Organophosphates Intoxication (Pesticides)

Organophosphates Intoxication

Pesticides can be defined as any substance or mixture of substances

intended for preventing, destroying, repelling, or mitigating pests.

Pests can be insects, rodents, weeds, and a host of other unwanted

Organisms

The four major classes (and their

target pests) are those of insecticides (insects), herbicides (weeds) fungicides (fungi, molds), and rodenticides (rodents), but there are

also acaricides (mites), molluscides (snails, other mollusks), miticides

(mites), larvicides (larvae), and pediculocides (lice). In addition,

for regulatory purposes, plant growth regulators, repellants, and

attractants (pheromones) often also fall in this broad classification of

chemicals.

For example, among insecticides, one can find organophosphorus

compounds, carbamates, organochlorines, pyrethroids, and

many other chemicals. Even within each of these subclasses, significant

differences can exist, as is the case, for example, of organochlorine

compounds such as DDT, aldrin, or chlordecone. Thus, detailed

knowledge of the toxicological characteristics of each chemical

is needed to properly evaluate their potential risks for nontarget

species.

Exposure to pesticides can occur via the oral or dermal routes or by

inhalation. From a quantitative perspective, oral exposure lies on the

extremes of a hypothetical dose–response curve. High oral doses,

leading to severe poisoning and death, are achieved as a result of pesticide

ingestion for suicidal intents, or of accidental ingestion, commonly

due to storage of pesticides in improper containers. Chronic

low doses on the other hand, are consumed by the general population

as pesticide residues in food, or as contaminants in drinking.

Workers involved in the production, transport, mixing

and loading, and application of pesticides, as well as in harvesting of

pesticide-sprayed crops, are at highest risk for pesticide exposure.

The dermal route is thought in this case to offer the greatest potential

for exposure, with a minor contribution of the respiratory route when

aerosols or aerial spraying are used

.

In the occupational setting, dermal

exposure during normal handling or application of pesticides,

or in case of accidental spillings, occurs in body areas not covered

by protective clothing, such as the face or the hands.

In the general population and in occupationally exposed

workers, a primary concern relates to a possible association between

pesticide exposure and increased risk of cáncer.

The World Health Organization

(WHO) estimated that there are around three million hospital

admissions for pesticide poisoning each year, that result in

around 220,000 deaths (WHO, 1990).

Indeed, among the 74 active ingredients

listed in Class 1A (Extremely hazardous) and Class 1B (Highly hazardous),

48 (65%) are insecticides, in particular organophosphates

Rodenticides are also highly toxic to rats, but do not

present the same hazard to humans. Indeed,warfarin, one of the most

widely used rodenticides, is the same chemical used as an effective

“blood thinner” (anticoagulant) for prevention of stroke and other

blood clot related conditions. Herbicides, again as a class, have generally

moderate to low acute toxicity, one exception being paraquat

(which has a low dermal toxicity but causes fatal effects when ingested).

Fungicides vary in their acute toxicity, but this is usually

low.

Reports of human poisonings worldwide confirm this analysis.

In Costa Rica between 1980 and 1986, 3330 individuals were

hospitalized for pesticide poisoning, and 429 died. Cholinesterase

inhibitors (organophosphates and carbamates) caused 63% of hospitalizations

and 36% of deaths, while paraquat accounted for 24%

of hospitalizations and 60% of deaths. Cholinesterase inhibitors

also caused more than 70% of occupational accidents.

Organophosphates, carbamates and paraquat were again involved in

the majority of cases

Insecticides play a most relevant role in the control of insect pests,

particularly in developing countries. All of the chemical insecticides

in use today are neurotoxicants, and act by poisoning the

nervous systems of the target organisms (Table 22-8).

