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CENTRAL NERVOUS SYSTEM DRUGS-1

CENTRAL NERVOUS SYSTEM DRUGS-1

  1. 5-HT1 agonists (triptans)
  2. 5-HT3 antagonists
  3. Antihistamine anti-emetics
  4. Antipsychotics – atypical
  5. Antipsychotics – typical
  6. Benzodiazepines
  7. Carbamazepine
  8. Dopamine antagonist anti-emetics
  9. Drugs for dementia
  10. Gabapentin and pregabalin, 

1. 5-HT1 agonists (triptans) 

EXAMPLES 
Sumatriptan, zolmitriptan

MECHANISM OF ACTION 
  • Selective activation of 5-HT1 receptors that are predominantly located in cranial blood vessel walls. 5-HT1 receptors mediate vasoconstriction thereby relieving symptoms that are believed to result from the dilatation of intra- and extracranial vessels.
INDICATIONS
  • Treatment of acute migraine. 
  • Cluster headache.
CAUTIONS AND CONTRA-INDICATIONS
  • Ischaemic heart disease or coronary vasospasm. 
  • Peripheral vascular disease. 
  • Previous stroke or TIA. 
  • Severe hypertension.
SIDE-EFFECTS 
  • Dizziness. 
  • Paraesthesia. 
  • Tinnitus. 
  • A transient rise in blood pressure. 
  • Tachycardia and palpitations. 
  • Very rarely, MI
MONITORING 
  • No specific drug monitoring required.
DRUG INTERACTIONS
  • Increased risk of CNS toxicity with SSRIs and MAOIs.
  • Plasma concentration of 5-HT1 agonists may be increased by macrolides and b blockers
IMPORTANT POINTS
  • 5-HT1 agonists are the preferred treatment for patients with migraines that do not respond to simple analgesia. 
  • 5-HT1 agonists should be used in the established headache phase of an attack; they are not suitable for migraine prophylaxis. 
  • Newer formulations include SC preparations, IN preparations and wafers for faster administration.

2. 5-HT3 antagonists 
EXAMPLES 
Ondansetron, granisetron

MECHANISM OF ACTION 
  • Selective5-HT3 receptor antagonists that act peripherally on vagal nerve endings of the GI tract and centrally in the CTZ. 
  • 5-HT3 receptors in the CTZ in the area post rema of the medulla contribute to the perception of nausea and the control of vomiting.
INDICATIONS
  • PONV
  • Nausea and vomiting associated with cytotoxic drugs (chemotherapy) and radiotherapy
CAUTIONS AND CONTRA-INDICATIONS. 
  • Prolonged QT interval and cardiac conduction defects
  • Hypersensitivity
SIDE-EFFECTS
  • GI disturbance (especially constipation due to increased large bowel transit time) 
  • Headache
  • Flushing
MONITORING 
  • No specific drug monitoring required
DRUG INTERACTIONS
  • Effects reduced by drugs that induce liver enzymes(phenytoin, carbamazepine, rifampicin). 
  • Increased risk of torsades de pointes with other drugs that prolong the QT interval
IMPORTANT POINTS
  • Establish a cause for emesis prior to prescribing an anti-emetic. 
  • Very effective anti-emetic agents, particularly for PONV. 
  • Careful monitoring of symptoms if these drugs are used in patients with subacute bowel obstruction due to effects on large bowel motility.

3. Antihistamine anti-emetics 

EXAMPLES 
Cyclizine, promethazine

MECHANISM OF ACTION 
  • H1 receptor antagonists directly inhibit the CTZ in the medulla. They possess anticholinergic and anti-emetic properties. 
  • Cyclizine also increases lower oesophageal sphincter tone and reduces the sensitivity of the labyrinthine apparatus.
INDICATIONS
  • Nausea and vomiting. 
  • Hyperemesis in pregnancy (promethazine is first line). 
  • Vomiting in labyrinthine disorders. 
  • Nausea associated with motion sickness
CAUTIONS AND CONTRA-INDICATIONS
  • Severe prostatic hypertrophy. 
  • Caution in patients at risk of closed-angle glaucoma
SIDE-EFFECTS
  • Drowsiness. 
  • Headache. 
  • Tachycardia (cyclizine). 
  • Psychomotor impairment. 
  • Antimuscarinic effects
MONITORING 
  • No specific drug monitoring required.
DRUG INTERACTIONS
  • Increased sedative effect with opiates
IMPORTANT POINTS
  • Establish a cause for emesis prior to prescribing an anti-emetic. 
  • Sedative effects of antihistamines are likely to be potentiated in liver disease. 
  • Cyclizine and promethazine are safe to prescribe in pregnancy

