One of the hallmarks of Huntington’s disease is the formation of inclusion bodies, or protein aggregates, among the brain’s striatal neurons. Credit: Kateryna Kon/Shutterstock
In seeing a room full of patients and families who had been affected by Huntington’s disease, Diana Miszczuk felt a wave of emotion wash over her.
“To really face the patients, it was emotional for the whole lab,” she says, reflecting on a meeting with a patient group more than ten years ago. Now Associate Director in CNS Pharmacology at Charles River, Miszczuk and her colleagues still carry memories of patients, some of whom became friends, as they develop a wide range of therapeutics to treat Huntington’s disease.
This debilitating, heritable condition is caused by >35 repeats of a trinucleotide sequence—a CAG repeat—in the Huntington (HTT) gene. The resulting mutant HTT protein elicits many toxic downstream effects, particularly in the central nervous system (CNS), leading to the gradual loss of neurons, failing motor skills and ultimately early death.
At the moment, Huntington’s disease is not curable. But researchers like Philip Mitchell, a Scientific Director at Charles River, who works with Miszczuk and oversees work on Huntington’s disease, are working hard to change this. As Mitchell explains, “The Huntington’s disease research community is very much focused on delaying the onset of the disease or slowing its progression by targeting pathological proteins through multiple approaches.”
Targeting Huntington’s disease
Therapeutic development for Huntington’s disease has largely focused on small molecules and antisense oligonucleotides (ASO). The latter are small strands of RNA or DNA that can be designed to target the HTT mRNA transcript and prevent the mutant protein from being made.
The potential of ASOs for treating genetic conditions like Huntington’s disease is great, but there remain significant hurdles in delivering ASOs across the blood-brain barrier and designing them to be selective for mutant protein transcripts. The recent failure of two ASOs in clinical trials, both of which failed to show clinical benefits for Huntington’s disease, underscores this point.1,2
Despite these hurdles, there’s a growing sense of hope in the Huntington’s disease community. The range of druggable targets is now broader than ever before. Small molecules can be used to target catalytic proteins, from histone deacetylases to spliceosome proteins, while ASOs can be developed against targets that are beyond the reach of small molecules.
A new class of small molecules
Working in collaboration with the Cure Huntington’s Disease Initiative (CHDI), a privately-funded, not-for-profit biomedical research organization devoted to developing therapeutics against Huntington’s disease, the team at Charles River recently published an article in the Journal of Medicinal Chemistry describing the optimization of a new class of small-molecule inhibitors that could mitigate Huntington’s disease.
Previous work from the team had identified a lead compound that could cross the blood brain barrier and target the DNA damage repair protein ataxia telangiectasia mutated (ATM).3 The protein, a kinase, has been shown to be hyperactive in Huntington's disease, and partial loss of ATM activity can slow the disease’s progression in animal models.4
While encouraging, a major hurdle in developing small molecules is the potential for off-target, non-specific binding. The team’s lead compound was no different. It showed affinity for a pro-autophagy kinase, Vsp34, whose inhibition may actually worsen symptoms of Huntington’s disease.
To overcome this drawback, the team compared the ATP binding domains of Vsp34 with that of ATM. That revealed a handful of differences that could be exploited to improve selectivity.
In light of these differences, the team progressed stepwise in the modification of their core compound, making it bulkier in some sections and adjusting its hydrophobicity in others. This yielded a series of compounds that demonstrated high selectivity for ATM at low nanomolar concentrations in in vitro phosphorylation assays.
Testing of the three most promising candidates in vivo showed they could penetrate the blood-brain barrier and held favorable pharmacokinetics. More pre-clinical work lies ahead.
“This is just the beginning,” Miszczuk explains. “We need to optimise compound exposure in the brain to ensure coverage over the IC50’s. But, I think the new core that we developed has some very promising properties.”
With CHDI and other partners, researchers at Charles River are continuing to explore potential therapeutics, from small molecules to ASOs. All the while, their work is done with the patient in mind. As Miszczuk puts it, “There is always a patient on the back end, very nearby for us.”