A combinatorial ledge: reverse transcriptase fidelity, total body viral burden, and the implications of multiple-drug HIV therapy for the evolution of antiviral resistance

Mathematical Models of HIV-1 Dynamics, Transcription, and Latency

HIV-1 latency is a major barrier to curing infections with antiretroviral therapy and, consequently, to eliminating the disease globally. The establishment, maintenance, and potential clearance of latent infection are complex dynamic processes and can be best described with the help of mathematical models followed by experimental validation. Here, we review the use of viral dynamics models for HIV-1, with a focus on applications to the latent reservoir. Such models have been used to explain the multi-phasic decay of viral load during antiretroviral therapy, the early seeding of the latent reservoir during acute infection and the limited inflow during treatment, the dynamics of viral blips, and the phenomenon of post-treatment control. Finally, we discuss that mathematical models have been used to predict the efficacy of potential HIV-1 cure strategies, such as latency-reversing agents, early treatment initiation, or gene therapies, and to provide guidance for designing trials of these novel interventions.

Insight into treatment of HIV infection from viral dynamics models

The odds of living a long and healthy life with HIV infection have dramatically improved with the advent of combination antiretroviral therapy. Along with the early development and clinical trials of these drugs, and new field of research emerged called viral dynamics, which uses mathematical models to interpret and predict the time-course of viral levels during infection and how they are altered by treatment. In this review, we summarize the contributions that virus dynamics models have made to understanding the pathophysiology of infection and to designing effective therapies. This includes studies of the multiphasic decay of viral load when antiretroviral therapy is given, the evolution of drug resistance, the long-term persistence latently infected cells, and the rebound of viremia when drugs are stopped. We additionally discuss new work applying viral dynamics models to new classes of investigational treatment for HIV, including latency-reversing agents and immunotherapy.

Occurrence of HIV eradication for preexposure prophylaxis treatment with a deterministic HIV model

The authors examine the human immunodeficiency virus (HIV) eradication in this study using a mathematical model and analyse the occurrence of virus eradication during the early stage of infection. To this end they use a deterministic HIV-infection model, modify it to describe the pharmacological dynamics of antiretroviral HIV drugs, and consider the clinical experimental results of preexposure prophylaxis HIV treatment. They also use numerical simulation to model the experimental scenario, thereby supporting the clinical results with a model-based explanation. The study results indicate that the protocol employed in the experiment can eradicate HIV in infected patients at the early stage of the infection.

Mathematical Models of HIV Latency

Viral latency is a major barrier to curing HIV infection with antiretroviral therapy, and consequently, for eliminating the disease globally. The establishment, maintenance, and potential clearance of latent infection are complex dynamic processes and can be best understood and described with the help of mathematical models. Here we review the use of viral dynamics models for HIV, with a focus on applications to the latent reservoir. Such models have been used to explain the multiphasic decay of viral load during antiretroviral therapy, the early seeding of the latent reservoir during acute infection and the limited inflow during treatment, the dynamics of viral blips, and the phenomenon of posttreatment control. In addition, mathematical models have been used to predict the efficacy of potential HIV cure strategies, such as latency-reversing agents, early treatment initiation, or gene therapies, and to provide guidance for designing trials of these novel interventions.

Efficacy of lamivudine combined with adefovir dipivoxil versus entecavir monotherapy in patients with hepatitis B-associated decompensated cirrhosis: A meta-analysis

Whether the combination of lamivudine (LAM) plus adefovir (ADV) de novo is more effective than entecavir (ETV) monotherapy in patients with HBV-associated decompensated cirrhosis is still controversial. We searched seven randomized controlled trials that included 411 patients in this meta-analysis. There are 205 and 206 patients in these two groups separately. The pooled risk ratio (RR) and mean difference (MD) were used to assess the treatment effects. ETV monotherapy significantly improved Child-Turcotte-Pugh (CTP) scores (MD = 0.33, 95%CI [0.21-0.44], P < .00001), and was associated with lower rates of serum creatinine increase compared LAM + ADV combination therapy (RR = 4.76, 95%CI [1.11-20.33], P = .04) at 48 weeks. The reduction of alanine aminotransferase (ALT) levels, HBV DNA levels, the rate of ALT normalization, undetectable HBV DNA, HBV e antigen (HBeAg) loss, HBeAg seroconversion and mortality were similar between the two groups. ETV is more effective than LAM + ADV in improving CTP scores at 48 weeks. Both of the LAM + ADV and ETV had similar efficacy in improving virological and biochemical parameters at 48 weeks of follow-up. Furthermore, use of these agents in decompensated HBV patients was generally safe and well tolerated at 48 weeks. However, the nephrotoxicity of ADV, and the potential adverse effects of ETV should be considered and monitored during prolonged therapy.

