Scientists at Weill Cornell Medical College say they have devised a new drug screen that capitalizes on the tendency of toxic compounds to alter the properties of the lipid bilayer that encases cells. The novel screen, called the Gramicidin-Based Fluorescence Assay (GBFA), repurposes an assay previously developed by principal investigator Olaf Andersen, M.D., and a former graduate student Helgi Ingólfsson. It tracks changes in the activity of a small protein (the gramicidin channel) coupled to a fluorescence signal as a way of monitoring changes in lipid bilayer properties, a correlate for toxicity.
The researchers presented a paper (“Predicting Ddrug Toxicity: Early Detection of Likely Failures in Drug Development”) on their screening method yesterday at the Biophysical Society Conference in Baltimore.
Changes to the properties of the lipid bilayer component of the cell membrane can alter the function of proteins embedded in the membrane: proteins that regulate critical functions such as transport of materials in and out of the cell and communication with other cells.
“As we gathered data, we began to notice a trend: molecules that significantly affected lipid bilayer properties were often indiscriminate modifiers of membrane protein function and thus tended to have an array of off-target effects,” said researcher Lea Sanford. That is, when compounds intended to influence a specific protein target also alter lipid bilayer properties, they may alter the function of numerous membrane proteins and thereby cause a cascade of usually unwanted off-target and side effects.
Sanford and her colleagues wanted to see whether changes in lipid bilayer properties were reliably correlated with a drug's toxicity to cells, or cytotoxicity. To test the hypothesis, they did a blinded screen of 134 compounds provided by The Rockefeller University's High-Throughput Screening Center. Indeed, the toxic compounds in the database, which had been characterized using high-content cytotoxicity screening, showed greater rates of fluorescence quenching.
The greater a molecule's bilayer-modifying effect (the faster the fluorescent signal decreased during the assay), the higher the likelihood it would be toxic. It appears that, as the team expected, the test could be used to identify probable cytotoxic drugs.
“This is our first study on toxicity and our results imply that the GBFA is a viable, cheap and straightforward option for predicting which compounds are likely to have off-target effects and potentially be cytotoxic, thus having the potential to indicate likely drug failures at an early stage in the development process,”pointed out Sanford. “We need to expand the library of compounds tested to more thoroughly vet the assay's use in this way.”
The researchers also emphasized that changes in bilayer properties are not inherently negative. In some cases, the off-target effects of a drug could make it useful for treating conditions other than the one for which it was originally developed.
“These results further support a mechanism by which amphiphiles [biologically active molecules that partition into lipid bilayers] exert their toxicity, namely by altering lipid bilayer physical properties and that such an in vitro measurement could be used as a warning sign for off-target biological effects in drug discovery efforts,” explained the scientists during their presentation at the conference.