Organophosphorus compounds (organophosphates and organo-phosphonates) are serine esterase and protease inhibitors. Organophosphates are widely used in agriculture as insecticides and acaricides, in industry and technology as softening agents and additives to lubricants. In 1990 a WHO task group noted that there may be 1 million serious unintentional pesticide poisonings each year and, on the basis of a survey of self reported minor poisoning, estimated there may be up to 25 million agricultural workers in the developing world suffering an episode of poisoning each year [Jeyaratnam, 1990].
    Some of the organophosphonates are declared as chemical warfare agents (combat anticholinesterase compounds; nerve agents) and have been recently used with devastateing consequences against civillians in Iraq.
    Sarin (GB) and VX were involved in terrorist attacks in Japan [Wiener & Hoffman, 2004], highlighting that the use of these compounds constitutes a major terrorist threat. The likelihood of the use of organophosphorus compounds by terrorist organization is related to the relative ease of production of these substances, certainlky within the means of even moderately sophisticated organizations.
     The inhibition of esterases (butyrylcholine: EC # 3.1.1.8 and acetylcholine: EC # 3.1.1.7) results from organophosphorus compounds reacting covalently with the active centre serine, i.e. by phosphorylation or phosphonylation [Levine, 1991; Bajgar, 2004]. The effects of poisoning with organophosphorus compounds are well known and have been described extensively; they are the consequence of an endogenous actylcholine poisoning [Namba, 1971; Namba et al, 1971; Zoch, 1971; Petroianu et al, 1998].
     The therapy of organophosphate poisoning is known by the catchy acronym A FLOP = Atropine, FLuids, Oxygen, Pralidoxime [Petroianu, 2005].
     Oximes are the only enzyme reactivators clinically available [Johnson et al, 2000]. Pralidoxime is used as an adjunct to atropine in the treatment of poisoning by most organophosphorus cholinesterase inhibitors. Clinically while atropine relieves muscarinic signs and symptoms pralidoxime is supposed to shorten the duration of the respiratory muscle paralysis by reactivation of cholinesterases [Johnson et al, 2000]. Clinical experience with pralidoxime (and other oximes) is disappointing [Peter & Cherian, 2000; Eddleston et al, 2002; Buckley et al, 2005].
    Pyridostigmine is a carbamate inhibitor of cholinesterases. Carbamates are known to confer some protection from the lethal effects of (some) organophosphorus compounds [Koster, 1946; Koelle, 1946; Berry & Davies, 1970]. Recently (February 5th, 2003) the FDA, based on animal experiments, approved for military combat medical use oral pyridostigmine for preexposure treatment (minimum 30 min) of some nerve agents (soman).
    The concept is to pre-emptively block the cholinesterase reversibly using the less potent reversible inhibitor (carbamate) in order to deny access to the active site of the enzyme to the more potent irreversible organophosphorus inhibitor (nerve gas) on subsequent exposure and thus facilitate enzyme reactivation with oxime treatment. The combined use of carbamate pretreatment followed by atropine, oxime and benzodiazepine was advanced early by the British [Gall, 1981].
     According to the package insert ( Pyridostigmine Bromide Package Insert, 2003( in order to derive benefit from the use of pyridostigmine, oxime treatment must follow. Also in the standard textbook of military medicine "Medical Aspects Of Chemical And Biological Warfare" in the chapter titled "Pretreatment for Nerve Agent Exposure" the authors state" Unfortunately, pyridostigmine by itself is ineffective as a pretreatment against subsequent nerve agent exposure and thus it is not a true pretreatment compound" [Dunn et al, 1997]. Similar views were also recently expressed by Israeli experts [Layish et al, 2005].
    The purpose of this paper is to review from a clinician's perspective the topic of poisoning with organophosphorus compounds and to attempt to debunk some of the myths and clarify some of the fuzzier issues related to the subject.
    

Myth # 1. Organophosphorus compounds were first developed by German scientists.


