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In the past decades a vast number of oximes have been synthesised in order to identify effective compounds for the reactivation of organophosphorus compound (OP)-inhibited acetylcholinesterase (AChE). Up to now, oxime efficacy has been tested primarily in animal experiments. However, the accretive evidence of substantial species differences regarding kinetic properties of human and animal AChE led to an increasing number of in vitro kinetic studies quantifying the reactivating potency of oximes. These data were shown to provide a basis for the selection of effective oximes and for defining adequate oxime doses in human OP poisoning. This review will discuss experimental and theoretical models for the in vitro assessment of oxime efficacy and will give an overview of the present status in the evaluation of oximes as antidotes against OP poisoning.

Abstract: Human poisoning by organophosphates bearing two methoxy groups, e.g. by malathion, paraoxon-methyl, dimethoate and oxydemeton-methyl, is generally considered to be rather resistant to oxime therapy. Since the oxime effectiveness is influenced not only by its reactivating potential but also by inhibition, aging and spontaneous reactivation kinetics, experiments were performed with human acetyl- (AChE) and butyrylcholinesterase (BChE) to determine the respective kinetic constants. The efficacy of obidoxime in reactivating dimethylphosphoryl-AChE was 40, 9 and 3 times higher than of HI 6, pralidoxime and HL 7, respectively. Aging (t 1/2 3.7 h) and spontaneous reactivation (t 1/2 0.7 h) occurred concomitantly, with the portion of the aged enzyme being dependent on the presence of excess inhibitor. Calculation of steady-state AChE activity in the presence of inhibitor and oxime revealed that obidoxime was superior to pralidoxime. In addition, organophosphate concentrations up to 10−6 M (paraoxon-methyl) and 10−4 M (oxydemeton-methyl) could be counteracted at clinically relevant oxime concentrations (10 μM). These data indicate that oximes may effectively reactivate human dimethylphosphoryl-AChE. Failure of oximes may be attributed to megadose intoxications and to prolonged time intervals between poison uptake and oxime administration. The potency of the oximes to reactivate dimethylphosphoryl-BChE was much lower and the spontaneous reactivation slower (t 1/2 9 h), while aging proceeded at a comparable rate. Thus, BChE activity determination for diagnosis and therapeutic monitoring may give no reliable information on AChE status.

1 The effect of pyridostigmine on cardiorespiratory func tion after oxime+atropine injection was investigated in tabun poisoned guinea-pigs and without tabun poisoning.2 The trachea, a carotid artery and jugular vein were can nulated in female urethane-anaesthetised Pirbright white guinea-pigs. After baseline measurements the ani mals received pyridostigmine (0.05 μmol kg-1) and 30 min later atropine (29.5 μmol kg-1) plus obidoxime, HI 6 or HL6 7 (30 or 100 μmol kg-1) or tabun (1.85 μmol kg-1= 5 x LD50) followed by oxime+atropine treatment (all i.v.). Erythrocyte, brain and diaphragm acetyl cholinesterase (AChE) activity were determined. Similar groups without pretreatment were included for compar ison.3 Pyridostigmine aggravated the oxime+atropine induced hypotension and prevented the increase in heart rate but not the respiratory stimulation. The pyridostigmine inhibited AChE recovered only in the 100 μmol kg-1kg oxime groups at the end of the experiment.4 In tabun poisoning, pyridostigmine reduced the oxime+atropine induced circulatory recovery and decreased the survival time and rate. It did not affect the therapeutic oxime+atropine effect on respiratory function.5 These results suggest that pyridostigmine enhances oxime+atropine related circulatory depression which may be the reason for the reduced efficacy of oxime+atropine treatment in tabun poisoning. The pos sible mechanisms are discussed.

