The synthesis and evaluation of (E)-styrylisatin analogues as inhibitors of monoamine oxidase B
Abstract
Monoamine oxidase B (MAO-B) inhibitors are currently clinically used in the symptomatic
treatment of Parkinson's disease (PO) and may also possess .neuroprotective activity. The
irreversible MAO-'B inhibitor, (R)-deprenyl, .is commonly used in PO treatment, usually in
combination with levodopa during dopamine replacement therapy. In contrast with
reversible inhibition, .enzyme recovery after irreversible inhibition involves de novo synthesis.
The relatively quick return of enzyme activity makes potent reversible MAO-B inhibitors safer
and more desirable.
Both isatin and caffeine are small molecules that have been reported to inhibit MAO-B.
!satin is a relatively good endogenous inhibitor (K = 3 µM) whereas caffeine is a weak
inhibitor (K1 = 650 µm) . The inhibitory potency of caffeine have been improved by
substitution at C-8 of the caffeine ring with a styryl side-chain. Addition of an electron
withdrawing substituent at C-3 of the phenyl ring produced structures with exceptional
reversible MAO-B .inhibitory potency, for example (E)-8-(3-chlorostyryl)caffeine (CSC) (K1 =
0.1 µM) .In this study we investigated whether styryl substitution of the lead compound,
isatin, at C-5 and C-6 will similarly enhance isatin's MAO-B inhibitory potency.
Preliminary computer modelling of the proposed (E)-5-sty.rylisatin and (E)-6-styrylisatin
analogues, as well as isatin, into the active site of recombinant MAO-B supported the
hypothesis of increased MAO-B inhibitory activity for these styryl analogues. The styryl sidechain
seems to be stabilised in the entrance cavity of the enzyme during binding, while the
isatin moiety is located in the substrate-binding cavity where it is involved in hydrogen
bonding. This duel binding mode is similar to that proposed for the styrylcaffeines and is
thought to facilitate the potent MAO-B inhibition of styrylcaffeines.
After successful synthesis of the (E)-5-styrylisatin and (E)-6-styrylisatin analogues, the
compounds were evaluated in vitro as reversible inhibitors of baboon liver MAO-B. Inhibitory
activity of the styrylisatin analogues were determined with a spectrophotometric method.
The inhibitory potencies for all the styrylisatin analogues were expressed in terms of the
concentration of the compound necessary for 50% inhibition of the enzyme in vitro (ICsa
value). Reversibility of inhibition was confirmed with a time-dependent inhibition study that
showed that the potencies of inhibition of MAO-B by the (E)-styrylisatin analogues were
independent of the time period for which the analogue was incubated with the enzyme.
The results confirmed that substitution with a styryl side-chain on C-5 of the isatin ring
resulted in reversible inhibitors of MAO-B that are exceptionally potent. For example, (E)-5-
(3-chlorostyryl)isatin (IC50 = 20.7 nM) was found to be 430 times more potent than the lead
,compound isatin (IC50 = 8.6 µM). Also, (E)-5-styrylisatin was found to have a IC50 value of
41.7 nM, which is approximately 210 times more potent as a MAO-B inhibitor than isatin.
The (E)-6-styrylisatin analogues showed moderate, reversible MAO-B inhibition. For
example, (E)-6-styrylisatin has an IC5o value of 436.8 nM. The styryl side-chain as well as
electronic and lipophilic properties seem to be important for potent inhibition of MAO-B by
these compounds.
In this study, the synthesised (E)-5-styry\isatin analogues, in particular (E)-5-(3-
chlorostyryl)isatin, showed potential as novel reversible MAO-B inhibitors. Synthesis and
evaluatin of a more extensive series of styrylisatin analogues would enable a Hansch-type
structure-activity relationship study that could identify the optimal physicochemical properties
for the compounds.
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