     Most people associate the birthplace of organophosphorus compounds with Germany. While the important German contribution starting with Willy Lange and Gerda von Krueger (Lange & Krueger,
     1932( and continued by the work of Schrader is undeniable, organophosphorus compounds were born 1854 in France, in the research laboratories of Adolphe Wurtz.
     A superb account of the history of the development of organophosphorus compound is given by Holmstedt [Holmstedt, 1963]. In brief, Wurtz asked two of his co-workers to look into the synthesis of phosphorus esters; Philippe de Clermont and M. Moschnine managed to synthesize independently the first organophosphorus compound, tetraethyl pyrophosphate (TEPP) [de Clermont, 1854; de Clermont, 1855]. Apparently, as it was customary at the time, de Clermont tasted the new compound, and despite the development of symptoms managed to survive and to present his work to the French Academy of Sciences.
      

Myth # 2. Organophosphorus compounds and organophosphates are synonyms

      Clinicians are not familiar with the chemical taxonomy and therefore are occasionally not using the different names appropriately. From a chemistry perspective the family of organophosphorus compounds (umbrella name) comprises organophosphates, organophosphonates and organophosphinates. The key to properly assigning a particular compound to the different classes are the bonds of the phosphorus atom. If no phosphorus to carbon (P-C) bond exists in the molecule then the compound is an organophosphate, as in TEPP (Fig. 2).

 

Organophosphates are mainly used for civilain purposes (e.g. in agriculture as pesticides) but their acute toxicity can be comparable to that of the organophosphonates, developed for military purposes. Organophosphonates have one P-C bond in the molecule as opposed to organophosphinates with two P-C bonds in the molecule (Fig. 3).

 

       Schrader is considered the father of nerve gases (which are in fact not gases but fluids) and therefore should be referred to as nerve agents or, as recently suggested, Combat Anti-Cholinesterase (CAChE) Agents.
      The classical Schrader nerve agents are tabun (GA), sarin (GB) and cyclo-sarin, also referred to as GF. Soman (GD) was developed by Richard Kuhn, another German scientist. The designation GC was apparently never assigned because at the time it was the custommary designation for gonococcus. GE is the ethyl-deriivative of sarin.
       There is some controversy with respect to the significance of the designation G: Gas, German, Gerhard (Schrader's first name) and Gruen (green) all have been suggested. Most probably the origin is related to Gruen, the code name of the nerve agent development program.
       The V series of nerve agents are not of German origin: The phosphoryl-thio-choline class of compounds was discovered independently by Ranaji Goshem of ICI (UK) and Lars-Erik Tammelin of the Swedish Institute of Defense Research in the late forties. This class of compounds is also sometimes known as Tammelin's esters.
     Again there is some controversy with respect to the significance of the designation V: venomous, viscous or victory.
   There are few pharmacodynamic differences between organophosphates and organophosphonates: both groups inhibit esterases and the symptoms resulting are quite simmilar. It appears that nerve agents ellicit more seizures as opposed to pesticides (organophosphates) where pulmonary symptoms dominate.
      The differences relate mainly to pharmacokinetic issues and a phenomenon called "ageing": subsequent to the inhibition via phosphorylation of the enzyme (by organophosphates) or phosphonylation (by organophosphonates) an organic moiety (leaving group) breaks away from the enzyme inhibitor complex, practically rendering the inhibition irreversible. With nerve agents the "ageing"occurs relatively rapidly as opposed to organophosphates where the phenomenon is so slow as to be mostly irrelevant. Within the nerve agents group soman ageing is very rapid (minutes) as opposed to sarin (5h), tabun (15 h) and VX ( 1-2 days) where ageing takes hours to days (Bajgar, 2004).