Abstract:  The new bispyridinium oximes HI 6 and HL 7 are promising antidotes against poisoning by highly toxic organophosphorus compounds, i.e. nerve agents. Until now, their ability to reactivate pesticide inhibited human acetylcholinesterase (AChE) has not been elucidated. For this purpose human erythrocyte AChE (EC 3.1.1.7) was inhibited (30 min) by chlorfenvinphos, dichlorvos, dicrotophos, heptenophos, mevinphos, monocrotophos, paraoxon, phosphamidon, trichlorfon, malaoxon, omethoate, oxydemeton-methyl or methamidophos by 85–98% of control. After removal of excess inhibitor, obidoxime, pralidoxime (2-PAM), HI 6 or HL 7 (10, 30 or 100 μmol/l) were added and the AChE activity was measured spectrophotometrically at various times thereafter (5–60 min). The oximes significantly, but not completely, reactivated organophosphate inhibited AChE. The velocity and extent of reactivation were dependent on the oxime and its concentration. In all cases obidoxime was superior to the three other oximes, followed by HL 7, 2-PAM and HI 6. In most cases obidoxime and HL 7 were most effective at 10 or 30 μmol/l while 2-PAM and HI 6 needed 100 μmol/l. These data suggest that 2-PAM, HI 6 and HL 7 are less patent than obidoxime in reactivating human AChE inhibited by organophosphate pesticides.

HLö 7 dimethanesulfonate (1-[[[4-(aminocarbonyl)pyridinio] methoxy] methyl] -2,4-bis [(hydroxyimino) methyl]pyridinium dimethanesulfonate) is a broad-spectrum reactivator against highly toxic organophosphorus compounds. The compound was synthesized by a new route with the carcinogenic bis(chloromethyl)ether being substituted by the non-mutagenic bis(methylsulfonoxymethyl)ether. The very soluble dimethanesulfonate of obidoxime was also prepared by this way. HLö 7 dimethanesulfonate is the first water-soluble salt of HLö 7 that should be suitable for the wet/dry autoinjector technology, because aqueous solutions of HLö 7 are not very stable (calculated shelf-life 0.2 years when stored at 8°C, 1 M solution, pH 2.5). The crystalline preparation contains 96% of thesyn/syn-isomer, less than 2% of thesyn/anti-isomer and some minor identified by-products. HLö 7 was very efficient in reactivating acetylcholinesterase (AChE) blocked by organophosphates as long as ageing did not prevent dephosphylation. HLö 7 was superior to HI 6 (1-[[[4-(aminocarbonyl)pyridinio]methoxy]methyl]-2-[(hydroxyimino)methyl]pyridinium dichloride) in reactivating soman and sarin-inhibited AChE from erythrocytes, and literature data indicate that HLö 7 exceeds HI 6 by far in reactivating tabun-inhibited AChE. In atropine-protected, soman-poisoned mice HLö 7 was three times more potent than HI 6 (protective ratio 5 versus 2.5), and in sarin-poisoned mice HLö 7 was 10 times more potent than HI 6 (protective ratio 8 for both oximes). In atropine-protected guinea-pigs HLö 7 was less effective than HI 6 (protective ratio: 2.3 versus 5.2 for soman; 5.2 versus 6.8 for sarin; 4.3 versus 3.8 for tabun). The mean survival time of anaesthetized guinea-pigs exposed to 5 LD50 soman (6.3 min) was increased by atropine (27 min) and atropine + HLö 7 (57 min). HLö 7 alone did not prolong the survival. The most impressive effect of HLö 7 was on respiration: 3 min after i.v. injection of HLö 7 and atropine, the depressed respiration increased rapidly to 60% of control and remained at that level during the observation period (60 min). With atropine alone, respiration recovered only slowly. Behavioural and physiologic parameters were determined in atropine-protected mice exposed to a sublethal soman dose. The running performance was significantly improved by HLö 7. Even central symptoms, e.g. hypothermia and convulsions, were decreased markedly by HLö 7 (evaluation 60 min after poisoning). The pharmacokinetic data for HLö 7 in male beagle dogs are similar to those of HI 6. After i.v. injection: t1/2α = 5 min; t1/2ß = 46 min; VD = 0.24 1/kg; Clp1 = 3.7 ml x min−1 x kg−1; Clren= 3.2 ml x min−1 x kg−1; renal excretion of unchanged HLö 7 = 86%. After i. m. injection: t1/2abs = 14 min; t1/2ß = 48 min; Vd = 0.27 1/kg; Clp1= 3.9 ml x min−1 x kg−1; Clren= 2.7 ml x min−1 x kg−1; renal excretion of unchanged HLö 7 = 76%; bioavailability >95%. Plasma protein binding was <5%; HLö 7 did not permeate into red cells. A dose of 20 μmol/kg was well tolerated both after i.v. and i.m. administration. In anaesthetized dogs (chloralose) HLö 7 i.v. (20 (imol/kg) showed marginal hypotensive effects, whereas 50 μmol/kg resulted in decreased mean blood pressure (−15%) and blood flow (−30%) without reflex tachycardia. One out of four dogs developed a circulatory shock syndrome with anuria. Respiration varied only transiently. Blood gases and pH were not influenced. Similar cardiovascular effects were observed in anaesthetized (urethane) guinea-pigs. In isolated guinea-pig hearts (Langendorff) sinus and ventricular heart rate were not influenced by HLö 7 <500 μM. HLö 7 antagonized both carbachol and nicotine effects. Red cell AChE was inhibited by HLö 7 by up to 50%; C50 about 100 μM. Previously, HLö 7 was shown to block ganglionic transmission (IC50= 500 μM), probably due to ion-channel blockade. These data indicate that HLö 7 combines ganglion blocking, anticholinergic and indirect cholinergic properties like other bispyridinium compounds. The results suggest that HLö 7 may be tolerated by man at a dose of 10 μmol/kg. Vital functions are not expected to be impaired. At such a dose (250–500 mg), which can be injected by an autoinjector, HLö 7 is expected to be superior to HI 6.

