Stereo-specific synthesis of analogs of nerve agents and their utilization for selection and characterization of paraoxonase (PON1) catalytic scavengers.

Fluorogenic organophosphate inhibitors of acetylcholinesterase (AChE) homologous in structure to nerve agents provide useful probes for high throughput screening of mammalian paraoxonase (PON1) libraries generated by directed evolution of an engineered PON1 variant with wild-type like specificity (rePON1). Wt PON1 and rePON1 hydrolyze preferentially the less-toxic R(P) enantiomers of nerve agents and of their fluorogenic surrogates containing the fluorescent leaving group, 3-cyano-7-hydroxy-4-methylcoumarin (CHMC). To increase the sensitivity and reliability of the screening protocol so as to directly select rePON1 clones displaying stereo-preference towards the toxic S(P) enantiomer, and to determine accurately K(m) and k(cat) values for the individual isomers, two approaches were used to obtain the corresponding S(P) and R(P) isomers: (a) stereo-specific synthesis of the O-ethyl, O-n-propyl, and O-i-propyl analogs and (b) enzymic resolution of a racemic mixture of O-cyclohexyl methylphosphonylated CHMC. The configurational assignments of the S(P) and R(P) isomers, as well as their optical purity, were established by X-ray diffraction, reaction with sodium fluoride, hydrolysis by selected rePON1 variants, and inhibition of AChE. The S(P) configuration of the tested surrogates was established for the enantiomer with the more potent anti-AChE activity, with S(P)/R(P) inhibition ratios of 10-100, whereas the R(P) isomers of the O-ethyl and O-n-propyl were hydrolyzed by wt rePON1 about 600- and 70-fold faster, respectively, than the S(P) counterpart. Wt rePON1-induced R(P)/S(P) hydrolysis ratios for the O-cyclohexyl and O-i-propyl analogs are estimated to be >>1000. The various S(P) enantiomers of O-alkyl-methylphosphonyl esters of CHMC provide suitable ligands for screening rePON1 libraries, and can expedite identification of variants with enhanced catalytic proficiency towards the toxic nerve agents.

Laboratory setup for long-term monitoring of the volatilization of hazardous materials: preliminary tests of O-ethyl S-2-(N,N-diisopropylamino)ethyl methylphosphonothiolate on asphalt.

Contrary to commonly used pesticides, the rate of volatilization of extremely toxic chemicals such as the nerve agent O-ethyl S-2-(N,N-diisopropylamino)ethyl methylphosphonothiolate (VX) cannot be readily obtained under environmental conditions due to its high mammalian toxicity that would require extraordinary precautions. An alternative is a laboratory setup that would be used to obtain environmentally relevant data required for risk assessment studies. In this paper we describe a newly designed climatic hood that enables control of temperature, humidity, and air velocity within less than +/- 0.5% fluctuations during continuous operation. The performance of the evaporation system togetherwith the sampling and analytical procedures produced a meaningful concentration profile of vapors obtained from a 15 mg sample of VX dispersed as small droplets over a 10 x 16 cm piece of asphalt road. The released vapors amounted to approximately 30% of the applied mass, and its time course was best fitted to a triexponential curve with rate constants changing over time from 2.2 to 0.03 h(-1). The asphalt enhanced a specific degradation pathway of VX that is relatively minor in aqueous solutions. Results provide the first data on the volatilization of VX from samples of asphalt road, and offer an insight into VX behavior in the environment.

Role of edrophonium in prevention of the re-inhibition of acetylcholinesterase by phosphorylated oxime.