Modelling HIV-1 2-LTR dynamics following raltegravir intensification

A model of reservoir activation and viral replication is introduced accounting for the production of 2-LTR HIV-1 DNA circles following antiviral intensification with the HIV integrase inhibitor raltegravir, considering contributions of de novo infection events and exogenous sources of infected cells, including quiescent infected cell activation. The model shows that a monotonic increase in measured 2-LTR concentration post intensification is consistent with limited de novo infection primarily maintained by sources of infected cells unaffected by raltegravir, such as quiescent cell activation, while a transient increase in measured 2-LTR concentration is consistent with significant levels of efficient (R0 > 1) de novo infection. The model is validated against patient data from the INTEGRAL study and is shown to have a statistically significant fit relative to the null hypothesis of random measurement variation about a mean. We obtain estimates and confidence intervals for the model parameters, including 2-LTR half-life. Seven of the 13 patients with detectable 2-LTR concentrations from the INTEGRAL study have measured 2-LTR dynamics consistent with significant levels of efficient replication of the virus prior to treatment intensification.

Selection of chronic hepatitis B therapy with high barrier to resistance

Antiviral drug resistance is a crucial factor that frequently determines the success of long-term therapy for chronic hepatitis B. The development of resistance to nucleos(t)ide analogues has been associated with exacerbations in liver disease and increased risk of emergence of multidrug resistance. The selection of a potent nucleos(t)ide analogue with a high barrier to resistance as a first-line therapy, such as entecavir or tenofovir, provides the best chance of achieving long-term treatment goals and should be used wherever possible. The barrier to resistance of a given nucleos(t)ide analogue is influenced by genetic barrier, drug potency, patient adherence, pharmacological barrier, viral fitness, mechanism of action, and cross-resistance. In countries with limited health-care resources, the selection of a therapy with a high barrier to resistance is not always possible and alternative strategies for preventing resistance might be needed, although limited data are available to support these strategies.

Optimal antiviral switching to minimize resistance risk in HIV therapy

The development of resistant strains of HIV is the most significant barrier to effective long-term treatment of HIV infection. The most common causes of resistance development are patient noncompliance and pre-existence of resistant strains. In this paper, methods of antiviral regimen switching are developed that minimize the risk of pre-existing resistant virus emerging during therapy switches necessitated by virological failure. Two distinct cases are considered; a single previous virological failure and multiple virological failures. These methods use optimal control approaches on experimentally verified mathematical models of HIV strain competition and statistical models of resistance risk. It is shown that, theoretically, order-of-magnitude reduction in risk can be achieved, and multiple previous virological failures enable greater success of these methods in reducing the risk of subsequent treatment failures.

Controlling the Evolution of Resistance

Evolution has long been understood as the driving force for many problems of medical interest. The evolution of drug resistance in HIV and bacterial infections is recognized as one of the most significant emerging problems in medicine. In cancer therapy, the evolution of resistance to chemotherapeutic agents is often the differentiating factor between effective therapy and disease progression or death. Interventions to manage the evolution of resistance have, up to this point, been based on steady-state analysis of mutation and selection models. In this paper, we review the mathematical methods applied to studying evolution of resistance in disease. We present a broad review of several classical applications of mathematical modeling of evolution, and review in depth two recent problems which demonstrate the potential for interventions which exploit the dynamic behavior of resistance evolution models. The first problem addresses the problem of sequential treatment failures in HIV; we present a review of our recent publications addressing this problem. The second problem addresses a novel approach to gene therapy for pancreatic cancer treatment, where selection is used to encourage optimal spread of susceptibility genes through a target tumor, which is then eradicated during a second treatment phase. We review the recent in Vitro laboratory work on this topic, present a new mathematical model to describe the treatment process, and show why model-based approaches will be necessary to successfully implement this novel and promising approach.

