Acetylcholine - Drug Design and Discovery - page 269

In 1993, 9-amino-1,2,3,4-tetrahydroacrine (tacrine, 16.9 ) was marketed as the i rst AChEI against
AD. It is a reversible, nonselective AChE/BuChE inhibitor, which also displays activities at monoam-
ine oxidases, potassium channels, and mAChR and nAChR subtypes. In fact, the “dirty” proi le of
tacrine has been proposed to contribute to its therapeutic effects. In 1997, the piperidine-based ligand
donepezil ( 16.10 ) was rationally designed and marketed for treatment of AD. Donepezil inhibits
AChE in a reversible and noncompetitively manner and displays a signii cant selectivity for AChE
over BuChE. Medicinal chemistry explorations into the “carbamate-stigmine” structure of phys-
ostigmine ( 16.11 ) from the Calabar bean ( Physostigma venenosum ) has given rise to several impor-
tant analogs, including eptastigmine ( 16.12 ), rivastigmine ( 16.13 , marketed in 2000), and phenserine
( 16.14 , currently in phase III trials), which all exhibit inhibitory activities at both AChE and BuChE.
Galanthamine ( 16.15 ), a phenantrene alkaloid originally isolated from Galanthus nivalis , was mar-
keted for treatment of AD in 2001. The natural product huperzine A ( 16.16 ) has been isolated from the
Chinese folk medicine Huperzia serrata , and it is a potent AChEI with no activity at the BuChE.
Substantial efforts in medicinal chemistry have gone into the optimization of existing AChEIs.
Developed hybrid compounds combining structural components from two AChEIs, such as the
tacrine/hyperzine A hybrid, huprine X ( 16.17 ), have displayed higher inhibitory potencies, and in
some cases different kinetic properties and/or AChE/BuChE selectivities, than their parent com-
pounds. In other hybrids, substructures of AChEIs have been combined with molecular compo-
nents of other drugs hereby giving rise to novel compounds, where the AChE activity has been
supplemented with activities at other neurotransmitter systems. For example, rivastigmine ( 16.13 )
constitutes the template of ladostigil ( 16.18 ), where the structure has been combined with the pro-
pylargyl group of the MAO-B inhibitor rasagiline, and of BCG 20-1259 ( 16.19 ), an inhibitor of both
AChE and the serotonin transporter. Another popular strategy has been the development of bivalent
ligands, such as the bivalent tacrine-indole ligand 16.20 , which displays a low picomolar IC 50 value
at AChE. Finally, the observation that regulation of the synaptic ACh concentrations appears to
become more and more dependent on BuChE as AD progresses has inspired the development of
completely selective BuChEIs, including several cymserine analogs ( 16.21 ) (Figure 16.5).
16.3.2 S UBSTRATE C ATALYSIS OF THE AChE AND L IGAND B INDING TO I T
At the molecular level, AChE is a 537 amino acids long protein composed of a 12-stranded mixed
b-sheet surrounded by 14 a-helices (Figure 16.6A). The hydrolysis of ACh in AChE takes place at
the bottom of a long and narrow gorge lined with numerous aromatic amino acid residues that
penetrates half into the enzyme. The active site is located ~20 Å from the surface of the enzyme and
is composed of two subsites. In the “catalytic anionic site” the choline moiety of ACh is stabilized by
a cation-p interaction between the quaternary amino group of ACh and the aromatic ring system of
the Trp 84 residues with minor contributions from the Glu 199 and Phe 330 residues, whereas the “ester-
atic subsite” contains a typical serine-hydrolase catalytic triad consisting of the residues Ser 200 ,
His 440 , and Glu 327 (Figure 16.6A). In addition, binding is stabilized by interactions between the car-
bonyl oxygen and the acetyl group of ACh with neighboring residues in AChE. Another binding site
for ACh, the “peripheral anionic site” (PAS), is located on the surface of the enzyme at the entrance
to the gorge, ~20 Å above the active site and contains the Trp 70 , Asp 72 , Tyr 121 , Trp 279 , and Phe 331 resi-
dues (Figure 16.6A). In addition to PAS being involved in processes important for other aspects of
AChE function, binding of ACh to PAS represents the i rst step in the ACh catalysis, as the trapping
of the substrate on its way to the active site enhances the catalytic efi ciency of the process. From the
PAS, ACh is transferred to the active site, where the catalysis occurs (Figure 16.7A).
The complex processes underlying ACh catalysis is rel ected in the diverse binding modes dis-
played by the reversible AChEIs both in terms of their binding sites and the mechanisms underlying
their inhibition. The “carbamoylating” inhibitors, compounds 16.11-16.14 , all contain a carbamate
group, which analogously to the ester group in ACh can be hydrolyzed by AChE (Figure 16.7B).
Thus, these AChEIs are split into their carbamate moiety and their stigmine ( 16.11 , 16.12 , and
16.14 ) or dimethylamino-a-methylbenzyl ( 16.13 ) moieties interacting with the Ser 200 residue and