Acetylcholinesterase inhibitors analogs

Mode of Action. All of the insecticidal carbamates are cholinergic, and poisoned insects and mammals exhibit violent convulsions and other neuromuscular disturbances. The insecticides are strong carbamylating inhibitors of acetylcholinesterase and may also have a direct action on the acetylcholine receptors because of their pronounced stmctural resemblance to acetylcholine. The overall mechanism for carbamate interaction with acetylcholinesterase is analogous to the normal three-step hydrolysis of acetylcholine however, is much slower than with the acetylated enzyme. 

In the synthesis of fluorinated analogs of the acetylcholinesterase inhibitor, huperzine A, it was necessary to accomplish reductive elimination of the diol 15-D to 15-E. Of the methods for diol reduction, which seems most compatible with the other functional groups in this compound ... 

Organophosphate insecticides with the P=S group are oxidatively desulfurated by cytochrome P450 monooxygenases of insects to their corresponding P=0 analogs. This reaction results in activation (increased toxicity), because the product, P=0, binds more tightly to the acetylcholinesterase than the parent compound and, thus, to more potent acetylcholinesterase inhibitors. For example, parathion is oxidatively desulfurated to paraoxon. 

Kozikowski, A. R Campiani, G. Tuckmantel, W. An approach to open-chain and modified heterocyclic-analogs of the acetylcholinesterase inhibitor huperzine-A through a bicyclo[3.3.1]nonane intermediate. Heterocycles, 1994, 39(1) 101-116. 

A. Classification and Prototypes The three major classes of insecticides are the chlorinated hydrocarbons (DDT and its analogs), acetylcholinesterase inhibitors (carbamates, organophos-phates), and the botanical agents (nicotine, rotenone, pyrethrum alkaloids). 

A diversity of thiolactams has been prepared from the corresponding lactams by use of LR or of the modified reagent (13). Monobactam analogs, e.g., the weakly antibacterial -thiolactam (93), have been prepared by thionation of suitably protected optically active /3-lactams with LR or Davy s reagent (2,4-bis (methylthio)-l,3,2,4-dithiadiphosphetane disulfide), deprotection and subsequent introduction of the side chain. Very recently, an efficient solid-phase synthesis of 1,3,4-trisubstituted /3-thiolactams has been described. The y-thiolactam (94) has been prepared as an intermediate for the synthesis of analogs of the acetylcholinesterase inhibitor huperzine B. Transformation of the quinoline alkaloid cytisine into its thio analog (95) enhanced its biological activity. ... 

As a result, the penicillin occupies the active site of the enzyme, and becomes bound via the active-site serine residue. This binding causes irreversible enzyme inhibition, and stops cell-wall biosynthesis. Growing cells are killed due to rupture of the cell membrane and loss of cellular contents. The binding reaction between penicillinbinding proteins and penicillins is chemically analogous to the action of P-lactamases (see Boxes 7.20 and 13.5) however, in the latter case, penicilloic acid is subsequently released from the P-lactamase, and the enzyme can continue to function. Inhibitors of acetylcholinesterase (see Box 7.26) also bind irreversibly to the enzyme through a serine hydroxyl. 

In a discovery project that is reminiscent of the discovery of captopril, scientists at Takeda created a hypothetical structure for the active site of acetylcholinesterase, based on SAR from previous biochemical and medicinal chemical work (141). The model consisted of (in addition to the serine protease-like catalytic machinery) an anionic binding site separating two discrete hydrophobic binding sites. This model was then used to design inhibitors of the enzyme (reviewed i n ref. 142). One set of analogs examined were based on the N-((o-phthalimidylalkyl)-iV-(a)-phenylalkyl)-amine (scaffold 66). An iterative process of testing. 

Nair, H. K. and Quinn, D. M. (1993) m-Alkyl a,a,a-trifluoroacetophenones A new class of potent transition state analog inhibitors of acetylcholinesterase. Bioorg. Med. Chem. Lett., 3, 2619-2622. 

Five novel carbonates/ designed as suicide (mechanism-based) inhibitors of acetylcholinesterase, were synthesized and evaluated against the enzyme in vitro and screened for insecticidal activity. The design strategy of inhibition was based on the isosteric relationship of carbonates to the ester of the natural substrate acetylcholine, and on the release of electrophilic quinone methides or alpha-chloroketones at the active site after enzymatic carbonate hydrolysis. Most coirpounds were inhibitory in vitro, with good specificity for acetylcholinesterase. Some showed modest insecticidal activity. Results of kinetic studies on one analog were consistent with mechanism-based inhibition. 

Because of the difficulties encountered, until recently, in obtaining pure acetylcholinesterase, studies of the kinetics of the interaction of this enzyme with substrates and inhibitors have been one of the primary tools for obtaining Information about its active sites. Since the classical studies of Wilson and Nachmansohn, the concept of "anionic" and "esteratlc" sites has become generally accepted. Evidence has now been presented by O Brien et al. , on the basis of inhibition studies with phosphate and phosphorothlolate analogs, that acetylcholinesterase contains a "hydrophobic patch" near the esteratlc site. 

The report that S-bromoacetyl coenzyme A is a potent inhibitor of choline acetylase , was followed by the recent report of a group of styrylpyrldine analogs also capable of inhibiting this enzyme OS, One of these, 4-(l—naphthylvinyl)pyridine is claimed to inhibit choline acetylase without affecting acetylcholinesterase, Quatemlzatlon of this compound permits it to inhibit both enzymes. 

As the intention was to synthesize new inhibitors of acetylcholinesterase, the anticholinesterase activity of the above-described phosphonates and phosphonothioates was studied. In 1978, Zakharova et al. examined the activity of dichlorovinyl-substituted carbaboranyl phosphonates 27-30 toward acetylcholinesterase and butyrylcholinesterase. As these compounds are analogs of 2,2-dichlorovinyl dimethyl phosphate (dichlorvos or DDVP) (26), their activity was compared with the activity of this known insecticide. As shown in Table 2.1, compounds 27-30 exhibit remarkably lower activity toward both enzymes than the phosphate DDVP. The replacement of an alkoxyl group with the bulky carborane moiety increases the steric demand of the compound and therefore hinders binding on the surface of the enzyme and the following phosphorylation of the hydroxyl group of serine. Additionally, the type of inhibition changed in snch a way that the compounds exhibit reversible instead of irreversible binding. The substituents at the second carbon atom of the carborane core have only minor influence on the inhibitor activity. 

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