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Luciferase Reporter Could Skew Drug Screening, Study Suggests

NEW YORK (GenomeWeb News) – Researchers should exercise caution when using the firefly luciferase enzyme as a reporter in drug screening assays, according to a new study by researchers at the National Institutes of Health.

The paper, which is scheduled to appear online this week in the Proceedings of the National Academy of Sciences, suggests that some compounds that inhibit firefly luciferase don't dim the enzyme's glow but actually make it appear brighter. That has implications for those using luciferase as a reporter during drug development, since increased luciferase activity is often associated with enhanced target protein activity.

"One wouldn't think that compounds that block firefly luciferase may actually increase its signal in some instances. But, in fact, that is exactly what our experiments found," senior author James Inglese, deputy director of the NIH Chemical Genomics Center, said in a statement. "This is a caveat that researchers must now consider when using firefly luciferase assays to screen for potential drugs."

Researchers frequently use firefly luciferase and other enzymes as reporters during drug screening. For instance, previous studies by researchers at the New Jersey-based company PTC Therapeutics identified a compound called PTC124 that appeared to suppress nonsense codon activity in screens using firefly luciferase as a reporter.

Based on those screens and follow-up studies in animal and cell culture models, PTC Therapeutics is pursuing clinical trials aimed at testing the compound's ability to treat some forms of cystic fibrosis and Duchenne muscular dystrophy disease. Roughly ten percent of CF cases and about 13 percent of DMD cases in the US are thought to be caused by such nonsense mutations.

But when Inglese and his team tested PTC124's ability to suppress nonsense codon activity with a different luciferase reporter produced by the sea pansy, Renilla reniformis, they failed to detect any suppression. "This suggests that the initial discovery of PTC124 may have been biased by its direct effect on the [firefly luciferase] reporter, implicating firefly luciferase as a molecular target of PTC124," the researchers wrote.

The NIH team stressed the need to consider such interactions and apply "appropriate control assays' in future drug development studies. And based on the results, they urged a "careful re-examination of subsequent studies obtained with PTC124 in more complex models systems such as the mdx mouse or Cfr-/- transgenetic mouse models and human testing should be considered.

In a statement released today, NIH noted that the latest results won't necessarily affect PTC124 clinical trials.

"It is important to note that studies of PTC124 in patients and animal models of cystic fibrosis and Duchenne muscular dystrophy have produced encouraging early results, so it is unclear what impact these findings will have on the current clinical trials," acting NIH Director Raynard Kington, who was not involved in the paper, said in a statement. "Still, this new work will improve screening procedures and make our efforts to understand biology and develop therapeutics for many diseases more effective."

While he said that the new study represents "interesting science," John Babiak, senior vice president of drug discovery at PTC Therapeutics, told GenomeWeb Daily News that he does not believe the results have any implications for the clinical applicability of PTC124.

"This is a common situation in high throughput screening," Babiak said, emphasizing that PTC Therapeutics' work on PTC124 goes "far beyond" such initial screening steps. He said the secondary and tertiary assays used to follow up on initial firefly luciferase screens have provided solid evidence that PTC124 can suppress nonsense mutations and lead to the desired read through in clinically relevant animal tissues.

PTC Therapeutics announced today that it has completed patient enrollment for clinical trials of PTC124, dubbed ataluren, for treating nonsense mutations in Duchenne and Becker muscular dystrophy. The one year, multi-center, randomized, double-blind trial is taking place at 37 sites and will involve 174 patients in the US, Europe, Canada, Australia, and Israel.