feasible. Additionally, while limited data exist for zalcitabine in true HAART combinations, data from small trials suggest a similar proportion of responders to standard HAART regimens.

New approaches to HIV protease inhibitor drug design II: testing the substrate envelope hypothesis to avoid drug resistance and discover robust inhibitors

PURPOSE OF REVIEW

Drug resistance results when the balance between the binding of inhibitors and the turnover of substrates is perturbed in favor of the substrates. Resistance is quite widespread to the HIV-1 protease inhibitors permitting the protease to process its 10 different substrates. This processing of the substrates permits the virus HIV-1 to mature and become infectious. The design of HIV-1 protease inhibitors that closely fit within the substrate-binding region is proposed to be a strategy to avoid drug resistance.

RECENT FINDINGS

Cocrystal structures of HIV-1 protease with its substrates define an overlapping substrate-binding region or substrate envelope. Novel HIV-1 protease inhibitors that were designed to fit within this substrate envelope were found to retain high binding affinity and have a flat binding profile against a panel of drug-resistant HIV-1 proteases.

SUMMARY

The avoidance of drug resistance needs to be considered in the initial design of inhibitors to quickly evolving targets such as HIV-1 protease. Using a detailed knowledge of substrate binding appears to be a promising strategy for achieving this goal to obtain robust HIV-1 protease inhibitors.

Efavirenz: shifting the HAART paradigm in adult HIV-1 infection

Efavirenz (DMP-266, Sustiva, Stocrin) is a member of the non-nucleoside class of HIV-1 reverse transcriptase inhibitors. It has demonstrated potent antiretroviral activity in treatment-naïve patients in combination with two nucleoside analogues or with a protease inhibitor. In nucleoside analogue-experienced patients, best activity has been seen with two nucleosides plus a protease inhibitor. An open-label comparison versus indinavir plus two nucleoside analogues in predominantly treatment-naïve patients, demonstrated that the efavirenz-based triple therapy provides at least similar antiviral effects, and possibly stronger effects, over 48 weeks. Furthermore, fewer patients discontinued efavirenz-based triple therapy than indinavir-based therapy because of adverse events. Side-effects associated with efavirenz include a maculopapular rash and central nervous system (CNS) disturbances (dizziness, vivid dreams, poor concentration, sleep disturbances) which generally occur, and resolve, within the first weeks of therapy. Efavirenz will be used as a combination agent with other antiretrovirals where therapy for HIV infection is indicated.

Triple nucleoside analogue antiretroviral therapy: expanding the treatment approaches for management of HIV infection

The expanding range of drug options for the management of persons with HIV infection and concerns about the long-term safety of some treatment approaches has precipitated the HIV treatment community to examine a wider range of treatment approaches. One new therapeutic approach is to consider using three nucleoside analogue reverse transcriptase inhibitors, particularly as initial therapy. Recent data comparing triple nucleoside analogue therapy with established 'standard of care' triple therapy regimens have demonstrated similar antiviral effects and tolerability for such regimens. Efficacy and tolerability of triple nucleoside therapy has been demonstrated up to 96 weeks of therapy. This approach has advantages over conventional two nucleoside plus either a protease inhibitor or non-nucleoside reverse transcriptase inhibitor in terms of drug interactions and maintenance of multiple class options. It creates regimens which are, for the most part, compact, twice daily and non-food dependent. Specifically, this approach has already become widely established for individuals for whom drug interactions exclude the use of protease or non-nucleoside antiretrovirals. Triple nucleoside analogue therapy is likely to represent an attractive option for both physicians and patients; its use in clinical practice is likely to further expand.

The role of combinations of HIV protease inhibitors in the management of persons with HIV infection

The current standard of care in antiretroviral therapy includes two nucleoside analogue reverse transcriptase inhibitors (NRTIs) plus a potent third agent, usually an HIV protease inhibitor (PI). However, around 20 - 30% of patients initiating therapy in clinical studies, and probably more in clinical practice, fail to achieve an optimal therapeutic response, a sustained undectectable viral load, using these regimens. Additionally, many triple therapy regimens currently require three times per day dosing, making treatment adherence difficult to sustain. Combinations of two PIs with or without NRTIs provide impressive reductions in viral load, with emerging data suggesting a higher proportion of patients on four drug regimens achieving below detection responses than those on three drug regimens. Additionally, pharmacokinetic interactions between PIs provide the potential for both dose reductions and twice daily dosing with PI combinations. However, limited resistance data are available from dual PI failures, and concerns regarding disturbances in fat metabolism, lipodystrophy and glucose intolerance remain obstacles to the widespread use of these regimens as initial therapy.