4. Antipsychotics – atypical 
EXAMPLES 
Clozapine, olanzapine, quetiapine, risperidone, amisulpride

MECHANISM OF ACTION 
  • Not a homogenous pharmacological class.
  • Atypical antipsychotics act predominantly via dopamine (D1 to D4) and 5-HT receptors.
INDICATIONS
  • Schizophrenia and other psychoses. 
  • Mania. 
  • Sedation. 
  • Anxiety and psychomotor agitation
CAUTIONS AND CONTRA-INDICATIONS
  • CNS depression and coma. 
  • Severe cardiovascular disease. 
  • Caution in patients with hepatic impairment. 
  • Caution in epilepsy. 
  • Caution in elderly patients and patients with risk factors for cerebrovascular disease (increased risk of stroke in older patients with dementia with olanzapine and risperidone)
SIDE-EFFECTS.
  • Sedation or agitation. 
  • Postural hypotension. 
  • Antimuscarinic effects. 
  • Weight gain. 
  • Neutropenia and agranulocytosis (clozapine only). 
  • Myocarditis and cardiomyopathy (clozapine only). 
  • Hyperglycaemia (particularly clozapine and olanzapine). 
  • Rarely, neuroleptic malignant syndrome
MONITORING 
  • Check FBC prior to starting clozapine and initially monitor counts weekly. 
  • Cardiovascular assessment prior to starting clozapine.
DRUG INTERACTIONS 
  • Antagonism of sympathomimetics, anticholinergics and antiepileptic drugs may occur (the latter may lower seizure threshold).
  • Enhanced hypotensive effect with antihypertensive agents. 
  • Increased risk of cardiac arrhythmias (including torsades de pointes) with other drugs that prolong the QT interval
IMPORTANT POINTS
  • Atypical antipsychotics are more effective in the treatment of negative symptoms of schizophrenia. 
  • Withdrawal of antipsychotic after long-term therapy should be gradual to avoid the risk of acute withdrawal or rebound psychosis. 
  • If a treatment is effective but compliance is poor, depot preparations may be considered

5. Antipsychotics – typical 

EXAMPLES 
Haloperidol, chlorpromazine, prochlorperazine, flupentixol

MECHANISM OF ACTION 
  • Not a homogenous pharmacological class. 
  • Mixed antagonists at muscarinic, histaminergic, dopaminergic, serotonergic and adrenergic receptors. 
  • Typical antipsychotics exert their predominant neuroleptic effect through blockade of dopamine D2 receptors.
INDICATIONS
  • Schizophrenia and other psychoses. 
  • Mania. 
  • Sedation. 
  • Anxiety and psychomotor agitation. 
  • Nausea and vomiting
CAUTIONS AND CONTRA-INDICATIONS
  • CNS depression and coma. 
  • Severe cardiovascular disease. 
  • Caution in patients with hepatic impairment. 
  • Caution in epilepsy.
SIDE-EFFECTS
  • Sedation or agitation. 
  • Extra-pyramidal symptoms. 
  • Postural hypotension. 
  • Cardiac arrhythmias (prolongation of QT interval). 
  • Antimuscarinic effects. 
  • Hyperprolactinaemia. 
  • Rarely, neuroleptic malignant syndrome.
MONITORING 
  • ECG before initiating treatment and then annually with some typical antipsychotics.
DRUG INTERACTIONS
  • Phenothiazines (e.g. chlorpromazine, prochlorperazine) may enhance the CNS depressant effect of opioids, anxiolytics, sedatives, hypnotics and alcohol. 
  • Antagonism of sympathomimetics, anticholinergics and antiepileptic drugs may occur (the latter may lower seizure threshold). 
  • Enhanced hypotensive effect with antihypertensive agents.
  • Increased risk of cardiac arrhythmias (including torsades de pointes) with other drugs that prolong the QT interval
IMPORTANT POINTS
  • Withdrawal of antipsychotics after long-term therapy should be gradual to avoid the risk of acute withdrawal or rebound psychosis.
  • If a treatment is effective but compliance is poor, depot preparations may be considered

