Synthesis and Structure Activity Relationships of a Series of Penicillin-Derived Pseudosymmetric Inhibitors of HIV-1 Proteinase

The synthesis from penicillin G of a series of potent pseudosymmetric inhibitors 11 a-k of HIV-1 proteinase is described. The 2-pyridyl substituted compounds 11a and 11j showed improved antiviral activity compared to their C2-symmetric counterparts 3 and 4.

Evolution of genotypic and phenotypic resistance during chronic treatment with the fusion inhibitor T-1249

T-1249 is a peptide HIV fusion inhibitor (FI) previously under development for use in FI-naive and experienced patients. Here we present prospectively planned longitudinal analyses of FI resistance during 48 weeks of T-1249 dosing in patients with extensive prior FI exposure. T1249-105 was a single-arm rollover study in patients with prior resistance to enfuvirtide (ENF) and 10 days of T-1249 functional monotherapy exposure. The phenotype and genotype of plasma virus envelopes were analyzed at baseline and at study weeks 8, 16, and 48. At study entry, viruses had a geometric mean decrease in susceptibility to ENF of 51.8-fold but to T-1249 of 1.8-fold; extensive genotypic resistance to ENF was observed. A median viral load response of - 1.5 log(10) copies/ml was observed at week 2 that was partially sustained (- 0.5 log(10) copies/ml) through 48 weeks. Resistance to T-1249 gradually increased to a geometric mean 92.7-fold decrease from FI-naive baseline; this occurred concomitant with further evolution of gp41 amino acids 36-45, most commonly the G36D (n = 6, 16%) or N43K (n = 9, 24%) substitutions. A novel substitution, A50V (n = 12, 32%), was also common, as were the N126K and S138A substitutions in heptad-repeat 2 (HR-2). These data point toward a primary role for the gp41 36-45 locus in modulating FI binding and suggest that residues in HR-2 may contribute in a more limited manner to development of peptide FI resistance. These data also point toward a substantial genetic barrier and fitness cost to development of resistance to next-generation fusion inhibitors.

Characterization of envelope glycoprotein gp41 genotype and phenotypic susceptibility to enfuvirtide at baseline and on treatment in the phase III clinical trials TORO-1 and TORO-2

Enfuvirtide (T-20) is the first entry inhibitor approved for treatment of HIV infection and acts by inhibiting conformational changes in the viral envelope protein gp41 that are necessary for fusion of the virus and host cell membranes. Here we present genotypic and phenotypic data on viral envelopes obtained at baseline (n = 627) and after 48 weeks of enfuvirtide treatment (n = 302) from patients in the TORO (T-20 versus Optimized Regimen Only)-1 and -2 phase III pivotal studies. The amino acid sequence at residues 36-45 of gp41 was highly conserved at baseline except for polymorphism of approximately 16% at position 42. Substitutions within gp41 residues 36-45 on treatment were observed in virus from 92.7% of patients who met protocol defined virological failure criteria and occurred in nearly all cases (98.8%) when decreases in susceptibility to enfuvirtide from baseline of greater than 4-fold were observed. Consistent with previous observations, a wide range of baseline susceptibilities (spanning 3 logs) was observed; however, lower in vitro baseline susceptibility was not significantly associated with a decreased virological response in vivo. Virological response was also independent of baseline coreceptor tropism and viral subtype.

Impact of human immunodeficiency virus type 1 gp41 amino acid substitutions selected during enfuvirtide treatment on gp41 binding and antiviral potency of enfuvirtide in vitro

Enfuvirtide (ENF), a novel human immunodeficiency virus type 1 (HIV-1) fusion inhibitor, has potent antiviral activity against HIV-1 both in vitro and in vivo. Resistance to ENF observed after in vitro passaging was associated with changes in a three-amino-acid (aa) motif, GIV, at positions 36 to 38 of gp41. Patients with ongoing viral replication while receiving ENF during clinical trials acquired substitutions within gp41 aa 36 to 45 in the first heptad repeat (HR-1) of gp41 in both population-based plasma virus sequences and proviral DNA sequences from isolates showing reduced susceptibilities to ENF. To investigate their impact on ENF susceptibility, substitutions were introduced into a modified pNL4-3 strain by site-directed mutagenesis, and the susceptibilities of mutant viruses and patient-derived isolates to ENF were tested. In general, susceptibility decreases for single substitutions were lower than those for double substitutions, and the levels of ENF resistance seen for clinical isolates were higher than those observed for the site-directed mutant viruses. The mechanism of ENF resistance was explored for a subset of the substitutions by expressing them in the context of a maltose binding protein chimera containing a portion of the gp41 ectodomain and measuring their binding affinity to fluorescein-labeled ENF. Changes in binding affinity for the mutant gp41 fusion proteins correlated with the ENF susceptibilities of viruses containing the same substitutions. The combined results support the key role of gp41 aa 36 to 45 in the development of resistance to ENF and illustrate that additional envelope regions contribute to the ENF susceptibility of fusion inhibitor-naïve viruses and resistance to ENF.