Saquinavir-soft gel: establishing saquinavir in HAART regimens

The initial formulation of the HIV protease inhibitor (PI) saquinavir (SQV) in hard gel capsules (Invirase) demonstrated significant clinical benefit in double and triple combination regimens in comparative studies. However, pharmacological limitations of this preparation resulted in fewer patients achieving optimal treatment responses than observed with other PIs. Changes in formulation and dosing to a new soft gelatin capsule formulation of SQV (SQV-SGC, Fortovase result in an average 8-fold increase in SQV exposure with SQV-SGC. Data from clinical trials indicate that this enhanced plasma exposure translates into more potent antiviral activity, with small comparative studies suggesting activity to be similar to other available PIs in triple therapy highly-active antiretroviral therapy (HAART) regimens. Additionally, SQV-SGC appears to be an excellent candidate for PI combination use. SQV-SGC appears well-tolerated with a low incidence of mainly gastrointestinal side-effects. The lower affinity to P450 isozymes of SQV means that the incidence of drug interactions is low, indeed probably lower than with other approved members of this class. Current obstacles to SQV-SGC's widespread use include only limited, short-term bid dosing data (outside of combination with ritonavir) and the pill burden of 18 capsules per day.

A re-evaluation of zalcitabine

Early in the HIV epidemic, zalcitabine (ddC) emerged as a nucleoside analogue reverse transcriptase inhibitor (NRTI) alternative to zidovudine (ZDV). However, a comparative study suggested ZDV monotherapy provided superior clinical benefit in treatment-naive patients with advanced immunodeficiency. Thus, ddC became most widely used in those patients no longer benefitting from or intolerant of ZDV. In ZDV-failed or -intolerant patients, ddC demonstrated similar benefit (or absence of benefit) to ddI monotherapy. In the first clinical end-point study of combination therapy, addition of ddC to on-going ZDV in patients substantially pre-treated with ZDV resulted in no overall benefit but some clinical advantage in a subset of patients with CD4 cell counts of 150 - 300/mm. Furthermore, initial studies of ddC, mostly performed in persons with advanced immunodeficiency and symptomatic HIV infection, indicated that 10 - 20% of ddC recipients developed a treatment-limiting peripheral neuropathy. Based on these early trials, a widespread perception that ddC was an antiviral with both limited activity and a potentially problematic safety profile evolved. More recent data suggest that the role of ddC requires re-evaluation. Indeed, the European Medicines Evaluation Agency (EMEA) has recently expanded the licencing claim of ddC stating that it "is indicated in HIV-infected adults in combination with other antiretroviral agents". The purpose of this short review is to discuss data that have shed new light on what in antiretroviral terms is an 'old' drug.

Reverse transcriptase mutations in HIV-1 infected patients treated with two nucleoside analogues: the SMART study

Resistance to nucleoside reverse transcriptase inhibitors (NRTIs) was studied in 527 HIV-1-infected patients, 342 responder and 185 non-responder to two NRTIs. Responders were followed for one year to assess the incidence of clinical failure. The prevalence of the 215Y/F substitution was higher among non-responder, compared to responder patients (33.7&#x0025 vs. 17&#x0025, P = 0.0005), whereas the prevalence of the 184V and of the 70R mutations was comparable between these two groups. The 74V substitution was never observed and the 75T mutation was detected in only two subjects non-responder to a stavudine including regimen. Reduced susceptibility to didanosine or stavudine was infrequent. Reduced susceptibility to zidovudine was observed in 25&#x0025 of individuals failing a zidovudine including regimen, whereas reduced susceptibility to lamivudine was detected in all subjects failing a lamivudine including regimen. In the prospective analysis, patients with undetectable viral load at enrollment had a lower incidence of failure rate over one year compared to those with detectable HIV-RNA at entry (P &#x003C 0.0001). A detectable viral load at enrollment was the only independent variable that predicted clinical failure over one year (P &#x003C 0.0001).