The German chemist Gerhard Schrader is credited for

the discovery of the general chemical structure of anticholinesterase

OP compounds, and for the synthesis of the first commercialized OP

insecticide [Bladan, containing TEPP (tetraethyl pyrophosphate) as

the active ingredient], and for one of the most known, parathion, in

Where X is the so-called “leaving group,” that is displaced when

the OP phosphorylates acetylcholinesterase (AChE), and is the most

sensitive to hydrolysis;R1 andR2 are most commonly alkoxy groups

(i.e., OCH3 or OC2H5), though other chemical substitutes are also

possible; either an oxygen or a sulfur (in this case the compound

should be defined as a phosphorothioate) are also attached to the

phosphorus with a double bond. Based on chemical differences,

OPs can be divided into several subclasses, which include phosphates,

phosphorothioates, phosphoramidates, phosphonates, and

others (Chambers et al., 2001).

For all compounds

that contain a sulfur bound to the phosphorus, a metabolic

bioactivation is necessary for their biological activity to be manifest,

as only compounds with a P=O moiety are effective inhibitors

of AChE.

This bioactivation consists in an oxidative desulfuration

mediated, mostly but not exclusively in the liver, by cytochromeP450 enzymes (CYPs), and leading to the formation of an “oxon,”

or oxygen analog of the parent insecticide (Fig. 22-2). Though this

reaction has been known for several decades, the exact CYP isoforms

involved have only been recently investigated, and available

data suggest that the overall picture is quite complex.

For example, diazinon is activated

by human hepatic CYP2C19, whereas parathion is activated

primarily by CYP3A4/5 and CYP2C8, and chlorpyrifos by CYP2B6. Of relevance to insecticidal OPs is thioether oxidation

(formation of a sulfoxide, S=O, followed by the formation of

a sulfone, O=S=O) which occurs in the leaving group moiety, and

is also catalyzed by CYPs. For example, in case of the OP disulfoton,

the sulfoxide and the sulfone are more potent inhibitors of

AChE than the parent compound. Noncatalytic hydrolysis of OPs also occurs when these compounds

phosphorylate serine esterases classified as B-esterases, that

are inhibited by OPs but cannot catalytically hydrolyze them. Examples

are the carboxylesterases (CarE) and butyrylcholinesterase

(BuChE), in addition to the OP target, AChE. CarE also performs

a catalytic hydrolysis of the carboxylic esters of malathion, and is

believed to be a major determinant of its low toxicity in mammals

Signs and Symptoms of Toxicity and Mechanism of Action OP

insecticides have high acute toxicity, with oral LD50 values in rat

often below 50 mg/kg, though for some widely used compounds

(e.g., chlorpyrifos, diazinon) toxicity is somewhat lower, due to effective

detoxication. One exception is malathion, which has an oral

LD50 in rat of >1 g/kg, due, as said, to rapid detoxication by CarE

For several OPs acute dermal toxicity is also high, with some exceptions

being azinphosmethyl and malathion (Murphy, 1986). The

primary target for OPs is AChE, a B-esterase whose physiological

role is that of hydrolyzing acetylcholine, a major neurotransmitter in

the central and peripheral (autonomic and motor-somatic) nervous

systems. Acetylcholine released from cholinergic nerve terminals is

disposed of solely through hydrolysis by AChE. In fact, in contrast

to other neurotransmitters (e.g., norepinephrine), it is choline, the

product of acetylcholine hydrolysis by AChE, that is taken up by

the presynaptic terminal. Hence, inhibition of AChE by OPs causes

accumulation of acetylcholine at cholinergic synapses, with overstimulation

of cholinergic receptors of the muscarinic and nicotinic

type. As these receptors are localized in most organs of the body, a

“cholinergic syndrome” ensues, which includes increased sweating

and salivation, profound bronchial secretion, bronchoconstriction,

miosis, increased gastrointestinal motility, diarrhea, tremors, muscular

twitching, and various central nervous system effects. When death occurs, this is believed to be due to respiratory

failure due to inhibition of respiratory centers in the brain stem,

bronchoconstriction and increased bronchial secretion, and flaccid

paralysis of respiratory muscles

Methylparathion, azynphosmehtyl, chlorpyrifos, diazinon, malathion,

diclorvos, methamidophos, sarvin

The first signs appears to be muscarinic, which may or may not be in combination with nicotinic signs. While respiratory failure is a hallmark of severe OP poisoning, mild poisoning

and/or early stages of an otherwise severe poisoning may

display no clear-cut signs and symptoms.