Measuring the effectiveness of antiretroviral agents

Considerable progress has been made recently in developing effective antiretroviral combination therapy that can suppress viral replication and delay disease progression in individuals infected with HIV. A range of up to 15 approved antiretroviral agents is now available, which target two different viral enzymes, while several agents are in clinical development. The rapid development and approval of antiretroviral agents, driven by the urgency of clinical need as well as the complexity of possible combinations, has precluded the extensive comparative clinical testing of regimens, which is necessary to establish the relative efficacy of various different agents. The lack of an appropriate animal model for HIV disease also increases reliance on in vitro measures. Several different in vitro and in vivo parameters have been defined in an attempt to quantify the effectiveness of antiretroviral agents, most importantly the 50% inhibitory and effective concentrations (IC50 and EC50). However, the clinical relevance of these measures is uncertain. Additionally, considerable variation exists in the usage of the terms 'IC50' and 'EC50' in recent publications in the literature. These issues pose interpretation problems to clinicians seeking information on the relative clinical efficacy of the agents. In this brief review, we attempt to clarify the different measures available and their potential utility for clinical decision-making, focusing particularly on the example of HIV protease inhibitors. There are many different quantifiable parameters that give information regarding the effectiveness of an antiviral drug. These include: inhibition of the viral target enzyme (inhibition constant, Ki); selectivity for viral versus host enzymes; inhibition of viral replication in cell culture (IC50); ratio of efficacy to cytotoxicity in vitro (therapeutic index); inhibition of viral replication or symptoms in an appropriate animal model of the disease (EC50); and the effect on surrogate markers, such as viral load or CD4 cell count, after administration to humans (in vivo EC50). Each of these different parameters gives valid information about the properties of an antiretroviral agent, which can help to build up a picture of its potential clinical utility relative to other drugs. However, to gain meaningful results, it is important to apply this information intelligently, understanding the limitations of each parameter.

HVR1 quasispecies analysis from a long-term culture of hepatitis C virus in Hep G2 derived cells grown in a haemodialysis cartridge

Studies on the in vitro hepatitis C virus (HCV) infection are hampered by the lack of an appropriate system to culture permissive cells to be continuously infected with HCV. Trypsinization required for cell passage can lead to possible temporary loss of permissiveness for infection, whereas refreshment of the medium can result in loss of infectious particles necessary for perpetuation of the infection; it is therefore very difficult to maintain a continuous HCV infection in cell cultures. A new infection method was designed and evaluated in order to prevent these unfavourable circumstances. A cell line derived from the human hepatoblastoma cell line Hep G2 was grown in the extracapillary space of a haemodialysis cartridge, in the presence of a HCV-positive inoculum, while the culture medium was recirculated through the intracapillary space, supplying the cells with nutrients and oxygen. HCV RNA could continuously be detected in the cells up to 77 days of culture. Sequence analysis of the HCV hypervariable region 1 (HVR1) revealed that 56% and 75%, respectively, of the clones obtained from the cells at day 20 and 40 after start of the infection were different from the clones obtained from the original inoculum and that certain nucleotide positions in this region were more susceptible to mutations, leading to an alteration in amino acid sequence. As none of these sequences were present in the clones from the inoculum, it is suggested that new HCV quasispecies have emerged as a result of viral replication in the hepatocytes in vitro. This system seems a valuable tool for the in vitro evaluation of antiviral drugs.

The potential for HIV fusion inhibition

In recent years, exciting developments in the understanding of the multi-step HIV-1 entry process have revealed potential approaches for therapeutic intervention that will compensate for the early disappointments of the soluble CD4 antigen approach. Although details of the HIV-1 fusion process are clearly complex, the proof-of-concept in the clinic provided by T-20 gives high hopes that new generation inhibitors of HIV fusion will be developed. This review highlights novel drug discovery technologies that are being employed in the search for such inhibitors.

Human immunodeficiency virus type 1 entry and chemokine receptors: a new therapeutic target

After the identification of CD4 as the primary receptor for human immunodeficiency virus (HIV) type 1 entry into cells of the immune system, it soon became clear that CD4 alone was not sufficient to establish a productive infection. The search for the second receptors or co-receptors started over 10 years ago, and it was not until 1996 that G protein-coupled 7-transmembrane receptors, CXCR4 and CCR5 were finally identified as the co-receptors for HIV-1 entry. The 7-transmembrane receptor family is a familiar therapeutic target for a number of diseases, and therefore these recent findings represent an exciting opportunity for new therapeutic approaches to the treatment of HIV-1 infection.

HIV protease genotype and viral sensitivity to HIV protease inhibitors following saquinavir therapy

OBJECTIVE

To examine the relationship between HIV protease genotype and altered protease inhibitor sensitivity of isolates from patients after therapy with saquinavir (SQV) in its hard gelatin formulation.

DESIGN

Forty-one post-therapy isolates and corresponding baseline samples were obtained from 37 patients in four different clinical trials after therapy with SQV for 16-147 weeks. Post-therapy isolates were selected on the basis of preliminary sequence or drug sensitivity data.

RESULTS

Fifteen out of 17 isolates without detectable Val-48 or Met-90 mutations retained sensitivity to SQV. (The remaining isolates showed only a marginal increase in median inhibitory concentration.) In addition, three out of 15 isolates with Met-90 retained sensitivity to all other protease inhibitors tested (indinavir, ritonavir, amprenavir, nelfinavir). Of the isolates showing reduced sensitivity to SQV, six out of 22 retained sensitivity to all other protease inhibitors, whereas only four out of 22 showed broad cross-resistance to all protease inhibitors tested. The reduction in sensitivity correlated closely with the presence of Val-48 or Met-90. Subsequent accessory substitutions were also linked to reduced sensitivity. However, significant linkage was observed only between mutations at residues 48 and 82 and between those at residues 82 and 74.

CONCLUSIONS

Recruitment of Val-48/Met-90 mutations was not found to be synonymous with cross-resistance. Indeed, the majority of isolates with these mutations retained sensitivity to at least one protease inhibitor (Val-48, 86%; Met-90, 77%). The recruitment of accessory mutations may occur only after the selection of key resistance mutations. Furthermore, Met-90 was found to be a poor marker of cross-resistance in SQV-treated patients.

