A study by British biotech Gentronix and GlaxoSmithKline appears to validate Gentronix’s new human cell-based genotoxicity assay and suggests that the technology could enable researchers to more accurately determine which candidate drug compounds are suitable for further development.
The paper, together with another pending publication by GSK scientists and additional undisclosed pharmaceutical beta-testers, provide evidence that the test could help clarify confusing results generated by existing genotoxicity assessment methods and may provide the assay with highly valuable pharma validation in advance of a 2007 commercial launch.
Gentronix, a 1999 spin-out of the University of Manchester Institute of Science and Technology, unveiled the assay last week at the Drug Discovery and Technology conference in Boston. Named GreenScreen HC, it is based on a similar technology called GreenScreen GC, which uses yeast cells and has been on the market since Gentronix was founded.
“If it’s a positive in our test, it is going to be a carcinogen, because we have had no false positives yet. This is a very strong prioritization tool.”
Both assays are based on the same core technology: a green fluorescent reporter gene that is expressed along with a specific gene known to be active during DNA repair and thus is a reliable indicator that DNA damage has occurred.
In the case of yeast, this gene is called RAD54. According to Gentronix, RAD54 is an evolutionarily conserved, recombinational DNA repair gene that is induced by all agents that mutate yeast.
According to Richard Walmsley, Gentronix’s founder and chief scientific officer, Gentronix has had a fair amount of success in selling the yeast-based GreenScreen GC to pharmaceutical and biotech customers as a tool for assessing the early-stage genotoxicity of candidate compounds. The most well-known customer is Johnson & Johnson, with whom Gentronix has published a research paper discussing the use of the yeast-based assay.
“But people kept saying, ‘Why don’t you do it with human cells or rodent cells?’” Walmsley told CBA News this week. “Well, they’re very different, more complex, and gene expression tends to be restorative rather than induced, among other things.”
Regardless, Walmsley, who is also a senior lecturer of life sciences at the University of Manchester, set out with graduate student Paul Hastwell to identify a similar gene in mammalian cells and develop a genotoxicity assay based on it.
“We managed to design a new trick around an old gene, I suppose,” Walmsley said. “It’s quite a well-known gene called GADD45. It is pretty much in the center of the cellular response to DNA damage and is important in processes like delaying cell division, triggering apoptosis, and increasing DNA repair.”
Gentronix subsequently developed a GFP assay for the gene using human lymphoblastoid cells and began internally testing compounds and achieving promising results as compared with existing genotoxicity assays. GSK got wind of the research and became interested in it, bringing Hastwell on board for several months to perform the tests at the heart of the new research paper. Soon afterwards, GSK hired Hastwell permanently, Walmsley said.
The results of the study will be published in the September print issue of the journal Mutation Research and are currently available online. Hastwell is first author on the paper, and Walmsley is corresponding author, with several additional GSK scientists contributing.
As detailed in the paper, the researchers used GreenScreen HC to test 75 well-characterized genotoxic and non-genotoxic compounds in a 96-well-plate format and found that the assay provided both high specificity and high sensitivity.
This is important, Walmsley said, because current genetic toxicity tests required by most regulatory authorities don’t have a combination of high specificity, high sensitivity, and a high-throughput format. These tests include the Salmonella mutagenicity assay, also known as the Ames test; mammalian cell tests such as a mammalian mutation assay; and image-based assays for micronucleus formation or chromosomal aberration.
“The Ames test is pretty reliable,” Walmsley said. “If it’s positive in Ames, it’s probably going to be a carcinogen. It’s about 65-percent certain. But the Ames test is bacteria-based and doesn’t have a lot of the features of mammalian cells — no mitosis, no chromosomes — so it’s not surprising that Ames misses things,” he said.
“Then there are these mammalian tests, and between those and the Ames test, you can get almost 100 percent of carcinogens as positives,” Walmsley added. “However, a very large number of non-carcinogens get positive results because the existing mammalian tests are sensitive but have low specificity.”
The resulting genotoxicity observed by these methods dooms many compounds to the scrap heap in early testing when in fact they may be viable candidates. Another option is animal testing, which is becoming less desirable for many reasons in pharmaceutical screening, not least of which is high cost.
This is where Gentronix believes its assay has potential – not necessarily to replace existing combinations of the Ames test and other mammalian assays, but as a way to clarify confusing results.
“In the paper, we talked about nine or so compounds that were falsely positive in in vitro tests and correctly negative in ours,” Walmsley said. “This gives you a way to sort out and prioritize candidates for screening. If it’s a positive in our test, it is going to be a carcinogen, because we have had no false positives yet. This is a very strong prioritization tool.”
According to Walmlsey, the test is also much more conducive to high-throughput studies than current genotoxicity tests.
As written in the Mutation Research paper, “the test should be of value both as a tool in the selection of candidate compounds for further development, where additional data may be required because of conflicting information from the in vitro test battery, or in product development areas where the use of animals is to be discontinued.”
In addition, the researchers wrote that as a microplate assay however, “it has the qualities of high throughput and low compound use that will facilitate its application in early screening for genotoxic liability.”
GreenScreen HC is currently in beta testing, Walmsley said. Besides GSK, which has an ongoing scientific collaboration with Gentronix to further validate the assay, two other undisclosed pharmas have signed on to become beta testers, and Gentronix expects two more to participate soon.
In addition, GSK is currently using the assay to test marketed pharmaceuticals, as opposed to known genotoxic or non-genotoxic compounds, and plans to publish a paper on these results sometime in the future.
“We started the program about a month ago,” Walmsley said. “We’re a small company, so if we get four testers prior to [the end of the year] then we’ll feel comfortable about the product we’re trying to sell.” He added that the company is aiming for a full-blown product release in early 2007.
According to the company, GreenScreen HC, like GreenScreen GC, is compatible with most bulk fluorescence readers, and “there is no reason, in principle, that the fluorescence data couldn’t be collected by imaging or FACS,” Walmsley said.
Also as with its yeast-based assay, Gentronix plans only to sell GreenScreen HC assay kits, which will include cell lines, associated media, and analysis software. However, the company is not ruling out offering some sort of contract research services based on its assays. The company currently licenses specific IP from GE Healthcare that allows it to sell the GFP-based assay worldwide
“At the moment, we can’t do service, but we are trying to renegotiate that,” Walmsley said. “But for now we intend to sell kits or have people go through other CROs.”