Polycyclic propargylamines as multi-functional neuro-protective agents

Abstract

The pathology of neurodegenerative disorders, such as Parkinson's disease (PD) and Alzheimer's disease (AD), is caused by the abnormal loss of neuronal cells in certain areas of the brain. It consequently causes an imbalance of certain neurotransmitter levels in the brain, giving rise to the characteristic signs and symptoms of these diseases. Ultimately it compromises the normal functionality and well being of the individual suffering from the disease, thus making it an absolute necessity to create compounds which would halt this neuronal breakdown process, but will also aid in treating the signs and symptoms. The abnormal death of neurons in the central nervous system of individuals suffering from neurodegenerative diseases, takes place by an intrinsic cell suicide program known as apoptosis. This process is triggered by several stimuli, and consists of numerous pathways and cascades, each one having an influence on the other, ultimately leading to the death of the cells. In PD and AD it has been shown that there are elevated levels of monoamine oxidase B (MAO-B), which not only acts indirectly as a trigger to the apoptotic process, but also gives rise to some of the signs and symptoms of the diseases. In the current study the approach was to develop multifunctional drugs, which would halt the neuronal breakdown process, but will also eliminate some of the signs and symptoms of diseases such as AD and PD. Keeping this in mind we focused on the structures of rasagiline, pentacyclo-undecane and amantadine. Rasagiline is a well known MAO-B inhibitor with promising neuroprotective activity, which can be attributed to its propargylamine moiety. The pentacyclo-undecane and amantadine polycyclic structures are highly non-polar compounds, with high potential to contribute in the transport of drugs across the blood-brain barrier and across cell membranes into cells. It was thus a rational decision to incorporate the structures of propargylamine and the polycyclic cages into the structures of the synthesised compounds. In synthesising the test compounds, several experimental procedures and methods, both conventional and modern, were utilised. In most of the synthetic routes either propargylamine, propargylbromide or ethynyl magnesium bromide were utilised together with pentacyclo-undecane or amantadine as reagents. The reaction of these substances afforded the target compounds. Most of the reactions took place without the necessity of a catalysing agent, even though an external source of energy was necessary to provide heat for the reactions to take place. Each compound was synthesised to evaluate the activity and benefit of the presence of a certain group of atoms in the molecule. Some of these groups include a terminal acetylene group, an acetylene group between two non-polar groups, a secondary propargylamine connected to a polycyclic cage and a tertiary propargylamine in an aza polycyclic structure. These synthesised compounds were evaluated for anti-apoptotic as well as MAO-B inhibiting activity. The anti-apoptotic activity were evaluated in vitro using the Desphiper™ kit, which marks changes in the mitochondrial membrane potential, that takes place during apoptosis. The quantitative and qualitative detection of these processes were done by means of flow cytometry, which made it possible to determine what percentage of the cells in the samples were still viable after treatment with the synthesised compounds. For this purpose SK-NBE(2) neuroblastoma cells were used, and apoptosis was induced using a serum-deprivation model. Besides their anti-apoptotic activity, the synthesised compounds were also evaluated in vitro as competitive inhibitors of MAO-B using a spectrophotometric assay that utilised MMTP, an analogue of the neurotoxin MPTP as substrate, with baboon liver mitochondria serving as enzyme source. The potency of MAO-B inhibition was expressed as percentage inhibition of the enzyme. In evaluating the activity of the synthesised compounds, there was one compound, 8- phenylethynyl-8-hydroxy-pentacyclo[5.4.1.02'6.03,10.05,9]undecane, that had significant MAO-B inhibiting activity as well as significant anti-apoptotic activity. The other compounds of the series all had moderate to weak anti-apoptotic activity, and showed limited MAO-B inhibition. Having moderate to weak anti-apoptotic activity, these compounds can be used as leadcompounds in the development and design of more potent inhibitors. It was confirmed that when linked to a polycyclic cage structure, propargylamine still displayed potent antiapoptotic activity. It was also determined that a terminal acetylene group, as well as an acetylene group between two non-polar groups had comparable anti-apoptotic activity. The polycyclic propargylamines thus have potential as neuroprotective agents and further investigation is necessary to determine what their maximum benefit can be in the treatment of neurodegenerative diseases.