Antidotes and Their Classification

Antidote
Prepared by: www.mcqsinpharmacology.com

Treatment of Poisoning:
Supportive care
Activated charcoal for serious
oral poisonings
Occasional use of specific
antidotes or dialysis
Only rare use of gastric
emptying

• The proper use of antidotes in the
intensive care setting, when
combined with aggressive supportive
care, may significantly reduce the
morbidity and mortality associated
with many severe poisonings.
 www.mcqsinpharmacology.com

ANTIDOTES
• According to WHO
“Antidote was defined as a therapeutic
substance used to counteract the toxic
action(s) of a specified xenobiotic.”
• antidotes reduce the overall burden of health
service in managing of poisoning cases
Supportive
therapy
correct
Antidote
↑↑
Pt.Survival

CLASSIFICATION OF ANTIDOTE
• According to mode of Action:

According to Site of Action:
1. Interacts with the poison to form a non toxic
complex that can be excreted:e.g. Chelators
2. Accelrates the detoxification of the poison:e.g. N-
acetylcystine,thiosulfate
3. Decrease the rate of conversion of posion into
toxic metabolite:e.g.Ethanol,Fomepizole
4. Compete the poison for certain receptors.:e.g.
Nalaxone
5. Block the receptor through which the toxic effect
of the poison is mediated e.g.Atropine
6. Bypass the effect of Poison:O2 in the treatment
of CO and cyanide toxicity
7. Antibodies to the poison : digiband and antivenoms

Physical Antidote:
Agent use to interfere with poison
through physical properties, not
change their nature
a) Adsorbing: The main example is
activated charcol
b) Coating: A mixture of egg & milk
make a coat over the mucosa.
c) Dissolving: 10% alcohol or glycine
for carbolic acid

CHARCOL:
(Universal Antidote)
• produced by heating pulverized carbonaceous
substances sawdust, peat, or coconut shells
• activation: Hot air to erode the internal
surfaces of the product and thereby increase its
adsorptive surface area.
• Adsorption results from weak intermolecular
(Van der Waals) forces
• AC can prevent systemic absorption of drugs
when given within 1-2 h of ingestion
• The optimal dose is probably a 40:1 ratio (by
weight) of charcoal to drug
• contraindicated for iron, lithium, potassium, and
ethanol overdose

Chemical Antidote:
• Interact specifically with a toxicant,
or neutralize the toxicant.
e.g. metal chelators combine with metals to form
complexes that can then be eliminated by the
kidneys
Mainly act by two mechanisms:
 Complex Formation:
Antidote make complex with the toxicant making it
unavailable to cross the membrane or to interact
with receptors
 DMSA(dimercaprol and dimercaptosuccinic
acid are sulfohydral compounds that bind metal
such as arsenic acid ,lead.

 Sp. Binding agents like EDTA, defroxamine and
D-pencillamine act by chelation of metal
forming more water soluble complex
 Antivenins and antibodies against digitoxin are
immunologicaly genrated agents that bind
specifically to the toxin or venom
 Metabolic conversion:
Detoxification to less toxic product
 Nitrite interact with hemoglobin and cyanide to
form cyanomethamoglobin , which is less toxic
than cyanide and interfare with the cyanide
access to cytochrome oxidase system.

Pharmacological antidote:
• counteract the effects of a poison by producing the opposite
pharmacological effects, e.g., ACHE inhibitors atropine
• Pharmacologic antidotes may
neutralize or antagonize the effects
of a toxicant.
• This type of antidote may act by
following 5 mechanism.

1. Preventing the formation of toxic
metabolites:
More effective when given immediately before
toxic metabolic activation
Example:
Ethanol and 4-methylpyrazole(4-MP) which
compete with the alcohol dehydrogenase which
prevent the formation of toxic intermediate from
ethylene glycol.
2. By Facilitation Of More Rapid Or Complete
Elimination Of A Toxicant :
Change the physiochemical nature of toxin,
allowing better glomerular filtration and prohibt
tubular reabsorption.
eg., molybdenum and sulfate for copper toxicity by
making water soluble complex,

3. By competing with the Toxicant’s action at a
receptor site:
a) Antagonism:
Competitive antagonism:
 Naloxone/Naltrexone: Opioid dependence,
longer action and affinity for mu receptor.
 Flumenazil: Antagonist for Benzodiazepine
 Atropine: organophosphate, carbamate and
other parasympathomimetic antidote.
It is also used to correct bradycardia caused by
morphine, digitalis, beta blockers etc

Non Competitive Antagonism:
 Calcium gluconate:
Used for Calcium channel blocker especialy
Verapamil
 Black widow spider bite
 Lead colic
 Oxalic acid
Paralidoxime :ChE activator act by breaking Alkyl
phosphate ChE bond. It is used in
organophosphate toxicity.
Diacetyl Monoxyime(DAM): action same as PAM
but with more BBB penetration.
Physostigmine: Counteract the anticholinergic
effect

4. By blocking receptors responsible for the
toxic effect :
The physiologic effect induced by a toxin is
prevented by an antidote, although the toxicant is
unchanged and may still be active.
Example:
atropine blocks the physiologic effect of
acetylcholine at cholinergic synapse and
neuromuscular junction ,and in organophosphate
toxicity

5. By aiding in the restoration of normal
function:
The antidote promotes return to normal function
by repairing a defect or enhancing a function that
correct the effect of poison.
Example:
Methylene blue:
In nitrite poisoning, methylene blue interact with
reduced NADPH to reduce the ferric iron of
methemoglobin back to ferrous ion in hemoglobin,
which can again transport oxygen
Acetylcysteine :
Acetylcysteien supplies the precursor amino acids
for glutathione, which serves as biologic
antioxident against acetaminphen toxicosis

Thanks
Questions ??

References:
• http://www.merckmanuals.com/professional/injuri
es_poisoning/poisoning/general_principles_of_p
oisoning.html
• J Med Toxicol. 2010 June; 6(2): 190–198.
Activated Charcoal for Acute Poisoning: One
Toxicologist’s Journey Kent R. Olson
• http://www.mdpoison.com/healthcareprofessiona
ls/toxtidbits.html
• NVMS Toxiology by Grey D. Osweiller

Antidotes and Their Classification

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