10 June 2011. Feeling sleepy? The amino acid behind the dreaded drowsiness that sets in after a big meal makes headlines in today’s Cell, where researchers report that tweaking a tryptophan degradation pathway with an oral compound relieved behavioral deficits and synaptic loss in Alzheimer’s and Huntington’s mouse models. Three additional studies made for a hodgepodge of recent news on therapeutic approaches being pursued across the field. A paper posted online May 25 in the Journal of Alzheimer’s Disease describes a cysteine protease inhibitor that lowers brain Aβ and relieves cognitive deficits in AD mice. In the May 6 Journal of Biological Chemistry, a study suggests that a histone deacetylase inhibitor approved for cancer treatment may help in certain cases of familial frontotemporal dementia. And, some food for thought for researchers preferring approaches au natural: A company developing an oral zinc-based therapy announced that it plans to conduct future trials with this compound in older AD patients while selling it at the same time.
For the Cell paper, researchers led by senior author Paul Muchowski, Gladstone Institute of Neurological Disease, San Francisco, and Robert Schwarcz, University of Maryland School of Medicine, Baltimore, fed AD and HD transgenic mice a compound that manipulates the kynurenine pathway for tryptophan breakdown. By inhibiting the enzyme kynurenine 3-monooxygenase (KMO), the compound (JM6) diverts the pathway toward a side reaction that produces a neuroprotective metabolite—kynurenate. AD and HD patients have reduced brain levels of kynurenate (Gulaj et al., 2010; Beal et al., 1992), and accumulate two neurotoxic kynurenine metabolites in their blood and brains (Guidetti et al., 2004; Heyes et al., 1992). The researchers decided to synthesize JM6, a “slow-release” version of a KMO inhibitor widely used in animal studies, because Muchowski’s group had shown that KMO-deficient yeast could resist the toxic effects of polyglutamine-expanded huntingtin protein (ARF related news story on Giorgini et al., 2005).
First author Daniel Zwilling and colleagues tested JM6 in pre-symptomatic AD mice (J20 strain) and found that the KMO inhibitor prevented spatial memory and anxiety deficits, and preserved synapses in the cortex and hippocampus. When given to R6/2 HD mice starting at early stages of disease, the compound helped the animals live longer, and prevented synaptic loss and microglial activation.
The authors hypothesize that when they give mice JM6, the KMO substrate kynurenine accumulates in the blood as peripheral monocytes break down tryptophan. Then, kynurenine gets pumped into the brain where astrocytes convert it to kynurenate. “The take-home message is that regulating the activity of KMO in the blood is sufficient to regulate brain levels of kynurenate,” Muchowski said in an interview with ARF. Underscoring the importance of the kynurenine pathway in neurodegeneration, a team led by Muchowski’s former postdoc Flaviano Giorgini showed that manipulating the pathway in various ways to increase kynurenate levels protected against disease in an HD fly model. Giorgini is now at the University of Leicester in the U.K., and reported the findings in a Current Biology paper published online June 2.
The possibility of alleviating both AD and HD with a single strategy is appealing, suggested Peter Reinhart, Proteostasis Therapeutics, Inc., Cambridge, Massachusetts, and Jeffrey Kelly, Scripps Research Institute, La Jolla, California, in a commentary accompanying the Cell paper. Reinhart and Kelly note, however, that future studies would need to address whether the approach works in animals with disease already well underway. Meanwhile, Muchowski said his lab is in the process of initiating safety and toxicology studies on their KMO inhibitor.
Another oral inhibitor—this one targeting cysteine proteases—looked promising in animal studies reported in the JAD paper. Greg Hook of American Life Science Pharmaceuticals, a small biotech company in San Diego, California, and colleagues report that the compound E64d reduced brain Aβ and memory deficits when fed to plaque-free, young AD transgenic mice or older animals with strong amyloid deposition. E64d also lowered brain Aβ levels in guinea pigs, whose amyloid precursor protein (APP) has the same β-secretase cleavage site as wild-type human APP.
Previously, the lab of Greg Hook’s wife and collaborator Vivian Hook at UCSD had shown that inhibitors of cathepsin B (CatB), a lysosomal cysteine protease, reduced brain amyloid in mice expressing the London APP mutant, which has the wild-type β-secretase site. The inhibitors did not work in mice expressing APP with the Swedish mutation, which is at the β-secretase site (Hook et al., 2008). (There is controversy in the field about cathepsin B being a bona fide β-secretase; see below). Those compounds were injected into the mouse brain. The current study tested an oral inhibitor—an approach more amenable to human studies.
“We got a big effect feeding E64d in chow to severely demented mice,” Hook told ARF. Furthermore, the compound seems safe, he said. It was developed in Japan in the 1980s to treat muscular dystrophy (Satoyoshi, 1992), failing Phase 3 because it lacked efficacy for that disease, the authors wrote. “We know its pharmacokinetics and its oral bioavailability,” Hook said. “It has overcome two of the biggest hurdles—druggability and toxic side effects—and is efficacious in AD animal models.”
The data are “intriguing,” noted Li Gan, at the Gladstone Institute for Neurological Disease. However, the mechanism by which E64d lowers Aβ is unclear because the compound inhibits many cysteine proteases besides cathepsin B, including the ubiquitously expressed calpain. To complicate matters, Gan and colleagues showed that cathepsin B-deficient AD mice had more plaques than AD mice with wild-type CatB (ARF related news story on Mueller-Steiner et al., 2006). This is the opposite of what one would expect if cathepsin B inhibition lowers Aβ, as suggested in the current study. Hook’s findings also run counter to recent work by Ralph Nixon’s group at the Nathan Kline Institute for Psychiatric Research, Orangeburg, New York. Nixon and colleagues reported reduced Aβ and plaques in AD mice with enhanced cathepsin activity owing to deletion of an endogenous inhibitor, cystatin B (Yang et al., 2011). In another recent paper by the Nixon lab (Lee et al., 2011), inhibitors of cathepsins, including E64, disrupted transport and caused dystrophic swelling in axons, urging caution for the E64d inhibition approach, Gan noted (see full comment below).