Analysis of low-frequency mutations associated with drug resistance to raltegravir before antiretroviral treatment

Raltegravir is highly efficacious in the treatment of HIV-1 infection. The prevalence and impact on virologic outcome of low-frequency resistant mutations among HIV-1-infected patients not previously treated with raltegravir have not been fully established. Samples from HIV treatment-experienced patients entering a clinical trial of raltegravir treatment were analyzed using a parallel allele-specific sequencing (PASS) assay that assessed six primary and six secondary integrase mutations. Patients who achieved and sustained virologic suppression (success patients, n = 36) and those who experienced virologic rebound (failure patients, n = 35) were compared. Patients who experienced treatment failure had twice as many raltegravir-associated resistance mutations prior to initiating treatment as those who achieved sustained virologic success, but the difference was not statistically significant. The frequency of nearly all detected resistance mutations was less than 1% of viral population, and the frequencies of mutations between the success and failure groups were similar. Expansion of pre-existing mutations (one primary and five secondary) was observed in 16 treatment failure patients in whom minority resistant mutations were detected at baseline, suggesting that they might play a role in the development of drug resistance. Two or more mutations were found in 13 patients (18.3%), but multiple mutations were not present in any single viral genome by linkage analysis. Our study demonstrates that low-frequency primary RAL-resistant mutations were uncommon, while minority secondary RAL-resistant mutations were more frequently detected in patients naïve to raltegravir. Additional studies in larger populations are warranted to fully understand the clinical implications of these mutations.

For whom the bell tolls? DING proteins in health and disease

DING proteins, identified mainly by their eponymous N-terminal sequences, are ubiquitous in living organisms. Amongst bacteria, they are common in pseudomonads, and have been characterised with respect to genetics and structure. They form part of a wider family of phosphate-binding proteins, with emerging roles in phosphate acquisition and pathogenicity. Many DING proteins have been isolated in eukaryotes, in which they have been associated with very diverse biological activities, often in the context of possible signalling roles. Disease states in which DING proteins have been implicated include rheumatoid arthritis, lithiasis, atherosclerosis, some tumours and tumour-associated cachexia, and bacterial and viral adherence. Complete genetic and structural characterisation of eukaryotic DING genes and proteins is still lacking, though the phosphate-binding site seems to be conserved. Whether as bacterial proteins related to bacterial pathogenicity, or as eukaryotic components of biochemical signalling systems, DING proteins require further study.

Virologic failure in first-line human immunodeficiency virus therapy with a CCR5 entry inhibitor, aplaviroc, plus a fixed-dose combination of lamivudine-zidovudine: nucleoside reverse transcriptase inhibitor resistance regardless of envelope tropism

The CCR102881 (ASCENT) study evaluated the antiviral activity of the novel CCR5 entry inhibitor aplaviroc plus a fixed-dose combination of lamivudine-zidovudine (Combivir) in drug-naïve human immunodeficiency virus type 1-infected subjects with only CCR5-tropic virus detected in plasma. Although the trial was stopped prematurely due to idiosyncratic hepatotoxicity, eight subjects met protocol-defined virologic failure criteria. Clonal analyses of the viral envelope tropism, aplaviroc susceptibility, and env sequencing were performed on plasma at baseline and at the time of virologic failure. Molecular evolutionary analyses were also performed. The majority of the subjects with virologic failure (six of eight) acquired the lamivudine resistance-associated mutation M184V, and none had evidence of reduced susceptibility to aplaviroc at the time of virologic failure, even at the clonal level. Six subjects with virologic failure maintained CCR5 tropism, while two exhibited a change in population tropism readout to dual/mixed-tropic with R5X4-tropic clones detected prior to therapy. Two evolutionary patterns were observed: five subjects had no evidence of population turnover, while three subjects had multiple lines of evidence for env population turnover. The acquisition of the M184V mutation is the primary characteristic of virologic failure in first-line therapy with aplaviroc plus lamivudine-zidovudine, regardless of the envelope tropism.