Correlation of response to treatment and HIV genotypic changes during phase III trials with saquinavir and reverse transcriptase inhibitor combination therapy

OBJECTIVES

Assessment of genotypic change in HIV protease during treatment with saquinavir (SQV) in combination with zidovudine (ZDV) and/or zalcitabine (ddC), to determine the influence of such changes on viral phenotype and response to treatment.

DESIGN

Virologic substudies of Phase III clinical trials NV14256 and SV14604.

METHODS

Population sequencing of HIV protease genes amplified from pre- and post-treatment plasma. Phenotyping of peripheral blood mononuclear cell (PBMC)-derived virus isolates, and genotyping of proviral DNA clones amplified from PBMC used in the expansion of virus isolates.

RESULTS

In both trials the incidence of Met90 remained at < or = 20% in subjects receiving SQV in combination with ddC (with or without ZDV) for 1 year. A Val48 substitution was observed in two out of 81 subjects after 24 weeks and in two out of 75 subjects after 48 weeks. In 12 out of 13 NV14256 subjects with viral load rebound during SQV monotherapy these substitutions were associated with the rebound. In subjects treated with SQV plus ddC, rebound was associated with SQV resistance in six out of 22 cases and ddC resistance in five out of 22 cases. The incidences of non-BRU residues at positions 10, 63 and 71 were increased significantly (P < 0.05, Fisher's exact test) after SQV treatment with or without ZDV. However, comparison of genotypic and phenotypic data showed that these changes were not associated with reduced sensitivity to SQV.

CONCLUSIONS

Virological failure during combination therapy can be due to resistance to either treatment drug, emphasising the need to change both the reverse transcriptase inhibitor and the protease inhibitor. Only Val48 and Met90 correlated directly with the development of reduced drug sensitivity during treatment with SQV in vivo.

Resistance to (-)-2',3'-dideoxy-3'-thiacytidine (3TC) in HIV-1 isolated from paediatric patients

We conducted detailed virological evaluations of 16 HIV-1-infected paediatric patients treated with 3TC (lamivudine) monotherapy. High-level phenotypic resistance against this compound (up to 2,500-fold) was seen in virtually all cases, usually within 8-12 weeks of initiation of therapy. This was concomitant with the appearance of the M184V mutation in viral reverse transcriptase, previously shown to be responsiblefor such resistance. Viral burden fell in virtually all cases after commencement of therapy, and remained below baseline in each instance studied, despite a rebound effect and the appearance of drug resistance. Viral isolates from some patients underwent a switch from a non-syncytium-inducing (NSI) to a syncytium-inducing (SI) phenotype during the course of the study, although no relationship was apparent between dose of drug employed, time to development of drug resistance or time of appearance of SI phenotype.

Phase I/II study of 3TC (lamivudine) in HIV-positive, asymptomatic or mild AIDS-related complex patients: sustained reduction in viral markers. The Lamivudine European HIV Working Group

OBJECTIVE

To evaluate the efficacy of 3TC (lamivudine), a synthetic nucleoside analogue that inhibits HIV reverse transcriptase in vitro, as treatment for HIV-positive, asymptomatic or mild AIDS-related complex patients.

DESIGN

Open-label, multinational and multicentre, non-comparative, escalating dose study.

METHODS

Patients who meet the selection criteria (n = 104) were enrolled in three European countries. Ten to 15 patients were included at each of the six dose levels of 3TC (0.5, 1.0, 2.0, 4.0, 8.0, 12.0 and 20.0 mg/kg daily in two divided doses every 12 h). Virological parameters--immune-complex dissociation (ICD) assay for HIV p24 antigenaemia, plasma HIV RNA load, whole blood assay and cellular viraemia--were evaluated at weeks 0, 4, 12 and 24.

RESULTS

Sustained reductions in HIV RNA load and in ICD p24 antigen levels were observed and maintained over the 12-week assessment period. Greater reductions were noted at higher doses but this trend did not reach statistical significance. In 38 patients, reductions of cell viraemia were significantly greater at 4 weeks for patients treated at higher doses of 3TC.

CONCLUSION

These virological data show that 3TC is a potent inhibitor of HIV replication in HIV-positive, asymptomatic or mild ARC patients as assessed by ICD p24 antigenaemia, plasma HIV RNA load and cell viraemia.

Benzophenone derivatives: a novel series of potent and selective inhibitors of human immunodeficiency virus type 1 reverse transcriptase

A series of benzophenone derivatives has been synthesized and evaluated as inhibitors of HIV-1 reverse transcriptase (RT) and the growth of HIV-1 in MT-4 cells. Through the use of the structure-activity relationships within this series of compounds and computational chemistry techniques, a binding conformation is proposed. The SAR also indicated that the major interactions of 1h with the RT enzyme are through hydrogen bonding of the amide and benzophenone carbonyls and pi-orbital interactions with the benzophenone nucleus and an aromatic function separated from the benzophenone by a suitable spacer group. The crystal structure of compound 1h has been determined. A number of compounds with potent inhibitory activity against HIV-1 RT and HIV in cellular assays at levels comparable with AZT and our efforts to identify a metabolically stable analogue are described.

Development of HIV-1 resistance to (-)2'-deoxy-3'-thiacytidine in patients with AIDS or advanced AIDS-related complex

OBJECTIVE

To determine the rate of development of in vitro HIV resistance to (-)2'-deoxy-3'-thiacytidine (3TC) and relate the effect of dose to emergence of resistance.

