Thiothixene

Indications

• FDA-approved for the management of schizophrenia in adults, which is the only indication carried on the current US label.
• A high-potency thioxanthene whose efficacy on positive symptoms is broadly comparable to haloperidol and other first-generation agents.
• Used off-label for acute agitation in psychotic illness and occasionally for delusional disorder when second-generation agents have failed.
• Not approved for dementia-related psychosis, where the boxed mortality warning and a poor risk-benefit balance apply.
• Not approved for bipolar mania, although first-generation antipsychotics are still used off-label in that setting.
• Rarely first-line today because second-generation agents carry less extrapyramidal and tardive risk at similar efficacy.

Mechanism of action

• Potent postsynaptic dopamine D2 receptor antagonist, with the thioxanthene nucleus closely related structurally to the phenothiazines.
• Mesolimbic D2 blockade reduces hallucinations and delusions, while nigrostriatal blockade produces the parkinsonism, dystonia and akathisia.
• Tuberoinfundibular D2 blockade removes dopaminergic inhibition of prolactin release, causing sustained hyperprolactinemia.
• Efficacy generally requires 60-80% striatal D2 occupancy, and extrapyramidal effects rise steeply once occupancy exceeds 80%.
• Moderate alpha-1 adrenergic blockade causes orthostasis, while antihistaminic and anticholinergic activity is lower than in low-potency agents.

Pharmacokinetics

• Well absorbed after oral administration as capsules; there is no long-acting injectable or oral concentrate on the US market.
• Highly lipophilic and extensively protein bound with a large volume of distribution, which prolongs effect after discontinuation.
• Elimination half-life exceeds 34 h with chronic dosing, which allows consolidation to a single daily dose once the patient is stable.
• Cleared by hepatic oxidation with biliary and renal elimination of metabolites; the pathways are less well characterized than for newer agents.
• No therapeutic plasma concentration range is established, so dosing is guided entirely by clinical response and extrapyramidal signs.

Dosing

• Milder presentations: start 2 mg three times daily and increase to 15 mg/day as needed based on response and tolerability.
• More severe presentations: start 5 mg twice daily, with the usual optimal dose falling between 20 and 30 mg daily.
• The maximum is 60 mg/day; doses above that rarely add benefit and reliably add extrapyramidal symptoms.
• Consolidate to a single bedtime dose once the patient is stable, taking advantage of the long elimination half-life.
• Start elderly patients at 1-2 mg daily and titrate slowly, given greater sensitivity to parkinsonism, falls and orthostasis.
• Taper over weeks when discontinuing to avoid cholinergic rebound, insomnia and emergent withdrawal dyskinesias.

Adverse effects

• Extrapyramidal symptoms dominate: akathisia, drug-induced parkinsonism and acute dystonia are frequent and often dose-limiting.
• Tardive dyskinesia accumulates at roughly 5% per year of exposure in adults and considerably faster in older patients.
• Hyperprolactinemia causes galactorrhea, amenorrhea, gynecomastia, sexual dysfunction and, over years, reduced bone density.
• Neuroleptic malignant syndrome is rare but potentially fatal, presenting with hyperthermia, rigidity, autonomic instability and raised creatine kinase.
• Sedation, orthostatic hypotension, dry mouth, constipation and blurred vision occur, though less than with low-potency phenothiazines.
• QT prolongation, lowered seizure threshold and lenticular or corneal deposits after prolonged high-dose therapy are also described.

Monitoring

• Abnormal Involuntary Movement Scale at baseline and every 6 months, or every 3 months in older or high-risk patients.
• Weight, body mass index, fasting glucose and lipids at baseline and periodically, since metabolic risk is lower but not absent.
• Prolactin when galactorrhea, amenorrhea, gynecomastia or sexual dysfunction appears, and bone density after prolonged elevation.
• ECG at baseline in patients with cardiac disease, electrolyte disturbance or other QT-prolonging medications.
• Temperature, rigidity and creatine kinase whenever neuroleptic malignant syndrome is suspected, with immediate discontinuation.

Interactions

• Additive sedation and respiratory depression with alcohol, benzodiazepines, opioids and other central nervous system depressants.
• Additive QT prolongation with methadone, citalopram, ondansetron, macrolides, fluoroquinolones and class III antiarrhythmics.
• Anticholinergic agents add to constipation, urinary retention and delirium risk while masking early extrapyramidal signs.
• Antagonizes levodopa and dopamine agonists, so avoid in Parkinson disease and dementia with Lewy bodies.
• Antihypertensives compound orthostatic hypotension, and smoking induces hepatic enzymes that can lower antipsychotic levels.

Special populations

• Pregnancy: first-generation agents have comparatively more reproductive data; third-trimester exposure can cause neonatal extrapyramidal signs.
• Lactation: thiothixene passes into human milk, so watch the infant for sedation, poor feeding and abnormal movements.
• Pediatric use is not recommended below 12 years, and adolescent use should generally favor second-generation agents.
• Geriatric: the boxed mortality warning applies in dementia, and tardive dyskinesia risk rises several-fold with age.
• Hepatic or renal impairment warrants slower titration and lower doses, though no formal dose adjustments are specified in the label.

Clinical pearls

• High potency means extrapyramidal effects, not weight gain; plan for akathisia and dystonia.
• Never use it for dementia-related psychosis; the boxed mortality warning is the reason.
• A half-life over 34 h lets stable patients take the whole dose at bedtime.