In the Journal of Biological Chemistry paper, researchers at the University of Texas Southwestern Medical Center, Dallas, describe a potential therapeutic strategy for frontotemporal dementia that, like the E64d approach, uses a compound with a prior safety record. Led by senior author Joachim Herz and lead author Basar Cenik, the team showed that the cancer drug suberoylanilide hydroxamic acid (SAHA), a histone deacetylase (HDAC) inhibitor, restored normal progranulin expression in cells from patients with frontotemporal dementia (FTD) caused by progranulin haploinsufficiency. In light of recent evidence that progranulin may tone down inflammation by blocking tumor necrosis factor (Tang et al., 2011), compounds that upregulate progranulin expression would be expected to be neuroprotective “not only in FTD but in all neurodegenerative syndromes in which neuroinflammation is a significant factor,” Herz suggested in an e-mail to ARF. Several companies are working on developing more brain-penetrant and more specific HDAC inhibitors than SAHA.
Finally, some approaches are trying to harness the power of basic nutrients, like zinc. People with Alzheimer’s or Parkinson’s have low blood levels of zinc (Brewer et al., 2010), and mice deficient in synaptic zinc develop an AD-like syndrome (ARF related news story on Adlard et al., 2010). Earlier this spring at the American Academy of Neurology (AAN) annual meeting in Honolulu, Hawaii, Adeona Pharmaceuticals of Ann Arbor, Michigan, presented Phase 2 trial data on its zinc cysteine tablet (reaZin), given daily to 57 AD and mild cognitive impairment (MCI) patients for six months. The treatment was safe and well tolerated, and achieved the trial’s primary outcomes of reducing serum levels of copper and increasing serum zinc. Six-month changes on three cognitive batteries (ADAS-Cog, CDR-SOB, MMSE) were reported to “favor the treatment group,” the poster authors wrote. Based on post-hoc analysis of age-related subgroups, Adeona recently announced plans to evaluate reaZin in a larger 12-month study of AD patients ages 70 and up. According to its press release, the company intends to develop the zinc tablet as a drug and, in parallel, market it as a prescription medical food.
“The results are interesting, but since the numbers are small and the trial only lasted six months, it is difficult to interpret. It is good that they are planning a bigger trial, and hopefully they will include biomarkers related to Aβ,” commented Colin Masters of the University of Melbourne, Australia.
Paul Aisen noted that, while the study addressed the product’s safety and bioavailability, “there is no meaningful information on efficacy.” Furthermore, the concept of medical foods is problematic, “particular for the AD field,” wrote Aisen, who directs the Alzheimer’s Disease Cooperative Study headquartered at the University of California, San Diego. “A product can be marketed as a medical food, even using phrases such as “FDA approved,” in a way that suggests there is evidence of efficacy, when in fact there is no such evidence.” (See also ARF series on medical foods.) John Breitner of McGill University, Montreal, Canada, agrees that the cognitive claims are not statistically founded. “I cannot agree with the authors' conclusion that these results provide a "strong trend toward cognitive benefit favoring the treatment group," he wrote in an e-mail to ARF (see full comment below).—Esther Landhuis.
Zwilling D, Huang SY, Sathyasaikumar KV, Notarangelo FM, Guidetti P, Wu HQ, Lee J, Truong J, Andrews-Zwilling Y, Hsieh EW, Louie JY, Wu T, Scearce-Levie K, Patrick C, Adame A, Giorgini F, Moussaoui S, Laue G, Rassoulpour A, Flik G, Huang Y, Muchowski JM, Masliah E, Schwarcz R, Muchowski PJ. Kynurenine 3-Monooxygenase Inhibition in Blood Ameliorates Neurodegeneration. Cell. 2011 Jun 1. Abstract
Campesan S, Green EW, Breda C, Sathyasaikumar KV, Muchowski PJ, Schwarcz R, Kyriacou CP, Giorgini F. The Kynurenine Pathway Modulates Neurodegeneration in a Drosophila Model of Huntington’s Disease. Curr Biol. 2011 Jun 2. Abstract
Reinhart PH, Kelly JW. Treating the Periphery to Ameliorate Neurodegenerative Diseases. Cell. 2011 Jun 10;145:1-2. Abstract
Hook G, Hook V, Kindy M. The Cysteine Protease Inhibitor, E64d, Reduces Brain Amyloid-β and Improves Memory Deficits in Alzheimer's Disease Animal Models by Inhibiting Cathepsin B, but not BACE1, β-Secretase Activity. J Alzheimers Dis. 2011 May 25. Abstract
Cenik B, Sephton CF, Dewey CM, Xian X, Wei S, Yu K, Niu W, Coppola G, Coughlin SE, Lee SE, Dries DR, Almeida S, Geschwind DH, Gao FB, Miller BL, Farese RV, Posner BA, Yu G, Herz J. Suberoylanilide Hydroxamic Acid (Vorinostat) Up-regulates Progranulin Transcription: Rational Therapeutic Approach to Frontotemporal Dementia. J Biol Chem. 2011 May 6;286(18):16101-8. Abstract