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Antiemetic Drugs: Mechanisms, Uses, and Side Effects

Understand the classification and pharmacokinetics of antiemetics, identify drugs for different causes of vomiting, and learn about adjuvant antiemetics. Explore the major classes of antiemetic drugs, their mechanisms of action, and side effects.

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Antiemetic Drugs: Mechanisms, Uses, and Side Effects

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  1. Antiemetics Prof. Hanan Hagar Pharmacology Department College of Medicine

  2. Learning objectives Classify the main different classes of antiemetic drugs according to their mechanism of action. Know the characteristic pharmacokinetics & dynamics of different classes of antiemetic drugs. Identify the selective drugs that can be used according to the cause of vomiting. Learn the adjuvant antiemetics. Describe the major side effects for the different classes of antiemetics.

  3. Vomiting • Is a complex series of integrated events culminating in the forceful expulsion of gastric contents through the mouth.

  4. Causes of nausea and vomiting a useful abbreviation for remembering causes of nausea and vomiting is VOMIT. Vestibular Obstruction or drugs like opiates Mind (dysmotility) Infection (irritation of gut) Toxins (taste and other senses)

  5. Causes of Vomiting Nausea and vomiting may be manifestations of many conditions and may occur due to stimulation of vomiting center that respond to inputs from: • Higher cortical centers stimulation (CNS) • Chemoreceptor trigger zone (CTZ) stimulation • Disturbance of vestibular system • The periphery (Pharynx, GIT) via sensory nerves

  6. 1. Stimulation of chemoreceptor trigger zone (CTZ) CTZ is an area of medulla that communicate with vomiting center to initiate vomiting. CTZ is physiologically outside BBB. CTZ Contains D2 receptors , 5 HT3 receptors, opioid receptors stimulated by the following in blood or CSF. : Drugs (chemotherapy, opioids, general anesthetics, digitalis, L-dopa,). Chemicals Toxins Radiation.

  7. 2. The periphery via sensory nerves GIT irritation, myocardial infarction, renal or biliay stones. 3. Disturbance of vestibular system by motion sickness (H1 & M1 receptors) 4. Higher cortical centers stimulation: emotional factors, nauseating smells or sights.

  8. Receptors Associated with Nausea and Vomiting

  9. Pathophysiology of Emesis Chemotherapy Drugs as opioids Anesthetics Endogenous toxins Nerves to somatic and visceral receptors Cerebral cortex Pain Smell Sight Thought ChemoreceptorTrigger Zone (CTZ) Vestibular nuclei Vomiting Centre (medulla) Motion sickness Muscarinic Histaminic H1 Muscarinic, 5 HT3 & Histaminic H1 (Outside BBB) 5 HT3 Dopamine D2 Opioid receptors Substance p Chemo & radio therapy Gastroenteritis Pharynx & GIT 5 HT3 receptors

  10. Chemical transmitters & receptors involved in vomiting include: Dopamine (D2) Serotonin (5 -HT3) Substance P (Neurokinin receptors, NK1) Opioid (Opioid receptors) Ach (Muscarinic receptors) Histamine (Histaminergic receptors H1)

  11. Consequences of vomiting Severe vomiting may result in : • Dehydration • Acid-base imbalance • Electrolyte depletion • Aspiration, pneumonia

  12. Classification of Antiemetic Drugs 5-HT3 antagonists D2 receptor antagonists NK1 antagonists H1-receptor antagonists Muscarinic receptor antagonists Cannabinoids Glucocorticoids

  13. Serotonin (5-HT3) antagonists Drugs as Ondansetron (zofran) Granisetron (Kytril) Orally or parenterally, have long duration of action The most potent antiemetic drugs Act by blocking 5-HT3 receptor centrally (in vomiting center, CTZ) and peripherally (5HT3 receptors on GI vagal afferents).