Virological and Immunological Responses to a Once-a-Day Antiretroviral Regimen with Didanosine, Lamivudine and Efavirenz

Current antiretroviral drugs cannot eradicate HIV infections, and persons living with HIV are often faced with very demanding daily therapeutic schedules that can induce poor adherence. More conveniently dosed and patient-friendly regimens are needed. We investigated, in this 48-week pilot study, a once-a-day highly active antiretroviral therapy regimen of didanosine, lamivudine and efavirenz. Seventy-five consecutive antiretroviral-naive subjects were enrolled. Over the 48-week period, plasma HIV-RNA levels declined sharply, with a median decrease at the end of the observation time >3.4 log copies/ml. The proportion of patients achieving a plasma HIV-RNA level below the limit of detection (50 copies/ml) was 77% (intention to treat analysis) at the end of the study period. The mean CD4 cell count increased steadily over time from 251 to 459 cells/μl. Antiviral efficacy was similar in patients with a baseline HIV-RNA level above or below 100 000 copies/ml. However, patients with a baseline CD4 cell count <200 cells/μl showed a significantly worse virological response than that observed in patients with higher baseline CD4 counts. Overall 15 patients interrupted therapy. In four cases treatment interruption was due to lack of treatment response; three additional patients were lost to follow-up or withdrew informed consent. Eight patients stopped therapy because of adverse events. The once-daily combination of didanosine, lamivudine and efavirenz resulted in sustained viral suppression and was well-accepted by patients. This regimen may offer advantages in selected difficult-to-treat populations, allows directly observed therapy and can be a safe and effective alternative in antiretroviral-naive patients. These encouraging pilot results need to be confirmed in a comparative clinical trial.

Substrate shape determines specificity of recognition for HIV-1 protease: analysis of crystal structures of six substrate complexes

The homodimeric HIV-1 protease is the target of some of the most effective antiviral AIDS therapy, as it facilitates viral maturation by cleaving ten asymmetric and nonhomologous sequences in the Gag and Pol polyproteins. Since the specificity of this enzyme is not easily determined from the sequences of these cleavage sites alone, we solved the crystal structures of complexes of an inactive variant (D25N) of HIV-1 protease with six peptides that correspond to the natural substrate cleavage sites. When the protease binds to its substrate and buries nearly 1000 A2 of surface area, the symmetry of the protease is broken, yet most internal hydrogen bonds and waters are conserved. However, no substrate side chain hydrogen bond is conserved. Specificity of HIV-1 protease appears to be determined by an asymmetric shape rather than a particular amino acid sequence.

Predictors of Virological Response in HIV-Infected Patients to Salvage Antiretroviral Therapy that Includes Nelfinavir

Different salvage strategies have been used to regain control in patients with HIV who have virological failure on combination antiretroviral therapy. We conducted a cohort study of 63 extensively antiretroviral pretreated patients who initiated nelfinavir as part of salvage therapy, to determine predictors of virological response. The maximum HIV RNA response was >0.5 log10 copies/ml reduction in 43 patients (68%), including 21 patients (33%) who had suppression to <500 copies/ml. Corresponding response rates at 24 weeks were 41 and 19%, respectively. Responders and non-responders could not be distinguished by mean baseline HIV RNA or CD4 cell count, duration of prior protease inhibitor (PI) use, introduction of an initial non-nucleoside reverse transcriptase inhibitor or the number of antiretroviral agents changed when nelfinavir was added, likely reflecting the homogeneity of the population studied. The only parameter predictive of response was virus genotype. Response rates were lower in patients with increasing numbers of primary ( P=0.045) or secondary ( P=0.001) PI mutations. The addition of increasing numbers of reverse transcriptase mutations further impaired response rates ( P=0.004).

