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Antipsychotics

Drugs in medicine

Drugs in medicine

Introduction

Antipsychotics are also sometimes called neuroleptics or major tranquilisers.They are typically used for the treatment of psychosis, including schizophrenia, and also in delirium – particularly in the elderly when related to dementia and in patient in intensive care. Occasionally quetiapine may be used for anxiety.

There are two types of antipsychotic drugs; typical antipsychotics (first generation) and atypical antipsychotics (second generation).
There is debate about how to classify a drug as typical or atypical, but atypical antipsychotics typically have fewer motor side effects. Also, the typical antipsychotics all tend to be from one class, whilst atypical ones have a wider range of mechanisms.
First generation anti-psychotics e.g. chlorpromazine, haloperidol, fluphenazine (depot (long acting) injection), clopenthixol
Atypical antipsychotics e.g. clozapine, risperidone, sertindole, aripiprazole, zotepine, olanzapine, quetiapine
 
Antipsychotic drugs generally have 4 important qualities:
The only real differences between the drugs are their differing efficacies in these four areas. Atypical antipsychotics also appear to be more effective at combating the negative symptoms seen in schizophrenia.

Mechanism of action

Most of the activity of these drugs is through blockade of dopamine receptors, although they have also been noted to block receptors for other neurotransmitters, including histamine, acetylcholine and 5-HT (serotonin). The activity at serotonin receptors is thought to affect their clinical efficacy, but the effect at other receptors is not, and is often responsible for side effects.
Dopamine receptors – there are 5 types of dopamine receptors (D1-D5), which are classed into 2 main categories – D1 and D2Antipsychotics mainly block D2 receptors. First generation drugs tend to have effect on D1 and D2, whilst atypical drugs have varying effects.
When the drug is first administered, dopaminergic neuron activity increases, but over time (after about 3 weeks), this activity decreases. This is thought to be related to the number of dopamine receptors – and that over time, due to the blockade, the number of receptors greatly increases, thus in an inverse relationship, dopaminergic neuron activity decreases.
The drugs take several weeks to become effective – even though the blockade of dopamine receptors is immediate.
You can also cause unwanted extrapyramidal effects if the blockade of dopamine receptors exceeds 80% (the therapeutic range is 65-80%).
Aytpical drugs have better compliance than their first generation counterparts – probably because they cause fewer side effects.

Clinical effects

Uses of antipsychotics

They are mainly used in schizophrenia and acute behavioural disturbances, but they may also be used as adjuvant therapy in many other psychiatric disorders, such as mania and psychotic depression.

Pharmakokinetics

Unwanted Effects

Extrapyramidal Effects – these result fro D2 blockade in the nigrostriatal pathways – examples of the side effects include:

Other side effects include:

NMS – neuroleptic malignant syndrome
Caused by an adverse reaction to neuroleptics. It is rare, but is a medical emergency and can be life-threatening. It results in death in about 10% of cases.
Treatment basically involves stopping the drug, and aggressively treating the hyperthermia e.g. with ice packs. May also often require circulatory and ventilator support. Benzodiazepines can be used to control agitation. It is also important to recognise the syndrome early to treat it as soon as possible.

References

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