We examined the role of edrophonium in the acceleration phenomenon using mouse wild-type and mutant D74N AChE inhibited with 7-(O,O-diethyl-phosphinyloxy)-1-methylquinolinium methylsulfate (DEPQ). With DEPQ-inhibited wild-type mouse acetylcholinesterase (AChE), the reactivation kinetic profile demonstrated one-phase exponential association only when 2-[hydroxyimino methyl]-1-methylpyridinium chloride (2-PAM) and 1-(2-hydroxy-iminomethyl-1-pyridinium)-1-(4-carboxy-aminopyridi nium)-dimethyl ether hydrochloride (HI-6) were used as reactivators. When 1,1[oxybis-methylene)bis[4-(hydroxyimino)methyl] pyridinium dichloride (LüH6) and 1,1-trimethylene bis(4-hydroxyimino methyl) pyridinium dichloride (TMB4) were used, the reactivation kinetic profile was biphasic in nature. Edrophonium had no effect on reactivation by 2-PAM and HI-6, but significantly accelerated LüH6- and TMB4-induced reactivation of DEPQ-inhibited wild-type mouse AChE. Comparison of the initial and overall reactivation rate constants with five oximes indicated that acceleration by edrophonium may be due to the prevention of re-inhibition of the reactivated enzyme by the phosphorylated oxime (POX) produced during the reactivation. With LüH6 and TMB4, about 2.5-fold increase in the reactivation rate constants was observed in the presence of edrophonium, but little or no effect was observed with the other three oximes. The initial reactivation rate constants were 5.4- and 4.2-fold of the overall rate constants with LüH6 and TMB4 as reactivators respectively, however, very little change was found between the initial and overall rate constants with the other three oximes. In experiments with D74N AChE, for which the inhibition potency of charged organophosphate (OP) was two to three orders less than wild-type enzyme, edrophonium had no effect on the reactivation by LüH6 and TMB4 and the time courses of reactivation were monophasic. The data from mutant enzyme substantiate the involvement of edrophonium in protecting POX re-inhibition of reactivated enzyme formed during the reactivation of OP-inhibited AChE.

Characterization of O,O-diethylphosphoryl oximes as inhibitors of cholinesterases and substrates of phosphotriesterases.

Reactivators of organophosphate (OP)-inhibited cholinesterases (ChEs) are believed to give rise to phosphorylated oximes (POX) that reinhibit the enzyme. Diethylphosphoryl oximes (DEP-OX) that were generated in situ were demonstrated in the past to be unstable, yet were more potent inhibitors of acetylcholinesterase (AChE) than the parent OPs. In view of the inconsistencies among reported results, and the potential toxicity of POXs, it seemed important to characterize authentic DEP-OXs, and to evaluate their interference with reactivation of diethylphosphoryl-ChE (DEP-ChE) conjugates. To this end, the diethylphosphoric acid esters of 1-methyl-2-pyridinium carboxaldehyde oxime (DEP-2PAM) and 1-methyl-4 pyridinium carboxaldehyde oxime (DEP-4PAM) were synthesized and chemically defined. The half-lives of DEP-2PAM and DEP-4PAM in 10 mM Tris buffer, pH 7.8, at 29 degrees were found to be 10 and 980 sec, respectively. The two DEP-OXs inhibited ChEs with the following ranking order: for DEP-2PAM, human butyrylcholinesterase (HuBChE, k(i) = 2.03 x 10(9) M(-1) min(-1)) > mouse AChE (MoAChE) approximately equal to fetal bovine serum AChE (FBS-AChE) approximately equal to equine BChE (EqBChE); for DEP-4PAM, HuBChE (k(i) = 0.71 x 10(9) M(-1) min(-1)) > EqBChE > MoAChE > FBS-AChE. A dialkylarylphosphate hydrolase (phosphotriesterase; PTE) from Pseudomonas sp. catalyzed the hydrolysis of DEP-4PAM with k(cat)/Km = 3.56 x 10(7) M(-1) min(-1) and Km = 0.78 mM. Reactivation of DEP-ChEs was enhanced by PTE when 4-PAM-based oximes were used as reactivators, whereas reactivation with 2-PAM-based oximes was not affected by PTE. This observation is attributed primarily to the short half-life of DEP-OXs derived from the latter oximes. Relatively low doses of PTE can detoxify large quantities of DEP-OXs rapidly, and thereby augment the efficacy of antidotes that contain the oxime function in position 4 of the pyridine ring.

Oxidative biodegradation of phosphorothiolates by fungal laccase.