      Clinical relevance:

      Soman inhibited cholinesterase ages rapidly (minutes): pre-exposure treatment strategies are therefore mandatory
       Nerve agents ellicit more readily seizures (Think GABA agonists)

Myth # 3. Organophosphorus compounds are causing bradycardia

       
      The pharmacodynamic effects of organophosphorus compounds (i.e. inhibition of esterases with subsequent development of an endogenous cholinergic poisoning) were recognized early by german scientists. The exact paternity of the observation is not known. Holmstedt writes: "In any event the parasympathomimetic effects of the nerve gases were clearly recognized by the German workers and atropine established as an antidote" [Holmstedt, 1963]. The clinical presentation of an endogenous cholinergic poisoning is sumarized by different mnemonics, as most of us would remember from medical school:
     SLUDGE stands for Salivation, Lacrimation, Urination, Defecation, Gastrointestinal-cramping and Emesis, Killer Bs: Bronchospasm, Bronchorrhea, Bradykardia,
   DUMBBELLS stands for Diarrhea, Urination, Miosis, Bradycardia, Bronchospasm & Bronchorrhea, Emesis, Lacrimation, Laxation and Salivation.
      Occasionally the heart rate and blood pressure in organophosphorous compounds exposure can be high. The catecholamine release from the adrenal medulla is under cholinergic control. As such the inhibition of esterases in organophosphorous compounds poisoning and the ensueing "endogenous acetylcholine poisoning" can present with elevated heart rate and blood pressure due to catechoamine release. In fact Namba [Namba, 1971], listing the signs and symptoms of 77 patients with parathion poisoning does not even mention bradycardia.
      Most patients with this type of poisoning present with tachycardia rather than bradycardia [Saadeh et al, 1997]. This is also in line with the clinical presentation of most terorist attack victims in Japan [Wiener & Hoffman, 2004].
      In our experience, working with mini-pigs, bradycardia due to muscarinic acetylcholine effects is seen only at a low dose/slow application rate organophosphorous compounds poisoning. At higher dose and faster application rate we have never observed bradycardia; the clinical picture has been that of a hypertensive crisis with mean arterial pressure of up to 220 mm Hg and heart rate over 150. Both norepinehrine levels and the clinical presentation after infusion of paraoxon in minipigs are "phaeochromocytoma-like" [Petroianu et al, 1998; Petroianu et al, 1999a]. The other clinical syndrome with a similar pathophysiology (massive release of catecholamines) is the obscure Irukandji syndrome: The Irukandji (Carukia barnesi) is a small jellyfish approx 2cm diameter bell, responsible the unusual and dramatic syndrome observed following stings in northern Australia, especially north Queensland (Fig.4) [Corkeron, 2003; Winkel et al, 2005].

 

      Some 25 years ago, Valero and Golan [Valero & Golan, 1967] suggested control of atropine-induced massive tachycardia in organophosphorous compounds poisoning by means of §-blockade. More recent work [Karalliedde & Senanayake, 1989] agreed on this point. Tachycardia is however, at times not (or not solely) due to therapeutic atropine application but also due to catecholamine release and thus present before atropine application. As such using §-blockade to control the heart rate might exacerbate the hypertension (unopposed a-adrenergic effects) at least in those cases were atropin is not the (only) culprit.
       For control of heart rate and blood pressure we used, with excellent results, magnesium infusion. This was advocated by James of South Africa for management of heart rate and blood pressure in phaeochromocytoma patients [James, 1985; James, 1989] (or as a matter of fact for any situation associated with excessive cathecholamine release) [James et al, 1989]. When the endogenous (adrenal) catecholamine reserves are exhausted the animals become hypotonic (and bradycardic) and need inotropic support.
     The effects of §1-adrenergic agonist administred in such situations are not opposed by magnesium [Prielipp et al, 1991] as they would have been by §-blockers. This and an inhibitory effect on synaptic acetylcholine release [Hutter & Kostial, 1954] are, in our view, further advantages of magnesium over §-blocker in the described setting.