96 weeks combination of adefovir dipivoxil plus emtricitabine vs. adefovir dipivoxil monotherapy in the treatment of chronic hepatitis B

BACKGROUND/AIMS

In order to prevent the occurrence of drug-resistant mutants associated with treatment for chronic hepatitis B virus (HBV) infection, combination therapy is being developed. To determine the efficacy of adefovir dipivoxil (ADV) plus emtricitabine (FTC) combination therapy in chronic HBV infection.

METHODS

Thirty treatment-nai ve, HBeAg-positive patients were randomized to combination ADV plus FTC (n=14) or ADV plus placebo monotherapy (n=16) for 96 weeks. HBV DNA was measured by polymerase chain reaction. Treatment was stopped in those with HBeAg seroconversion.

RESULTS

The median decrease in HBV DNA at week 96 was higher in the combination group (-5.30 vs. -3.98 log(10)copies/ml, p=0.05). More patients in the combination group had normalization of alanine aminotransaminase and HBV DNA<300 copies/ml at week 96 when compared with the monotherapy group [11 of the 14 patients (78.6%) vs. 6 of the 16 patients (37.5%), p=0.03]. However, HBeAg seroconversion at week 96 was similar in the 2 groups [2/14 (14.3%) vs. 4/16 (25.0%), p=NS]. No ADV or FTC resistance was detected at week 96. In those with HBeAg seroconversion, 50.0% had post-treatment relapse.

CONCLUSIONS

Combination ADV plus FTC resulted in more potent suppression of HBV DNA over 96 weeks of therapy.

Minor-drug-resistant HIV populations and treatment failure

Treatment efficacy in those infected with HIV is eventually compromised by the development of resistance to antiretroviral drugs. To delay resistance in patients it is critical to better understand drug-resistance mechanisms and to accurately detect drug-resistant mutations prior to treatment. Minor-drug-resistant viruses are present in chronically infected patients, treatment-failure patients and those recently infected with resistant viruses through transmission. They are often present at levels below 20%, therefore conventional genotypic and phenotypic assays cannot detect them. Accumulating data indicate that minor-resistant viruses that are present before treatment can lead to drug resistance and poor treatment responses in patients. Detailed characterization of such minor-resistant populations using highly sensitive assays may have a profound impact on the treatment of HIV-infected individuals.

Hepatitis C virus infection--pathobiology and implications for new therapeutic options

Despite progress in therapeutic approaches for the elimination of hepatitis C, chronic hepatitis C virus infection remains an important cause of liver disease. Therapeutic intervention with the currently available interferon-based treatment regimens is quite successful, but treatment is difficult to tolerate and is contraindicated in many patients. A better understanding of the HCV biology, immunopathology, and liver disease will help to design better therapeutic strategies. The American Association for the Study of Liver Diseases sponsored a single-topic conference on hepatitis C virus infection on March 4 and 5, 2005, to enhance our current knowledge in the areas of basic and clinical research related to antiviral and immunomodulatory therapies in hepatitis C disease. The faculty consisted of 23 invited experts in the field of viral hepatitis. The program was divided into four sections including: (a) replicative mechanisms and models; (b) viral-host interactions; and (c) antiviral drug development and new strategies; and (d) back to the bedside-current issues. This report summarizes each of the presentations sections.

Molecular mechanism of a thumb domain hepatitis C virus nonnucleoside RNA-dependent RNA polymerase inhibitor

