Viruses and Host Cells in Drug Tests

Screens for drugs against targets that consist of several subunits, or that assemble into linked pathways, require the generation of designer cells with multiple expression cassettes. Although complicated in the generation, the advantage of the established assay is isolation of the target from overlapping biochemical pathways in the native context and the opportunity for robust and fast quantification of drug effects via coupled reporter genes.

Desired screen complexity that can be solved with designed cells is also illustrated by assays directed against interactions among cells, or between host cells and parasites, in the search for therapeutics against infectious diseases. The range and variability of examples from the area of screening for antivirals against HIV illustrate the versatility of designed cells. Cell-based drug screens help in medium-throughput screens of libraries with hundreds of compounds, in small-scale applications for detailed characterization of selected promising drug candidates, and in the evaluation of the results from clinical trials with virus isolates from patients treated with new compounds. Drug screening against HIV is discussed as an example because of the complexity of HIV pathogenesis and the large amount of knowledge already available. Lessons learned here may be applicable to screens for drugs against other complex infectious diseases or tumor processes involving specific transactivation events.

Among the very first drugs to be described against HIV was the nucleoside analogue AZT (azidothymidine or zidovudine), and this is still in use today [61]. This compound was synthesized [62] well before discovery of HIV, and its suitability against HIV was demonstrated in what can be considered the first stage towards establishment of antiviral drug screens with designed cells. For this, T cells were cloned and immortalized with HTLV-I virus (an oncogenic human retrovirus in a different genus than HIV) that was released from co-cultivated lethally irradiated natural tumor cells. An immortalized clone with high susceptibility to HIV (at that time still called HTLV-III/LAV) was selected for studies where HIV infection was allowed in the presence or absence of AZT. The effect of AZT was quantified by counting the number of viable cells and of cells positive for viral antigens after 8-10 days of culture. Such experiments are time-consuming, and provide good confirmation ofobserved effects, but they are not suitable for medium-throughput screens of large libraries. Furthermore, promising but unstable drugs may not be identified in screens that require days to develop. Finally, the virus-induced cytopathic effect and potential toxicity of a drug may interfere and overlap, further complicating the interpretation of results. All of these caveats can be addressed in screens with designed cells.

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