METHODS

HIV-infected men and non-pregnant women, aged > or = 18 years, with a CD4 count < or = 300 x 10(6)/l cells were followed in a Phase I/II study, in which they were evaluated for tolerance to 3TC and effect of this agent with regard to viral susceptibility. Peripheral blood and plasma samples were collected at regular intervals for analysis. HIV was isolated using umbilical cord blood mononuclear cells as targets. These cells were also used in determinations of median inhibitory drug concentration. Specific amplification of the 184 mutation site, associated with HIV resistance to 3TC, was performed by polymerase chain reaction, using specific primer pairs, on DNA harvested from infected peripheral blood mononuclear cells (PBMC) of donors or, alternatively, on DNA that had been reverse transcribed from plasma-associated HIV RNA.

RESULTS

Phenotypic resistance was detected in approximately one-third of individuals studied, who were followed between 8 and 56 weeks. Development of 3TC resistance occurred independently of dose, although time of first appearance of resistant HIV-1 variants appeared reduced at high 3TC doses. Amino-acid changes at codon 184 in HIV-1 reverse transcriptase were associated with, and preceded, the development of phenotypic 3TC resistance. Most commonly, a Met to Ile substitution appeared transiently before being superceded by a Val substitution at codon 184.

CONCLUSIONS

In vitro resistance to 3TC developed in a high proportion of subjects who received prolonged monotherapy with this drug. The development of resistance to 3TC was associated with appearance of mutated viral forms and the disappearance of wild-type virus, with regard to codon 184, in both patient plasma and PBMC.

Immunomodulatory and antiviral activities of 2',3'-dideoxy-beta-L-cytidine and 2',3'-dideoxy-beta-L-5-fluorocytidine

Two dideoxynucleosides, 2',3'-dideoxy-beta-L-cytidine and 2',3'-dideoxy-beta-L-5-flurocytidine, containing unnatural L-configuration in their sugar moieties, were synthesized and assayed for antiviral activities. Both compounds were shown to possess potent anti-human immunodeficiency virus type 1 and antihepatitis B virus activities, while demonstrating no anti-herpes simplex viruses 1 and 2 activity. These two compounds exhibited in vitro cellular toxicities for several leukocytic cell lines and were shown to inhibit phytohemagglutinin-stimulated human peripheral blood mononuclear leukocyte proliferations. At inhibitory concentrations, both compounds caused accumulations of cells in the S phase. While demonstrating no obvious morphological toxicity in vivo in mice at concentrations of 75 and 150 mg/kg, 2',3'-dideoxy-beta-L-5-fluorocytidine- treated animals were shown to have considerable increases in CD4/CD8 double positive T lymphocyte population in their blood circulation.

Synthesis and structure-activity relationships of a series of penicillin-derived HIV proteinase inhibitors: heterocyclic ring systems containing P1' and P2' substituents

As an extension of our earlier work based upon a single penicillin-derived thiazolidine moiety we have found that the decahydroisoquinoline grouping, also present in Ro 31-8959, is an effective replacement for one of the thiazolidine units in C2 symmetric penicillin-derived dimers. Reaction of racemic epoxide 6 with [3S-[3 alpha, 4a alpha, 8a alpha]]-decahydro-N-(1,1-dimethylethyl)-3- isoquinolinecarboxamide gave diasteroisomers 34a and 34b. The stereochemistry of the hydroxyl grouping of 34a was determined to be (S). Reaction of the amines derived from 34a and 34b with thiazolidine 8a gave 50 and 51, respectively. Compound 50 was a potent inhibitor of HIV proteinase (IC50 = 23 nM) with antiviral activity against HIV-1 in vitro (EC50 C8166 cells = 50 nM). However, a poor pharmacokinetic profile in the dog for compound 50 and its analogues, in keeping with earlier studies on penicillin-derived dimers in three species, precluded their development as potential antivirals.

Discovery and analysis of a series of C2-symmetric HIV-1 proteinase inhibitors derived from penicillin

In order to identify a suitable peptide substrate for human immunodeficiency virus-1 (HIV-1) proteinase, a range of peptides from various cleavage sites within the gag-pol polyprotein were assayed by HPLC for specific cleavage. The peptide with the optimal combination of favorable kinetics and good solubility was based on the N-terminus cleavage site of HIV-1 proteinase (KQGTVSFNF*PQIT). The HPLC assay, using the above peptide, was developed into a rapid isocratic method in order to analyze inhibition kinetics. An assay suitable for high-throughput screening was developed using a radioactively labeled peptide with the same sequence, coupled to a solid phase. Using this assay, a C2-symmetric HIV-1 proteinase inhibitor derived from penicillin was discovered during random screening of a compound library. A chemical synthesis program developed this structure into a series of potent inhibitors. The lead structures were highly selective for HIV-1 proteinase with good antiviral activity in vitro against HIV and no cytotoxicity. The HPLC assay was used to demonstrate that these compounds are competitive tight-binding inhibitors of HIV-1 proteinase.

Synthesis and anti-HIV-1 activity of a series of imidazo[1,5-b]pyridazines

A series of substituted imidazo[1,5-b]pyridazines have been prepared and tested for inhibitory activity against the reverse transcriptase of HIV-1 (RT) and their ability to inhibit the growth of infected MT-4 cells. Crystal data are reported on two compounds, 15c and 33. From the structure-activity relationships developed within this and other series, it is proposed that key features of the interaction with RT include hydrogen-bond acceptor and aromatic pi-orbital bonding with the imidazopyridazine nucleus and a benzoyl function separated from the heterocycle by a suitable spacer group. Exceptional activity against the reverse transcriptase of HIV-1 (IC50 = 0.65 nM) was obtained with a 2-imidazolyl-substituted derivative, 7-[2-(1H-imidazol-1- yl)-5-methylimidazo-[1,5-b]pyridazin-7-yl]-1-phenyl-1-heptanone (33) which is attributed to additional binding of the imidazole sp2 nitrogen atom. A number of the compounds in this series also inhibit the replication of HIV-1 in vitro in MT-4 and C8166 cells at levels observed with the nucleoside AZT.