       Clinical relevance:

       Presence of tachycardia does not exclude organophosphorous compounds poisoning.
       Presence of tachycardia is not indicative of sufficient atropine administration
       Do not titrate atropine dose to heart rate
       (drying of mucous membranes is probably a better end-point)
       Do not use (b-blockers to control tachycardia
      Magnesium i.v. is the drug of choice for heart rate control

     

Myth # 4. Organophosphorus compounds are causing lung edema

      One of the therapeutic aspects sometimes ignored in organophosphorous compounds exposure is that of fluid replacement. In organophosphorous poisoning there is a massive and rapid haemoconcentration as reflected by haematocrit increase. This rather fulminant development, while similar to the haemoconcentration one sees in massive venous air embolism, might not be familiar to all emergency medicine practitioners. The drastically increased fluid consumption caused by organophosphorous compounds is most probably due to alteration of biologic membranes and thus to fluid extravasation and to concomitant massive activation of secretory glands with subsequent "consumption" of fluid.
      Close haematocrit control as a guide for volume therapy is appropriate [Petroianu et al, 1998]. While it is normally (very) wise to stay out of the dispute over the "right" or "wrong" replacement fluid, in this special case we wish to suggest to give lactated Ringerصs solution the benefit of the doubt. In vitro at least lactate seems to offer some protection against organophosphorous compounds induced inhibition of the esterases [Petroianu et 1999b; Petroianu et al, 2000]. Results in vivo (minipigs) were however disappointing [Maleck et al, 2002].
      Irrespective of whether lactate confers advantages or not, the fluid replacement needs of organophosphorous compounds exposed patients are very high and agressive substitution is appropriate. The lungs are filled with fluid due to excessive activation of secretory glands (bronchorrhea) and not to left ventricular failure or volume overload. The word used to describe the situation is pseudo-edema.

      Clinical relevance:

       Agressive fluid substitution is needed
       Do not relly on central venous pressure monitoring only
       Monitor serially the hematocrit
    

Myth # 5. Reactivator (oxime) therapy works

     The therapy of organophosphate poisoning is known by the catchy acronym A FLOP = Atropine, FLuids, Oxygen, Pralidoxime [Petroianu, 2005]. Pralidoxime, developed by Irwin B. Wilson in North America in the fifties, was the first cholinesterase reactivators to become clinically available [Wilson & Ginsburg, 1955]. The contribution of Wilson to the development of treatment strategies for nerve agent exposure was highlighted by Alston: "Wilson did not sketch the pralidoxime molecule as an analog of some serendipitously discovered prior drug. Instead, he applied his theory of enzyme action in order to design a peerless pharmaceutical. As Wilson predicted, organophosphorus-poisoned cholinesterase is not completely "dead." Instead, the poisoned enzyme retains catalytic ability to transfer its blocking organophosphorus group away from its enzyme active site and onto pralidoxime" [ Alston, 2005]. Clinically, while atropine relieves muscarinic signs and symptoms oximes are supposed to shorten the duration of the respiratory muscle paralysis by reactivation of cholinesterases [Johnson et al, 2000]. Clinical experience with oximes is however disappointing [Peter & Cherian, 2000; Eddleston et al, 2002; Buckley et al, 2005]. 
      Over the years new potential reactivators were developed by different groups. Methoxime (MMC-4) was synthesized and tested by Hobbiger and Sadler in the UK. Currently, this reactivator is used by the Czech Army after nerve agent exposure [Hobbiger & Sadler, 1959]. Obidoxime, the standard oxime in Western Europe, was developed by Luettringhaus and Hagedorn in Germany [Luettringhaus & Hagedorn, 1964].
      Jiri Bielavsky, a synthetic chemist working in the Department of Toxicology at the Faculty of Military Health Sciences (University of Defence, Hradec Kralove, Czech Republic) is the "father" of BI-6, while the K-series of reactivators were developed later by Kucùa at the same institution (Bajgar, 2004). Their chemical structures were derived from the structures of existing esterase reactivators, especially pralidoxime, obidoxime and the Hagedorn (Ilse) oxime HI-6 (Fig. 5).

 

 HI-6 autoinjectors were issued to Canadian forces involved in the 1991 Gulf War.
       From a chemical point of view, the newly developed oximes are bisquaternary symmetric (K-33 and methoxime) or asymmetric (K-27, K-48 and BI-6) pyridinium aldoximes with the functional aldoxime group at position two (K-33 and BI-6) or four (K-27, K-48 and methoxime) at the pyridine rings (Fig. 6).