A new pyranoindole class of small-molecule inhibitors was studied to understand viral resistance and elucidate the mechanism of inhibition in hepatitis C virus (HCV) replication. HCV replicon variants less susceptible to inhibition by the pyranoindoles were selected in Huh-7 hepatoma cells. Variant replicons contained clusters of mutations in the NS5B polymerase gene corresponding to the drug-binding pocket on the surface of the thumb domain identified by X-ray crystallography. An additional cluster of mutations present in part of a unique beta-hairpin loop was also identified. The mutations were characterized by using recombinant replicon variants engineered with the corresponding amino acid substitutions. A single mutation (L419M or M423V), located at the pyranoindole-binding site, resulted in an 8- to 10-fold more resistant replicon, while a combination mutant (T19P, M71V, A338V, M423V, A442T) showed a 17-fold increase in drug resistance. The results of a competition experiment with purified NS5B enzyme with GTP showed that the inhibitory activity of the pyranoindole inhibitor was not affected by GTP at concentrations up to 250 microM. Following de novo initiation, the presence of a pyranoindole inhibitor resulted in the accumulation of a five-nucleotide oligomer, with a concomitant decrease in higher-molecular-weight products. The results of these studies have confirmed that pyranoindoles target the NS5B polymerase through interactions at the thumb domain. This inhibition is independent of GTP concentrations and is likely mediated by an allosteric blockade introduced by the inhibitor during the transition to RNA elongation after the formation of an initiation complex.

Management of Antiviral Resistance in Patients with Chronic Hepatitis B

A meeting of physicians and scientists involved in the management of chronic hepatitis B (CHB) was held to review current scientific data regarding antiviral resistance in hepatitis B virus (HBV) infection. The goals of the meeting were to describe current treatments for CHB, discuss emerging issues in HBV drug resistance and to delineate patient monitoring, including markers for resistance, during administration of antiviral therapy. The aim of this review article is to provide treating physicians with a framework for the management of CHB in the context of antiviral resistance. Definitions of primary and secondary antiviral treatment failure can be used to aid monitoring and early diagnosis of drug resistance before disease progression occurs as a consequence of viral breakthrough. Primary antiviral treatment failure is defined as failure of a drug to reduce HBV DNA levels by ≥1xlog10 IU/ml within 3 months following initiation of therapy, and secondary antiviral treatment failure as a rebound of HBV replication of ≥1xlog10 IU/ml from nadir in patients with an initial antiviral treatment effect (≥1xlog10 IU/ml decrease in serum HBV DNA). Confirmation of antiviral drug failure can be established by sequencing the HBV DNA polymerase and identifying specific genetic markers of antiviral drug resistance. In addition to virological assays, HBV resistance can be assessed from a clinical perspective including increased serum alanine aminotransferase levels and the development of systemic symptoms or signs of liver failure. Potential strategies to prevent the emergence of resistance and how to manage drug-resistant HBV once it emerges are discussed.

Dynamics of HIV-1 recombination in its natural target cells

Genetic recombination is believed to assist HIV-1 diversification and escape from host immunity and antiviral therapies, yet this process remains largely unexamined within the natural target-cell populations. We developed a method for measuring HIV-1 recombination directly that employs reporter viruses bearing functional enhanced yellow fluorescent protein (YFP) and enhanced cyan fluorescent protein (CFP) genes in which recombination produces a modified GFP gene and GFP fluorescence in the infected cells. These reporter viruses allow simultaneous quantification of the dynamics of HIV-1 infection, coinfection, and recombination in cell culture and in animal models by flow-cytometric analysis. Multiround infection assays revealed that productive cellular coinfection was subject to little functional inhibition. As a result, generation of recombinants proceeded according to the square of the infection rate during HIV-1 replication in T lymphocytes and within human thymic grafts in severe combined immunodeficient (SCID)-hu (Thy/Liv) mice. These results suggest that increases in viral load may confer a compounding risk of virus escape by means of recombinational diversification. A single round of replication in T lymphocytes in culture generated an average of nine recombination events per virus, and infection of macrophages led to approximately 30 crossover events, making HIV-1 up to an order of magnitude more recombinogenic than recognized previously and demonstrating that the infected cell exerts a profound influence on the frequency of recombination.

