Pimavanserin

Indications

• Hallucinations and delusions associated with Parkinson's disease psychosis in adults, which is its only approved indication.
• Chosen precisely because it does not block dopamine receptors and therefore does not worsen motor function in Parkinson disease.
• Guidelines position it alongside low-dose quetiapine and clozapine after reducing dopaminergic medication and treating reversible causes.
• Off-label use in dementia-related psychosis without Parkinson disease is not supported and runs directly into the boxed warning.
• Off-label interest in Lewy body dementia psychosis exists, but trial results have been mixed and the drug is not approved for it.

Mechanism of action

• Selective inverse agonist and antagonist at serotonin 5-HT2A receptors, with lesser activity at 5-HT2C and none at 5-HT2B.
• It has no measurable affinity for dopamine D2 receptors, which is why motor symptoms of Parkinson disease do not worsen.
• No meaningful muscarinic, histaminergic, or adrenergic binding, so there is little sedation, orthostasis, or anticholinergic effect.
• The 5-HT2A hypothesis of Parkinson disease psychosis holds that visual hallucinations arise from cortical serotonergic dysregulation.
• Because the mechanism is purely serotonergic, it does nothing for the positive symptoms of primary schizophrenia.

Pharmacokinetics

• Bioavailability is about 80 percent and absorption is not clinically affected by food, so it can be taken at any consistent time.
• Metabolized principally by CYP3A4 and CYP3A5 to the active metabolite AC-279, with CYP2J2 and CYP2D6 playing minor roles.
• Half-life is about 57 hours for parent drug and roughly 200 hours for the active metabolite, so steady state takes about 12 days.
• The long half-life explains both the delayed onset of benefit and the persistence of adverse effects after stopping.
• Not recommended in severe renal impairment with creatinine clearance under 30 mL/min or in hepatic impairment.

Dosing

• The recommended dose is 34 mg once daily as a single capsule or as two 17 mg tablets, with no titration required.
• Reduce to 10 mg once daily when a strong CYP3A4 inhibitor such as ketoconazole or itraconazole is used concomitantly.
• Avoid strong CYP3A4 inducers including rifampin, carbamazepine, phenytoin, and St. John's wort, since exposure falls substantially.
• Before starting, reduce or simplify dopaminergic therapy where possible and treat delirium, infection, and metabolic causes of psychosis.
• Allow at least 4-6 weeks at full dose before judging response, since the long half-life delays maximal effect.
• No adjustment is needed for mild to moderate renal impairment, and no formal taper schedule is specified.

Adverse effects

• Peripheral edema occurs in about 7 percent and confusion or a confusional state in about 6 percent, both more than with placebo.
• Nausea, constipation, gait disturbance, and hallucination are the other commonly reported treatment-emergent effects.
• QTc prolongation averages 5-8 ms, which matters mainly when other QT-prolonging drugs are already in use.
• Older patients with dementia show increased mortality and serious adverse events, which is the substance of the boxed warning.
• Because there is no dopamine blockade, extrapyramidal worsening, prolactin elevation, and metabolic changes are not expected.

Monitoring

• Baseline ECG in patients with cardiac disease, bradycardia, electrolyte disturbance, or concurrent QT-prolonging medication.
• Assess cognition and confusion at follow-up, since worsening confusion is a common reason to stop the drug in this population.
• Check for new or worsening peripheral edema, particularly in patients with heart failure or venous insufficiency.
• Reassess benefit at 6 weeks and periodically thereafter, and discontinue if hallucinations and delusions have not improved.
• Review renal and hepatic function before starting, since both severe renal and any hepatic impairment make the drug unsuitable.

Interactions

• Strong CYP3A4 inhibitors such as ketoconazole, itraconazole, clarithromycin, and ritonavir require reducing the dose to 10 mg daily.
• Strong CYP3A4 inducers markedly reduce exposure and concomitant use should be avoided rather than compensated with a higher dose.
• Avoid combining with Class IA and III antiarrhythmics, methadone, moxifloxacin, and other drugs that prolong the QT interval.
• Additive confusion and anticholinergic burden when combined with amantadine, trihexyphenidyl, or other anticholinergic Parkinson agents.
• There is no contraindication other than hypersensitivity, and pimavanserin does not antagonize levodopa or dopamine agonists.

Special populations

• Pregnancy and lactation data are essentially absent, which is of limited practical relevance in the typical Parkinson disease population.
• Safety and effectiveness have not been established in pediatric patients and there is no plausible pediatric indication.
• The trial population was elderly with Parkinson disease, so geriatric use is the norm, but dementia-related psychosis alone is excluded.
• Not recommended when creatinine clearance falls below 30 mL/min, since exposure rises without a defined reduced dose.
• Hepatic impairment has not been studied and use is not recommended in patients with significant liver disease.

Clinical pearls

• No dopamine blockade, so it is the one antipsychotic that will not worsen parkinsonism.
• Allow 4-6 weeks before judging response; the metabolite half-life is roughly 200 hours.
• It has no role in schizophrenia or in dementia psychosis outside Parkinson disease.