 

    Oximes, unfortunately, are not equally effective against all available organophosphorus compounds. While the newer oximes, especially the kukoximes K-27 and K-48 are excellent at reactivating esterases inhibited by ethyl organophosphates (paraoxon), they are probabely only marginally better than the conventional ones at reactivating methyl organophosphates (Petroianu et al, 2005(. With respect to enzyme inhibited by nerve gases the kukoximes K-27 and K-48 are the choice for tabun exposurewhile HI-6 appears to be best for soman exposure (Kassa, 2002; Calic et al, 2005(. There is a clear demand for "broad spectrum" cholinesterase reactivators with a higher efficacy than the available oximes.
 
      Clinical relevance:

      The new kucoximes K-27 and K-48 are excellent at reactivating esterases inhibited by ethyl organophosphates (e.g. paraoxon)
     No available oxime reactivator is good at reactivating esterases inhibited by methyl organophosphates
      There is a clear demand for "broad spectrum" cholinesterase reactivators with a higher efficacy than the available oximes.

       
Myth # 6. Carbamate (pyridostigmine) pre-treatment works in most cases

     Pyridostigmine  (Fig. 7)

 

 is a carbamate inhibitor of cholinesterases. Carbamates are known to confer some protection from the lethal effects of (some) organophosphorus compounds [Koster, 1946; Koelle, 1946; Berry & Davies, 1970(. Recently (February 5th, 2003) the FDA, based on animal experiments, approved for military combat medical use oral pyridostigmine for preexposure treatment (minimum 30 min) of soman. While pyridostigmine might be beneficial for preexposure treatment of other nerve agents as well, with soman inhibited cholinesterase ageing so rapidly (minutes) preexposure treatment is absolutely necessary.
      The concept is to pre-emptively block the cholinesterase reversibly using the less potent reversible inhibitor (carbamate) in order to deny access to the active site of the enzyme to the more potent irreversible organophosphorus inhibitor (nerve gas) on subsequent exposure and thus facilitate enzyme reactivation with oxime treatment. The combined use of carbamate pretreatment followed by atropine, oxime and benzodiazepine was advanced early by the British [Gall, 1981].
      According to the pyridostigmine package insert (Pyridostigmine Bromide Package Insert, 2003] in order to derive benefit from the use of pyridostigmine, oxime treatment must follow. Also in the standard textbook of military medicine "Medical Aspects Of Chemical And Biological Warfare" in the chapter titled "Pretreatment for Nerve Agent Exposure" the authors state" Unfortunately, pyridostigmine by itself is ineffective as a pretreatment against subsequent nerve agent exposure and thus it is not a true pretreatment compound" [Dunn et al, 1997]. Similar views were also recently expressed by Israeli experts [Layish et al, 2005].
     Recently we performed a prospective, controlled animal (rat) study to quantify in vivo the effect of pyridostigmine pretreatment on survival in rats exposed to the organophosphate paraoxon with and without subsequent reactivator (pralidoxime) treatment. Paraoxon is a highly toxic non-neuropathic ethyl organophospate.
      Group 1 received 1 µMol paraoxon (إLD75), group 2 received 1 (Mol pyridostigmine followed 30 min later by 1µMol paraoxon, group 3 received 1 µMol pyridostigmine followed 30 min later by 1µMol paraoxon and 50 µMol pralidoxime while group 4 received 1µMol paraoxon and 50 µMol pralidoxime.
       Each group contained six rats. The experiment was repeated twelve times. All substances were applied ip. The animals were monitored for 48 hours and mortality (survival time) was recorded. Mortality was analysed using Kaplan-Myer plots. Both pyridostigmine and pralidoxime statistically significantly decreased organophosphate induced mortality in the described model. While the same applies to their combination the decrease in mortality when using both (pyridostigmine and pralidoxime) is less than that achieved with their single use (but not significantly so) (Fig. 8).