  14. Uses of 5-HT3 antagonists First choice for prevention and treatment of moderate to severe emesis: Chemotherapy-induced nausea and vomiting (CINV) especially cisplatin (highly emetogenic anticancer). Post-radiation NV& Post-operative NV Their effects is augmented by combination with corticosteroids or NK1 antagonists.

  15. Side effects Well tolerated Headache, dizziness and constipation minor ECG abnormalities (QT prolongation)

  16. D2 receptor antagonists block D2 dopamine receptors in the CTZ Two types exist: Prokinetics drugs Neuroleptics (antipsychotics)

  17. D2 receptor antagonists Prokinetics drugs Domperidone: oral Metoclopramide: oral, i.v Are prokinetic agents ( increased GI motility & gastric emptying).

  18. Uses Antiemetics (blocking D2 receptors in CTZ) Effective against vomiting due to cytotoxic drugs, gastroenteritis, surgery, toxins, uremia, radiation Prokinetic Gastroesophageal reflux disease (GERD) Gastroparesis (impaired gastric emptying after surgery).

  19. Metoclopramide crosses BBB but domperidone cannot (both have antiemetic effects as CTZ is outside BBB). Side effects (only for metoclopramide): Dyskinesia (extra-pyramidal side effects), Galactorrhea, menstrual disorders, impotence Postural hypotension (α-blocking action). Sedation, drowsiness

  20. Other D2 receptor antagonists Neuroleptics (Antipsychotics) Chlorpromazine (CPZ), droperidol Acts centrally to block D2 receptors in CTZ used for postoperative vomiting and chemotherapy-induced emesis. Side effects: Extra pyramidal symptoms Sedation Postural hypotension

  21. Neurokinin1 (NK1) receptor antagonists Aprepitant • Acts centrally as substance P antagonistby blocking neurokinin 1 receptors in vagal afferent fibers in STN and area postrema. • Orally • Usually combined with 5-HT3 antagonists and corticosteroids in chemotherapy-induced nausea and vomiting and post- operative NV. N.B. STN is Solitary Tract Nucleus

  22. H1-receptor antagonists Include drugs as diphenhydramine, promethazine meclizine, cyclizine Used for Motion sickness Morning sickness in pregnancy Promethazine: severe morning sickness of pregnancy(if only essential). Side effects: Prominent sedation, hypotension, Anticholinergic effects (dry mouth, dilated pupils, urinary retention, constipation).

  23. Muscarinic receptor antagonists • Hyoscine (scopolamine) • Orally, injection, patches • Used as transdermal patches for prevention of motion sickness (applied behind the external ear before an insult). • Acts centrally by reduce impulses from vestibular nuclei • Not in chemotherapy-induced vomiting

  24. Side effects: • Sedation • Tachycardia, blurred vision, dry mouth, constipation, urinary retention (atropine-like actions).

  25. Glucocorticoids • Dexamethasone - methylprednisolone • Used in chemotherapy-induced vomiting • combined with 5-HT3 antagonists or NK1 receptor antagonists.

  26. Glucocorticoids Side effects: Hyperglycemia Hypertension Cataract Osteoporosis Increased intraocular pressure Increased susceptibility to infection Increased appetite & obesity

  27. Cannabinoids Nabilone, dronabinol mechanism of action not understood. act at central cannabinoid receptors. Used in vomiting due to cytotoxic drugs (adjuvant therapy). Limited use due to side effects Side effects: Euphoria, dysphoria, sedation, hallucination.

  28. Summary The choice of antiemetic drug depends on the etiology Motion sickness Muscarinic antagonists Antihistaminics Vomiting with pregnancy (morning sickness) avoid all drugs in the first trimester Pyridoxine (B6) Promethazine (only if absolutely essential in late pregnancy.

  29. Post operative nausea & vomiting D2- antagonists 5-HT3 antagonists Vomiting due to cytotoxic drugs. 5-HT3 antagonists D2- antagonists NK1 antagonists Glucocorticoids Cannabinoids (rare)

  30. Thank youQuestions ?

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