Considerations in the choice of protease inhibitor-sparing regimens in initial therapy for HIV-1 infection

The current standard of care for initial antiretroviral therapy is a combination of 2 nucleoside analogues with a third agent, a protease inhibitor (PI), a non-nucleoside reverse transcriptase inhibitor or a third nucleoside analogue. Recent data heralding the arrival of potent PI-sparing regimens for initial anti-retroviral therapy and concerns regarding PI-related metabolic disturbances have led to significant shifts in treatment practices. Protease inhibitor-sparing regimens with optimal antiviral activity may have several advantages over PI-based therapy for initial or prolonged therapy. These advantages include more convenient, non-food dependent dosing regimens, lower tablet volume, fewer drug interactions, central nervous system penetration and the maintenance of PIs as an option for second line therapy. However, no data currently exist directly comparing triple therapy regimens based on the three leading PI-sparing agents, efavirenz, nevirapine or abacavir. All these agents have been compared in randomized controlled studies in treatment naïve patients to triple therapy with the PI indinavir. In these studies, similar responses to indinavir were observed with nevirapine or abacavir regimens, whereas superiority was observed with efavirenz. Limited data in high viral load patients treated with nevirapine based regimens currently exist. However, the superiority of efavirenz over indinavir based regimens was also observed in comparative data in this patient subset. PI-sparing approaches appear generally well tolerated with few individuals discontinuing in clinical studies due to adverse drug events. The majority of adverse events with efavirenz and nevirapine occur within the first month, are predictable and are manageable without therapy interruption. Similarly, apart from a rare (3%) hypersensitivity reaction, which requires therapy cessation without rechallenge, adverse effects with abacavir are uncommon.

Human immunodeficiency virus drug resistance testing: state of the art in genotypic and phenotypic testing of antiretrovirals

Antiretroviral drugs have significantly reduced death rates from the acquired immunodeficiency syndrome in the United States. They are highly effective in reducing viral replication, but their utility is threatened by rapid development of drug resistance. Although antiretroviral drug resistance testing is available by either genotyping or phenotyping, no consensus guidelines have been published regarding the appropriate use or interpretation of these new tests. Even though their role in clinical practice is not defined, it is important for clinicians to become familiar with relative advantages and disadvantages of genotypic and phenotypic testing and various mechanisms of antiretroviral resistance.

An anti-HIV strategy combining chemotherapy and therapeutic vaccination

Combination chemotherapy using potent anti-retroviral agents has led to significant advances in the clinical management of human immunodeficiency virus (HIV) disease. However, the emergence of multiple drug-resistant mutants, the high need for compliance to adhere to demanding drug-dosing schemes, and the remaining toxic side-effects of drugs make the perspective of life-long treatment unattractive and possibly unrealistic. Therefore, means must be sought to shorten the time span during which treatment is necessary. Such means could be to stimulate an efficient immune response during the period of low virus load and restored CD4 + cell levels, which might be capable of keeping the virus under long-lasting control after treatment is stopped. Here we tested this concept of combined chemotherapy/ therapeutic vaccination in a non-human primate model. Rhesus macaques chronically infected with the chimeric simian/human immunodeficiency virus (SHIV) containing the HIV type 1 (HIV-1) HXBc2 gene for reverse transcriptase (RT) in the genomic background of simian immunodeficiency virus (SIV)(mac239) (RT-SHIV) were treated with (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA), a potent anti-HIV drug. When virus load had decreased significantly, we immunized with SIV genes env, gag/pol, rev, tat, and nef inserted in two different expression vector systems. Four weeks after the second immunization, drug treatment was stopped. Animals were monitored to determine if virus load stayed low or if it increased again to the original levels and if CD4+ T-cell levels remained stable. Humoral and cellular immune responses were also measured. This combined chemotherapy/ therapeutic vaccination regimen induced a significant reduction in the steady-state level of viremia in one out of two chronically infected rhesus macaques. Chemotherapeutic treatment alone did not achieve reduction of viremia in two chronically infected animals. The nature of the immune responses assumed to have been induced by vaccination in one out of the two monkeys remains to be elucidated.

