Thioridazine (sold under the brand names Mellaril or Melleril) is a first-generation antipsychotic drug belonging to the phenothiazine drug group and was previously widely used in the treatment of schizophrenia and psychosis. The branded product was withdrawn worldwide in 2005 because it caused severe cardiac arrhythmias. However, generic versions are still available in the United States.
Indications
Its primary use in medicine is for the treatment of schizophrenia.
It was also tried with some success as a treatment for various psychiatric symptoms seen in people with
dementia,
but chronic use of thioridazine and other anti-psychotics in people with dementia is not recommended.
Side effects
Thioridazine prolongs the
QT interval in a dose-dependent manner.
It produces significantly less extrapyramidal side effects than most first-generation antipsychotics, likely due to its potent anticholinergic effect.
Its use, along with the use of other typical antipsychotics, has been associated with degenerative retinopathies (specifically retinitis pigmentosa).
It has a higher propensity for causing
anticholinergic side effects coupled with a lower propensity for causing extrapyramidal side effects and sedation than
chlorpromazine, but also has a higher incidence of hypotension and cardiotoxicity.
It is also known to possess a relatively high liability for causing orthostatic hypotension compared to other antipsychotics. Similarly to other first-generation antipsychotics it has a relatively high liability for causing prolactin elevation. It is moderate risk for causing weight gain.
As with all antipsychotics thioridazine has been linked to cases of tardive dyskinesia (an often permanent neurological disorder characterised by slow, repetitive, purposeless and involuntary movements, most often of the facial muscles, that is usually brought on by years of continued treatment with antipsychotics, especially the first-generation (or
typical) antipsychotics such as thioridazine) and neuroleptic malignant syndrome (a potentially fatal complication of antipsychotic treatment).
Blood dyscrasias such as
agranulocytosis,
leukopenia and
neutropenia are possible with thioridazine treatment.
Thioridazine is also associated with abnormal retinal pigmentation after many years of use.
Thioridazine has been correlated to rare instances of clinically apparent acute cholestatic liver injury.
Pharmacology
Thioridazine has the following binding profile:
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The ratio of 5-HT2A to D2 receptor binding is believed to dictate whether or not most antipsychotics are atypical or typical. In thioridazine's case its ratio of 5-HT2A to D2 receptor binding is below the level that's believed to be required for atypicality despite its relatively low extrapyramidal side effect liability in practice. |
Believed to play a role in the weight gain-promoting effects of antipsychotics. |
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Likely the receptor responsible for the orthostatic hypotension known to occur in individuals on thioridazine. |
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This receptor is believed to be the chief receptor responsible for the anticholinergic side effects of thioridazine (e.g. dry mouth, constipation, blurred vision, etc.). Likely plays a role in thioridazine's low extrapyramidal side effect liability as anticholinergic drugs such as benzatropine are routinely given to treat extrapyramidal side effects resulting from antipsychotic treatment. |
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Believed to be the receptor responsible for the therapeutic effects of antipsychotics. |
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| Likely involved in thioridazine's cardiac effects. |
| Likely responsible for the sedating effects of thioridazine. |
| Regulates the release of hydrochloric acid into the stomach. |
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Note: The Binding affinities given are towards cloned human receptors unless otherwise specified
Acronyms used
HB – Human brain receptor
RC – Cloned rat receptor
ND – No data
Metabolism
Thioridazine is a
racemic compound with two
, both of which are metabolized, according to Eap et al., by CYP2D6 into (
S)- and (
R)-thioridazine-2-sulfoxide, better known as
mesoridazine,
and into (
S)- and (
R)-thioridazine-5-sulfoxide.
Mesoridazine is in turn metabolized into
sulforidazine.
Thioridazine is an inhibitor of CYP1A2 and CYP3A4.
History
Thioridazine was first synthesized in 1958 by Swiss pharmaceutical company
Sandoz.
It quickly entered widespread clinical use, as there were few alternatives for treating schizophrenia. In 2005, thioridazine was voluntarily discontinued by its manufacturer,
Novartis, worldwide because it caused severe cardiac arrhythmias. However, generics remain on the market in some countries.
Generic forms of thioridazine however remain on the market in a few countries, usually with restrictions. For example, in the US it is restricted to patients who have taken at least 2 other antipsychotics that either failed or caused serious side effects.
Antimicrobial activity
Thioridazine is known to kill extensively drug-resistant tuberculosis
and to make methicillin-resistant
Staphylococcus aureus sensitive to β-lactam antibiotics.
A possible mechanism of action for the drug's antibiotic activity is via the inhibition of bacterial secretion pumps. The β-lactam antibiotic resistance is due to the secretion β-lactamase, a protein that destroys antibiotics. If the bacteria cannot secrete the β-lactamase, then the antibiotic will be effective.
more broadly have also been used in combination with other drugs to treat infections caused by pathogenic free living ameoba (PFLA).
Synthesis
Note: Same sidechain used for
mesoridazine and
sulforidazine.
The alkylation of 2-Picoline 109-06-8 (
1) with formaldehyde gives 2-Pyridineethanol 103-74-2 (
2). Forming the quat salt with methyl iodide 74-88-4 leads to 2-(2-hydroxyethyl)-1-methyl-pyridinium iodide 56622-15-2 (
3). Catalytic hydrogenation in the presence of hydrochloric acid leads to 2-(2-Chloroethyl)-1-Methylpiperidine 50846-01-0 (
4). Alkylation of 2-Methylthiophenothiazine 7643-08-5 (
5) in the presence of sodium hydride base completed the synthesis of
Thioridazine (
6).
Further reading
External links
