Mellaril - General Information
A phenothiazine antipsychotic used in the management of psychoses, including schizophrenia, and in the control of severely disturbed or agitated behavior. It has little antiemetic activity. Mellaril has a higher incidence of antimuscarinic effects, but a lower incidence of extrapyramidal symptoms, than chlorpromazine. (From Martindale, The Extra Pharmacopoeia, 30th ed, p618)
Pharmacology of Mellaril
Mellaril is a trifluoro-methyl phenothiazine derivative intended for the management of schizophrenia and other psychotic disorders. Mellaril has not been shown effective in the management of behaviorial complications in patients with mental retardation.
Mellaril for patients
Patients should be informed that Mellaril has been associated with potentially fatal heart
rhythm disturbances. The risk of such events may be increased when certain drugs are given
together with Mellaril. Therefore, patients should inform the prescriber that they are
receiving Mellaril treatment before taking any new medication.
Given the likelihood that some patients exposed chronically to neuroleptics will develop
tardive dyskinesia, it is advised that all patients in whom chronic use is contemplated be
given, if possible, full information about this risk. The decision to inform patients and/or
their guardians must obviously take into account the clinical circumstances and the competency
of the patient to understand the information provided.
Reduced cytochrome P450 2D6 isozyme activity, drugs which inhibit this isozyme (e.g., fluoxetine and paroxetine), and certain other drugs (e.g., fluvoxamine, propranolol, and pindolol) appear to appreciably inhibit the metabolism of thioridazine. The resulting elevated levels of thioridazine would be expected to augment the prolongation of the QTc interval associated with Mellaril and may increase the risk of serious, potentially fatal, cardiac arrhythmias, such as torsade de pointes-type arrhythmias. Such an increased risk may result also from the additive effect of co-administering Mellaril with other agents that prolong the QTc interval. Therefore, Mellaril is contraindicated with these drugs as well as in patients, comprising about 7% of the normal population, who are known to have a genetic defect leading to reduced levels of activity of P450 2D6.
Drugs that Inhibit Cytochrome P450 2D6
In a study of 19 healthy male subjects, which included 6 slow and 13 rapid hydroxylators of debrisoquin, a single 25 mg oral dose of thioridazine produced a 2.4-fold higher Cmax and a 4.5- fold higher AUC for thioridazine in the slow hydroxylators compared to rapid hydroxylators. The rate of debrisoquin hydroxylation is felt to depend on the level of cytochrome P450 2D6 isozyme activity. Thus, this study suggests that drugs that inhibit P450 2D6 or the presence of reduced activity levels of this isozyme will produce elevated plasma levels of thioridazine. Therefore, the co-administration of drugs that inhibit P450 2D6 with Mellaril and the use of Mellaril in patients known to have reduced activity of P450 2D6 are contraindicated.
Drugs that Reduce the Clearance of Mellaril® through Other Mechanisms
Fluvoxamine: The effect of fluvoxamine (25 mg b.i.d. for one week) on thioridazine steady state concentration was evaluated in 10 male in-patients with schizophrenia. Concentrations of thioridazine and its two active metabolites, mesoridazine and sulforidazine, increased three-fold following co-administration of fluvoxamine. Fluvoxamine and Mellaril should not be co-administered.
Propranolol: Concurrent administration of propranolol (100-800 mg daily) has been reported to produce increases in plasma levels of thioridazine (approximately 50%-400%) and its metabolites (approximately 80%-300%). Propranolol and Mellaril should not be co-administered.
Pindolol: Concurrent administration of pindolol and thioridazine have resulted in moderate, dose-related increases in the serum levels of thioridazine and two of its metabolites, as well as higher than expected serum pindolol levels. Pindolol and Mellaril should not be co-administered.
Drugs That Prolong the QTc Interval
There are no studies of the co-administration of Mellaril and other drugs that prolong the QTc interval. However, it is expected that such co-administration would produce additive prolongation of the QTc interval and, thus, such use is contraindicated.
Mellaril (thioridazine HCl) use should be avoided in combination with other drugs that are known to prolong the QTc interval and in patients with congenital long QT syndrome or a history of cardiac arrhythmias.
Reduced cytochrome P450 2D6 isozyme activity drugs that inhibit this isozyme (e.g., fluoxetine and paroxetine) and certain other drugs (e.g., fluvoxamine, propranolol, and pindolol) appear to appreciably inhibit the metabolism of thioridazine. The resulting elevated levels of thioridazine would be expected to augment the prolongation of the QTc interval associated with Mellaril and may increase the risk of serious, potentially fatal, cardiac arrhythmias, such as torsade de pointes-type arrhythmias. Such an increased risk may result also from the additive effect of co-administering Mellaril with other agents that prolong the QTc interval. Therefore, Mellaril is contraindicated with these drugs as well as in patients, comprising about 7% of the normal population, who are known to have a genetic defect leading to reduced levels of activity of P450 2D6.
In common with other phenothiazines, Mellaril is contraindicated in severe central nervous system depression or comatose states from any cause including drug induced central nervous system depression. It should also be noted that hypertensive or hypotensive heart disease of extreme degree is a contraindication of phenothiazine administration.
Additional information about Mellaril
Mellaril Indication: For the treatment of schizophrenia and generalized anxiety disorder.
Mechanism Of Action: Mellaril blocks postsynaptic mesolimbic dopaminergic D1 and D2 receptors in the brain; blocks alpha-adrenergic effect, depresses the release of hypothalamic and hypophyseal hormones and is believed to depress the reticular activating system thus affecting basal metabolism, body temperature, wakefulness, vasomotor tone, and emesis.
Drug Interactions: Not Available
Food Interactions: Not Available
Generic Name: Thioridazine
Synonyms: Thioridazine Hydrochloride; Thioridazine Chloride; Thioridazin; Thoridazine Hydrochloride; Thioridazine Hcl
Drug Category: Antipsychotics; Dopamine Antagonists; Phenothiazines
Drug Type: Small Molecule; Approved
Toxicity (Overdose): LD50=956-1034 mg/kg (Orally in rats); Agitation, blurred vision, coma, confusion, constipation, difficulty breathing, dilated or constricted pupils, diminished flow of urine, dry mouth, dry skin, excessively high or low body temperature, extremely low blood pressure, fluid in the lungs, heart abnormalities, inability to urinate, intestinal blockage, nasal congestion, restlessness, sedation, seizures, shock
Protein Binding: 95%
Half Life: 21-25 hours
Dosage Forms of Mellaril: Tablet Oral
Chemical IUPAC Name: 10-[2-(1-methylpiperidin-2-yl)ethyl]-2-methylsulfanylphenothiazine
Chemical Formula: C21H26N2S2
Thioridazine on Wikipedia: https://en.wikipedia.org/wiki/Thioridazine
Organisms Affected: Humans and other mammals