Several novel analogues of tacrine have been synthesized and tested for their ability to inhibit acetylcholinesterase, butyrylcholinesterase, and neuronal uptake of 5-HT (serotonin) and noradrenaline. Changes in the size of the carbocyclic ring of tacrine produced modest potency against cholinesterase enzymes. Addition of a fourth ring resulted in compounds with marked selectivity for acetylcholinesterase (AChE) over butyrylcholinesterase (BChE): e.g. 6-amino-4,5-benzo-5H-cyclopenta[1,2-b]-quinoline (14a) had an IC50 of 0.35 microM against AChE and 3.1 microM against BChE. Some tetracyclic compounds are 100-400 times more active than tacrine as inhibitors of neuronal uptake of serotonin, in particular 13-amino-6,7-dihydro-5H-benzo-[3,4]cyclohepta[1,2-b]quinoline (18), which had an IC50 of 20 nM. These compounds would be expected to facilitate both cholinergic and monoaminergic transmission. They should be worth investigating in models of memory impairment.
Syntheses and reactions of N-tosylphenacylamines with bases have been systematically examined. Monomeric C-methoxy-imines are available from some of these reactions. C-Methoxyphenacylarylamines, made by two routes, were converted into monomeric imines on treatment with noble-metal catalysts. The boron trifluoridecatalysed reactions of aryl aldehydes with sulphonamides provide a new and convenient route to N-sulphonylarylimines.
Three man synthetic routes to analogues of tacrine are described: reaction of anthranilonitriles with cyclohexanone and other ketones, reaction of various anilines with alpha-cyanoketones, and reactions involving anilines and cyclic beta-ketoesters. Although tacrine has a wide range of pharmacological effects, it is best known as an inhibitor of cholinesterase enzymes. Many of the analogues that have been made have not been tested against acetylcholinesterase or butyrylcholinesterase activity. Consequently, there is limited information from which a detailed understanding of structure-activity relationships can be derived. However, some halogenated derivatives are not only more potent acetylcholinesterase inhibitors than tacrine, they are also more selective for acetylcholinesterase than for butyrylcholinesterase.
The emergence of resistant strains of Plasmodium falciparum and Mycobacterium tuberculosis underscores the need for novel drugs that are effective against these microorganisms. As part of our screening programme of the flora of Puerto Rico, we tested a number of ethanol extracts of higher plants for antiplasmodial and antimycobacterial activities. A total of 40 extracts belonging to 23 plant families and 37 species were tested for antiplasmodial activity. Five extracts demonstrated activity against Plasmodium falciparum in vitro (50%-100% parasite suppression at 5 microg/mL). Another 63 extracts belonging to 30 plant families and 50 species were tested in vitro against Mycobacterium tuberculosis. Two extracts were found to be active, Ficus citrifolia and Pisonia borinquena (85% or more inhibition of microbial growth at 100 microg/mL of extract).
From the bulbs and leaves of Hymenocallis expansa (Amaryllidaceae), three alkaloid constituents were identified: (+)-tazettine, (+)-hippeastrine, and (-)-haemanthidine. These alkaloids demonstrated significant cytotoxicity when tested against a panel of human and murine tumor cell lines.
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