 

      While certainly further work using different organophosphorus compounds and animal species are needed before a final conclusion is reached, our animal data does not support the combined use of pyridostigmine and pralidoxime in paraoxon exposure.

       Clinical relevance:
 
       Pre-treatment with pyridostigmine (military setting) is probably advantageous only with nerve agents inducing rapid esterase ageing (soman)
 

Myth # 7. Other pre-treatment regimens are superior

    Many of the drugs used clinically for a variety of purposes are weak inhibitors of cholinesterases. Best known among these are some of the substituted benzamides (metoclopramide,tiapride,sulpiride) and histamin-2 receptor blocker (ranitidine, nizatidine) (Graham & Crossley, 1995; Chemnitius et al, 1996; Fontaine & Reuse, 1980; Hansen & Bertl, 1983 a & b; Laine-Cessac et al, 1993; Kounenis et al, 1994). We speculated that a weak inhibitor of cholinesterases applied at high dose might offer similar or superior benefits with less side effects than the standard pre-treatment drug pyridostigmine. The concept was tested with promising results initially in vitro and the pharmacokinetic data derived from these experiments is presented in table 1.
      The key element shown is the so called IC50 shift. The IC50 is the concentration of the inhibitor (organophosphorus compound) at which the enzyme activity is reduced to 50% of the base-line activity. While IC50 values are dependent on the experimental setting if the measurements are performed under identical conditions than the results are comparable.
      For the determination of the shift IC50 determinations are repeated in the absence of and then in the presence of increasing concentrations of the substance used as possible pre-treatment. The calculated IC50 values are plotted against the concentrations of the protective substance to obtain an IC50 shift curve. For the graphical representation and calculations the SlideWrite(TM) (Advanced Graphics Software Inc, Encinitas, CA-USA) software is used (equation y=a0 + a1 x) where a1 represents the slope (tangent; tg a) of the IC50 shift graph. The IC50 shift (tg a) is expressed as nM shift per microM pre-treatment substance. Figure 9

 

 shows such an IC50 shift curve obtained in vitro using ranitidine.
       The putative mode of protective action of weak cholinesterase inhibitors -when administered in excess- is competition for the enzyme with the more potent organophosphate, so that the enzyme is occupied by the weak inhibitor instead of the potent one (organophosphate or phosphonate) and thus -less inhibited.
       Interestingly no protective effect of pyridostigmine pre-treatment could be demonstrated for paraoxon (ethyl organophosphate). Figure 10

 

shows the IC50 shift curve obtained in vitro using the carbamate pyridostigmine as pre-treatment. Here the slope is downward (negative tg a) indicating a potentiation of the inhibition.
      Encouraged by the described in vitro results in vivo experiments in rats using tiapride and ranitidine as pre-treatment were performed. Paraoxon was used as the cholinesterase inhibitor. Figures 11 & 12 show Kaplan Meier plots derived from those experiments.

 

       Tiapride: the protection derived from using tiapride pre-treatment is essentially identical with that derived from using pyridostigmine pre-treatment.
       Ranitidine: the protection derived from using ranitidine instead of pyridostigmine is inferior and thus it can not be recommended as carbamate substitutes.
       When interpreting these results one must bear in mind that It is extremely difficult -if at all possible- to extrapolate results obtained using an organophosphate to organophosphonates. Same difficulties apply when interpreting rodent data: the cholinesterase activity in rat plasma is partially due to soluble AChE, unlike human plasma which is essentially all BChE. Therefore, our measurements of AChE activity after selective inhibition of BChE reflect both the membrane bound and the soluble component present in rat blood [Thompson & Richardson, 2004].