Abstract: Cyclohexylmethylphosphonofluoridate␣(cyclosarin) is a highly toxic organophosphate, which was shown to be rather resistant to conventional oxime therapy. To give more insight into the inhibition, reactivation and aging kinetics, human acetyl-(AChE) and butyrylcholinesterase (BChE) were inhibited by cyclosarin (k 2 of 7.4 and 3.8 * 108 M−1 min−1, respectively; pH 7.4, 37 �C) and reactivated with obidoxime, pralidoxime and three experimental oximes. The new oxime HL� 7 (1-[[[4-aminocarbonyl)-pyridinio]-methoxy]-methyl]-2,4-bis-[(hydroxyimino)methyl] pyridinium dimethanesulphonate) was shown to be superior to the other oximes. At oxime concentrations anticipated to be relevant in humans, obidoxime and pralidoxime were extremely weak reactivators of AChE. Aging velocity of BChE was almost fourfold higher compared to AChE (ka of 0.32 h−1 and 0.08 h−1, respectively). A substantial spontaneous reactivation was observed with AChE. These results support previous in vivo findings that obidoxime and pralidoxime are insufficient antidotes in cyclosarin poisoning. By contrast, HL� 7 was shown to be an extremely potent reactivator of human AChE and BChE,␣which supports its position as a broad-spectrum oxime.

