Huntington’s disease is a neurodegenerative disorder caused by mutations in the CAG tract of huntingtin. Several studies in HD cellular and rodent systems have identified disturbances in cyclic nucleotide signaling, which might be relevant to pathogenesis and therapeutic intervention. To investigate whether selective phosphodiesterase (PDE) inhibitors can improve some aspects of disease pathogenesis in HD models, we have systematically evaluated the effects of a variety of cAMP and cGMP selective PDE inhibitors in various HD models. Here we present the lack of effect in a variety of endpoints of the PDE subtype selective inhibitor SCH-51866, a PDE1/5 inhibitor, in the R6/2 mouse model of HD, after chronic oral dosing.
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Larry Park
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To evaluate the potential of memantine as a therapeutic agent for Huntington’s disease (HD) we have undertaken a series of in vitro, ex vivo and whole animal studies to characterize its pharmacokinetics (PK) and pharmacodynamics (PD) in rats and mice. Results from these studies will enable determination of memantine exposures needed to engage the related functional PD marker and help predict the dose regimen for clinical trials to test its proposed mechanism of action; the selective blockade of extrasynaptic, but not synaptic, NMDA receptors. The studies reported here describe the PK of memantine in rats and mice at low (1 mg/kg) and high (10 mg/kg) doses. Our studies indicate that the clearance mechanisms of memantine in rats and mice are different from those in human, and that clearance needs to be taken into account when extrapolating to the human. In rats only, there is a significant metabolic contribution to memantine clearance at lower dose levels. While memantine is primarily cleared renally in all three species, the proportion of total systemic clearance above the glomerular filtration rate (GFR) is much higher in rats and mice (~13, 4.5, and 1.4 times higher than GFR in rats, mice, and humans, respectively), suggesting that the contribution of active transport to memantine elimination in rats and mice is more significant than in the human. In rats and mice, memantine had a short half-life (100). In the human, the half-life of memantine was reported to be very long (60-80 h) with a Cmax/Cmin ratio at steady state concentrations of ~1.5. A small change in the clearance of memantine – for example due to renal impairment or competition for the elimination pathway with a co-administered drug – will likely affect exposure and, therefore, the selectivity of memantine on NMDA receptors . The PK differences observed between these species demonstrate that the PK in mice and rats cannot be directly extrapolated to the human. Further, the relationship between the plasma concentration (and therefore dose) needed to elicit a mechanism-related in vivo functional effect (PD readout) while maintaining the selectivity of the extrasynaptic blockade of the NMDA receptors needs to be established before clinical trials can be appropriately planned.
Homozygosis for the rd1 mutation in the Pbe6b gene results in the loss of the rod beta-subunit of the cyclic GMP phosphodiesterase and, eventually, of all rod and cone photoreceptors. The R6/2 mouse line is a widely used model of Huntington’s disorder (HD). The original line was made available on a mixed background obtained by crossing, via ovarian transplant, female R6/2 (on a B6CBA mixed background) with male B6CBAF1/J mice. As the CBA/J strain used in the US is homozygous for the rd1 mutation and the breeding scheme does not ensure heterozygosis for the mutation, a significant percentage of the offspring on this mixed background is expected to be homozygous for the rd1 mutation. We investigate here the effect of rd1 homozygosis on motor function and examined the effects of the mutation on the R6/2 phenotype. Homozygosis for the rd1 mutation resulted in increased activity in the open field test and reduced rotarod test performance. In addition, rd1 mutation absence or heterozygosis reduced the differences between the R6/2 and the WT mice. Our recommendation for the neurodegeneration field, and for all mouse studies in general, is to carefully control homozygosis for retinal degeneration mutation, even when using tests of motor function.
Mouse models of Huntington’s disease (HD) were trained to acquire one of two simple instrumental responses (a lever press or a nosepoke) to obtain food reinforcement. Animals from several HD strains revealed apparently progressive deficits in this task, being significantly less able than littermate controls to perform the required responses, at ages where motor function is only mildly affected. These data could provide a simple way to measure learning deficits in these mouse models, likely related to the characteristic pattern of neural damage observed in HD mouse models.