|Parkinson's disease (PD) is a severe and debilitating age-related neurodegenerative
disease, with symptoms of resting tremor, akinesia and rigidity. The symptoms of PD
are due to the selective loss of dopaminergic neurons in the substantia nigra. Three
drug targets were identified for this study aimed at developing new drugs for the
symptomatic treatment and/or prevention of PD. These included the dopamine
transporter (DAT), monoamine oxidase (MAO) B enzyme and the N-methyl-D-aspartate
(NMDA) receptor/ion channel complex. In addition, the parent compound,
NGP1-01 (1), was evaluated in two in vivo models of neurodegenerative disease,
which included the MPTP Parkinsonian mouse model (as a model for chronic
neurodegenerative processes) and a stroke model - middle artery occlusion (MCAO)
induced focal cerebral ischemia model - in mice (as a model for acute
Memantine is a polycyclic mine that is used clinically to treat PD and Alzheimer's
disease. It is a low-affinity uncompetitive NMDA receptor antagonist and is well
tolerated clinically. Antagonism of the NMDA receptor prevents Ca2+ overload in
neuronal cells, thereby preventing neuronal cell death. The importance of Ca2+ in
neuronal cell death led us to evaluate another polycyclic mine NGPl-01 (1) and its
derivatives for possible Neuro protective activity. These compounds were previously
characterized as L-type calcium channel antagonists.
NGP1-01 (1) and its derivatives were evaluated for their effects on [3H]dopamine ([3H]DA) release and uptake Inhibition in murine striatal synaptosomes, as well as for
inhibition of baboon liver monoamine oxidase (MAO) B. NGP1-01 (1) blocked the
uptake of [3H]DA with an IC50 of 57 µM, while another compound, 8-phenylethyl-
8,11-oxapentacyclo[5.4.0.02,6.03,10.05,9]undecane(9), blocked uptake at an IC50 value
of 23 µM. These values were comparable to that of amantadine (IC50; 82 µM),
another polycyclic mine that is currently used in parkinsonian therapy. MAO-B
inhibition for this group was weak, with less than 50% inhibition at 300 µM for any of
the compounds in the series.
NGP1-01 (1) and its derivatives were then further evaluated in a functional
biochemical assay to assess NMDA antagonism. The IC50 values for the reference
compounds MK-801 and memantine were 1.23µpM and 3.05 µM, respectively, for
blocking NMDA-mediated 45Ca2+ influx into whole brain murine synaptoneurosomes.
NGP1-01 (1) proved to be the most potent experimental compound with an IC5-0 of
2.98 µM, while 8-amino-pentacyclo[5.4.0.02,6.03,10.05,9]undecane (8) had a similar
IC50 of 4.06 µM. NGP1-01 (1) was further shown to be a non-competitive NMDA
Comparative molecular field analysis (CoMFA) and comparative molecular similarity
index analysis (CoMSIA) techniques were used to provide models to predict the PCP
binding affinity of NGP1-01 and other polycyclic mines on the NMDA receptor.
The resulted r2 of the CoMFA and CoMSIA models were 0.976 and 0.883
respectively. Radioligand binding studies with [3H]MK-801 of [3H]TCP however,
showed little or no displacement of the ligands by pentacycloundecylamines,
indicating that they bind to a novel site on the NMDA receptor.
Since NGP1-01 (1) was shown to have NMDA receptor antagonist activity, it was
evaluated in a battery of behavioural tests in the MPTP-mouse model using retired
breeder C57BL/6 male mice, to determine whether this compound might alleviate
behaviour deficits in this model. The behavioural tests included fore-paw stride
length, pole test (bradykinesia), catalepsy, rearing and movement. Biochemical
confirmation of MPTP-induced toxicity was done by measuring striatal DA levels
using HPLC with electrochemical detection while the amount of DAT present was
determined by Western Blot. NGP1-01 was found to attenuate bradykinesia and
catalepsy to a small extent.
NGP1-01 (1) was also evaluated in the middle artery occlusion (MCAO) induced
focal cerebral ischemia model in mice to evaluate the Neuro protective potential in
stroke. In this study, NGP1-01 (1), at a dose of 20 mg/kg ip injected 30 min before
MCAO, reduced the edema volume by 87%, and the infarct volume by 58%,
compared to control.
In conclusion, the pentacycloundecylamines represent a new class of compounds that
might have utility in the prevention and/or treatment of neurodegenerative diseases
through their interaction with a variety of drug targets involved in these diseases.