1 Invitro studies with human erythrocyte acetylcholin esterase (AChE) and the mouse diaphragm model were performed to unravel the various microscopic reaction parameters that contribute to the dynamic equilibrium of AChE inhibition, ageing and reactivation. These data may help to define more precisely the indications and limitations of oxime therapy in organophosphate (OP) poisoning.2 Diethylphosphoryl-AChE resulting from intoxications with parathion, chlorpyrifos, chlorfenvinphos, diazi non and other OPs is characterized by slow sponta neous reactivation and low propensity for ageing. This kind of phosphorylated enzyme is particularly sus ceptible to reactivation by oximes.3 None of the oximes tested (pralidoxime, obidoxime, HI 6 and HLö 7) can be regarded as a universally suitable reactivator. Obidoxime turned out to be the most potent and most efficacious oxime in reactivating AChE inhibited by various classes of OP insecticides and tabun. Obidoxime, however, was inferior to HI 6 against soman, sarin, cyclosarin and VX. Pralidoxime was generally less potent.4 The kinetic data of reactivation established for diethylphosphoryl-AChE of human red cells indicate that the usually recommended dosage to attain a plasma concentration of 4 μg/ml does not permit exploitation of the full therapeutic potential of the oximes, in particular of pralidoxime. However, in suicidal mega-dose poisoning, oximes, even at optimal plasma concentrations, may be unable to cope with the fast re-inhibition of reactivated AChE in the first days following intoxication.5 It is suggested that oximes be administered by continuous infusion following an initial bolus dose as long as reactivation can be expected and until permanent clinical improvement is achieved.

Abstract: The treatment of poisoning by highly toxic organophosphorus compounds (nerve agents) is unsatisfactory. Until now, the efficacy of new potential antidotes has primarily been evaluated in animals. However, the extrapolation of these results to humans is hampered by species differences. Since oximes are believed to act primarily through reactivation of inhibited acetylcholinesterase (AChE) and erythrocyte AChE is regarded to be a good marker for the synaptic enzyme, the reactivating potency can be investigated with human erythro‐cyte AChE in vitro. The present study was undertaken to evaluate the ability of various oximes at concentrations therapeutically relevant in humans to reactivate human erythrocyte AChE inhibited by different nerve agents. Isolated human erythrocyte AChE was inhibited with soman, sarin, cyclosarin, tabun or VX for 30 min and reactivated in the absence of inhibitory activity over 5–60 min by obidoxime, pralidoxime, HI 6 or HL� 7 (10 and 30 μM). The AChE activity was determined photometrically. The reactivation of human AChE by oximes was dependent on the organophosphate used. After soman, sarin, cyclosarin, or VX the reactivating potency decreased in the order HL� 7 > HI 6 > obidoxime > pralidoxime. Obidoxime and pralidoxime were weak reactivators of cyclosarin-inhibited AChE. Only obidoxime and HL� 7 reactivated tabun-inhibited AChE partially (20%), while pralidoxime and HI 6 were almost ineffective (5%). Therefore, HL� 7 may serve as a broad-spectrum reactivator in nerve agent poisoning at doses therapeutically relevant in humans.

Background: Acute organophosphorus (OP) pesticide poisoning is a major clinical problem in the developing world. Textbooks ascribe most deaths to respiratory failure occurring in one of two distinct clinical syndromes: acute cholinergic respiratory failure or the intermediate syndrome. Delayed failure appears to be due to respiratory muscle weakness, but its pathophysiology is unclear. Aim: To describe the clinical patterns of OP-induced respiratory failure, and to determine whether the two syndromes are clinically distinct. Design: Prospective study of 376 patients with confirmed OP poisoning. Methods: Patients were observed throughout their admission to three Sri Lankan hospitals. Exposure was confirmed by butyrylcholinesterase and blood OP assays. Results: Ninety of 376 patients (24%) required intubation: 52 (58%) within 2 h of admission while unconscious with cholinergic features. Twenty-nine (32%) were well on admission but then required intubation after 24 h while conscious and without cholinergic features. These two syndromes were not clinically distinct and had much overlap. In particular, some patients who required intubation on arrival subsequently recovered consciousness but could not be extubated, requiring ventilation for up to 6 days. Discussion: Respiratory failure did not occur as two discrete clinical syndromes within distinct time frames. Instead, the pattern of failure was variable and overlapped in some patients. There seemed to be two underlying mechanisms (an early acute mixed central and peripheral respiratory failure, and a late peripheral respiratory failure) rather than two distinct clinical syndromes.