The interaction of OPs with AChE has been studied in much

detail. OPs with a P=O moiety phosphorylate an hydroxyl group on

serine in the active (esteratic) site of the enzyme, thus impeding its

action on the physiological substrate (Fig. 22-3). The first reaction

leads to the formation of a Michaelis complex, while a subsequent

reaction leads to phosphorylatedAChE (Table 22-10). Rates of these

two reactions, that are usually very rapid, indicate the affinity of the

enzyme for a given OP. The bond between the phosphorus atom and

the esteratic site of the enzyme is much more stable than the bond

between the carbonyl carbon of acetate (in acetylcholine) at the same

enzyme site. While breaking of the carbon–enzyme bond is complete

in a few microseconds, breaking of the phosphorus-enzyme

bond can take from a few hours to several days, depending on the

chemical structure of the OP.

Phosphorylated AChE is hydrolyzed

by water at a very slow rate (Fig. 22-3; Table 22-10), and the rate

of “spontaneous reactivation” depends on the chemical nature of

the R substituents. Reactivation decreases in the order demethoxy

> diethoxy _ diisopropoxy (Gallo and Lawryk, 1991). Whereas

water is a weak nucleophilic agent, certain hydroxylamine derivatives,

known as oximes, can facilitate dephosphorylation of AChE,

and are utilized in the therapy of OP poisoning

Poisoning treatment: In case of dermal

exposure, contaminated clothing should be removed, and the skin

washed with alkaline soap. In case of ingestion, procedures to reduce absorption from the

gastrointestinal tract do not appear to be very effective (Lotti, 2001).

Atropine represents the cornerstone of the treatment for OP poisoning;

it is a muscarinic receptor antagonist, and thus prevents the

action of accumulating acetylcholine on these receptors. Atropine

is preferably given intravenously, though the intramuscular route

is also effective. The best clinical approach is to administer doses

of atropine large enough to achieve evidence of atropinization, i.e.,

flushing, dry mouth, changes in pupil size, bronchodilation, and increased

heart rate; atropinization should be maintained for at least 48

hours (Lotti, 2001). Indicative doses of atropine are 1 mg or 2–5 mg

in case of mild or moderate poisoning, respectively. Higher doses

by continuous infusion may be required in severe cases. Overdosage

with atropine is rarely serious in OP poisoned patients. Oximes, such as pralidoxime (2-PAM) are also used in the therapy

of OP poisoning. 2-PAM contains a positively charged atom

capable of attaching to the anionic site of AChE, and facilitate. dephosphorylation of the enzyme (Fig. 22-4), thus restoring the

catalytic site of AChE to its function. However, this chemical reaction

occurs only when the phosphorylated AChE has not undergone

aging. Dosing regimens for various oximes depend on the

specific compound and the severity of OP poisoning. For example,

for pralidoxime chloride, an initial 1 g dose given intravenously is

recommended, followed after 15–30 minutes by another 1 g if no

improvement is seen. If still no improvement is seen, an infusion of

0.5 g/h can be started (Lotti, 2001).

OPs bearing two methoxy groups (malathion, methylparathion,

dimethoate) is considered to be rather resistant to oxime therapy.

Biochemical Measurements In addition to synapses, AChE is

also present in red blood cells (RBC). Additionally, BuChE, also

known as pseudo-cholinesterase, is found in plasma. The physiological

functions of these enzymes are yet to be discovered. Nevertheless because activity of both enzymes is usually inhibited upon

exposure to OPs, their measurement is widely used as an indication

of exposure, and/or biological effect of OPs. Neither measurement is specific for a

certain OP, and indeed, other insecticides, such as carbamates, also

inhibit AChE and BuChE. Several analytical methods are available to measure OPs and

their metabolites in body fluids; the parent compound is measured

in blood, whereas metabolites are measured in urine.