6. Benzodiazepines 

EXAMPLES 
Diazepam, lorazepam, chlordiazepoxide, midazolam, temazepam

MECHANISM OF ACTION 
  • Bind to benzodiazepine receptors that are coupled to GABA receptors. This increases theaffinity of GABA to its receptor and opens Cl- channels resulting in hyperpolarisation of the cell membrane, thereby preventing further excitation.
INDICATIONS
  • Short-term use in anxiety or insomnia. 
  • Acute alcohol withdrawal. 
  • Sedation. 
  • Status epilepticus. 
  • Muscle spasm
CAUTIONS AND CONTRA-INDICATIONS
  • Respiratory depression. 
  • A neuromuscular disease affecting muscles of respiration. 
  • Acute pulmonary insufficiency. 
  • Should not be used as monotherapy to treat depression (with or without anxiety)
SIDE-EFFECTS
  • Drowsiness and lightheadedness. 
  • Dependence. 
  • Confusion. 
  • Amnesia.
MONITORING 
  • Monitor respiratory effort and effect (respiratory rate and oxygen saturation).
DRUG INTERACTIONS
  • Should not be taken with alcohol due to increased sedative effect.
IMPORTANT POINTS 
  • Effect of excessive benzodiazepine use can be reversed by flumazenil. This is administered via IV route. 
  • Chlordiazepoxide can be used to lessen the symptoms of alcohol withdrawal. 
  • Chlordiazepoxide is given for limited time(3–10days) in reducing doses but should not be given if a patient is likely to continue drinking alcohol.

7. Carbamazepine 
MECHANISM OF ACTION 
  • Use-dependent blockade of voltage-gated Naþchannels responsible for the propagation of the action potential. Thus carbamazepine preferentially blocks the excitation of neurones that are firing repeatedly.
INDICATIONS
  • Epilepsy. 
  • Prophylaxis of bipolar disorder. 
  • Trigeminal neuralgia.
CAUTIONS AND CONTRA-INDICATIONS. 
  • AV conduction abnormalities (if not paced). 
  • History of bone marrow suppression. 
  • Acute porphyria.
SIDE-EFFECTS 
  • Nausea and vomiting. 
  • Drowsiness. 
  • Generalised erythematous rash. 
  • Cardiac conduction disturbances. 
  • Leucopenia.
MONITORING 
  • No specific drug monitoring required.
DRUG INTERACTIONS
  • Plasma levels can be enhanced by drugs inhibiting Cytochrome P450, including isoniazid, verapamil and diltiazem. 
  • Plasma levels can be reduced by drugs that potentiate Cytochrome P450, including phenytoin, phenobarbitone and theophylline. 
  • Reduces the anticoagulant effect of warfarin. 
  • Anticonvulsant effect antagonised by antipsychotics.
IMPORTANT POINTS
  • Treatment should be started at a low dose with small incremental increases every 2 weeks.
  • Carbamazepine can be used in the prophylaxis of bipolar disorder if symptoms are not responding to lithium

