Emery Ewing posted an update 1 week, 2 days ago
HD is a lethal autosomal dominant neurodegenerative disease caused by expansion of a stretch of CAG-encoded glutamines near the N-terminus of huntingtin, a protein whose mutant form accumulates as nuclear and cytoplasmic inclusions in the brain of HD patients. The disease is a progressive disorder with severe psychiatric, cognitive, and motor impairments. Mutant HTT confers a particular vulnerability to the medium spiny neurons of the corpus striatum, as well as subsets of cortical neurons in the motor, frontal, and occipital cortices, and in other brain regions such as the hypothalamus. Age of onset in BYL719 PI3K inhibitor humans is inversely correlated to the size of the CAG expansion, with expansions.39 CAGs in the HTT gene resulting in complete penetrance of the disease. The cellular and biological pathways affected by mHTT are widespread, including transcriptional dysregulation, disruption of energy homeostasis, impairment of protein turnover by the ubiquitin-proteasome system and the autophagy-lysosomal system, and impairment of synaptic transmission and plasticity. HDAC inhibition has been proposed as a therapeutic strategy for HD. Indeed, broad-spectrum HDAC inhibitors partially rectify the transcriptional dysregulation in HD cell and animal models, enhance the degradation of mHTT by altering the acetylation state of key residues within the protein, and improve cognition through enhancement of learning and memory processes. Thomas et al showed that HDACi 4b has a therapeutic effect in the R6/2 HD mouse model. The R6/2 strain used in this study expresses the exon 1 HTT protein with an expanded polyglutamine region of,300 repeats, and manifests a delayed phenotype compared to the better characterised R6/2 model that has a shorter polyglutamine expansion. The R6/2300Q mice exhibit significant deficits in motor behaviour by 12 weeks of age, striatal atrophy, and survive 6 to 7 months. A short pharmacodynamic study successfully ameliorated gene expression abnormalities in these mice and showed increased histone H3 acetylation in association with selected down-regulated genes. In a chronic efficacy study, 4b was complexed to 2- hydroxypropyl-b-cyclodextrin and diluted in drinking water and given to mice from 4 months of age. However, the expected differences in oral versus parenteral administration were not addressed in the Thomas et al. study. While this precludes direct correlation between the pharmacodynamic studies and the results of the efficacy trial, these mice showed improved motor performance and overall appearance and an amelioration of body weight loss. Gross brain weight and striatal volume were also improved on termination of the study at 6 months of age. The successful use of 4b in treating R6/2 mice loosely correlates with an earlier report, in which the hydroxamic acid HDAC inhibitor SAHA was administered in drinking water to R6/2 mice that harbour the smaller polyglutamine repeat and exhibit a more aggressive phenotype. These animals also showed significant improvement in motor dysfunction as assessed by rotarod performance and grip strength, but this improvement was offset by the failure of both wild type and R6/2 mice to gain weight at the maximum tolerated dose, suggestive of a narrow therapeutic window. Subsequent reports provided an intriguing explanation for the efficacy and well-tolerated effects of the pimelic diphenylamidebased inhibitors. A common feature of these HDAC inhibitors is an acylated ortho-phenylene diamine unit, which is thought to interact with the zinc ion of HDACs. Compounds from this series are selective for HDAC1, HDAC2, and HDAC3 over other HDAC isoforms, with no activity reported against HDAC Class IIa enzymes and only weak activity reported against HDAC8. Furthermore, they appear to bind to the catalytic site of these HDACs via a unique binding mode not shared with other hydroxamic acid based inhibitors; a time-dependent increase in affinity with an extremely slow off rate has been observed. These exciting findings plus the report of in vivo efficacy and a neuroprotective profile in the R6/2 HD model prompted us to synthesize and evaluate 4b to independently validate this finding in the more widely used lower CAG repeat length R6/2 model, and also potentially in other HD rodent models. We had two objectives: to further validate the finding that preferential central HDAC3 inhibition is of therapeutic relevance in R6/2 and other HD models, and to assess the suitability of 4b or structurallyrelated compounds for translation into clinical trials. We therefore evaluated the pharmacology, ADME, and pharmacokinetic properties of 4b in mice, adhering as closely as possible to the original design for the in vivo work. Our in vitro biochemical selectivity profiling is largely in agreement with the previous reports. However, we identified a physicochemical instability of 4b that constitutes a potential liability of all published compounds containing an acyl phenylenediamine ‘warhead’: cyclisation of this zinc-binding moiety to an inactive benzimidazole product was observed, which was especially efficient under acidic conditions such as during the solubilisation process of acidic salts of 4b. We suspect that similar solubilisation conditions may have been used in the previous study as part of the oral formulation preparation procedures. Importantly, our in vitro and in vivo ADME evaluation showed high metabolic turnover of 4b and very low brain penetration due to high systemic clearance and efficient transport out of the brain since 4b acts as a Pgp substrate.