New Findings Raise Questions about Process Used to Identify Experimental Drug for Rare Genetic Diseases

A study by National Institutes of Health (NIH) researchers has revealed surprising new insights into the process used to initially identify an experimental drug now being tested in people with cystic fibrosis and muscular dystrophy. Researchers emphasized the clinical implications of their findings are unclear, but said the results suggest more work may be needed to make sure the screening process to select promising agents was not flawed by its effects on a firefly enzyme used as a marker. The study was published in the Proceedings of the National Academy of Sciences (PNAS). Over the past several years, an experimental drug called PTC124 has generated excitement among those seeking treatments for inherited diseases caused by a type of genetic alteration that leads to production of abnormally short proteins. Scientists refer to such alterations as nonsense mutations. About 10 percent of cystic fibrosis cases and about 15 percent of Duchenne muscular dystrophy cases are thought to arise from nonsense mutations. In addition, nonsense mutations may affect a substantial portion of the approximately 25 million Americans suffering from other rare, genetic disorders. The enzyme that makes fireflies glow, called luciferase, is widely used in biomedical experiments and in high-throughput screening often utilized to discover drugs. In a positive reaction, the tested material literally lights up. PTC Therapeutics of South Plainfield, N.J., identified PTC124 through a cell-based screening system that used firefly luciferase to gauge the power of chemical compounds to enable cells with nonsense mutations to produce normal, full-length proteins. In the PTC screen, a particular group of test compounds elicited very bright signals from the firefly enzyme. Company researchers interpreted the bright signals to mean that such compounds were highly active - that they had enabled the cellular machinery to efficiently read through nonsense mutations and increase the production of full-length, functional protein. One of those compounds was then optimized, also using firefly luciferase tests, to develop the experimental drug PTC124. The news about PTC124 was particularly encouraging because previous research had suggested that restoring the ability to produce even low levels of full-length protein might ease symptoms of cystic fibrosis and possibly other diseases caused by nonsense mutations. The drug used in that previous work, however, an antibiotic called gentamicin, is difficult to administer and can lead to hearing loss and life-threatening kidney damage. Additional testing in animals and healthy human volunteers showed that PTC124 seemed safe. Moreover, PTC124 was shown to increase the amount and function of critical proteins in animal and cell-culture models that do not rely on firefly luciferase as a marker, namely, the cystic fibrosis transmembrane conductance regulator protein in models of cystic fibrosis and the dystrophin protein in models of Duchenne muscular dystrophy.