An agent that keeps young neurons alive appears to do the same for aging neurons that are taking a beating in mouse models of both Parkinson’s disease (PD) and amyotrophic lateral sclerosis (ALS). Two companion studies report as much in the October 1 Proceedings of the National Academy of Sciences USA online. Researchers developing the compound are looking for collaborations to test it in mouse models of other neurodegenerative conditions, including Alzheimer’s disease and traumatic brain injury. Meanwhile, chemists are tweaking the molecule in the hopes of producing an ideal drug candidate for human trials.
"These two papers offer hope that we can develop a neuroprotective drug to treat human neurodegenerative disease," said Andrew Pieper, senior author on both papers together with Steven McKnight and Joseph Ready at the University of Texas Southwestern Medical Center in Dallas. The drug they tested came out of a previous in-vivo screen. McKnight and Pieper, who recently moved to the University of Iowa Carver College of Medicine in Iowa City, tested 1,000 compounds as they searched for chemicals that would support survival of maturing hippocampal neurons in mice. Pieper named his top candidate P7C3 because it was the third compound in the seventh pool of 10 he analyzed (see ARF related news story on Pieper et al., 2010).
P7C3 is a carbazole, consisting of three carbon rings. Chemists in Ready’s lab have since swapped out molecular elements to create nearly 300 variants of P7C3 (MacMillan et al., 2011). A20, the twentieth such analogue, shows the most promise, rescuing more young hippocampal neurons than P7C3. In the current two studies, the researchers saw that A20 also saves struggling neurons in diseases of aging.
In the ALS study, first author Rachel Tesla gave A20 to mice expressing the mutant ALS gene superoxide dismutase 1 (SOD1). Staining spinal cord sections from treated and control mSOD1 mice, she found that the former possessed up to twice as many spinal motor neurons as the latter. She also found that when given A20, mice walked more normally and hung on to a rotating rod longer than did untreated animals. However, the compound did not lengthen the life of the mice. Similar improvements in symptoms, but not survival, have been reported for other studies with these severely ill mice, McKnight noted (see ARF related news story on Da Cruz et al., 2012; Rouaux et al., 2007).
First author Héctor De Jesús-Cortés led the PD study. He used chemical toxins to poison dopaminergic neurons in mice and the worm C. elegans. Analysis of mouse brain sections showed that A20 saved up to 60 percent of dopaminergic neurons in the substantia nigra that were killed by the toxin, MPTP, in control mice. In the nematodes, A20 saved 80 percent of the neurons normally poisoned by the toxin. A20-treated worms also moved more than their untreated counterparts. Because the drug worked similarly in mice and worms, McKnight hopes it acts on a conserved biochemical pathway that people might possess as well. However, the research is a long way from showing if that is indeed true, as treatments that work in animals have been notoriously difficult to translate to humans.
The results are exciting, said Jay Baraban of Johns Hopkins University in Baltimore, Maryland, who was not involved in the research. The big question is, What is the mechanism by which P7C3 and A20 save neurons? The team so far has not managed to recapitulate neuroprotection in vitro, Ready said, making it difficult to dissect the drugs’ targets.
P7C3’s chemical structure bears a passing resemblance to Dimebon, an antihistamine approved for use in Russia since 1983, and which has been purported to have neuroprotective effects. Dimebon reportedly looked promising in early clinical trials for Alzheimer’s, conducted in Russia (see ARF related news story), but subsequently failed to reach its endpoint in international Phase 3 trials for AD (see ARF related news story) and Huntington’s disease (see ARF related news story). De Jesús-Cortés and Tesla tested Dimebon side by side with their compounds in the PD and ALS models. Dimebon had no effect. "We gave it every chance and it does not have a lick of activity," McKnight said. Recently, however, researchers in Samuel Gandy’s lab at Mount Sinai Medical Center, New York, reported that Dimebon was protective in models of synucleinopathy (Steele et al., 2012) and AD (see Steele et al., 2012), leading the authors to suggest that the chemical could serve as a scaffold for building more potent neuroprotective compounds. Based on the experiments the Texas researchers conducted, Ready said he believes the carbazoles are better candidates.
In addition, Ready thinks he can improve his molecules further. He would like to improve their bioavailability and reduce potential toxicity. Once satisfied with the chemical makeup, the team is interested in finding a partner for clinical studies.
In the meantime, they are seeking collaborators for more preclinical work. "It is really important for other people, not associated with us, to see if [A20] works," McKnight said. Scientists in Miami are testing the drug in a mouse model of traumatic brain injury, and the Texas researchers are interested in sending it to Alzheimer’s researchers, too.
Tesla R, Wolf HP, Xu P, Drawbridge J, Estill SJ, Huntington P, McDaniel L, Knobbe W, Burket A, Tran S, Starwalt R, Morlock L, Naidoo J, Williams NS, Ready JM, McKnight SL, Pieper AA. Neuroprotective efficacy of aminopropyl carbazoles in a mouse model of amyotrophic lateral sclerosis. Proc Natl Acad Sci U S A. 2012 Oct 1. Abstract
De Jesús-Cortés H, Xu P, Drawbridge J, Estill SJ, Huntington P, Tran S, Britt J, Tesla R, Morlock L, Naidoo J, Melito LM, Wang G, Williams NS, Ready JM, McKnight SL, Pieper AA. Neuroprotective efficacy of aminopropyl carbazoles in a mouse model of Parkinson disease. Proc Natl Acad Sci U S A. 2012 Oct 1. Abstract
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