Recent progress in herpes simplex virus immunobiology and vaccine research

Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) cause prevalent, chronic infections that have serious outcomes in some individuals. Neonatal herpes may occur when the infant traverses the cervix during maternal genital herpes. Genital herpes is a major risk factor for human immunodeficiency virus type 1 transmission. Considerable efforts have been made to design and test vaccines for HSV, focusing on genital infection with HSV-2. Several protein subunit vaccines based on HSV-2 envelope glycoproteins have reached advanced-phase clinical trials. These antigens were chosen because they are the targets of neutralizing-antibody responses and because they elicit cellular immunity. Encouraging results have been reported in studies of treatment of HSV-seronegative women with a vaccine consisting of truncated glycoprotein D of HSV-2 and a novel adjuvant. Because most sexual HSV transmission occurs during asymptomatic shedding, it is important to evaluate the impact of vaccination on HSV-2 infection, clinically apparent genital herpes, and HSV shedding among vaccine recipients who acquire infection. There are several other attractive formats, including subunit vaccines that target cellular immune responses, live attenuated virus strains, and mutant strains that undergo incomplete lytic replication. HSV vaccines have also been evaluated for the immunotherapy of established HSV infection.

Viral dynamics in chronic hepatitis B patients treated with lamivudine, lamivudine-famciclovir or lamivudine-ganciclovir

BACKGROUND

Prolonged nucleoside analogue therapy has been shown to reduce viral replication and normalize serum transaminases in the majority of chronic hepatitis B patients. However, from a theoretical point of view, monotherapy with lamivudine (a cytosine nucleoside analogue) will probably not result in eradication of hepatitis B virus. A prolonged course of lamivudine therapy would be needed to clear the virus from the liver. The occurrence of mutations, in combination with continuing low-grade viral replication in a number of patients, will prevent elimination of the virus from the liver. However, combination therapy with more than one nucleoside analogue could possibly overcome the disadvantages of monotherapy.

PATIENTS AND METHODS

In this study, we report on 12 patients who were evaluated by means of a mathematical model during lamivudine monotherapy and lamivudine-famciclovir and lamivudine-ganciclovir therapy.

RESULTS

There was no difference in the parameters representing blocking of viral production (epsilon = 93%, 95% and 86%, respectively), turnover of free virus (half-life of 16 h, 10 h and 12 h, respectively) and turnover of infected hepatocytes (half-life of 9 days, 7 days and 4 days, respectively) between the lamivudine, lamivudine-famciclovir and lamivudine-ganciclovir treatment groups.

CONCLUSIONS

Although our study group is small, we do not think the drug combinations used offer a major advantage over lamivudine monotherapy. Different combinations of nucleoside analogues need to be studied in order to obtain a major breakthrough in this treatment strategy.

Antiretroviral prophylaxis of perinatal HIV-1 transmission and the potential impact of antiretroviral resistance

Since 1994, trials of zidovudine, zidovudine and lamivudine, and nevirapine have demonstrated that these antiretroviral drugs can substantially reduce the risk of perinatal HIV-1 transmission. With reductions in drug price, identification of simple, effective antiretroviral regimens to prevent perinatal HIV-1 transmission, and an increasing international commitment to support health care infrastructure, antiretrovirals for both perinatal HIV-1 prevention and HIV-1 treatment will likely become more widely available to HIV-1-infected persons in resource-limited countries. In the United States, widespread antiretroviral usage has been associated with increased antiretroviral drug resistance. This raises concern that drug resistance may reduce the effectiveness of perinatal antiretroviral prophylaxis as well as therapeutic intervention strategies. The purpose of this article is to review what is known about resistance and risk of perinatal HIV transmission, assess the interaction between antiretroviral resistance and the prevention of perinatal HIV-1 transmission, and discuss implications for current global prevention and treatment strategies.

Nucleoside analogues for chronic hepatitis B

Hepatitis B virus replicates inside the hepatocyte through an intermediate step of reverse transcription mediated by the viral polymerase. We describe five nucleoside/nucleotide analogues that interfere with the replication mechanisms of the hepatitis B virus. The resemblance of nucleoside analogues to natural nucleosides may lead to direct cytotoxicity. Therefore, antiviral activity should always be interpreted in the light of cellular toxicity. In addition, prolonged therapy with a nucleoside analogue may induce mutations in the viral polymerase, causing structural and configurational changes of the polymerase resulting in a decreased affinity for the nucleoside analogue. Subsequently, the mutated virus is capable of renewed replication during continued antiviral pressure of the nucleoside analogue. The best antiviral strategy in the future is probably combination therapy, either with several nucleoside analogues or with a nucleoside analogue and interferon.