Organophosphorus (OP) insecticides and nerve agents that contain P-S bond are relatively more resistant to enzymatic hydrolysis. Purified phenol oxidase (laccase) from the white rot fungus Pleurotus ostreatus (Po) together with the mediator 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) displayed complete and rapid oxidative degradation of the nerve agents VX and Russian VX (RVX) and the insecticide analog diisopropyl-Amiton with specific activity: k(sp) = 2200, 667 and 1833 nmol min(-1) mg(-1), respectively (pH 7.4, 37 degrees C). A molar ratio of 1:20 for OP/ABTS and 0.05 M phosphate at pH 7.4 provided the highest degradation rate of VX and RVX. The thermostable laccase purified from the fungus Chaetomium thermophilium (Ct) in the presence of ABTS caused a 52-fold slower degradation of VX with k(sp) = 42 nmol min(-1) mg(-1). The enzymatic biodegradation products were identified by 31P-NMR and GC/MS analysis.

Combined effect of organophosphorus hydrolase and oxime on the reactivation rate of diethylphosphoryl-acetylcholinesterase conjugates.

Reactivation of inhibited acetylcholinesterase (AChE) is essential for rapid recovery after organophosphate (OP) poisoning. However, following administration of an oxime reactivator, such as pralidoxime mesylate (P2S), in patients poisoned with certain diethylphosphorothioate pesticides, no reactivation is observed, presumably due to reinhibition by circulating anti-cholinesterase OPs. Pretreatment alone with organophosphorus hydrolases (OPH) that are capable of rapidly hydrolyzing OPs was demonstrated, in animals, to confer significant protection against OP toxicity. One strategy to augment the potentially therapeutic scope of OPHs is a combined post-exposure treatment consisting of a drug(s) commonly used against OP toxicity and a suitable hydrolase. In this study, we examined the in vitro ability of OPH from Pseudomonas sp. (OPHps) to prevent reinhibition of P2S-reactivated AChE by excess OPs. The kinetic parameters of the reactivation of a series of diethylphosphoryl-AChE (DEP--AChE) conjugates, obtained by the use of various diethylphosphates, were determined and compared with the rates of reactivation in the presence of OPHps, with and without the OP inhibitors in the reactivation medium. Extrapolation of the in vitro results to in vivo conditions suggests that an OPHps concentration as low as 1 microgram/mL blood would result in a 100-fold decrease in the concentration of circulating anti-AChE pesticides within less than one blood-circulation time, thereby minimizing reinhibition of the reactivated enzyme. Thus, for DEP-based pesticides, the combination of P2S-OPH treatment can significantly improve clinical recovery after OP intoxication. In addition, it is shown here for the first time that an OPH can effectively hydrolyze quaternary ammonium-containing OPs. This indicates that hydrolysis of phosphorylated oximes, toxic side products of oxime treatment, may also be accelerated by OPHs.

Synthesis and antimuscarinic activity of 2-[N-(ethyl)-(N-beta-hydroxyethyl)]aminoethyl 2,2-diphenylpropionate, a metabolite of aprophen.

The preparation of 2-[N-(ethyl)-(N-beta-hydroxyethyl)]amino-ethyl 2,2-diphenylpropionate (1), a metabolite of aprophen [2-diethylaminoethyl 2,2-diphenylpropionate], is described. Hydrolysis of [2-(2-chloroethyl)ethylamino]ethyl acetate hydrochloride (2) in a basic solution, followed by acidic pH adjustment, gave the ethylcholineaziridinium ion (3) that upon treatment with 2,2-diphenylpropionic acid produced 1 in a 56% yield. Synthetic 1 was found to possess antimuscarinic activities, but was approximately 10-fold less potent than the parent compound aprophen.

Protection against tabun toxicity in mice by prophylaxis with an enzyme hydrolyzing organophosphate esters.

We demonstrate here the correlation between protection afforded by pretreatment alone with parathion hydrolase purified from Pseudomonas sp. against tabun toxicity in mice and the kinetic parameters which are assumed to determine the in vivo detoxification of tabun by the same enzyme. Results show that 15 and 22 micrograms of parathion hydrolase per animal conferred a protective ratio of 3.94 and 5.65 respectively, against tabun toxicity, without post-exposure treatment.

Muscarinic receptor subtype specificity of (N,N-dialkylamino)alkyl 2-cyclohexyl-2-phenylpropionates: cylexphenes (cyclohexyl-substituted aprophen analogues).

