A small-molecule therapeutic lead for Huntington's disease: Preclinical pharmacology and efficacy of C2-8 in the R6/2 transgenic mouse

Abstract

Huntington's disease (HD) is a progressive neurodegenerative disease caused by a glutamine expansion within huntingtin protein. The exact pathological mechanisms determining disease onset and progression remain unclear. However, aggregates of insoluble mutant huntingtin (mhtt), a hallmark of HD, are readily detected within neurons in HD brain. Although aggregated polyglutamines may not be inherently toxic, they constitute a biomarker for mutant huntingtin useful for developing therapeutics. We previously reported that the small molecule, C2-8, inhibits polyglutamine aggregation in cell culture and brain slices and rescues degeneration of photoreceptors in a Drosophila model of HD. In this study, we assessed the therapeutic potential of C2-8 in the R6/2 mouse model of HD, which has been used to provide proof-ofconcept data in considering whether to advance therapies to human HD. We show that, at nontoxic doses, C2-8 penetrates the blood-brain barrier and is present in brain at a high concentration. C2-8-treated mice showed improved motor performance and reduced neuronal atrophy and had smaller huntingtin aggregates. There have been no prior drug-like, non-toxic, brain-penetrable aggregation inhibitors to arise from cell-based high-throughput screens for reducing huntingtin aggregation that is efficacious in preclinical in vivo models. C2-8 provides an essential tool to help elucidate mechanisms of neurodegeneration in HD and a therapeutic lead for further optimization and development.huntingtin ͉ neuroprotection ͉ protein aggregates H untington's disease (HD) is a fatal autosomal dominant neurodegenerative disease caused by a glutamine-coding CAG expansion within exon 1 of the huntingtin gene (1). HD is characterized clinically by progressive motor, psychiatric, cognitive, and functional decline resulting from neurodegeneration particularly in the striatum and cerebral cortex (2-4). As a result of its N-terminal polyglutamine expansion, mutant huntingtin is misfolded and deposited as a component of insoluble protein aggregates that persist in neuronal nuclei, perikarya, and processes (5-7). Although most evidence suggests that huntingtin's toxicity resides in its soluble protein-protein interactions, huntingtin aggregates mark the presence of misfolded mutant huntingtin in vulnerable cell populations and are a hallmark of HD in humans (7) and in genetic in vivo (6,8,9) and in vitro (10) models of HD. Antioxidant and energy buffering compounds, such as coenzyme Q10 (11, 12), creatine (13-15), and cystamine (16, 17), which are neuroprotective in HD transgenic mice, significantly reduce huntingtin aggregates. Aggregation occurs in vivo in the presence of expressed polyglutamine-containing proteins, such as huntingtin or huntingtin fragments and adaptation to high-throughput screening has enabled the identification of selective inhibitors (18,19). The small molecule C2-8 was identified in a high-throughput screen using a yeast-based aggregation assay and was observed to partially inhibit polyglutamine-mediated aggreg...

The R6 lines of transgenic mice: A model for screening new therapies for Huntington's disease

“…Furthermore, C2-8 significantly reduced neuronal atrophy and the size of striatal NIIs. However, C2-8 treatment had no effect on body weight loss and survival of these transgenic mice (Chopra et al, 2007).…”

“…Therefore, the same group subsequently assessed its therapeutic potential in R6/2 mice (Chopra et al, 2007). C2-8 (administered orally starting at 3 weeks of age) penetrates the blood-brain barrier and is present in the brain at a high concentration.…”

Huntingtin Aggregation Kinetics and Their Pathological Role in aDrosophilaHuntington’s Disease Model

“…Longer poly(Q) tracts have faster in vitro aggregation kinetics and result in earlier disease onset (Scherzinger et al 1999). Similarly, treatments that suppress aggregation, including chaperone overexpression (Carmichael et al 2000) and administration of small molecule aggregation inhibitors (Chopra et al 2007), have been shown to decrease neurodegeneration. Live imaging demonstrates that Htt aggregates can sequester and alter kinetics of trafficked organelles and proteins such as synaptic vesicles (Sinadinos et al 2009) and transcription factors (Chai et al 2002).…”

Neuroprotection for Huntington’s Disease: Ready, Set, Slow

“…Commercial and academic efforts that screen compound libraries against elements of the HD phenotype suitable for in vitro or cell-based assays are also bringing forward families of novel compounds to be sorted for efficacy, toxicity, and drugability (i.e., the potential for pharmaceutical delivery of the compound). 1,2 These approaches are expanding the preclinical segment of the pipeline of potential neuroprotective treatments and are beginning to provide new compounds suitable for early-phase clinical testing.…”