| Summary: | Parkinson’s disease (PD) is a neurodegenerative disorder that is characterised by a
reduction of dopamine concentration in the striatum due to degeneration of dopaminergic
neurons in the substantia nigra. Currently, first line treatment of PD includes the use of
dopamine precursors, dopamine agonists and inhibitors of enzymatic degradation of
dopamine, in an effort to restore dopamine levels and/or its effects. However, all these
therapeutic strategies are only symptomatic and unfortunately do not slow, stop or reverse
the progression of PD.
From the discovery of adenosine A2A receptor-dopamine D2 receptor heteromers and the
antagonistic interaction between these receptors, the basis of a new therapeutic approach
towards the treatment of PD emerged. Adenosine A2A receptor antagonists have been
shown to decrease the motor symptoms associated with PD, and are also potentially
neuroprotective. The possibility thus exists that the administration of an adenosine A2A
antagonist may prevent further neurodegeneration. Furthermore, the antagonism of
adenosine A1 receptors has the potential of treating cognitive deficits such as those
associated with Alzheimer's disease and PD. Therefore, dual antagonism of adenosine A1
and A2A receptors would be of great benefit since this would potentially treat both the motor
as well as the cognitive impairment associated with PD.
The affinities (Ki-values between 0.6 mM and 38 mM) of a series of 1,4-dihydropyridine
derivatives were previously illustrated for the adenosine A1, A2A and A3 receptor subtypes by
Van Rhee and co-workers (1996). These results prompted this pilot study, which aimed to
investigate the potential of the structurally related 3,4-dihydropyrimidin-2(1H)-ones
(dihydropyrimidones) and 2-amino-1,4-dihydropyrimidines as adenosine A1 and A2A
antagonists.
In this pilot study, a series of 3,4-dihydropyrimidones and 2-amino-1,4-dihydropyrimidines
were synthesised and evaluated as adenosine A1 and A2A antagonists. Since several
adenosine A2A antagonists also exhibit MAO inhibitory activity, the MAO-inhibitory activity of
selected derivatives was also assessed. A modified Biginelli one pot synthesis was used for
the preparation of both series of compounds under solvent free conditions. A mixture of a β-
diketone, aldehyde and urea/guanidine hydrochloride was heated for an appropriate time to
afford the desired compounds in good yields. MAO-B inhibition studies comprised of a
fluorometric assay where kynuramine was used as substrate. A radioligand binding protocol
described in literature was employed to investigate the binding of the compounds to the adenosine A2A and A1 receptors. The displacement of N-[3H]ethyladenosin-5’-uronamide
([3H]NECA) from rat striatal membranes and 1,3-[3H]-dipropyl-8-cyclopentylxanthine
([3H]DPCPX) from rat whole brain membranes, was used in the determination of A2A and A1
affinity, respectively.
The results showed that both 3,4-dihydropyrimidones and 2-amino-1,4-dihydropyrimidines
had weak adenosine A2A affinity, with the p-fluorophenyl substituted dihydropyrimidone
derivative (1h) in series 1, exhibiting the highest affinity for the adenosine A2A receptor (28.7
μM), followed by the p-chlorophenyl dihydropyrimidine derivative (2c) in series 2 (38.59 μM).
Both series showed more promising adenosine A1 receptor affinity in the low micromolar
range. The p-bromophenyl substituted derivatives in both series showed the best affinity for
the adenosine A1 receptor with Ki-values of 7.39 μM (1b) and 7.9 μM (2b). The pmethoxyphenyl
dihydropyrimidone (1d) and p-methylpneyl dihydropyrimidine (2e) derivatives
also exhibited reasonable affinity for the adenosine A1 receptor with Ki-values of 8.53 μM
and 9.67 μM, respectively. Neither the 3,4-dihydropyrimidones nor the 2-amino-1,4-
dihydropyrimidines showed MAO-B inhibitory activity.
Comparison of the adenosine A2A affinity of the most potent derivative (1h, Ki = 28.7 μM)
from this study with that of the previously synthesised dihydropyridine derivatives (Van Rhee
et al., 1996, most potent compound had a Ki = 2.74 mM) reveals that an approximate 100-
fold increase in binding affinity for A2A receptors occurred. However, KW6002, a known A2A
antagonist, that has already reached clinical trials, has a Ki-value of 7.49 nM. The same
trend was observed for adenosine A1 affinity, where the most potent compound (1b) of this
study exhibited a Ki-value of 7.39 μM compared to 2.75 mM determined for the most potent
dihydropyridine derivatives (Van Rhee et al., 1996). N6-cyclopentyladenosine (CPA), a
known adenosine A1 agonist that was used as a reference compound, however had a Kivalue
of 10.4 nM. The increase in both adenosine A1 and A2A affinity can most likely be
ascribed to the increase in nitrogens in the heterocyclic ring (from a dihydropyridine to a
dihydropyrimidine) since similar results were obtained by Gillespie and co-workers in 2009
for a series of pyridine and pyrimidine derivatives. In their case it was found that increasing
the number of nitrogens in the heterocyclic ring (from one to two nitrogen atoms for the
pyridine and pyrimidine derivatives respectively) increased affinity for the adenosine A2A and
adenosine A1 receptor subtypes, while three nitrogen atoms in the ring (triazine derivatives)
were associated with decreased affinity. It thus appears that two nitrogen atoms in the ring
(pyrimidine) are required for optimum adenosine A1 and A2A receptor affinity. The poor adenosine A2A affinity exhibited by the compounds of this study can probably be
attributed to the absence of an aromatic heterocyclic ring. The amino acid, Phe-168 plays a
very important role in the binding site of the A2A receptor, where it forms aromatic - -
stacking interactions with the heterocyclic aromatic ring systems of known agonists and
antagonists. Since the dihydropyrimidine ring in both series of this pilot study was not
aromatic, the formation of aromatic - -stacking interactions with Phe-168 is unlikely.
In conclusion, the 3,4-dihydropyrimidone and 2-amino-1,4-dihydropyrimidine scaffolds can
be used as a lead for the design of novel adenosine A1 and A2A antagonists, although further
structural modifications are required before a clinically viable candidate will be available as
potential treatment of PD. === MSc (Pharmaceutical Chemistry), North-West University, Potchefstroom Campus, 2014
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