Monoamine oxidase B inhibition and neuroprotection: studies on selective adenosine A2A receptor antagonists.

The principal therapeutic agents used in the management of Parkinson's disease (PD) enhance nigrostriatal dopaminergic flux through either replenishment of depleted dopamine stores or the action of dopaminergic agonists. Adenosine A2A receptor antagonists (e.g., KW-6002) may provide symptomatic relief in PD and perhaps also may display neuroprotective properties based on studies in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of nigrostriatal neurodegeneration. A second class of compounds that is neuroprotective in the MPTP model comprises inhibitors of the outer mitochondrial flavoenzyme monoamine oxidase B (MAO B), one of the two forms of MAO that regulate levels of brain neurotransmitter substances, including dopamine. In this article, data are presented that document the overlapping A2A antagonist and MAO B inhibitory properties of several 2-styrylxanthinyl derivatives. A limited structure-activity analysis of these compounds and structurally related analogs is provided. The results raise the possibility that a single structure may offer the combined benefits of two pharmacologic strategies, each with symptomatic and potential neuroprotective benefits, for the management of PD.

Inhibition of monoamine oxidase B by selective adenosine A2A receptor antagonists.

Adenosine receptor antagonists that are selective for the A(2A) receptor subtype (A(2A) antagonists) are under investigation as possible therapeutic agents for the symptomatic treatment of the motor deficits associated with Parkinson's disease (PD). Results of recent studies in the MPTP mouse model of PD suggest that A(2A) antagonists may possess neuroprotective properties. Since monoamine oxidase B (MAO-B) inhibitors also enhance motor function and reduce MPTP neurotoxicity, we have examined the MAO-B inhibiting properties of several A(2A) antagonists and structurally related compounds in an effort to determine if inhibition of MAO-B may contribute to the observed neuroprotection. The results of these studies have established that all of the (E)-8-styrylxanthinyl derived A(2A) antagonists examined display significant MAO-B inhibitory properties in vitro with K(i) values in the low micro M to nM range. Included in this series is (E)-1,3-diethyl-8-(3,4-dimethoxystyryl)-7-methylxanthine (KW-6002), a potent A(2A) antagonist and neuroprotective agent that is in clinical trials. The results of these studies suggest that MAO-B inhibition may contribute to the neuroprotective potential of A(2A) receptor antagonists such as KW-6002 and open the possibility of designing dual targeting drugs that may have enhanced therapeutic potential in the treatment of PD.

8-(3-Chlorostyryl)caffeine may attenuate MPTP neurotoxicity through dual actions of monoamine oxidase inhibition and A2A receptor antagonism.

Caffeine and more specific antagonists of the adenosine A(2A) receptor recently have been found to be neuroprotective in the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) model of Parkinson's disease. Here we show that 8-(3-chlorostyryl)caffeine (CSC), a specific A(2A) antagonist closely related to caffeine, also attenuates MPTP-induced neurotoxicity. Because the neurotoxicity of MPTP relies on its oxidative metabolism to the mitochondrial toxin MPP(+), we investigated the actions of CSC on striatal MPTP metabolism in vivo. CSC elevated striatal levels of MPTP but lowered levels of the oxidative intermediate MPDP(+) and of MPP(+), suggesting that CSC blocks the conversion of MPTP to MPDP(+) in vivo. In assessing the direct effects of CSC and A(2A) receptors on monoamine oxidase (MAO) activity, we found that CSC potently and specifically inhibited mouse brain mitochondrial MAO-B activity in vitro with a K(i) value of 100 nm, whereas caffeine and another relatively specific A(2A) antagonist produced little or no inhibition. The A(2A) receptor independence of MAO-B inhibition by CSC was further supported by the similarity of brain MAO activities derived from A(2A) receptor knockout and wild-type mice and was confirmed by demonstrating potent inhibition of A(2A) receptor knockout-derived MAO-B by CSC. Together, these data indicate that CSC possesses dual actions of MAO-B inhibition and A(2A) receptor antagonism, a unique combination suggesting a new class of compounds with the potential for enhanced neuroprotective properties.