Genetic polymorphism near HIV-1 reverse transcriptase resistance-associated codons is a major obstacle for the line probe assay as an alternative method to sequence analysis

The performance of the line probe assay (LIPA) for the detection of mutations conferring resistance to nucleoside inhibitors of HIV-1 reverse transcriptase was evaluated in comparison with sequence analysis. The tests were undertaken on plasma samples from 63 patients (61 receiving combination therapy and 2 without treatment at the time of inclusion). In 27 cases (43%) which included codons 41, 69, 70, 74, 184 and 215, the sequence of the RT gene was distinct from the hybridization probes used in LIPA. Correspondingly, LIPA gave uninterpretable results in 15, 30 and 41% of cases for codons 184, 215 and 41, respectively. Overall, the concordance between LIPA and sequence analysis varied from 52% (codons 41 and 215) to 85% (codon 70). These data show that the polymorphism of the nucleotide sequence near resistance-associated codons is a major shortcoming of LIPA.

A risk-benefit assessment of HIV protease inhibitors

The use of triple regimens, often called highly active antiretroviral therapy (HAART), generally involving 2 nucleoside analogues and an HIV protease inhibitor, have been endorsed as the standard of care for persons with HIV initiating therapy by a number of sets of international guidelines. The widespread availability of protease inhibitor-containing regimens has been associated with a dramatic drop in the incidence of new AIDS events and mortality throughout the developed world. Use of HAART regimens, particularly in treatment-naïve individuals, is also associated with dramatic reductions in HIV RNA load, rises in CD4+ cell numbers and improvements in some aspects of immune function. However, protease inhibitor therapy is associated with a range of adverse effects, which varies between agents, and regimens frequently involve inconvenient administration schedules and disruption to patient's lives. Thus, the undoubted benefits of antiretroviral therapy come at some cost in terms of both physical and psychological morbidity to the recipient. In assessing an individual for therapy, consideration of the risk of disease events and the benefit of therapy in reducing or preventing these events must be weighed against the potential of therapy to cause morbidity. Using these criteria, we suggest that an individual with a 3 year risk of disease progression of less than 10% (based on CD4+ cell count and HIV RNA load) is more likely to a experience a morbidity if treated with HAART than if left untreated and monitored. For individuals with higher risks of HIV progression the risk versus benefit of initiating therapy may, in many cases, still be in favour of no therapy and continued observation. This will vary depending on the individuals risks (such as family and past medical history) and on the choice of agents in the regimen, some regimens having greater risks than others.

Saquinavir delays the emergence of zidovudine resistance in HIV-1 seropositive patients treated with combination therapy

During a randomized double-blind study to assess the antiviral activity of saquinavir (SQV) alone or in combination with zidovudine (ZDV), the emergence of phenotypic resistance was evaluated in 44 patients treated with SQV (13 subjects), ZDV (14 subjects), and SQV plus ZDV (17 subjects). A significant (P< 0.05) lower CD4+ cell count and higher HIV RNA copy number at entry were found in six patients who developed resistant viral strain (3 to ZDV and 3 to SQV) during the first 4 months of treatment. After 1 year, drug-resistant strains (12 to ZDV and 14 to SQV) were isolated in 26 out of 37 patients. A significant higher number of patients treated with ZDV alone (10/13) harbored ZDV-resistant strains compared to patients treated by combination therapy (2/13); whereas more than 50% of patients had SQV-resistant strains aside from treatment. Early SQV-resistant strains were isolated in a limited number of patients treated with SQV alone (3/13). The rates of emergence of resistant strains during ZDV or SQV monotherapies are comparable. Combination therapy may delay the emergence of phenotypic resistance to either drugs in the short term and to ZDV, but not to SQV, at least after 1 year.

1998 revision to the British HIV Association guidelines for antiretroviral treatment of HIV seropositive individuals. BHIVA Guidelines Writing Committee

When the British HIV-1 Association (BHIVA) guidelines on the treatment of HIV-seropositive individuals with antiretrovirals were published in The Lancet in April 1997, it was clear that they would require updating on a frequent basis. The guidelines have been useful in ensuring that viral-load testing and combination therapy is widely available in the UK. However, standards of treatment are rapidly changing as new evidence becomes available. Since formulation of the guidelines, data from two large clinical endpoint studies have been presented that show superior clinical benefit for the use of triple therapy compared with dual therapy in treatment of both naive individuals and patients who have been given zidovudine. Here we update the BHIVA guidelines with a consensus drawn from a wide range of UK medical opinion. The guidelines include input from groups representing individuals living with HIV-1. A more detailed reflection of these views may be found in publications such as the National AIDS Manual and the AIDS Treatment Project's Doctor fax.