Applied Evolution

▪ Abstract  Evolutionary biology is widely perceived as a discipline with relevance that lies purely in academia. Until recently, that perception was largely true, except for the often neglected role of evolutionary biology in the improvement of agricultural crops and animals. In the past two decades, however, evolutionary biology has assumed a broad relevance extending far outside its original bounds. Phylogenetics, the study of Darwin's theory of “descent with modification,” is now the foundation of disease tracking and of the identification of species in medical, pharmacological, or conservation settings. It further underlies bioinformatics approaches to the analysis of genomes. Darwin's “evolution by natural selection” is being used in many contexts, from the design of biotechnology protocols to create new drugs and industrial enzymes, to the avoidance of resistant pests and microbes, to the development of new computer technologies. These examples present opportunities for education of the public and for nontraditional career paths in evolutionary biology. They also provide new research material for people trained in classical approaches.

Investigation of some Hypericum species native to Southern of Brazil for antiviral activity

Three plant species, Hypericum connatum, Hypericum caprifoliatum, Hypericum polyanthemum (Guttiferae), growing in Southern of Brazil were chemically investigated and tested for their antiviral activity against feline immunodeficiency virus (FIV). The chemical analysis revealed the presence of polyphenolic compounds such as tannins and flavonoids. Hypericin was not detected in these species. The aqueous extract (AE), the aqueous extract with low tannin concentration (LTCAE) and the methanolic extract (ME) were tested for their cytotoxic properties in concentrations of 50-150 microg/ml. AE was toxic to CRFK for the three species in all concentrations. LTCAE and ME varied between different concentrations being not toxic or allowing 80% of cell growth. LTCAE and ME (10-50 microg/ml) were analyzed for antiviral activity by inhibition of CPE and measuring FIV genome from cell culture supernatant. LTCAE of all species in this work did not cause any inhibition of FIV. Although no difference was seen in CPE, a lower number of viral particles in the supernatant was observed when FIV infected cells were treated with ME of H. connatum. These results suggest that some plants of the genus Hypericum from Southern Brazil contain compounds with potential antiviral activity against lentiviruses.

Fluorometric determination of 2'-beta-fluoro-2',3'-dideoxyadenosine 5'-triphosphate, the active metabolite of a new anti-human immunodeficiency virus drug, in human lymphocytes

A sensitive precolumn derivatization method has been developed to measure the 5'-triphosphate of 2'-beta-fluoro-2',3'-dideoxyadenosine (F-ddA, lodenosine), a new anti-HIV drug, in human lymphocytes by HPLC using fluorescence detection. Reaction of chloroacetaldehyde with F-ddA triphosphate in extracts from human lymphocytes produces a highly fluorescent etheno adduct. This derivative is then separated and quantitated by reverse-phase paired-ion chromatography. Degradation of natural nucleic acid ribosides, such as ATP, using periodate oxidation simplifies the chromatogram and minimizes interference with detection of the target analyte. This method, modeled using cultured MOLT-4 T-lymphocytes, achieves a linear detector response for peak area measurements over the range 2.5 to 22.5 pmol (50-450 nM using 50 microl sample). Analyte recovery is greater than 90%, and the method achieves a limit of detection and limit of quantitation of 1.4 and 2.5 pmol per HPLC injection (50 microl sample containing cellular extract from 2.5 x 10(6) cells), respectively. Application of this method to measure F-ddATP in peripheral blood mononuclear cells from HIV-infected patients treated with F-ddA at 3.2 mg/kg twice daily for 22 days shows F-ddATP levels which range from 1.5 to 3.5 pmol/10(6) cells.

Evolutionary and immunological implications of contemporary HIV-1 variation

Evolutionary modelling studies indicate less than a century has passed since the most recent common ancestor of the HIV-1 pandemic strains and, in that time frame, an extraordinarily diverse viral population has developed. HIV-1 employs a multitude of schemes to generate variants: accumulation of base substitutions, insertions and deletions, addition and loss of glycosylation sites in the envelope protein, and recombination. A comparison between HIV and influenza virus illustrates the extraordinary scale of HIV variation, and underscores the importance of exploring innovative HIV vaccine strategies. Deeper understanding of the implications of variation for both antibody and T-cell responses may help in the effort to rationally design vaccines that stimulate broad cross-reactivity. The impact of HIV-1 variation on host immune response is reviewed in this context.

