Chapter 6 - Antidotes Flashcards
Definition of antidote
Antidotes are agents with a specific action against the activity or effect of a poison
Limitations to antidote effectiveness
Antidotes may be ineffective or inadequate if general decontamination of the animal is not carried out first.
Some animals may be less tolerant than others to certain antidotes due to species or age.
Mechanism of action of chemical antidotes
Chemical antidotes specifically interact with or neutralize the toxicant (few act in this manner)
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Complex formation: antidotes bind to the toxicant, unavailable to cross membranes.
- Dimercaprol and dimercaptosuccinic acid (DMSA): sulfhydryl compounds that bind metals (e.g. As, Pb)
- Specific binding agents: e.g. EDTA, deferoxamine D-penicillamine act by chelation of the metal, form a water-soluble complex
- Antivenins against snake venoms and antibodies against digitoxin bind specifically.
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Metabolic conversion:
- Nitrite interacts with hemoglobin and cyanide to form cyanmethemoglobin (less toxic and interferes with cyanide access to cytochrome oxidase system)
- Thiosulfate provides sulfur, interacts with cyanide to form thiocyanate (readily excreted in urine)
Mechanism of action of pharmacologic antidotes
- Prevention of toxic metabolite formation: e.g. ethanol and 4-methylpyrazole (4-MP) compete with alcohol dehydrogenase from etyhlene glycol (antifreeze)
- Enhancement of toxicant excretion: e.g. molybdenum and sulfate form a water-soluble complex with copper that is readily excreted in urine.
- Competition for receptor sites: e.g. naloxone blocks the action of opioids by competing for the same receptor sites.
- Blockade of receptors responsible for the toxic effect: atropine blocks the physiologic effect of acetylcholine (ACh) at cholinergic synapses and neuromuscular junctions.
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Restoration of normal function:
- in nitrite poisoning, methylene blue interacts with reduced nicotinamide adenine dinucleotide (NADPH) to reduce the ferric iron of methemoglobin back to ferrous ion in hemoglobin, which can again transport oxygen.
- Acetylcysteine supplies the precursor amino acids for glutathione, which serves as a biologic antioxidant against acetaminophen toxicosis.
Acetylcysteine as an antidote:
- Usage
- Absorption, metabolism, excretion
- Effects from overdosage
- Acetylcysteine is a reducing agent that counteracts acetaminophen toxicosis.
- most effective given 8-16h of ingestion of toxicant
- maximum plasma concentrations within 2-3 hours after oral dosing, remain for 4-6 hours
- metabolites: cysteine, cystine, glutathione, sulfate
- OD: Unlikely. Possible anaphylactic. LD50 dogs > 1000mg/kg
Ammonium molybdate and ammonioum tetrathiomolybdate as antidotes:
Usage and mechanism of action
Molybdate treatments promote excretion of copper in sheep.
Use after the accumulation and sudden release of copper from the liver which causes an acute hemolytic crisis in sheep.
Antivenins as antidotes:
Usage and adverse effects
- Antivenins act against crotalid snake venoms.
- Adverse effects: product derived from equine serum: hypersensitivy or anaphylaxis. Animals should be skin-tested for potential reactions before admin. and epinephrine or diphenhydramine should be available to counteract possible anaphylatic reactions.
Ascorbic acid as antidote:
use, absorptio/excretion and adverse effects.
- Ascorbic acid reverses methemglobinemia in cats
- the drug is rapidly absorbed and distributed throughout the body, including in milk.
- AE:
- acidification of urine, which may affect disposition of other drugs
- inteference with oral absorption of iron.
Atropine sulfate as an antidote:
Use, absortion and excretion, effects of overdosage
Atropine sulfate counteracts anticholinesterase poisoning.
Atropine’s half-life is short (approx. 3 hours), dosage may need to be repeated.
Effects of OD: the drug crosses the blood-brain barrier, and OD can result in CNS disturbances (e.g. depression, disorientation), dry mucuous membranes and GI hypomotility.
Calcium disodium EDTA as an antidote:
Usage, absorption/excretion, adverse effects
Calcium disodium EDTA is used in chelation therapy of lead and zinc toxicoses.
EDTA is rapidly excreted in urine, with up to 95% found in urine within 24 h.
EDTA may chelate other essential divalent cations such as calcium, magnesium and copper, but affinity for these is less than for lead.
Dimercaprol (British antilewisite BAL) as an antidote:
Usage, limitations
Dimercaprol (BAL) is the classic antidote for arsenic poisoning and may also be used in mercury, antimony and lead toxicosis.
Dimercaprol has limited efficacy in animals with clinical signs.
Ethanol (20%) as an antidote:
Usage, absorption/excretion, adverse effects
Ethanol (20%) alleviates the effects of ethylene glycol ingestion and toxicosis by preventing its metabolsim to acidic intermediates.
Ethanol is rapidly metabolized and must be repeatedly administered. Most effective if given within 4 hours if ethylene glycol ingestion.
AE: CNS depression caused by ethylene glycol and its resultant acidosis may be increased by teh alcohol. Ethanol acts as a diuretic and increases fluid replacement needs.
Methylene blue as an antidote:
Usage, absorption/excretion
Methylene blue is a reducing agent that corrects methemoglobinemia from nitrate, nitrite, or chlorate toxicosis in large animals.
Urinary excretion is the main route of elimination, and the drug colors the urine blue or green as teh oxidation product (methylene blue sulfone) is formed.
4-MP as an antidote:
Usage, absorption/excretion
4-MP is an alternative therapy for ethylene glycol toxicosis.
4-MP is most effective when given soon after exposure (3-8h post ingestion). In cats does not effectively inhibit ethylene glycol metabolism unless given concurrently with the ethylene glycol.
Homemade by adding 5g of 4-MP to 50ml of polyethylene glycol and 46ml of sterile water to make 100ml. The solution is then filter sterilized through a 0.22um filter.
D-penicillamine as an antidote:
Usage, absorption/excretion, adverse effects
D-penicillamine is effective against copper and lead toxicoses.
Penicillamine is beta-beta-dimethylcysteine, a monothiol chelating agent with affinity for copper, lead and mercury. It is excreted in the urine.
AE: in humans: allergic reactions, moderate proteinuria, leukopenia, vomiting.