8. Dopamine antagonist anti-emetics 
EXAMPLES 
Domperidone, metoclopramide

MECHANISM OF ACTION 
  • The anti-emetic effect is due to a combination of prokinetic activity, up-regulation of the parasympathetic nervous system and antagonism of dopamine D2 receptors in the CTZ.
INDICATIONS
  • Nausea and vomiting
CAUTIONS AND CONTRA-INDICATIONS
  • Phaeochromocytoma. 
  • Prolactin-releasing pituitary tumour. 
  • GI obstruction. 
  • Perforation.
SIDE-EFFECTS
  • Extra-pyramidal symptoms.
  • Hyperprolactinaemia. 
  • Neuroleptic malignant syndrome. 
  • Rashes. 
  • Tardive dyskinesia. 
  • Confusion. 
  • Drowsiness.
MONITORING 
  • No specific drug monitoring required.
DRUG INTERACTIONS
  • Metoclopramide increases plasma levels of ciclosporin and NSAIDs. 
  • Ketoconazole increases the risk of arrhythmias when given with domperidone
IMPORTANT POINTS
  • Always establish a cause for vomiting prior to prescribing anti-emetics. 
  • Dopamine antagonists can cause severe extrapyramidal side-effects when given to young adults and elderly patients. 
  • Dopamine antagonists are particularly useful in vomiting secondary to chemotherapy and radiotherapy

9. Drugs for dementia 

MECHANISM OF ACTION 
  • Donepezil, galantamine and rivastigmine – inhibition of acetylcholinesterase thereby preventing the breakdown of ACh; the cholinergic hypothesis suggests that reduced ACh synthesis is a key aetiological factor in Alzheimer’s disease. Galantamine also acts as a partial agonist at nicotinic receptors.
  • Memantine – NMDA receptor antagonist that blocks the effects of pathologically elevated levels of glutamate that may contribute to neuronal dysfunction in Alzheimer’s disease.
INDICATIONS
  • Mild to moderate dementia in Alzheimer’s disease. 
  • Mild to moderate dementia in Parkinson’s disease (rivastigmine only)
CAUTIONS AND CONTRA-INDICATIONS
  • Caution in cardiac disease. 
  • Caution in asthma/COPD. 
  • Caution in patients susceptible to peptic ulcers . 
  • Caution in renal impairment.
SIDE-EFFECTS
  • GI disturbance. 
  • Gastric or duodenal ulcers. 
  • Drowsiness and confusion. 
  • Rarely, arrhythmias and SA node/AV node block.
MONITORING 
  • Monitor MMSE(Mini-mental state examination) score every 6 months – treatment should only be continued while the score remains at or above 10 (out of 30).
DRUG INTERACTIONS
  • Increased risk of arrhythmias with agents that reduce heart rate (e.g. b blockers, digoxin, amiodarone).
  • Effects antagonised by antimuscarinics
IMPORTANT POINTS 
  • Exclude reversible causes of cognitive impairment prior to initiating treatment (e.g. hypothyroidism). 
  • Drugs for dementia should only be started by specialists. 
  • The clinical benefits of acetylcholinesterase inhibitors are modest with only about a 10% improvement in short-term memory, language and praxis abilities at best.

10. Gabapentin and pregabalin 

MECHANISM OF ACTION
  • The exact mechanism is unclear. 
  • Gabapentin and pregabalin are analogues of GABA. Their clinical benefits are not mediated by the action on GABA receptors within the CNS but through binding to a subunit on T-type Ca2+ channels and selectively inhibiting the release of various neurotransmitters. Pregabalin is a higher potent analogue of gabapentin in chronic pain control.
INDICATIONS
  • Partial seizures with or without secondary generalisation (as monotherapy or adjunct). 
  • Neuropathic pain.
CAUTIONS AND CONTRA-INDICATIONS
  • Hypersensitivity
SIDE-EFFECTS
  • GI disturbance. 
  • Weight gain. 
  • Hypertension. 
  • Dizziness and drowsiness. 
  • Leucopenia. 
  • Visual disturbances (e.g. diplopia).
MONITORING 
  • No specific drug monitoring required.
DRUG INTERACTIONS
  • Increased risk of CNS depression with opiates.
  • Antacids containing aluminium and magnesium can reduce gabapentin bioavailability
IMPORTANT POINTS
  • Abrupt withdrawal can cause anxiety, insomnia, pain and increases the risk of seizures in epileptics. Lower doses should be considered in the elderly and patients with renal impairment.
  • Gabapentin should be started at a low dose and gradually increased inincrementsover2–3 days. Sedation, confusion and ataxia have been reported with rapid titration.







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