Typical antipsychotics, which include phenothiazines and non-phenothiazines, can be broken down into smaller classifications.
Many clinicians believe that the phenothiazines should be treated as three distinct drug classes because of the differences in the ad-verse reactions they cause:
§ Aliphatics primarily cause sedation and anticholinergic effects. They’re low potency drugs that include chlorpromazine.
§ Piperazines primarily cause extrapyramidal reactions and in-clude fluphenazine decanoate, fluphenazine enanthate, fluphena-zine hydrochloride, perphenazine, and trifluoperazine.
§ Piperidines primarily cause sedation and anticholinergic and cardiac effects; they include mesoridazine and thioridazine.
Based on their chemical structure, nonphenothiazine antipsy-chotics can be divided into several drug classes, including:
§ butyrophenones, such as haloperidol and haloperidol decanoate
§ dibenzoxazepines such as loxapine
§ dihydroindolones such as molindone
§ diphenylbutylpiperidines such as pimozide
§ thioxanthenes, such as thiothixene and thiothixene hydro-chloride.
Although phenothiazines are absorbed erratically, they’re very lipid-soluble and highly protein-bound. Therefore, they’re distrib-uted to many tissues and are highly concentrated in the brain.
Like phenothiazines, nonphenothiazines are absorbed errati-cally, are lipid-soluble, and are highly protein-bound. They’re also distributed throughout the tissues and are highly concentrated in the brain.
All phenothiazines are metabolized in the liver and excreted in urine and bile. Because fatty tissues slowly release accumulated phenothiazine metabolites into the plasma, phenothiazines may produce effects up to 3 months after they’re stopped.
Nonphenothiazines are also metabolized in the liver and ex-creted in urine and bile.
Although the mechanism of action of phenothiazines isn’t fully un-derstood, researchers believe that these drugs work by blocking postsynaptic dopaminergic receptors in the brain.
The mechanism of action of nonphenothiazines resembles that of phenothiazines.
The antipsychotic effect of phenothiazines is due to receptor blockade in the limbic system. Their antiemetic effect is due to re-ceptor blockade in the chemoreceptor trigger zone located in the brain’s medulla.
Phenothiazines also stimulate the extrapyramidal system (motor pathways that connect the cerebral cortex with the spinal nerve pathways).
Phenothiazines are used primarily to:
· treat schizophrenia
· calm anxious or agitated patients
· improve a patient’s thought processes
· alleviate delusions and hallucinations.
Other therapeutic uses have been found for phenothiazines:
§ They’re administered to treat other psychiatric disorders, such as brief reactive psychosis, atypical psychosis, schizoaffective psychosis, autism, and major depression with psychosis.
§ In combination with lithium, they’re used in the treatment of pa-tients with bipolar disorder, until the slower-acting lithium pro-duces its therapeutic effect.
§ They’re prescribed to quiet mentally challenged children and ag-itated geriatric patients, particularly those with dementia.
§ The preoperative effects of analgesics may be boosted with their addition.
§ They’re helpful in the management of pain, anxiety, and nausea in patients with cancer.
As a group, nonphenothiazines are used to treat psychotic disor-ders. Thiothixene is also used to control acute agitation. Haloperi-dol and pimozide may also be used to treat Tourette syndrome.
Phenothiazines interact with many different types of drugs and may have serious effects:
· Increased CNS depressant effects, such as stupor, may occur when phenothiazines are taken with CNS depressants.
· CNS depressants may reduce phenothiazine effectiveness, re-sulting in increased psychotic behavior or agitation.
· Taking anticholinergic drugs with phenothiazines may result in increased anticholinergic effects, such as dry mouth and constipa-tion. By increasing phenothiazine metabolism, anticholinergic drugs may also reduce the antipsychotic effects of phenothiazines.
· Phenothiazines may reduce the antiparkinsonian effects of lev-odopa.
· Concurrent use with lithium increases the risk of neurotoxicity.
· Concurrent use with droperidol increases the risk of extrapyra-midal effects.
· The threshold for seizures is lowered when phenothiazines are used with anticonvulsants.
· Phenothiazines may increase the serum levels of TCAs and beta-adrenergic blockers. Thioridazine can cause serious, even fatal, cardiac arrhythmias when combined with such drugs as fluvoxam-ine, propranolol, pindolol, and fluoxetine that inhibit the cyto-chrome P-450 2D6 isoenzyme, or drugs known to prolong the QTc interval. (See Adverse reactions to typical antipsychotics.)
Nonphenothiazines interact with fewer drugs than phenothiazines. Their dopamine-blocking activity can inhibit levodopa and may cause disorientation in patients receiving both medications. Halo-peridol may boost the effects of lithium, producing encephalopa-thy (brain dysfunction).