An automated time-of-drug-addition assay to routinely determine the mode of action of HIV-1 inhibitors.

Cell-based high-throughput screening campaigns are widely used to identify novel antiviral compounds, for example, against human immunodeficiency virus type 1 (HIV-1). Typically, these assays enable identification of compounds that potentially target any viral or cellular factor involved in the viral replication cycle. Unraveling the mechanism of action of these active compounds is an important step to facilitate further drug development. Time-of-addition (TOA) assays are an elegant tool to achieve this goal by comparing the TOA profile of novel compounds with those of well-studied reference compounds. Downscaling to a 384-well format and automation significantly increase the capacity of the TOA assay, enabling compound handling around the clock. Mechanical liquid dispensing with optimized time points for compound addition ensures robustness (Z'>0.8) and maximal resolution in profiling novel antiviral compounds. The presented methodology has been optimized for routine use and allows for fully automated high-throughput screening to support the process in search for novel inhibitors of HIV-1.

A fluorescence-based high-throughput screening assay to identify HIV-1 inhibitors.

Highly active antiretroviral therapy (HAART) dramatically increases the long-term survival rates of human immunodeficiency virus type 1 (HIV-1) infected patients. Yet, poor adherence to therapy, adverse effects and the occurrence of resistant viruses can compromise the efficacy of HAART regiments. Therefore, there remains a clear unmet medical need for novel drugs and treatment options. In this chapter, we describe an HIV-1 antiviral high-throughput screening assay based on an HIV-1 permissive T lymphoblastoid MT4 cell line, stably transfected with a construct carrying an HIV-1 long terminal repeat promoter driving the expression of a reporter gene (enhanced green fluorescent protein). This assay runs in a 384-well format and enables the identification of HIV-1 inhibitors during a high-throughput screening campaign. In parallel, a cytotoxicity assay is performed to evaluate the compound-related in vitro toxicity.

A homogeneous time-resolved fluorescence assay to identify inhibitors of HIV-1 fusion.

The human immunodeficiency virus type 1 (HIV-1) initiates infection through sequential interactions with CD4 and chemokine coreceptors unmasking the gp41 subunit of the viral envelope protein. Consequently, the N-terminal heptad repeats of gp41 form a trimeric coiled-coil groove in which the C-terminal heptad repeats collapse, generating a stable six-helix bundle. This brings the viral and cell membrane in close proximity enabling fusion and the release of viral genome in the cytosol of the host cell. In this chapter, we describe a homogeneous time-resolved fluorescence assay to identify inhibitors of HIV-1 fusion, based on the ability of soluble peptides, derived from the N- and C-terminal domains of gp41, to form a stable six-helix bundle in vitro. Labeling of the peptides with allophycocyanin and the lanthanide europium results in a Föster resonance energy transfer (FRET) signal upon formation of the six-helix bundle. Compounds interfering with the six-helix bundle formation inhibit the HIV-1 fusion process and suppress the FRET signal.

Identification of HIV-1 reverse transcriptase inhibitors using a scintillation proximity assay.

The human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) converts the viral single-stranded RNA into double-stranded DNA. The inhibition of reverse transcription in the viral life cycle has proven its efficacy as a clinically relevant antiviral target, but the appearance of resistance mutations remains a major cause of treatment failure and stresses the continuous need for new antiviral compounds. In this chapter, we describe an HIV-1 RT scintillation proximity assay (SPA) to identify inhibitors of the RT. The assay uses an RNA/DNA (poly(rA)/oligo(dT)) template/primer bound to SPA beads, which contain scintillant. Reverse transcriptase extends the primer by incorporating [(3)H]dTTP and dTTP, which results in light production by the scintillant in the bead. Compounds that inhibit reverse transcriptase will prevent the incorporation of tritiated dTTP resulting in a reduction of emitted light compared to the untreated controls.

Design, synthesis, and antiviral evaluation of purine-ß-lactam and purine-aminopropanol hybrids.

