Pharmacodynamics of Antihelminthics
Session 17: Pharmacodynamics of Antihelminthics
Total Session Time: 120 minutes
Prerequisites
None
Learning Tasks
By the end of this session students are expected to be able to:
Describe mechanism of action of Antihelminthics
Describe drug interactions associated with Antihelminthics
Describe side effects of Antihelminthics
Describe contraindications of Antihelminthics
Resources Needed:
Flip charts, marker pens, and masking tape
Black/white board and chalk/whiteboard markers
Computer and projector
SESSION OVERVIEW
Step
Time
Activity/
Content
Step
Time
Activity/
Content
Step
Time
Method
Content
Method
Method
1
1
05 minutes
05 minutes
Presentation
Introduction, Learning Tasks
Introduction, Learning Tasks
2
2
45 minutes
45 minutes
Presentation/
Mechanism of Action of Antihelminthics
Mechanism of Action of Antihelminthics
2
2
45 minutes
45 minutes
Buzzing
Mechanism of Action of Antihelminthics
Mechanism of Action of Antihelminthics
Buzzing
3
3
20 minutes
20 minutes
Presentation/
Drug Interactions Associated With
Drug Interactions Associated With
3
3
20 minutes
20 minutes
brainstorming
Antihelminthics
Antihelminthics
brainstorming
Antihelminthics
Antihelminthics
4
4
20 minutes
20 minutes
Presentation
Side Effects of Antihelminthics
Side Effects of Antihelminthics
5
5
20 minutes
20 minutes
Presentation/
Contraindications of Antihelminthics
Contraindications of Antihelminthics
5
5
20 minutes
20 minutes
Brainstorming
Contraindications of Antihelminthics
Contraindications of Antihelminthics
Brainstorming
6
6
05 minutes
05 minutes
Presentation
Key Points
Key Points
7
7
05 minutes
05 minutes
Presentation
Evaluation
Evaluation
PST 05104 Pharmacology & Therapeutics 141 NTA Level 5 Semester 1 Facilitator Guide
SESSION CONTENTS
STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)
READ or ASK students to read the learning tasks and clarify
ASK students if they have any questions before continuing.
STEP 2: Mechanism of Action of Antihelminthics (45 minutes)
Activity: Buzzing (5 minutes)
ASK students to pair up and buzz on the following question for 2 minutes
How do antihelminthics produce their pharmacological effects?
ALLOW few pairs to respond and let other pairs add on points not mentioned
WRITE their response on the flip chart/board
CLARIFY and SUMMARIZE by using the content below
Benzimidazoles
Albendazole
o Albendazole like other benzimidazoles acts by binding to and interfering with the synthesis of the parasite's microtubules and also by decreasing glucose uptake. Affected parasites are expelled with the feces.
o Benzimidazoles have a selective inhibitory action on helminthic microtubular function, being 250-400 times more potent in helminths than in mammalian tissue. The inhibitory concentration is lower
Mebendazole
Has same mechanism of action with Albendazole discussed above
Thiabendazole
Has same mechanism of action with Albendazole discussed above
Pyrantel pamoate
o It acts as a depolarizing neuromuscular blocking agent, causing persistent activation of the parasite‘s nicotinic receptors.
o The paralysed worms are then expelled from the intestinal tract of the host.
Ivermectin
o Ivermectin targets the parasite's y-aminobutyric acid (GABA) receptors.
o Chloride efflux is enhanced and hyperpolarization occurs, resulting in paralysis of the worm.
Praziquantel
PST 05104 Pharmacology & Therapeutics 142 NTA Level 5 Semester 1 Facilitator Guide
Praziquantel acts by increasing membrane permeability to ca2+.This causes increased contraction of the musculature and eventually results in paralysis and death of the worm (SPASTIC PARALYSIS).
It has been suggested that praziquantel (at slightly high concentrations ) modifies the parasite so that it becomes susceptible to hosts normal immune responses by causing
tegumental damage hence worm destruction.
It is associated with IL-4 and a type 2 (TH2) response
Piperazine
Piperazine inhibits neuromuscular transmission in the worm, probably by acting like GABA the inhibitory neurotransmitter on GABA gated chloride channels in the
nematode muscle hence flaccid paralysis.
The paralysed worms are expelled live
Niclosamide
Act by inhibiting anaerobic phosphorylation of ADP by the mitochondria of the parasite ,an energy producing process that is dependent on carbon dioxide (co2 )
fixation thus resulting into decreased glucose uptake and glycogen synthesis
The scolex and a proximal segment are irreversibly damaged by the drug. The worm separates from the intestinal wall and is expelled. Neither the larvae nor the ova are affected.