A series of penicillin derived C2-symmetric inhibitors of HIV-1 proteinase: synthesis, mode of interaction, and structure-activity relationships

The C2-symmetric diester 1 was identified by random screening as a novel inhibitor of HIV-1 proteinase. This led to the preparation of a series of related more potent amides from readily accessible penicillins. Many of the compounds showed potent antiviral activity in HIV-1-infected MT-4 cells and an ability to inhibit syncytia formation in infected C8166 cells, with no evidence of cytotoxicity. The compounds showed no activity against other aspartyl proteinases (renin, pepsin, and cathepsin D). Structure-activity relationships support a symmetrical interaction with the enzyme. Pharmacokinetic evaluation of the ethylamide 3 revealed it was subject to rapid plasma clearance and had low oral bioavailability.

Synthesis and structure-activity relationships of a series of penicillin-derived HIV proteinase inhibitors containing a stereochemically unique peptide isostere

A series of HIV-1 proteinase inhibitors was synthesized based upon a single penicillin derived thiazolidine moiety. Reaction of the C-4 carboxyl group with (R)-phenylalaninol gave amide 10 which was a moderately potent inhibitor of HIV-1 proteinase (IC50 = 0.15 microM). Further modifications based on molecular modeling studies led to compound 48 which contained a stereochemically unique statine-based isostere. This was a potent competitive inhibitor (Ki = 0.25 nM) with antiviral activity against HIV-1 in vitro (5 microM). Neither modification to the benzyl group in an attempt to improve interaction with the S2' pocket, nor introduction of a hydrogen bond donating group to interact with residue Gly48' resulted in improved inhibitory or antiviral activity.

High-level resistance to (-) enantiomeric 2'-deoxy-3'-thiacytidine in vitro is due to one amino acid substitution in the catalytic site of human immunodeficiency virus type 1 reverse transcriptase

Passage of human immunodeficiency virus type 1 in the presence of increasing 2'-deoxy-3'-thiacytidine (3TC) concentrations results in high-level (> 100-fold) 3TC-resistant viruses. All 3TC-resistant viruses possess a substitution at the second codon (from a methionine into an isoleucine) at position 184 within the highly conserved motif (YMDD) of human immunodeficiency virus type 1 reverse transcriptase. 3TC-resistant viruses were cross-resistant to the (-) enantiomer of the fluorinated derivative of BCH-189 but remained susceptible to 2',3'-dideoxyinosine and 2',3'-dideoxycytidine. The susceptibilities of the 3TC-resistant viruses to the (+) enantiomers of BCH-189 and the fluorinated derivative of BCH-189 demonstrate an enantiomeric specificity for viruses selected under these conditions. Introduction of an isoleucine substitution at codon 184 into a background of two known 3'-azido-3'-deoxythymidine resistance mutations (amino acids 41 and 215) restored the susceptibility of this virus to 3'-azido-3'-deoxythymidine.

The same mutation that encodes low-level human immunodeficiency virus type 1 resistance to 2',3'-dideoxyinosine and 2',3'-dideoxycytidine confers high-level resistance to the (-) enantiomer of 2',3'-dideoxy-3'-thiacytidine

Variants of human immunodeficiency virus type 1 that display 500- to 1,000-fold resistance to the (-) enantiomer of 2'-deoxy-3'-thiacytidine and approximately 4- to 8-fold resistance to 2',3'-dideoxycytidine and 2',3'-dideoxyinosine have been generated through in vitro selection with the former compound. The polymerase regions of several of these resistant viruses shared a codon alteration at site 184 (ATG-->GTG; methionine-->valine), a mutation previously associated with low-level resistance to 2',3'-dideoxycytidine. The biological relevance of this mutation for the (-) enantiomer of 2'-deoxy-3'-thiacytidine was confirmed by site-directed mutagenesis with the HXB2-D clone of human immunodeficiency virus type 1.

Cellular metabolism of (-) enantiomeric 2'-deoxy-3'-thiacytidine

The metabolism of (-) enantiomeric 2'-deoxy-3'-thiacytidine (3TC) was examined in human immunodeficiency virus type 1 (HIV-1)-infected and mock-infected human cells. 3TC 5'-triphosphate levels accumulated comparably in HIV-1-infected and mock-infected phytohaemagglutinin-stimulated peripheral blood lymphocytes (PBL) and reached 40% or more of total intracellular 3TC metabolites after 4 hr. The rate of decay of 3TC triphosphate in HIV-1-infected and mock-infected PBL measured as a half-life (T1/2) ranged from 10.5 to 15.5 hr. 3TC did not significantly affect metabolism of deoxynucleotides in the U937 cell line, and was shown to be resistant to the action of human platelet pyrimidine nucleoside phosphorylase.

(-)-2'-deoxy-3'-thiacytidine is a potent, highly selective inhibitor of human immunodeficiency virus type 1 and type 2 replication in vitro

The (-)-enantiomer of 2'-deoxy-3'-thiacytidine (3TC) was found to be a potent and selective inhibitor of human immunodeficiency virus types 1 (HIV-1) and 2 (HIV-2) in vitro. We determined its antiviral activity against a number of laboratory strains of HIV-1 and HIV-2 in a range of CD4-bearing lymphocyte cell lines (mean 50% inhibitory concentration [IC50] range, 4 nM to 0.67 microM). 3TC was also active against a range of HIV-1 strains in peripheral blood lymphocytes (mean IC50 range, 2.5 to 90 nM). The IC50 for cytotoxicity in seven lymphocyte cell cultures, including human peripheral blood lymphocytes, ranged from 0.5 to 6 mM. 3TC had no detectable antiviral activity against a range of other viruses or in cells chronically infected with HIV-1 or HIV-2. The effects of time of addition of the compound and varying the multiplicity of infection on the antiviral activity of 3TC were determined. The results showed that 3TC is a potent and selective inhibitor of HIV-1 and HIV-2 replication in vitro.

