| Summary: | Neurodegenerative diseases include common and debilitating disorders such as
Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD) and
post-stroke neurodegeneration. Among these disorders, PD and AD have especially
drawn attention because of their devastating impact on the elderly, their families, the
health care system and society. These disorders are characterised by progressive and
irreversible loss of neurons from specific regions of the brain. Among the
mechanisms implicated to be responsible for neuronal cell death we focused our
interest on excitotoxicity, which initiates a cascade of events resulting in neuronal
injury as a result of excessive influx of Ca2+ through the N-methyl-D-aspartate
receptor (NMDAR) and voltage gated calcium channels (VGCC).
The focus of the current study was to develop a novel group of multifunctional
therapeutic agents that can be used in the treatment and/or prevention of
neurodegenerative diseases. Several triquinylamine derivatives containing an
endocyclic nitrogen atom (ma-bridged) and select side chains were synthesised by
thermal [2 + 2] cycloreversion of the symmetric cage compound
pentacyclo[5.4.0.02,6.03,10.05,9]undecane-8.11-dione. To be able to perform the thermal
fragmentation reactions we designed and built a pyrolysis apparatus based on
descriptions found in the literature. With this technology, the reaction was optimised
to a yield approaching 80%. The symmetric cis-syn-cis triquinane scaffold obtained
from the thermal fragmentation reaction underwent subsequent catalytic reduction,
amination and hydride reduction to generate the series of compounds under study.
The synthesised compounds were characterised using NMR, MS and IR techniques.
Reductive amination produced very low yields of the desired products and we were
also unable to isolate the N-methyl derivative, N-methyl-3,11-matricyclo[
6.3.0.02,6]undecane. A computational study was initiated to explain the
apparent selectivity in the formation of the benzylamine derivative over the
methylamine derivative. The computational study revealed that a significantly lower
energy of formation and more favourable HOMO-LUMO overlap (indicating a stronger covalent bond) might be an explanation for the selectivity in formation of the
3-benzyliminotricyclo[6.3.0.02,6]undecanIe-11-one intermediate over that of the 3-
methyliminotricyclo [6.3.0.02,6]undecane-11-one intermediate.
In order to afford protection against excitotoxicity, new drug candidates have to
attenuate the induced Ca2+ flux through the L-type calcium channels and demonstrate
binding affinity for the NMDAR channels. Fluorescent microscopy was utilized to
monitor Ca2+ flux through the L-type calcium channel after KCI-induced
depolarisation (KCI at 140 mM) in the Mag-fura-2/AM preloaded N2a mouse
neuroblastoma cell line. The N-(3-methoxybenzyl)-3,11-azatricyclo[
6.3.0.02,6]undecane compound proved to be the most potent experimental
compound with a reduction in fluorescence of approximately 55.9% and was the only
compound that showed a statistically significant (p < 0.05) attenuation of Ca2+ flux.
This data indicate that the introduction of electronic effects, such as the inductive
effect with significant electron-withdrawing properties of the N-(3-methoxybenzyl)-
3,11-azatricyclo[6.3.0.02,6]undecane compound, significantly influences the L-type
calcium channel binding characteristics. The N-(phenylpropyl)-3,11-azatricyclo[
6.3.0.02,6]undecane showed a reduction in fluorescence of 42.9 %, which
indicated that, an increase in chain length leads to a commensurate increase in activity
when compared to the benzylamine derivatives.
Radioligand binding studies were used to measure the displacement of [3H]MK-801
from NMDA/glycine-activated murine synaptoneurosomes by the triquinylamine
derivatives. The study indicated N-benzyl-3,11-azatricyclo[6.3.0.02,6]undecane to be
the compound with the highest affinity, with an IC50 value of 1.93 uM (p < 0.05),
which is comparable to the clinically used drug memantine(IC50 = 0.54 uM). None of
the other triquinylamine compounds tested showed significant displacement, which
indicated that these compounds do not strongly interact with the PCP binding site and
possibly have a different site of interaction within the NMDA receptor/ion channel
complex.
Our results demonstrate that the triquinylamines have the ability to simultaneously
block both major neuronal calcium channels. Different binding characteristics were
however found to be important for the two channels. Of the structure-activity relationship parameters studied, geometric or steric constraints for interaction both at
the VGCC and NMDAR channel appear to be dominant. However, binding
characteristics for the VGCC were greatly improved with the introduction of
inductive electronic effects. We conclude that the triquinylamines tested represent a
novel group of dual-mechanistic agents that have potential as therapeutic agents in the
treatment of neurodegenerative diseases. === Thesis (M.Sc. (Pharmaceutical Chemistry))--North-West University, Potchefstroom Campus, 2007.
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