Oxamoniquine
The drug undergoes ATP dependent enzymatic activation to unstable phosphate ester
which dissociates to yield a reactive carbocation which then alkylates DNA.
Has some anticholinergic effects on the worm.
Diethylcarbamazine
Modifies parasite membrane so that it becomes susceptible to the host normal immune
response.
Causes progressive paralysis of the worm due to decreased ATP resulting from
prevention of ADP phosphorylation.
may also interfere with parasite arachidonate mechanism
Levamisole
It has nicotinic like action, stimulating and subsequently blocking the neuromuscular junctions.
The paralysed worms are then passed in the faeces. The ova are not killed.
PST 05104 Pharmacology & Therapeutics 143 NTA Level 5 Semester 1 Facilitator Guide
STEP 3: Drug Interactions Associated with Antihelminthics Drugs (20 minutes)
Activity: Brainstorming (5 minutes)
Ask students to brainstorm on the following question:
What drug interactions associated with of antihelminthics drugs?
ALLOW few students to respond
WRITE their responses on the flip chart/ board
CLARIFY and SUMMARISE by using the content below
Antihelminthics do not have significant drug interactions.
STEP 4: Side Effects of Antihelminthics (20 minutes)
Benzimidazoles: Albendazole and others.
o Generally well tolerated. Attributing side effects to drug can be challenging, as many side effects are consistent with the host response to dead or dying parasites rather than the drug itself.
o GI side effects may be experienced, although in some cases these may be largely the result of passage of the worm.
o Mebendazole only: Serious and Rare: Agranulocytosis, alopecia, and elevated hepatic enzymes have been reported at high doses, although mechanisms have not been established for any of these side effects.
Praziquantel
o Serious and Rare: CNS effects (seizures, changes in mental status, intracranial hypertension) may occur in the treatment of neurocysticercosis.
o These effects are believed to be caused by inflammatory reactions that occur because of the dead parasites.
o Corticosteroids may be coadministered with praziquantel to reduce inflammation.
PST 05104 Pharmacology & Therapeutics 144 NTA Level 5 Semester 1 Facilitator Guide
STEP 5: Contraindications of Antihelminthics (20 minutes)
Activity: Brainstorming (5 minutes)
Ask students to brainstorm on the following question:
What are the contraindications of Antihelminthic Drugs?
ALLOW few students to respond?
WRITE their responses on the flip chart/ board
CLARIFY and SUMMARISE by using the content below
Benzimidazoles
Mebendazole and albendazole
o It is contraindicated in pregnant women, because it has been shown to be embryo toxic and teratogenic in experimental animals. However, Mebendazole and albendazole have each demonstrated teratogenic effects in animals. They are used in pregnancy, but only after a careful risk-benefit assessment.
Praziquantel
o Ocular cysticercosis: The type of reaction described later in the discussion of neurocysticercosis can be irreversible if it occurs in the eye.
STEP 6: Key Points (5 minutes)
Antihelminthics have different mechanism of action
Most antihelminthics do not exhibit significant drug interactions
The choice of a drug is dependent on the type of the worm
STEP 7: Evaluation (5 minutes)
What are contraindications of antihelminthics?
What is the mechanism of action of Ivermectin
What are adverse effects of antihelminthics?
PST 05104 Pharmacology & Therapeutics 145 NTA Level 5 Semester 1 Facilitator Guide
References
Katzung, B. G. (2018). Basic and clinical pharmacology. New York: Mcgraw Hill Education.
Santos, R. R., Rang, H. P., Dale, M. M., Ritter, J. M., & Flower, R. J. (2007). Rang & Dale Farmacologia. Rio de Janeiro: Elsevier.
Tripathi, K. (2018). Essentials of Medical Pharmacology. Place of publication not identified:
Jaypee Brothers Medical P.
Ministry of Health and Social Welfare. (2013). Standard Treatment Guidelines & National Essential Medicines List Tanzania Mainland (4th ed.). Dar es salaam, Tanzania government printers.
Sally S.R, Jeanne C.S. (2000). Introductory Clinical Pharmacology (6th ed) New York, Lippincott Williams and Wilkins.
School of Pharmaceutical sciences. (2011).Tanzania Pharmaceutical Handbook (2nd ed.).
Dar es Salaam, ARDHI University press.
The Royal Pharmaceutical Society of Great Britain. (2007). Martindale, the Extra Pharmacopoeia (5TH ed). London, pharmaceutical press.
The Royal Pharmaceutical Society of Great Britain. 2009. British National Formulary (59th ed). London, BMJ Group and RPS Publishing.
PST 05104 Pharmacology & Therapeutics 146 NTA Level 5 Semester 1 Facilitator Guide
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