Resistance of hepatitis B virus to antiviral drugs: current aspects and directions for future investigation

Despite the existence of vaccines, chronic hepatitis B virus (HBV) infection remains a major health problem worldwide. Interferon therapy successfully controls infection in only a small percentage of chronically infected individuals. The recent approval of the nucleoside analogue lamivudine for the treatment of chronic HBV infection has ushered in a new era of antiviral therapy. While lamivudine is highly effective at controlling viral infection short-term, prolonged therapy has been associated with an increasing incidence of viral resistance. Thus, it appears that lamivudine alone will not be sufficient to control chronic viral infection in the majority of individuals. In addition to lamivudine, several new nucleoside and nucleotide analogues that show promising antihepadnaviral activity are in various stages of development. Lamivudine resistance has been found to confer cross-resistance to some of these compounds and it is likely that resistance to newer antivirals may also develop during prolonged use. Drug resistance therefore poses a major threat to nucleoside analogue-based therapies for chronic HBV infection. Fortunately, combination chemotherapy (antiviral therapy with two or more agents) can minimize the chance that resistance will develop and can be expected to achieve sustained reductions in viral load, provided that suitable combinations of agents are chosen. Here we review the basis of drug resistance in HBV, with emphasis on aspects that are likely to affect drug choice in future.

Combination chemotherapy for hepatitis B virus: the path forward?

Hepatitis B virus (HBV) was identified as a cause of viral hepatitis more than 30 years ago and hepatitis B vaccines have been available for almost 20 years, but HBV infection continues to be a global health problem, responsible for about 1.2 million deaths annually. By the end of this year, almost 400 million people--about 5% of the world's population and more than ten times the number infected with human immunodeficiency virus (HIV)--will be infected with HBV. Chemotherapy remains the only treatment option for controlling chronic HBV infection once acquired, but none of the many different chemotherapeutic strategies used in the past has proven consistently successful. Prospects for successful treatment of HBV have improved dramatically during the past decade due to the development of new, well tolerated and efficacious anti-HBV drugs, and to advances in our understanding of HBV replication and pathogenesis. The newer anti-HBV drugs are capable of reducing viral loads very rapidly, but the initial response is invariably followed by very much slower elimination of residual virus. As more effective anti-HBV drugs become available, the emergence of drug resistance during the slower phase of HBV elimination will probably become the most significant obstacle in the way of eventual control of HBV infection. Experience with HIV indicates that combination chemotherapy may suppress or eliminate drug resistance and methods for pre-clinical and clinical assessment of anti-HBV drug combinations are being developed. Basic research into mechanisms of drug action and interaction should assist in the design and optimisation of combination chemotherapy for HBV infection, for which additional new anti-HBV drugs will undoubtedly be required in future.

Factors associated with the successful modification of antiretroviral therapy. HIV Outpatient Study Investigators

OBJECTIVES

To assess the characteristics of medication regimen modification and the influence of a commercial genotypic resistance assay on the short-term (3-12 weeks) viral load response (> or = 0.5 log reduction) in HIV-1-infected patients extensively treated with antiretroviral therapy (ART).

METHODS

A nested cohort study was performed in two clinics from the HIV Outpatient Study of 96 persons with a HIV-1 viral load of 10(4) log copies/ml or greater taking at least two antiretroviral medications.

RESULTS

Successful modification was associated with adding at least two new medications [relative risk (RR), 1.5; 95% confidence interval (CI), 1.1-2.2], adding a drug from a previously unused class of agents (RR, 2.0; CI, 1.4-2.9), the initiation of a non-nucleoside reverse transcriptase inhibitor (NNRTI) (RR, 1.7; CI, 1.2-2.4), but not substituting a protease inhibitor or the use of a commercial genotypic resistance assay.

CONCLUSION

Incorporating a drug from a previously unused class or changing at least two new medications, but, within the confines of this study, not using a commercial genotypic resistance assay, was associated with the successful modification of ART as measured by a reduction in viral load.