An impressive data set that appeared yesterday in Neuron shows that a specific M1 muscarinic receptor agonist can reverse both cognitive deficits as well as the amyloid and tau pathology in the APP/PS1/tau triple transgenic mice mouse model of AD. The publication, from Frank LaFerla and colleagues from the University of California at Irvine and Abraham Fisher at the Israel Institute for Biological Research in Ness Ziona, advances a story that garnered significant attention when the investigators presented their preliminary results on the small molecule AF267B at the 2004 Society for Neurosciences meeting in San Diego (see ARF related news story).
The compound was licensed at that time to TorreyPines Therapeutics, Inc. (formerly Neurogenetics, Inc.) in San Diego, who made their own news this week by announcing that they have initiated a second Phase I trial of the compound in humans. The trial, which is expected to yield results as soon as next June, might give an early indication of whether the compound is capable of affecting Aβ or tau metabolism or cognitive function in humans.
For years, Fisher and his colleagues have synthesized and studied series of small molecules in an effort to prove his hypothesis that agonists that are highly selective for M1 muscarinic acetylcholine receptors could treat the symptoms of AD, as well as change its course (see, for example, Fisher et al., 2003 and Fisher et al., 1998). Muscarinic agonists have been tested before in vitro and in humans, but none have made it all the way to a useful AD drug, and early agonists failed in clinical trials. All along, Fisher has insisted that the failure was due to the compounds’ inadequate M1 selectivity and poor pharmacokinetics. AF267B provided a chance to experiment with a rigid acetylcholine mimic that is highly selective, orally available, and penetrates the blood-brain barrier.
At the 2004 SfN conference, Antonella Caccamo from the LaFerla lab presented a poster showing that daily intraperitoneal injections of AF267B in triple transgenic AD mice diminished the animals’ plaque pathology, intraneuronal pathology, and tau pathology in cortex and hippocampus. Their experiments with ELISA and Western blots indicated decreases in soluble and insoluble Aβ formation, associated with a decrease in C99 (the product of β-secretase cleavage) and an increase in C83 (the APP fragment released by α-secretase cleavage). Measuring steady-state levels, Caccamo and colleagues found a decrease in BACE, an increase in ADAM-17, and no change in ADAM-10 (see ARF related news story). AF267B also diminished phospho-reactive tau as detected with the antibody AT8. Finally, the compound reversed the memory retention deficit in the water maze.
The new paper, which features Caccamo as first author, presents all this data, and in addition shows that AF267B activates ERK and protein kinase C (PKC) in vivo, consistent with the known effects of M1 agonists to stimulate these enzymes in vitro. Other studies have shown that M1 receptor stimulation in vitro increases the production of αAPP via activation of ERK and PKC, and the new results suggest that AF267B activates the same pathway.
In a similar vein, the new work shows that amelioration of tau hyperphosphorylation is associated with a decrease in GSK3β kinase activity in brains of AF267B-treated mice. Throughout the work, the researchers compare the effects of AF267B with the M1 antagonist dicyclomine, and they consistently find that the antagonist has the expected, opposite effect on the mice.
In contrast to its effects in the cortex and hippocampus, AF267B does not reduce plaque load, Aβ, or tauopathy in the amygdala. That is consistent with its failure to correct deficits in amygdala-based fear conditioning in these mice. The authors suggest that this state of affairs may reflect the absence of ADAM-17 in the amygdala, compared to the cerebellum and hippocampus.
At TorreyPines Therapeutics, AF267B is being developed under the name of NGX267. A first Phase I trial, completed at the end of 2005, established the safety of single doses of the compound in healthy adult volunteers. Earlier this week, the company reported the initiation of a second Phase I trial that will measure the safety and tolerability of single and multiple doses in 65 healthy elderly subjects. In the study, the company will also assess several biomarkers (levels of Aβ1-42, total tau, and phospho-tau in cerebrospinal fluid) as well as cognitive function.
The results, expected in June 2006 will determine how development will proceed, said Neil Kurtz, the president and CEO of TorreyPines Therapeutics. "So far the drug is demonstrating good pharmaceutical properties. Based on the data from our second Phase I trial, we will decide whether to move forward with NGX267 for symptomatic treatment, or for disease modification or progression. If we get the expected changes in the cerebrospinal fluid biomarkers, it will provide a strong rationale to move forward in disease modification or progression trials,” Kurtz told ARF.—Pat McCaffrey