Purine-ß-lactam chimera were prepared as a novel class of hybrid systems through N-alkylation of 6-benzylamino- or 6-benzyloxypurine with (ω-haloalkyl)-ß-lactams, followed by reductive ring opening of the ß-lactam ring by LiEt(3)BH to provide an entry into the class of purine-aminopropanol hybrids. Both new types of hybrid systems were assessed for their antiviral activity and cytotoxicity, resulting in the identification of eight purine-ß-lactam hybrids and two purine-aminopropanol hybrids as promising lead structures.

MAPPIT as a high-throughput screening assay for modulators of protein-protein interactions in HIV and HCV.

The discovery of novel antivirals for HIV and HCV has been a focus of intensive research for many years. Where the inhibition of critical viral enzymes by small molecules has proven effective for many viruses, there is considerable merit in pursuing protein-protein interactions (PPIs) as targets for therapeutic intervention. The mammalian protein-protein interaction trap (MAPPIT) is a two-hybrid system used for the study of PPIs. The bait and prey proteins are linked to deficient cytokine receptor chimeras, where the bait and prey interaction and subsequent ligand stimulation restores JAK-STAT signaling, resulting in reporter gene expression controlled by a STAT3-responsive promoter. We report the use of MAPPIT as a high-throughput screening assay for the discovery of inhibitors or stimulators of the Vif-APOBEC3G interaction and the reverse transcriptase heterodimerization (RTp66-RTp51) for HIV and the NS4A-NS3 interaction for HCV.

A time-resolved fluorescence assay to identify small-molecule inhibitors of HIV-1 fusion.

Fusion of host cell and human immunodeficiency virus type 1 (HIV-1) membranes is mediated by the 2 "heptad-repeat" regions of the viral gp41 protein. The collapse of the C-terminal heptad-repeat regions into the hydrophobic grooves of a coiled-coil formed by the corresponding homotrimeric N-terminal heptad-repeat regions generates a stable 6-helix bundle. This brings viral and cell membranes together for membrane fusion, facilitating viral entry. The authors developed an assay based on soluble peptides derived from the gp41 N-terminal heptad-repeat region (IQN36) as well as from the C-terminal region (C34). Both peptides were labeled with fluorophores, IQN36 with allophycocyanin (APC) and C34 with the lanthanide europium (Eu3+). Formation of the 6-helix bundle brings both fluorophores in close proximity needed for Förster resonance energy transfer (FRET). Compounds that interfere with binding of C34-Eu with IQN36-APC suppress the FRET signal. The assay was validated with various peptides and small molecules, and quenching issues were addressed. Evaluation of a diversified compound collection in a high-throughput screening campaign enabled identification of small molecules with different chemical scaffolds that inhibit this crucial intermediate in the HIV-1 entry process. This study's observations substantiate the expediency of time-resolved FRET-based assays to identify small-molecule inhibitors of protein-protein interactions.

Development of a homogeneous screening assay for automated detection of antiviral agents active against severe acute respiratory syndrome-associated coronavirus.

The severity and global spread of the 2003 outbreak of the severe acute respiratory syndrome-associated coronavirus (SARS-CoV) highlighted the risks to human health posed by emerging viral diseases and emphasized the need for specific therapeutic agents instead of relying on existing broadly active antiviral compounds. The development of rapid screening assays is essential for antiviral drug discovery. Thus, a screening system for anti-SARS-CoV agents was developed, which evaluated compound potency, specificity and cytotoxicity at the initial screening phase. Cell lines were engineered to constitutively express an enhanced green fluorescent protein (EGFP) and used to detect (1) antiviral potency in SARS-CoV infection tests; (2) antiviral specificity in tests using the porcine coronavirus transmissible gastroenteritis virus (TGEV); and (3) cytotoxicity in the same assays without virus challenge. The assay system involves minimal manipulation after assay set-up, facilitates automated read-out and minimizes risks associated with hazardous viruses. The suitability of this assay system in drug discovery was demonstrated by screening of 3388 small molecule compounds. The results show that these assays can be applied to high-throughput screening for identification of inhibitors selectively active against SARS-CoV.