Thioridazine

Thioridazine (sold under the brand name Mellaril) is a first-generation, or typical, antipsychotic that was used to treat schizophrenia. It works mainly by blocking dopamine D2 receptors in the brain.

Its use has declined sharply because it can prolong the QT interval on an electrocardiogram in a dose-related way, which raises the risk of dangerous heart-rhythm changes. Because of this, thioridazine carries a boxed warning and is generally reserved for patients who have not responded to other antipsychotics.

The key gene affecting thioridazine metabolism is CYP2D6. This gene encodes a liver enzyme that breaks thioridazine down and clears it from the body.

When CYP2D6 activity is reduced, thioridazine is cleared more slowly and builds up to higher blood levels. Higher levels are directly tied to a greater risk of QT prolongation, so CYP2D6 status is an important safety consideration for this drug.

Patients are classified by CYP2D6 metabolizer status: ultrarapid, normal, intermediate, or poor. Poor metabolizers clear thioridazine much more slowly, so the drug reaches higher concentrations and the risk of serious heart-rhythm problems rises. For this reason, the FDA labeling contraindicates thioridazine in people who are known CYP2D6 poor metabolizers or who take strong CYP2D6 inhibitors.

Ultrarapid Metabolizer

• Effect on drug levels: Faster clearance leading to lower plasma concentrations.
• Clinical consequence: No specific FDA guidance; the standard dose is typically used.
• Side effects: Lower exposure, though efficacy should still be monitored clinically.

Normal Metabolizer

• Effect on drug levels: Standard plasma concentrations.
• Clinical consequence: Typical response at standard doses.
• Side effects: Expected, dose-related risk of QT prolongation and other adverse effects.

Intermediate Metabolizer

• Effect on drug levels: Modestly reduced clearance with somewhat higher exposure.
• Clinical consequence: No specific FDA guidance; the standard dose is typically used, with monitoring.
• Side effects: Possible mild increase in exposure-related effects.

Poor Metabolizer

• Effect on drug levels: Markedly reduced clearance resulting in much higher plasma concentrations.
• Clinical consequence: Significantly increased risk of QT prolongation and serious arrhythmia. Thioridazine is contraindicated in poor metabolizers, and an alternative medication is recommended.
• Side effects: High risk of severe, potentially life-threatening heart-rhythm events.

Indeterminate / Not Available

• Effect: Unknown impact on drug metabolism and levels.
• Clinical consequence: No specific guidance; follow standard dosing with clinical monitoring.

CYP2D6 Dosing Guideline

Examples of other antipsychotic medications include haloperidol, risperidone, aripiprazole, and quetiapine. These are provided for illustrative purposes only and are not direct recommendations. Some antipsychotics also have their own pharmacogenetic considerations, so choice of therapy should always be made with a clinician.

• FDA – Table of Pharmacogenetic Associations