Anti-Alzheimer¹s Drug Mechanism Revealed With X-Rays

Using x-rays produced by the National Synchrotron Light Source at Brookhaven National Laboratory, a team of scientists has gained new insight into the effects of a newly approved drug, called rivastigmine, in the treatment of Alzheimer¹s disease ­ a debilitating brain causing memory loss and other cognitive deficits in about 10 percent of the elderly. The new results, which may provide important information for generating improved drugs for this as-yet incurable neurodegenerative disease, were published in the March 19, 2002, issue of the American Chemical Society journal Biochemistry.

³We were very surprised by our results,² says Joel Sussman, a structural biologist at the Weizmann Institute of Science in Rehovot, Israel, and the lead author of the study. ³They show that we can safely treat Alzheimer¹s disease with much lower quantities of rivastigmine, thus minimizing unwanted adverse effects.²

Though the drug is currently available under the trade name Exelon, its mechanism of action at the atomic level had not been studied until now. So, the team of scientists, composed of Sussman¹s team and scientists from Novartis, a pharmaceutical company based in Basel, Switzerland, decided to take a close look at how the drug helps to slow the memory loss of Alzheimer¹s patients.

One of the main pathological phenomena in Alzheimer¹s disease is the deterioration of nerve cells releasing acetylcholine, a chemical that helps to carry messages among brain cells. The inadequate supply of acetylcholine in Alzheimer¹s patients is compounded by the action of an enzyme called acetylcholinesterase (AChE), which breaks down acetylcholine in the body at the rapid rate of 20,000 molecules per second.

The desired effect of potential Alzheimer¹s treatments, such as rivastigmine, is to inhibit AChE long enough to offset the absence of acetylcholine. But rivastigmine and other anti-Alzheimer¹s drugs have side effects and may merely slow deterioration rather than halt it.

To look at the action of the drug over time, Israel Silman, a neurochemist at the Weizmann Institute of Science and a co-investigator on the study, together with Pazit Bar-On, a joint graduate student with Sussman and Silman, tested the drug on various types of AChE, extracted from an electric ray, the fruit fly, and human beings. ³We wanted to see how long it takes AChEs to go back to normal, or become Œreactivated,¹ after being inhibited by the drug,² Silman says.

The scientists were very surprised to notice an ³extremely low reactivation² of the AChEs from all three organisms. ³Inhibition of AChE by rivastigmine appears to be almost irreversible, with little reactivation over a period of days,² Silman says.

To explain what happens at the molecular level, the scientists took ³snapshots² of rivastigmine while it was binding to AChE, using a method called x-ray crystallography. They projected x-rays produced by Brookhaven National Laboratory¹s National Synchrotron Light Source (NSLS) on crystals of rivastigmine combined with AChE. They then determined the structure of the complex rivastigmine-AChE by looking at how the x-rays scattered off the crystal. By reconstructing the positions of these scattered x-rays, the scientists established a molecular map that revealed the locations of all the atoms of AChE and rivastigmine in three dimensions.

³When we looked at this map, things became clearer,² Sussman says. ³We had suspected that rivastigmine was binding very tightly to AChE, preventing surrounding fluid ­ mainly water ­ from breaking this bond quickly, as it usually does.²

By looking closely at the AChE ³active site² ­ the part to which rivastigmine binds to AChE ­ Sussman and his colleagues noticed that rivastigmine was broken in two, each part being ensconced comfortably in the active site (Figure 1). The scientists also precisely determined how each part was bound to the surrounding AChE atoms and moved other AChE atoms, which slowed down reactivation of AChE (Figure 2).

³The x-ray molecular maps allow us to see how every atom of rivastigmine interacts with the atoms of AChE¹s active site,² Silman says. ³This information will be important in designing new chemicals that will target specific atomic sites in AChE, possibly leading to better drugs that last longer and have less undesirable effects on Alzheimer¹s patients.²

³I am very excited by the perspectives offered by these results,² Sussman says. ³By fine-tuning the properties of anti-Alzheimer¹s drugs or their targets at the molecular level, we can truly hope to find a cure for Alzheimer¹s disease in the future.²

BEAMLINE

X12C

 

PUBLICATION

P. Bar-On, et al., "Kinetic and Structural Studies on the Interaction of Cholinesterases with the Anti-Alzheimer Drug Rivastigmine," Biochemistry, 41, 3555-3564 (2002).

SCIENCE WRITER: Patrice Pages

FOR MORE INFORMATION, CONTACT: Joel Sussman
or visit http://www.weizmann.ac.il/~joel/