The separated enantiomers of 2'-deoxy-3'-thiacytidine (BCH 189) both inhibit human immunodeficiency virus replication in vitro

Racemic 2'-deoxy-3'-thiacytidine (BCH 189) is a dideoxycytidine analog having a sulfur atom in place of the 3' carbon. The enantiomers of BCH 189 have been resolved and found to be equipotent in antiviral activity against human immunodeficiency virus types 1 and 2. However, the (-)-enantiomer (3TC) is considerably less cytotoxic than the (+)-enantiomer.

Molecular biology and the control of viral vaccines

The live attenuated vaccines against poliomyelitis developed by Sabin are among the safest and most effective antiviral vaccines in current use and have eliminated poliomyelitis as a public health problem in many countries. They form the main basis of the WHO intention to eliminate poliomyelitis from the world by the year 2000, and the molecular basis of their attenuated phenotype and some of its virological consequences are increasingly clear. Nonetheless, the data reviewed here show how poorly understood their mechanism of action is in use. Our studies raise the possibility of in vitro neurovirulence tests and may help to identify features of particular importance in the attenuation of the virus for human rather than simian recipients. On the other hand it is clear that when a live virus is used as a vaccine it is not possible to control it in the same way that genetically engineered products may be controlled in so far as replication in the recipient makes it possible for the live vaccine strain to alter in ways which may or may not be undesirable.

Virus excretion and mutation by infants following primary vaccination with live oral poliovaccine from two sources

The excretion of virus by 30 children was followed over a period of 28 days after their first vaccination with live oral poliovaccine. The type 1 and type 2 strains were isolated at similar high frequencies throughout this period, while the type 3 strain was not excreted by most children after day 2 post vaccination. Mutations in the 5' non-coding region associated with the attenuated phenotype reverted most rapidly for type 3 and least rapidly for type 1. The data are consistent with different degrees of selection against the attenuating mutations in the three serotypes in the gut, but imply that reversion is required for prolonged excretion. The findings also provide a possible explanation for the reported distribution of the rarely occurring vaccine associated cases between recipients and contacts for the three serotypes. A statistically significant difference was not observed between vaccine from the two main suppliers to the UK market.

Translation deficiency of the Sabin type 3 poliovirus genome: association with an attenuating mutation C472----U

Previous studies have shown that the genome of Sabin type 3 poliovaccine strain (P3/Leon 12a1b) possesses a diminished translation efficiency as compared to genomes of closely related neurovirulent strains, the neurovirulent progenitor (P3/Leon/37), or a revertant (P3/119/70) of the vaccine (Y.V. Svitkin, S.V. Maslova, and V.I. Agol, 1985, Virology 147, 243-252). Here we attempted to evaluate the contribution of each mutation in the genome of the vaccine to this translation deficiency. Recombinants between P3/Leon 12a1b and P3/Leon/37 or P3/119/70 were constructed in vitro and their RNAs were translated in a cell-free system derived from Krebs-2 cells. The results show that of 10 nucleotide differences between the genomes of P3/Leon 12a1b and P3/Leon/37 9 have minor or no effect on translation and that the only mutation of significance is C472----U which is known to reduce the neurovirulence of the virus. Reversion from uridine to cytosine at position 472 in type 3 poliovaccine upon replication in the human gut resulted in an increase of both translation efficiency of polio RNAs and neurovirulence of corresponding strains. The data provide evidence for a common nucleotide sequence regulatory element for protein synthesis of the virus and its neurovirulence. In vitro translation assays may therefore prove to be useful for detection of attenuating mutations in the 5' noncoding region of poliovirus genome. The apparent involvement of the translation mechanism in the expression of neurovirulent or attenuated phenotype of poliovirus is briefly discussed.

Nucleotide sequence of a neurovirulent variant of the type 2 oral poliovirus vaccine

Infectious cDNAs of the Sabin type 2 poliovirus vaccine virus and a vaccine-derived neurovirulent type 2 strain (P2/117) have been cloned in Escherichia coli. Nucleotide sequence analysis revealed that P2/117 differs from the vaccine strain by just 23 point mutations. Three occur in the 5' noncoding region. The remainder result in a total of 5 coding changes located in VP1, VP4, 2B, and 3D. The likely role of these mutations in the evolution to neurovirulence is discussed.

Use of a monoclonal antibody specific for wild-type yellow fever virus to identify a wild-type antigenic variant in 17D vaccine pools

Monoclonal antibodies (MAbs) against the Asibi wild-type strain of yellow fever (YF) virus were prepared and characterized. One of the MAbs (designated MAb 117) was shown, by cross-immunofluorescence tests with flaviviruses, to be specific for wildtype YF virus. This MAb was used in indirect immunofluorescence tests to identify wild-type antigenic variants in several different YF vaccine pools. Simultaneously, a vaccine-specific MAb prepared previously (MAb 864) was used to identify YF strain 17D vaccine type variants in the wild-type Asibi virus preparation. One variant, isolated by plaque purification from a 17D vaccine pool, possessed the wild-type epitope and was neurovirulent in infant mice whereas other variants, lacking the wild-type epitope but with vaccine-specific epitopes (identified by MAb 411), were avirulent in infant mice. Avirulent variants were able to infect mice and induce antibody. Virus-specific antigen was still detected in the brains of these mice 4 weeks after inoculation, suggesting that persistent infections were developing. These results to the potential risk of selection of wild-type variants in YF vaccine preparations. They also point to the potential risk of selection of wild-type variants in YF vaccine preparations and re-emphasize the need for modernization of techniques and more effective control measures to be taken during the production of YF vaccine.

