Fallopius of Padua, the 16th-century anatomist and physician, famously complained, “When soporifics are weak, they are useless, and when strong, they kill.” Western medicine has come a long way in the centuries since, but anesthesiologists are only beginning to understand how the drugs of their trade work at the most fundamental level.
“We don’t understand how general anesthetics work in any detail,” said Roderic Eckenhoff, MD, the Austin Lamont Professor of Anesthesiology and Critical Care at the Perelman School of Medicine at the University of Pennsylvania, in Philadelphia. “We don’t know the molecular targets that they need to engage to create their effects.”
To find out, Dr. Eckenhoff is leading a multi-institution team, along with researchers at Thomas Jefferson University, Temple University, Drexel University, Rutgers University, the University of Pittsburgh and Penn, in five connected projects. Each has the dual aim of identifying the precise binding sites where anesthetics interact with proteins in the neuronal membrane, and characterizing those interactions in detail. In addition to anesthesiologists, the group includes electrophysiologists, biophysicists, computational physicists, and structural and molecular biologists, who approach the question of how anesthetics work from different angles.
“If we can use the parable of the blind man and the elephant, we’re sort of each seeing a different bit of this problem,” Dr. Eckenhoff toldAnesthesiology News. “But we’re then able to assemble it back to what’s really happening—what the mechanisms really are.”
Inhaled anesthetics produce a variety of effects: analgesia, immobility and amnesia, along with hypnosis and the alteration of blood pressure. Researchers know that the drugs work by regulating the activity of particular proteins in the neuronal membrane, but not which proteins are involved or how specific drugs interact with them (Figure). The likeliest candidates are voltage-gated ion channels, which control the flow of sodium and potassium ions throughout the neuronal membrane, said Manuel Covarrubias, MD, PhD, an electrophysiologist and professor at Thomas Jefferson University’s Farber Institute for Neurosciences, in Philadelphia.
“The idea is to identify within these particular types of ion channels, which have been implicated in general anesthesia, to identify binding sites that will allow us to find the best fit for the drug, the way a hand fits in a glove,” Dr. Covarrubias said. “To do that, we induce mutations in these ion channels in specific regions that we guess are the binding sites.
“Within those regions, we have identified specific residues that are possible binding sites for anesthetics,” Dr. Covarrubias added. That information is crucial to creating anesthetics that target the binding sites specifically, therefore reducing or eliminating the toxicity of present drugs, such as desflurane and sevoflurane.
In addition to identifying the binding sites, the researchers also are using nuclear magnetic resonance spectroscopy to study the interaction between the anesthetics and the binding sites in real time. To complement and enhance that work, they also are creating computer simulations to model the activity of these systems over time spans as long as a microsecond—“an eon” in terms of brain activity, Dr. Eckenhoff said. Finally, they are using shallow-angle x-ray scattering and neutron spectroscopy to confirm the electrophysiologic observations and the test hypotheses derived from the computer simulations.
The project, Interaction of Inhaled Anesthetics with Macromolecules, is the continuation of a collaboration that began 15 years ago. The researchers recently received $8.6 million in renewed funding for the next five years from the National Institutes of Health.
“We’re trying to get to the very basic level,” Dr. Eckenhoff said. “If you don’t know what the binding site looks like, and what atoms are needed to bind, it’s hard to do anything besides empirically alter the drug. For us to intelligently alter it—to predictably alter it—we really need to understand, at the atomic level, what’s going on.”