A compound that partially activates α7 nicotinic acetylcholine receptors (nAChRs) has shown promise at surprisingly low doses in early Alzheimer’s disease clinical trials, and scientists are now starting to understand why. Researchers at EnVivo Pharmaceuticals, Watertown, Massachusetts, report that their α7nAChR agonist, EVP-6124, acts by sensitizing the α7nACh receptor to its natural ligand, acetylcholine. “Low levels of EVP allow the receptors to respond more potently,” Gerhard Koenig of EnVivo told ARF before presenting the data at the 4th World Congress on Controversies in Neurology (CONy), held 28-31 October 2010 in Barcelona, Spain. His colleague, Dana Hilt, reported the same findings today at the Clinical Trials on Alzheimer’s Disease (CTAD) conference in Toulouse, France. If EVP-6124’s pro-cognitive effects hold up in ongoing Phase 2 trials, the proposed mechanism suggests the compound could be given with existing cholinergic drugs as a combination therapy that allows AD patients to benefit from low doses of each, reducing side effects.
One of 17 nAChR subunits identified to date, the α7 receptor subunit assembles into homopentameric α7 nACh receptors. These ligand-gated ion channels occur primarily in learning and memory hubs, including the hippocampus, cortex, and amygdala. Early work at EnVivo established that EVP-6124 binds selectively to α7 receptors, and Phase 1 clinical trials demonstrated the compound has good bioavailability and pharmacokinetics. Subsequently, as described on a poster at the 2009 International Conference on Alzheimer’s Disease in Vienna, the compound looked promising in a small Phase 2 trial of mild to moderate AD patients who were taking cholinesterase inhibitors. In that 28-day study, people taking EVP-6124 showed improvement in certain tasks within the CogState and NTB cognitive batteries, which some believe to be more sensitive than the ADAS-Cog for mild AD (see ARF related conference story).
For a closer look at the compound’s mechanism of action, the researchers expressed human α7 nAChR in frog oocytes. First, the scientists exposed those oocytes to increasing amounts of EVP-6124 (ranging from 3 to 1,000 nanomolar) and measured evoked electrical currents. They determined that 10-30 nanomolar was needed, at minimum, for channel opening. In this system, EVP-6124 maximally activates the receptors about half as strongly as acetylcholine, indicating that the compound functions as a partial agonist.
The surprise came in subsequent studies that measured electrical currents evoked by acetylcholine in the presence of increasing amounts of EVP-6124. To establish the baseline response to natural ligand, the scientists spiked the oocytes with 40 micromolar acetylcholine. The cells were then exposed to 40 micromolar acetylcholine and, in parallel, low concentrations of EVP-6124.
At just 0.3 nM EVP-6124—more than 30-fold lower than the concentration required for channel opening but at levels patients were exposed to during the earlier Phase 2 trial—the response to low levels of acetylcholine doubled, Koenig told the CONy audience. When applied to the oocytes at 3-10 nM, the agonist still potentiates the response to acetylcholine, but these higher concentrations cause receptor desensitization, whereas the 0.3 and 1 nM treatments did not.
Koenig and colleagues extended these in vitro findings by testing whether EVP-6124 could reduce cognitive deficits in rats with memory loss induced by the anticholinergic drug scopolamine. The studies used an object recognition test, where an animal exposed to a familiar and unfamiliar object spends far more time sniffing the novel object. However, if the rats forget the familiar object, they give equal attention to both. In the EnVivo studies, rats that got 0.03 mg/kg EVP-6124 along with scopolamine showed no signs of memory preservation, exploring novel and “familiar” objects equally well, but received maximum benefit at 0.3 mg/kg. “At that concentration, the compound has completely reversed the memory deficit induced by scopolamine,” Koenig said. “If that were a human trial, it would be a complete success.”
In another set of experiments using the same object recognition task, scopolamine-treated rats were given both EVP-6124 and low doses of donepezil, an acetylcholinesterase inhibitor commonly prescribed to AD patients. This time, the α7 compound rescued memory loss at just 0.03 mg/kg—the same concentration that gave a mere hint of benefit in the earlier experiments without donepezil. “We take a sub-threshold dose of donepezil, and a sub-threshold dose of EVP, and they work synergistically,” Koenig said. Moreover, EVP-6124 had cognitive benefits in the small Phase 2 trial at steady-state brain concentrations in the same range as those found to potentiate the α7 response to acetylcholine.
What might be going on at the cellular level? Normally, two molecules of acetylcholine are needed to open the ion channel, and EVP-6124 binds α7 at the same site as acetylcholine. “It probably puts the receptor in a sensitized state,” Koenig said. “EVP-6124 effectively allows the channel to open at lower acetylcholine concentrations.”
Keith Wesnes, an expert in cognition batteries at United BioSource Corporation, Goring, U.K., chaired the CONy session in which Koenig initially reported the data. “It was an encouraging presentation,” Wesnes wrote in an e-mail to ARF. “The compound has been well characterized, the promising preclinical findings have been confirmed in early volunteer studies, and some [pro-cognitive] effects have been seen in patients with schizophrenia (see ARF related conference story).” Wesnes was intrigued by the compound’s potential synergistic action with donepezil in animals treated with sub-threshold doses of each. The compound is entering a larger Phase 2b study, “which will be the big test, of course,” Wesnes noted (see full comment below).
The Phase 2b trial is a multinational, six-month trial testing 0.3, 1, and 2 mg/day doses of EVP-6124 in mild to moderate AD patients who are unmedicated, or taking donepezil or rivastigmine. EnVivo hopes to recruit about 300 to 400 people for the study. Enrollment began earlier this fall, and should be completed by late 2011, Koenig said.—Esther Landhuis.