Intertypic genomic rearrangements of poliovirus strains in vaccinees

In vivo intertypic RNA recombination has previously been observed in excreted type 3 poliovirus isolates from a normal asymptomatic primary vaccinee. This study examines isolates from additional primary vaccinees to determine whether intertypic recombination is a general occurrence in excreted polioviruses. T1 RNase oligonucleotide finger-printing and limited dideoxy primer extension RNA sequencing demonstrated no evidence of intertypic recombination among type 1 or type 2 excreted strains. However, vaccinees excreting type 3 strains for long periods of time eventually produced recombinant strains involving either type 1 or type 2 poliovirus. Moreover, a characteristic time course of appearance of excreted type 3 intertypic recombinant polioviruses was established. Type 3/type 1 and type 3/type 2 recombinant strains appeared at Days 10-11 with a single crossover site in the gene for nonstructural protein 2C. Type 3/type 2/type 3 complex recombinant strains with an additional crossover site in the polymerase gene replaced type 3/type 2 strains at approximately Day 28. A significant portion of the genome of the type 3 Sabin vaccine strain is thus replaced during long-term excretion by vaccinees, and the appearance of some genomic arrangements coincided with a base deletion at the 3' terminus.

Antigenic analysis of yellow fever virus glycoproteins: use of monoclonal antibodies in enzyme-linked immunosorbent assays

A sensitive enzyme-linked immunosorbent assay with biotin and streptavidin (B/SA ELISA) was developed for the specific detection of yellow fever (YF) viruses. Monoclonal antibodies (MCA) specific for the envelope (E) glycoprotein or the non-structural glycoprotein (NV3) of YF virus-infected cell lysates were tested by antigen and antibody capture B/SA ELISA against YF viruses and a large number of other flaviviruses. In an antigen capture assay, with suckling mouse brain antigens, MCA directed against the envelope glycoprotein clearly differentiated YF viruses from any other flaviviruses, wild-type YF viruses from YF vaccine viruses and YF17D-204 substrains from other YF vaccine viruses. Specific MCA could identify YF viruses in an antibody capture assay with concentrated YF-infected tissue culture supernatant medium as the solid phase. On the basis of the principles established above, MCA binding profiles of purified YF virus strains were compared. No qualitative differences in immunoreactivity could be detected between closely related strains of YF virus. An antibody capture assay on infected tissue culture monolayers was developed to enable in situ detection of YF viruses using MCA specific for both the envelope glycoprotein and the non-structural NV3 glycoprotein.

Topographical analysis of epitope relationships on the envelope glycoprotein of yellow fever 17D vaccine and the wild type Asibi parent virus

Monoclonal antibodies (MCA), with defined molecular specificity, were used in a competition binding enzyme-linked immunosorbent assay (ELISA) to locate the relative positions of the epitopes on the envelope glycoprotein of yellow fever 17D vaccine virus and its wild type parent virus, Asibi (AS). Five topographically distinct antigenic domains were defined on the E glycoprotein of the 17D vaccine. Three of these (A, B, and C) were represented by one MCA each, a fourth (D) was represented by two MCA, and a fifth domain (E) comprised a major cluster of at least five overlapping epitopes. Asibi virus also possessed domain E which is proposed to be a conserved antigenic region within the envelope glycoprotein of all flaviviruses. Domains A and C were not represented on Asibi virus and one epitope, situated proximal to the E domain, showed structural alterations in physical overlap. Functional activities were assigned to physically mapped epitopes by haemagglutination inhibition (HAI), virus neutralisation (N), and passive protection in mice. The HAI and N functions were not necessarily linked but only MCA with N activity were able to protect mice passively against lethal infection. All domains demonstrated a heterogeneous range of biological properties dependent upon the virus strain rather than the epitope.

Neutralizing (54K) and non-neutralizing (54K and 48K) monoclonal antibodies against structural and non-structural yellow fever virus proteins confer immunity in mice

The capacity of monoclonal antibodies to protect mice passively against yellow fever (YF) virus infection was investigated. Both neutralizing (54K-specific) and non-neutralizing (54K- and 48K-specific) antibodies protected mice against challenge with the RMP substrain of YF virus. Average survival times of mice inoculated intracerebrally with a standard lethal dose of YF virus differed according to the strain used: thus mice inoculated with the most neurovirulent viruses, FNV and Asibi, survived for 6 X 50 and 7 X 65 days respectively, and those with RMP virus survived for 15 X 75 days. The capacity of antibodies to protect mice passively against virus challenge was directly related to virus neurovirulence. Possible mechanisms and the significance of protection by antibodies against non-structural proteins that do not mediate neutralization, are discussed.