A series of aprophen [(N,N-diethylamino)ethyl 2,2-diphenylpropionate] analogues, called cylexphenes, were synthesized with alterations in (1) the chain length of the amine portion of the ester, (2) the alkyl groups on the amino alcohol, and (3) a cyclohexyl group replacing one of the phenyl rings. The antimuscarinic activities of these analogues were assessed in two pharmacological assays: the inhibition of acetylcholine-induced contraction of guinea pig ileum, and the blocking of carbachol-stimulated release of alpha-amylase from rat pancreatic acinar cells. These two tissues represent the M3(ileum) and M3(pancreas) muscarinic receptor subtypes. In addition, the analogues were also evaluated for their competitive inhibition of the binding of [3H]NMS to selected cell membranes, each containing only one of the m1, M2, m3, or M4 muscarinic receptor subtypes. The m1 and m3 receptors were stably transfected into A9 L cells. The replacement of one phenyl group of aprophen with a cyclohexyl group increased the selectivity of all the analogues for the pancreatic acinar muscarinic receptor subtype over the ileum subtype by more than 10-fold, with the (N,N-dimethylamino)propyl analogue exhibiting the greatest selectivity for the pancreas receptor subtype, over 30-fold. The cylexphenes also showed a decrease in potency in comparison to the parent compound when examined for the binding of [3H]NMS to the M2 subtype. In agreement with the pharmacological data obtained from the pancreas, the (N,N-dimethylamino)propyl cylexphene 3 demonstrated the greatest selectivity for the m3 subtype, and additionally showed a preference for the m1 and M4 receptor subtypes over the M2 receptor subtype in the binding assay. Thus, this compound showed a potent selectivity according to the pharmacological and binding assays between the muscarinic receptor subtypes of the pancreas and ileum. In both the pharmacological and binding assays, the potency of the analogues decreased markedly when the chain length and the bond distance between the carbonyl oxygen and protonated nitrogen were increased beyond three methylene groups. When the structures of these analogues were analyzed using a molecular modeling program, the bond distance between the carbonyl oxygen and protonated nitrogen was deduced to be more important for the antagonist activity than subtype specificity.

Isolation and identification of beta-hydroxyethylaprophen: a urinary metabolite of aprophen in rats.

The metabolism of the anticholinergic drug aprophen was studied in rats after oral administration via stomach intubation. beta-Hydroxyethylaprophen, a major urinary metabolite of aprophen, was isolated and identified by normal-phase high-performance liquid chromatography and electron ionization mass spectrometry. More than 22% of the parent drug was recovered and quantified over a 72-h collection period. Results show that 2,2-diphenylpropionic acid, another major metabolite of aprophen which lacks anticholinergic properties, was also isolated and identified in this study. Experiments are currently underway to synthesize and test the anticholinergic properties of beta-hydroxyethylaprophen in mammals.

Aprophit: an irreversible antagonist for muscarinic receptors.

The development of selective irreversible ligands has proven to be an invaluable technique for the isolation, purification and characterization of many receptor proteins. An isothiocyanato-derivative of the muscarinic antagonist aprophen was synthesized and evaluated as a potential irreversible ligand for muscarinic receptors. This compound (aprophit) displaced [3H]N-methylscopolamine binding from rat cerebral cortex with a Ki of 3.1 x 10(-7) M. The inhibition was concentration-dependent and could not be reversed by extensive washing. Aprophit inhibited the acetylcholine-stimulated release of catecholamines from isolated, perfused guinea pig adrenal glands in a concentration-dependent manner. This inhibition was not reversed by perfusing the tissue with Locke's solution and was not due to a non-selective acylation by the isothiocyanate function. The data suggest that aprophit is selectively acylating muscarinic receptor proteins and thus may be useful in their further characterization.

Binary antidotes for organophosphate poisoning: aprophen analogues that are both antimuscarinics and carbamates.

