30 September 2011. Scientists have developed an arsenal of experimental compounds targeting the protein aggregates that accumulate in the neurons of patients with Huntington’s disease (HD) and Alzheimer’s disease (AD), including antibodies, small molecules, peptides, and proteins. Another class of agents, known as peptoids, now joins the fray.
Peptoids, which are oligomers of modified glycines, have been tested as possible antimicrobials and cancer drugs, but had not yet made their foray into neurodegenerative disease therapy. The first such report comes from researchers at the University of Texas Southwestern Medical Center in Dallas. In the September 22 Chemistry & Biology, they describe a peptoid that binds to huntingtin, the protein that causes HD. The compound both reduces the protein’s ability to clump together and improves motor symptoms in a mouse model of the disease. “We are not claiming this is a cure for Huntington’s, but peptoids are one modality we can use that has not been explored in the neurodegenerative field,” said senior author Ilya Bezprozvanny. “We have applied the approach to Huntington’s as a proof of principle, and we are now trying to do the same for Alzheimer’s.” There are currently no effective treatments to alter the course of either HD or AD.
HD is a neurodegenerative disorder caused by the repetition in the huntingtin gene of a cytosine-adenine-guanine trinucleotide, which encodes the amino acid glutamine. The mutation results in huntingtin proteins that contain long polyglutamine (polyQ) tracts that tend to stick to one another, causing the mutant proteins to aggregate. “There are several approaches nowadays for targeting mutant huntingtin,” said Cristina Sampaio, chief clinical officer at the CHDI Foundation, Inc. in Princeton, New Jersey. One such approach, which is close to reaching clinical trials, is to use RNA interference to specifically stop the production of mutant huntingtin and not the wild-type protein (see ARF related news story on Hu et al., 2009). Another way is to use molecules that block the rogue huntingtin from aggregating or interacting with other proteins. Small molecules and peptides have been used for this purpose (see ARF related news story on Chopra et al., 2007), but according to Bezprozvanny, peptoids could have some advantages over these other agents.
“Small molecules are usually too small to prevent a protein as large as huntingtin from aggregating. For them to have an effect, you have to use very high concentrations,” he said. Peptoids are larger than small molecules, and have been shown to be more effective in preventing aggregation. Peptides also bust aggregation, but are rapidly degraded by proteases in vivo, whereas their artificial chemical structure makes peptoids fairly resilient. “The peptoid approach has a lot of potential, but whether it will be better than others remains to be seen,” said Sampaio.
Peptoids came on the neurodegenerative disease scene just recently. Earlier this year, Thomas Kodadek at the Scripps Research Institute in Jupiter, Florida, reported using a peptoid library to fish out antibodies specific to AD that might have diagnostic potential (see ARF related news story on Reddy et al., 2011). Other groups have also tried developing peptoid-based diagnostic tools for neurodegenerative diseases (Yam et al., 2011; Gao et al. 2010).
Bezprozvanny and colleagues decided to apply the peptoid technology to therapy, choosing HD as a model system. Collaborating with Kodadek, first author Xuesong Chen and colleagues generated a library consisting of 60,000 different peptoids, each six units long, and screened them for binding to huntingtin. They isolated one peptoid, called HQP09, that in vitro bound the mutant protein harboring 82 glutamine residues much more strongly than wild-type huntingtin. They also developed a smaller derivative of HQP09, dubbed HQP9_9, which they reasoned might have better potential for biological applications and drug development.
Chen et al. tested the two peptoids in vitro and in primary striatal cultures derived from YAC128 mice, which carry a polyQ-expanded human huntingtin gene on a yeast artificial chromosome (Slow et al., 2003). They showed that both HQP9 and HQP9_9 reduced mutant huntingtin aggregation in vitro and stabilized abnormal calcium signaling in neurons, which Bezprozvanny’s group had previously shown to play a role in HD pathogenesis (Bezprozvanny, 2009). They also protected the neurons from glutamate-induced toxicity. In general, HQP9 did better in vitro, and HQP9_9 did better in the cell culture assays.
To test whether these peptoids could improve symptoms of HD, Bezprozvanny and colleagues first gave HQP09_9 to wild-type and YAC128 mice. Initially, the researchers administered the peptoid subcutaneously, and with that saw no improvement in symptoms, perhaps because HQP09_9 was unable to cross the blood-brain barrier. They then administered the larger HQP09 peptoid, which is better at binding huntingtin, directly in the brains of eight- to nine-month-old mice by continuous intracerebroventricular infusion with an implanted mini-pump that delivered 2.5 mg of peptoid over one month. Following treatment, the YAC128 mice performed better on a beam-walk test than did untreated littermates, and proved to have less protein aggregates in the brains of these mice.
The next step will be to optimize HQP09 or its smaller derivative. “We can add a carrier or nanoparticle that makes delivery to the brain more effective. We can also play around with the chemical structure. There is a lot of room for improvement,” said Bezprozvanny. In the meantime, his group has applied a similar strategy to finding a peptoid that will bind to the amyloid-β42 (Aβ42) peptide. They have identified some candidates, but that work has yet to be published.
Other groups have developed small molecules that can prevent amyloid aggregates from forming (see ARF related news story on Sievers et al., 2011) and some Aβ aggregation inhibitors are in clinical trials (see ARF related news story and Salloway et al., 2011). So far, however, none has succeeded in Phase 3. Bezprozvanny has filed patent applications for the development of HD and AD drugs using peptoid technology and is looking for industrial partners to help bring this work to the clinic.—Laura Bonetta.
Chen X, Wu J, Luo Y, Liang X, Supnet C, Kim MW, Lotz GP, Yang G, Muchowski PJ, Kodadek T, Bezprozvanny I. Expanded polyglutamine-binding peptoid as a novel therapeutic agent for treatment of Huntington’s disease. Chemistry & Biology 2011; 18:1. Abstract