See also Part 1.
7 April 2011. With effective symptomatic treatments in hand, many Parkinson’s researchers have now set their sights on loftier goals. Experimental treatments aim to restore lost brain function and arrest disease progression. Researchers are exploring diverse strategies, from cell replacement (see Part 1 of this series), to gene therapy, to small molecules that could counteract pathological processes. Scientists at the 10th International Conference on Alzheimer’s and Parkinson’s Diseases, held 9-13 March 2011 in Barcelona, Spain, unveiled a number of approaches, most of them still preclinical.
Growth Factor Therapy
An alternative to replacing lost cells is providing growth factors that will protect or even restore neurons. One way to do this is to engineer stem cells to pump out growth factors. Eldad Melamed at Tel Aviv University, Israel, described how his team converted mesenchymal stem cells to astrocytes in vitro. These cells then secrete glial-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF). The researchers injected these newly generated astrocytes into two different rodent models produced by lesioning the brain with toxins. In both cases, rodents who received astrocytes walked better than those who received undifferentiated mesenchymal stem cells, Melamed said. The researchers obtained similar results in mouse models of multiple sclerosis (see Barhum et al., 2010). The approach has been approved for clinical trials with ALS patients in Israel, Melamed said, and an upcoming small Phase 1/2 trial will be conducted in collaboration with startup company Brainstorm Cell Therapeutics in Petach Tikva, Israel.
Scientists can also deliver growth factors through viral vectors. The gene therapy approach recently saw its first Phase 2 success for PD, with the enzyme glutamic acid decarboxylase (see ARF related news story). At AD/PD, Raymond Bartus at Ceregene, Inc., San Diego, California, updated the crowd on CERE-120, which uses an adenovirus to deliver the growth factor neurturin. Initial Phase 2 trials failed to show benefit (see ARF related news story on Marks et al., 2010). By looking more closely at a monkey model, Bartus said, his team discovered that the growth factor was not reaching the substantia nigra (see Bartus et al., 2011). Ceregene developed a new dosing paradigm, adding injections directly into the substantia nigra, and has now completed Phase 1 testing of this method. Toxicity results were good, and the trial is proceeding into Phase 2, Bartus said.
In a slightly different approach, researchers led by John Forsayeth at the University of California in San Francisco are interested in whether high levels of GDNF would restore brains ravaged by PD. They used adenovirus to deliver GDNF into the putamen of rhesus monkeys whose brains they had lesioned with the toxin MPTP six months earlier. The monkeys’ clinical rating improved and has stayed stable out to two years, Forsayeth said. Improved dopamine turnover and innervation of the putamen indicates a partial restoration of the dopamine system, Forsayeth said, although the treatment did not reconstruct the lesioned regions. A Phase 1 safety trial with 24 participants will start soon, Forsayeth said, adding that, to deliver the virus, the scientists will employ the same MRI-guided stereotactic system approved by the FDA for electrode implantation. This system targets the injection with remarkable accuracy, Forsayeth said, allowing genes to be delivered in a reproducible way without off-target effects or leakage into the CSF.
Blocking α-Synuclein Deposits
Scientists believe that α-synuclein pathology causes both motor and non-motor symptoms of PD. One way to directly attack the disease, therefore, would be to block or break up α-synuclein aggregates. Though not yet in clinical trials, many scientists are pursuing this strategy. I think this should receive high priority, Jeffrey Kordower at Rush University Medical Center, Chicago, Illinois, told ARF. Kordower is also one of the founders of Ceregene. Brit Mollenhauer at the Paracelsus-Elena-Klinik, Kassel, Germany, concurs, saying, This is a good focus for research. I put a lot of hope in it. Kordower pointed out, however, Since α-synuclein is all over the brain by end-stage PD, the ability to get widespread delivery of some agent will be a challenge.
Luke Esposito at ProteoTech, Inc. in Kirkland, Washington, described the screening process his company went through to find their lead anti-aggregant candidate, dubbed Synuclere, which evokes a French dessert perhaps as much as the desired allusion to clear. After identifying a plant compound with high potency against aggregated α-synuclein, the scientists screened variants in vitro on cultured cells and in one-year-old transgenic α-synuclein mice. The compound reversed α-synuclein aggregation in a dose-dependent manner, inhibited β-sheet structures, and worked at a concentration one-tenth that of the aggregated target, Esposito said. It also shows good drug-like properties and no toxicity. After six months of 50 mg/kg/day treatment with this compound, the mice had 80 percent less accumulated α-synuclein in the cortex and substantia nigra, 70 percent less α-synuclein oligomers, and were better able to walk on a beam than untreated mice were, Esposito said. A backup compound gave similar results, he added.