Prophylaxis against organophosphate poisoning can be achieved by pretreatment with physostigmine or pyridostigmine, which are carbamates, and aprophen, which is an anticholinergic agent. Thus, a series of aprophen analogues was synthesized with carbamyl substitutions on the phenyl rings (carbaphens). The rationale behind this design is that such compounds might exhibit most of the therapeutic characteristics of aprophen, as well as the ability to protect prophylactically by chemically masking cholinesterase enzymes. Compounds 4 (dimethylhydroxycarbaphen), 15 (dimethylcarbaphen), and 16 (monomethylcarbaphen) were found to inactivate human butyrylcholinesterase in a time-dependent manner with potencies similar to those of physostigmine or pyridostigmine, and the latter two exhibited almost the same antimuscarinic profile as aprophen. In contrast to the potent inactivation of butyrylcholinesterase by these compounds, marginal inactivation of acetylcholinesterase activity was observed, and only at much higher drug concentrations. The noncarbamylated analogues had no effect on the activity of either cholinesterase. The carbaphen compounds are hence prototype drugs that can interact with either muscarinic receptors or butyrylcholinesterase. Furthermore, these compounds are prodrugs, since after carbamylation of the cholinesterase, the leaving group 14 (hydroxyaprophen) is a potent antimuscarinic itself.

In vitro and in vivo protection of acetylcholinesterase against organophosphate poisoning by pretreatment with a novel derivative of 1,3,2-dioxaphosphorinane 2-oxide.

Covalent molecular combinations of a cyclic phosphate (dioxaphosphorinane) and a potential leaving group, such as 3-(trimethylammonio)phenol iodide (TMPH), suggested the synthesis of O-[3-(trimethylammonio)phenyl]-1,3,2-dioxaphosphorinane 2-oxide iodide (TDPI). TDPI inhibited acetylcholinesterase (AChE) (ki = 8.4 x 10(3) M-1 min-1) via the formation of an unstable covalent intermediate. TDPI-inhibited AChE hydrolyzed spontaneously with t1/2 approximately equal to 10 min. Butyrylcholinesterase (BuChE) was also inhibited by TDPI (ki = 1.8 x 10(4) M-1 min-1), but the inhibited BuChE was more stable (greater than 10 times) than the corresponding AchE-TDPI conjugate. Pretreatment of mice with TDPI conferred protection against 22 LD50's of paraoxon and 5 LD50's of soman, provided that treatment with anticholinergics and an oxime followed administration of these anticholinesterase poisons. Correlation between in vitro and in vivo observations suggests that the main protection of AChE conferred by TDPI results from temporary masking of the active site of the enzyme. The acute toxicity of TDPI was found to be 444 mg/kg (sc, mice), whereas analogous carbamates and a noncyclic phosphate also displaying antidotal properties are greater than 170 times more toxic.

Formation of an unstable covalent intermediate during the inhibition of electric-eel acetylcholinesterase with 1,3,2-dioxaphosphorinane 2-oxides.

The kinetics of interaction of eel acetylcholinesterase (EC 3.1.1.7) with 1,3,2-dioxaphosphorinane 2-oxides were investigated. It was demonstrated that the rate of spontaneous re-activation as well as the re-activation profile in the presence of 2-pyridine aldoxime methiodide of the inhibited enzyme are irrespective of the leaving group of three inhibitors and exhibit the same values. The dissociation constant of the corresponding Michaelis complex was evaluated by two independent methods and the results were found to be in close agreement. It was shown that the active site is essential for interaction between the enzyme and the various dioxaphosphorinanes. The mixed anhydride of diethyl phosphoric acid and 2-hydroxy-1,3,2-dioxaphosphorinane 2-oxide behaves exactly as would be predicted from a typical diethyl phosphate inhibitor. Enxyme that was incubated with the cyclic acid or the corresponding methyl ester recovered immediately upon extensive dilution. Inhibition of enzyme in the presence of high concentratasions of the corresponding 2-chloro and 2-fluoro derivatives decreased the regeneration rates as well as the maximal amount of the re-activated enzyme. This observation could not be explained in terms of a classical aging process. On the basis of the kinetics observations it is suggested that an unstable covalent phospho-enzyme intermediate is formed during the reaction between acetylcholinesterase and 1,3,2-dioxaphosphorinane 2-oxides.

Sugar-oximes, new potential antidotes against organophosphorus poisoning.

In attempt to improve distribution and transport qualities of antidotes against organophosphorus poisoning, a new series of pyridine aldoximes linked to glucose moiety were synthesized and studied both in vivo and in vitro. Preliminary results describing the biological activity of the new compounds are presented and discussed in this report.