The Intermediate Syndrome A second distinct manifestation of

exposure to OPs is the so-called intermediate syndrome, which was

first conceptualized by clinicians in Sri Lanka involved in the treatment

of suicide attempts (Senanayake and Karalliedde, 1987). The

intermediate syndrome is seen in 20–50% of acute OP poisoning

cases, and has been observed following exposure to a large variety

of OPs. The syndrome develops one to several days after the

poisoning, during recovery from cholinergic manifestations, or in

some cases, when patients are completely recovered from the initial

cholinergic crisis. Prominent features of the intermediate síndrome. are a marked weakness of respiratory, neck, and proximal limb muscles.

Mortality due to respiratory paralysis and complications ranges

from 15–40%, and recovery in surviving patients usually takes up to

15 days. The intermediate syndrome is not a direct effect of AChE

inhibition, and its precise underlying mechanisms are unknown. One

hypothesis is that muscle weakness may result from nicotinic receptor

desensitization due to prolonged cholinergic stimulation (Lotti,

2001).

Organophosphate-Induced Delayed Polyneuropathy A few

OPs may also cause another type of toxicity, known as

organophosphate-induced delayed polyneuropathy (OPIDP). Signs

and symptoms include tingling of the hands and feet, followed by

sensory loss, progressive muscle weakness and flaccidity of the distal

skeletal muscles of the lower and upper extremities, and ataxia.

These may occur 2–3 weeks after a single exposure, when signs of

both the acute cholinergic and the intermediate syndromes have subsided.

OPIDP can be classified as a distal sensorimotor axonopathy.

Neuropathological studies in experimental OPIDP have evidenced

that the primary lesion is a bilateral degenerative change in distal

levels of axons and their terminals, primarily affecting larger/longer

myelinated central and peripheral nerve fibers, leading to breakdown

of neuritic segments and the myelin sheats (Ehrich and Jortner,

2001). OPIDP is not related to AChE inhibition.

However, only OPs whose chemical structure

leads to aging of phosphorylated NTE (by a process analogous

to that described for AChE) can cause OPIDP.

most recent expert reviews tend to conclude that

the balance of evidence does not support the existence of clinically

significant neuropsychological effects, neuropsychiatric abnormalities,

or peripheral nerve dysfunction in humans chronically exposed

to low levels of OPs. OPs as a class are not considered to be mutagenic, and there

is little evidence that they may be carcinogenic. Immunotoxicity of

OPs has been suggested from in vitro or high dose animal studies,

but evidence in humans is lacking

Carbamate insecticides have a variety of chemical structures

(Fig. 22-7), but all derive from carbamic acid, the majority being

N-methylcarbamates. They present different degrees of acute oral

toxicity, ranging from moderate to low toxicity such as carbaryl, extremely high toxicity, such as aldicarb (Fig. 22-7). Dermal toxicity

is lower, but skin penetration is increased by organic solvents

and emulsifiers present in most formulations. For the most

part, the metabolites are devoid of biological activity, but this is

not always the case. For example, two metabolites of aldicarb, the

sulfoxide and the sulfone, are more potent anticholinesterases than

the parent compound (Risher et al., 1987). The mechanism of toxicity

of carbamates is analogous to that of OPs, in that they inhibit

AChE. However, inhibition is transient and rapidly reversible, because

there is rapid reactivation of the carbamylated enzyme in the

presence of water (Table 22-10). Additionally, carbamylated AChE

does not undergo the aging reaction. However, differently

from OPs, acute intoxication by carbamates is generally resolved

within a few hours. AChE inhibitors and do not require

metabolic bioactivation, and enzyme activity returns to control levels

within two hours. The treatment of carbamate intoxication relies

on the use of the muscarinic antagonist atropine. Use of oximes

is generally not recommended, as 2-PAM has been shown to aggravate

the toxicity of carbaryl. Yet, oximes may

have beneficial effects in case of other carbamates such as aldicarb

(Ecobichon, 2001b). There are several cases of human poisoning associated

with exposure to various carbamates, in particular carbaryl. Carbamates can

inhibit NTE, but because carbamylated NTE cannot age, they are

thought to be unable to initiate OPIDP. Embryotoxicity or fetotoxicity are observed only at maternally

toxic doses