Cell-based assays that use electrical impedance-sensing technology may provide a useful tool for measuring the cytopathic effects of viral infection, and may thus be a viable option for high-throughput screening of compounds that inhibit this process, according to a recent research paper by scientists at the University of Louisville.
The research paper adds to the growing number of favorable reviews of cell-based assay platforms based on electrical impedance technology, which may be good news for Applied Biophysics — whose instrument the Louisville researchers used — as well as for companies marketing similar products, such as Acea Biosciences and MDS Sciex.
Eugenia Wang, a professor of biochemistry and molecular biology at Louisville, and graduate student Morgan McCoy, who is currently the virology laboratory supervisor in the Livestock Disease Diagnostic Center at the University of Kentucky, co-authored the research paper, which will appear in the December 2005 print edition of the Journal of Virological Methods.
In the paper, Wang and McCoy describe how they used Applied Biophysics' electric cell-substrate impedance sensing (ECIS) platform to quantify in real time the cytopathic effect of influenza A virus-infected canine cells. Applied Biophysics has been selling a relatively low-throughput, 8-well version of its instrument since 1991, but recently unveiled a 96-well version (see CBA News, 8/15/2005). The Louisville researchers used the lower-throughput version in their experiment.
"When you're measuring cytopathic effect and using that as your basis for showing that one drug is inhibiting virus replication or progression, with the ECIS system you can measure the cytopathic effect in real time, from start to finish."
Charles Keese, co-founder and president of Troy, NY-based Applied Biophysics, said that the company has primarily been looking at using the ECIS platform for wound-healing assays, but that measuring the cytopathic effect of viral infection is an intriguing area.
"We've always known that this would be useful for cytopathic effects of viral infection, but we didn't have the time to pursue that," he said. "So we were delighted when Louisville picked up the instrument and that was one of their interests in using it.
"Louisville may have gone the route of patenting this application, and we hope that they have, because that's something we could back and license from them," Keese added.
Calls to Louisville's Wang were not returned in time for this publication. Applied Biophysics has a US patent protecting the basic technology, and a patent pending for wound healing applications.
Keese also said that he knows Wang's lab is applying for funding to further pursue this application, and that Applied Biophysics would likely partner with the lab in this endeavor.
Balancing Srength with Weakness
In short, electric cell-substrate impedance works by measuring minute perturbations in plated cells using a non-invasive electrical current that is applied to the cell medium via thin electrodes in the bottom of the well plate. The instrument is suitable for many simple cell-based assays such as those that measure cell health, apoptosis, or migration.
"I think that what this technology has in leaps and bounds over a lot of other technologies is that when you're measuring cytopathic effect and using that as your basis for showing that one drug is inhibiting virus replication or progression, with the ECIS system you can measure the cytopathic effect in real time, from start to finish," McCoy told CBA News.
McCoy said that other methods exist for determining cytopathic effect, but most — such as MTT cell-proliferation assays or time-lapse microscopy — are time-consuming, cumbersome, or expensive.
"When doing an MTT assay you must do an exhaustive number of different time-point replicates because you are taking replicates out of the incubator every half-hour or hour and destroying the cells in order to run them through the assay protocol," McCoy said. "Since with ECIS you are monitoring the exact same cells in real time, you don't have to perform as many replicates."
McCoy added that time-lapse microscopy might also be a viable option, but that this method "is extremely expensive" and "still leaves a lot to interpretation. ECIS, however, provides numbers so you can easily quantify your data," he said.
McCoy's comments about the expensive nature of time-lapse microscopy also underscore another advantage of using electrical impedance sensing: It is label-free. Many researchers feel that such assays perturb the cells less than microscopy — which involves labeling cellular structures with optical reporters — thus resulting in a more realistic assay outcome.
Of course, the strength of the label-free approach is also a weakness: researchers can only see whether cytopathic effect is occurring or being inhibited by candidate compounds. Additional studies would be necessary to understand the mechanism involved or even to verify that the compound was actually negating the infection process.
Other companies, such as Acea Biosceiences and MDS Sciex, have been attempting to market a cell-based assay technology based on electrical impedance sensing.
Acea's product, called RT-CES (Real-Time Cell Electronic Sensing), was introduced in mid-2004, and is based on similar concepts as the Applied Biophysics platform (see CBA News, 1/18/2005). Last week, Xiaobo Wang, Acea's chief technology officer and vice president of R&D, told CBA News that Acea has investigated the use of RT-CES for quantifying cytopathic effects of viral infections and verified that it works. Wang declined to comment on his company's level of interest in pursuing such an application.
"We have been looking at this, we certainly have some customers looking at it, and we have been doing some internal experiments, as well," Wang said.
MDS Sciex more recently joined the fray with its CellKey system, which is based on cellular dielectric spectroscopy, and which the company introduced in August (see CBA News, 8/15/2005). MDS Sciex has since publicized several scientific collaborations, with drug makers such as Roche, Amgen, Johnson & Johnson, and AstraZeneca, in poster presentations and scientific talks at various meetings.
Calls to MDS Sciex were not returned in time for this publication, but Applied Biophysics' Keese said that he believes the company is mainly pursuing higher-throughput applications such as GPCR screening with its platform. MDS Sciex said as much in August when product manager Anne Ferguson told CBA News that the company would "initially market CellKey to the secondary screening market based on its belief that the instrument can detect the activation of specific GPCRs and tyrosine kinase receptors."
— Ben Butkus ([email protected])