 

         Clinical relevance:
 
         Tiapride is a possible pre-treatment drug in pesticide exposure


Myth # 8. Organophosphorus compounds affect coagulation

        Based on experiments performed in vitro it was stated that "The generally low potency of ..... organophosphates for blood-clotting factors and digestive enzymes suggests that associated toxic effects are unlikely at sublethal doses" (Quistad & Casida, 2000). This contradicts the clinical experience which indicates that alterations in coagulation tend to contribute to the pathology. Jastrzebski et al. presented a case of suicidal poisoning with organophosphate pesticide, associated by acute activation of blood coagulation where heparin treatment efficiently inhibited this activation (Jastrzebski et al, 1994). Ziemen disagrees and writes "In nine patients suffering from organophosphate intoxication, platelet function and blood coagulation parameters were investigated. Thrombocyte function was impaired in all patients, characterized by a diminished platelet shape change. Platelet shape change was also inhibited in rats and after oral administration of 10 mg/kg parathion. Thrombocytopenia and coagulation abnormalities (diminished fibrinogen, plasminogen and anti-thrombin III) were more pronounced in cases with severe intoxication. In five of nine patients a marked bleeding tendency was observed. The bleeding tendency in organophosphate intoxication is probably mainly caused by the defective platelet function. Patients with this intoxication should receive heparin only for special indications".
        We assessed the in vitro effects of paraoxon (POX) on human blood coagulation by fibrin monomer concentration measurements and thrombelastographic determinations. Increasing doses of POX dissolved in alcohol (POX + ALO) or alcohol (ALO) only in corresponding quantities were added to blood drawn from six human volunteers. In both series (POX + ALO and ALO-only) FM concentrations increased in comparison to the baseline levels. No statistically significant differences exist, however, between FM measurements performed on blood with POX + ALO and those performed on blood with ALO-only. No coagulation-activating effect of POX in vitro was demonstrable; the changes seen in vitro are due to the ALO used as a vehicle.
       The thrombelastographic parameters showed several changes in the POX + ALO series as dosage increased. At high POX levels, reaction time r and clot formation time k became longer than in the baseline measurements, the clot formation rate alpha and the maximum amplitude MA were reduced. The TEG changes indicate a hypocoagulable state, probably due to the POX effect on platelet function and/or inhibition of clotting factors (serine proteases) (Petroianu et al,1997).
      We also assessed the in vivo effects of paraoxon (POX) on blood coagulation of mini pigs by measuring the partial thromboplastin time (PTT), prothrombin time (PT), fibrinogen, factor V, factor VII, factor VIII, antithrombin III, protein C, and platelet count. The mini pigs were randomly assigned to a POX-treatment group (n = 9) receiving 54 mg POX kg(-1) BW(-1) or the control group (n = 9). Measurements were carried out over a period of 150 min after poisoning. The exposure to POX did not have any influence on measurements of PT, factor VIII, factor VII, factor V, antithrombin III, protein C, or fibrinogen compared to the control group evaluated by rank order test (ROT) during the time of observation (150 min). Changes seen in the intrinsic coagulation followed a biphasic pattern corresponding to an early sympathomimetic phase with PTT-shortening and a decrease of the platelet count, and a late vagal phase, with PTT-prolongation.
       The hypercoagulability seen in the sympathomimetic phase is probably due to a massive release of catecholamines from the adrenals. Previous studies showed in vitro no coagulation activating effect of POX. The hypocoagulability in the vagal phase shown by the PTT-protongation is probably due to POX influencing platelet function or its inhibition of clotting factors, which are serine proteases, or a combination of the two) (Petroianu et al,1999a; Ziemen, 1984).

         Clinical relevance:

         Monitor coagulation & platelet function

Myth # 9. Most organophosphorus compounds are neuropathic

       
      Neuropathy target esterase (NTE) or neurotoxic esterase is a membrane-bound protein that hydrolyzes phenyl valerate. The enzyme is operationally defined as that component of esteratic activity against phenyl valerate that is resistant to inhibition by paraoxon but sensitive to inhibition by mipafox. A small proportion of phenyl valerate esterases (ca. 15% in hen brain) are resistant to paraoxon, while circa 80% are sensitive to mipafox. This fraction is defined as NTE.
      The inhibition and ''aging'' of the phosphorylated or phosphonylated NTE, is highly correlated with the initiation of organophosphorus induced delayed neurotoxicity (OPIDN). Not all organophosphorus compounds that inhibit NTE cause OPIDN, but all organophosphorus compounds that cause OPIDN inhibit NTE (Johnson, 1975a & b). Clinical signs and pathology first appear between 2 and 4 weeks following organophosphorus compound exposures.
       In humans, OPIDN is a neurological syndrome presenting as a flacid paresis that develops distally in the lower extremity and spreads to the thighs and upper extremity. In later stages, signs and symptoms of central nervous system injury, such as spasticity and ataxia become evident, while the the symptoms of peripheral neuropathy recede (Abou Donia, 1981). The term "dying-back axonopathy is a usefull generic term describing the pattern of nerve fibre degeneration in OPIDN (Schaumburg & Spencer ,1979).
        Despite the fact that NTE is by deffinition paraoxon resistant, It was repeatedly suggested that high dose exposure to parathion or its oxon can cause OPIDN (Petry, 1951; Petty, 1958; De Jager et al, 1981: Besser et al, 1993). In order to assess clinically whether or not high-dose intravenous paraoxon causes OPIDN in mini pigs, 14 mini pigs were anaesthesized, intubated and mechanically ventilated. In a first set of experiments eight pigs received 1 mg paraoxon /kg body weight dissolved in alcohol. Two control animals received alcohol in a corresponding amount. After infusion of paraoxon, survival of the animals during the acute phase of intoxication was achieved by intensive-care support, using appropriate drugs and fluids according to a pre-established protocol. The mini pigs were extubated 1036 ± 363 min later (mean ±SD). The pigs were observed prior to paraoxon application and for 6 weeks thereafter for any abnormalities and/or signs of OPIDN, such as leg weakness, ataxia and paralysis. Observations were graded on a scale for three categories (position, motor deficiency, reaction), with a maximal cumulative score of 9. In a second set of experiments (four additional pigs) larger paraoxon doses were used (3, 9, 27 and 81 mg /kg body weight). After recovering from general anaesthesia/surgery, within 2 weeks all animals reached the initial score on the scale. We concluded that high-dose i.v. paraoxon exposure does not induce OPIDN in mini pigs during the 6-week observation period. These results are in line with earlier publications [Soliman et al, 1982] One can speculate that previously published case reports of paraoxon-induced OPIDN-like symptoms were due to hypoxic damage sustained during the acute phase of the intoxication (Petroianu et al, 2001).

      Clinical relevance:

       Oxygenation during the acute phase is paramount.

       Conclusion
       
      Exposure to organophosphorus compounds continues to be a major global problem. Understanding of the patho-physiology of the event allows provision of a rationale intensive care treatment regimen. The main pharmacology tools are atropine, oxygen, fluids and inotropes. Continuous monitoring of oxygenation, coagulation and perfusion will guide the clinician in his/her choice of additional drugs and procedures to be used. The magic bullet antidote (enzyme reactivator) is not (yet) clinically available.
       When the number of organophosphorus compound exposed patients is high and the ability to provide appropriate intensive care treatment is exhausted, the use of enzyme reactivators becomes quintessential. None of the clinically available substances is satisfactory and the introduction of new oxime reactivators is eagerly awaited.

      
      

References

1-Abou-Donia MB (1981). Organophosphorus ester-induced delayed neurotoxicity. Annu Rev           Pharmacol Toxicol 21: 511-548.

 2- Alston TA (2005). Pralidoxime Rescues both Muscarinic and Nicotinic Systems. Anesth Analg 101: 926-927

 3- Al-Suwaidi AAM, AMH Al-Rahoomi, K Arafat, MY Hasan, G Petroianu (2005). Sulpiride Effect In Vitro On Inhibition Of Acetyl-Cholinesterase by Paraoxon. 34th Annual ACCP Meeting, Rockville, Maryland (abstract # 115). J Clin Pharmacol 45: 1095

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