Armin Giese at Ludwig-Maximilians-Universitet München, Germany, discussed a preclinical candidate with broad anti-aggregation effects. His team searched for compounds that inhibit prion aggregates as well as α-synuclein. They identified a class of 3,5-diphenyl-pyrazole derivatives active at concentrations below one micromolar in cell culture. The lead compound, ANLE138B, stopped propagation of all prion strains tested in vitro, Giese said, indicating it is not strain-specific. ANLE138B also gets into the brain well, Giese said, reaching three times the concentration in brain as in blood. When prion-infected mice late in the incubation period were fed 5 mg/day of ANLE138B, the compound blocked prion deposition, prevented cell death, and prolonged survival up to 10 weeks. Mice survived even longer when the treatment was started earlier. The compound is also active against α-synuclein, preventing oligomers from forming. When transgenic mice that express human A30P α-synuclein ate ANLE138B, the animals had less α-synuclein deposition at 16 months, lived up to 10 weeks longer, and had nearly normal motor abilities, Giese said. The compound lessened dopaminergic neuron death in lesioned rodent brains. The results support the ideas that different kinds of protein aggregates have common structural features and that inhibiting aggregation can be therapeutic, Giese said.
Other Small-Molecule Approaches
Kevin Barnham at the University of Melbourne, Australia, described a different tactic for inhibiting harmful α-synuclein aggregates. Drawing on data showing that nitrated α-synuclein is more toxic than un-nitrated forms (see Yu et al., 2010), Barnham’s team looked for small-molecule inhibitors of nitration. They found that an organic molecule containing copper, Cu(II)-diacetyl-bis(N4-methylthiosemicarbazone), or Cu-ATSM, blocked both nitration and oligomerization of α-synuclein. Barnham’s team tested the molecule in four animal models of PD, including one transgenic line and three toxin models. In all four models, the copper-containing molecule prevented dopaminergic neuron death and improved dopamine metabolism, Barnham said, and the mice recovered normal motor skills and memory. Nitration may be a factor in other neurodegenerative diseases, Barnham said, noting that the drug also extends lifespan and reduces inflammation in a SOD1 ALS mouse model. Cu-ATSM accumulates in the brain, but the researchers have not yet analyzed the pharmacokinetics, Barnham said. Thus, it remains to be seen whether the molecule will make a good drug in people.
In Alzheimer’s disease, agonists of the M1 muscarinic acetylcholine receptor have been shown to reverse cognitive problems and amyloid and tau pathologies in mice (see ARF related news story). The same strategy works in PD mice, said Abraham Fisher at the Israel Institute for Biological Research in Ness-Ziona. Fisher is a co-organizer of AD/PD. In a collaboration, Eliezer Masliah at the University of California in San Diego, Manfred Windisch at JSW Life Sciences, Grambach, Austria, and Fisher discovered that the M1 selective muscarinic agonist AF102B clears α-synuclein pathology. When transgenic α-synuclein mice were given 2.5 mg/kg/day of the drug by intraperitoneal injection for three months, α-synuclein deposits and inflammation decreased and dopaminergic neurons survived. Fisher noted that the data have been reproduced by both labs in the U.S. and Austria. Additionally, Windisch saw the same results after chronic treatment with another M1 agonist, AF267B, Fisher said. The effects appear to be mediated by the M1 receptor, since chronic treatment with the relatively selective M1 antagonist, dicyclomine, elevated α-synuclein deposits in the transgenic mice. Developing additional low-molecular-weight compounds that target the M1 receptor should be a priority, Fisher suggested.
Immunotherapy approaches, which harness the body’s immune system to clear harmful deposits, are less advanced for PD than for AD. One reason for this, Kordower told ARF, is because α-synuclein aggregates are intracellular, not extracellular, as are Aβ deposits. This heightens the challenge for clearance. However, some evidence suggests that α-synuclein does not just remain hidden inside cells, but gets released and even propagates from cell to cell, Masliah said (see, e.g., Lee et al., 2010). For example, the protein accumulates at pre-synaptic sites and can be found in the extracellular space, Masliah said, suggesting it may be released from synapses. Trans-synaptic transmission has been demonstrated in cell culture, and other data indicate that cells can disgorge α-synuclein through exocytosis. The findings fit with observations from both mice and people that grafted neurons in the striatum become peppered with α-synuclein from the host tissue.
Antibodies might be able to block α-synuclein transfer, Masliah suggested. His team tested this idea by injecting different anti-synuclein antibodies into transgenic PD model mice for six months. Treated mice had less α-synuclein in their neuropil and learned better in a water maze. Antibodies against the C-terminus of α-synuclein gave the best response, Masliah said. These antibodies facilitated the phagocytosis and autophagy of α-synuclein. To show that the antibodies were specifically blocking synaptic transmission, Masliah’s group used a culture model containing neurons in two adjoining chambers. They found an antibody, 1H7, that dose-dependently lowered the amount of α-synuclein transferred between chambers. They then tested the antibody in vivo, using an α-synuclein-null mouse. By injecting a lentivirus expressing α-synuclein into a specific brain region, the researchers could observe trans-synaptic propagation through the brain. Antibody 1H7 reduced this transmission and improved axonal stability, Masliah said. As before, treated mice showed better memory in a water maze test. Antibody treatment shows potential for arresting the spread of Parkinson’s pathology, Masliah concluded.
This is Part 2 of a two-part series. See also Part 1.
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