Conditions for haemolysis by flaviviruses and characterization of the haemolysin

The 17D vaccine strain of yellow fever virus (YF 17D) was used to establish the optimal conditions for lysis of chick erythrocytes. Tissue culture-grown, polyethylene glycol-concentrated virus showed peak activity at pH 5.4 in citrate buffer when incubated at 37 degrees C. A further two- to fourfold increase in titre was obtained by pretreatment of the chick erythrocytes with 250 micrograms/ml trypsin. These conditions were also shown to be optimal for Japanese encephalitis (JE), West Nile (WN) and dengue-2 (den2) viruses. The ratio of haemagglutination (HA) titre to haemolysis (HL) titre approximated to unity, suggesting that the two functions are associated with the same molecule although as separable entities since selective inactivation of the HL activity of the virus was accomplished using 60 micrograms/ml trypsin. HL could be demonstrated at neutral pH if the chick erythrocytes were first subjected to treatment with acidic pH buffer. The effect on the virus envelope is thus not the sole contribution of a low pH environment to optimal HL. Hyperimmune rabbit antiserum prepared against purified YF 17D virions inhibited HA and HL if added before agglutination had occurred by the virus but when added after agglutination had taken place it showed specific anti-HL activity. Monoclonal antibodies that inhibited HA (HAI) by YF 17D did not inhibit HL (HLI) activity when applied after agglutination had taken place. Moreover, monoclonal antibodies specific for the 54K glycoprotein of YF virus but without HAI activity also had no effect on HL when added either before or after agglutination. As yet, we have been unable to identify a monoclonal antibody displaying specific anti-HL activity but all those directed against the 54K envelope glycoprotein possessing HAI activity showed HA to be a prerequisite for HL.

Examination of the immunological relationships between flaviviruses using yellow fever virus monoclonal antibodies

Monoclonal antibodies prepared against vaccine strains of yellow fever (YF) virus were initially characterized by fluorescence microscopy of Vero cells infected with YF virus strain 17D. When similarly tested against representatives of all flavivirus subgroups, the antibodies produced a wide spectrum of reactions ranging from the monospecific to the broadly cross-reactive; at least five antigenic domains in the YF virus envelope glycoprotein were identified. Monoclonal antibodies differentiated between YF virus vaccine strains (17D, 17DD, FNV), wild-type viruses and plaque variants selected from a 17D pool. One isolate from a patient with YF was antigenically similar to the Brazilian vaccine strain 17DD. Several of the antibodies reacting with the YF viral envelope glycoprotein in biological tests identified the 54K envelope glycoprotein; 45K and 26K polypeptides in YF 17D virus-infected cells were also identified by radioimmunoprecipitation and polyacrylamide gel electrophoresis. Neither of these polypeptides was found in uninfected cells. They may represent short-lived precursors of the 54K protein, post-translational cleavage or breakdown products. Other antibodies reacted with a 48K polypeptide in virus-infected cell lysates. This may be the non-structural NV3 protein described for YF virus. Its appearance on the surface of unfixed infected cells, but not on released virions, was demonstrated by fluorescence microscopy.

Use of the biotin-streptavidin interaction to improve flavivirus detection by immunofluorescence and ELISA tests

The specificity and sensitivity of immunofluorescence microscopy and ELISA tests were compared both with and without the use of biotinylated anti-species antibody and streptavidin, conjugated with either fluorescein isothiocyanate or horse radish peroxidase. In the biotin-streptavidin system monoclonal and polyclonal antibodies against yellow fever virus had titres between ten- and fifty-fold higher and background readings, particularly in ELISA tests, were noticeably reduced. Plaque variants of yellow fever virus, selected for their apparent loss of antigenic determinants against some monoclonal antibodies in conventional fluorescence tests, were found to possess the antigenic determinants when tested with the more sensitive system.

Single mutations at many sites within the DNA polymerase locus of herpes simplex viruses can confer hypersensitivity to aphidicolin and resistance to phosphonoacetic acid

Aphidicolin, a tetracyclic diterpenoid which inhibits the DNA polymerase-alpha activities of many eukaryotic cells, inhibited herpes simplex virus growth and DNA synthesis in infected cultures and the activity of the virus DNA polymerase in vitro. A wide range of stable aphidicolin sensitivities was represented amongst a collection of virus strains with no prior exposure to this drug, but viruses with polymerase mutations selected for resistance to phosphonoacetic acid (PAA) or to acycloguanosine typically showed increased sensitivity to aphidicolin. Of 16 unrelated PAA-resistant variants, 7 were hypersensitive to aphidicolin. A number of mutants with temperature-sensitive (ts) lesions in the polymerase gene also showed increased aphidicolin sensitivity (e.g. HSV-1[mP17]tsH) or aphidicolin hypersensitivity (e.g. HSV-1[KOS]tsD9, tsC4). Resistance or hypersensitivity of virus growth and DNA synthesis in vivo were correlated with resistance or hypersensitivity of virus DNA polymerase reactions in vitro. Resistance phenotypes were closely linked to the polymerase gene during recombination with outside markers. Moreover, the selection of aphidicolin-resistant mutants from hypersensitive variants with independent PAA resistance or ts mutations in the polymerase gene could result in co-selection for PAA-sensitive and ts+ phenotypes. Confirmation that multiple independent mutations could determine aphidicolin hypersensitivity was obtained by studies of recombination between independent hypersensitive variants. Aphidicolin-resistant recombinant progeny were formed with recombination frequencies (0.4 to 2.6%) compatible with intragenic events. With parental hypersensitive variants which were products of limited PAA selection, or with the ts polymerase mutations, aphidicolin-resistant recombinants were PAA-sensitive and/or ts+. The segregation of other markers (ts, plaque morphology) amongst recombinant progeny permitted the orientation of multiple determinants of PAA resistance and aphidicolin hypersensitivity with respect to other markers in the polymerase gene and in other genes. The nature of residues determined at any one of a constellation of separate sites within the polymerase locus can determine resistance or sensitivity to antiviral drugs and aphidicolin hypersensitivity associated with changes at the polymerase locus facilitates high resolution genetic analysis of this locus.