Pimavanserin is a potent 5-HT2A inverse agonist and 5-HT2C inverse agonist, with 5-fold greater affinity for the 5-HT2A receptor.1 Although antagonists block agonist actions at the receptor site, inverse agonists reduce the level of baseline constitutive activity seen in many G protein-coupled receptors. This medication is FDA approved for treating hallucinations and delusions associated with Parkinson’s disease (PD) psychosis (Table 1).1
In the pivotal 6-week clinical trial, pimavanserin significantly reduced positive symptoms seen in PD patients with psychosis (effect size = 0.50), with no evident impairment of motor function.2 Only 2 adverse effects occurred in ≥5% of pimavanserin-treated patients and at ≥2 times the rate of placebo: peripheral edema (7% vs 3% for placebo) and confusion (6% vs 3% for placebo). There was a mean increase in the QTc of 7.3 milliseconds compared with placebo in the pivotal phase III study.
Despite numerous developments in the pharmacotherapeutics of psychotic disorders, patients with psychosis related to PD previously responded in a robust manner to only 1 antipsychotic, low-dosage clozapine (mean effect size, 0.80),2 with numerous failed trials for other atypical antipsychotics, including quetiapine.3,4 The pathophysiology of psychosis in PD patients is not related to dopamine agonist treatment, but is caused by the accumulation of cortical Lewy body burden, which results in loss of serotonergic signaling from dorsal raphe neurons. The net effect is up-regulation of postsynaptic 5-HT2A receptors.5 Psychosis is the most common cause of nursing home placement among PD patients without dementia.6
Receptor blocking. Based on the finding that clozapine in low dosages acts at 5-HT2A receptors,7 pimavanserin was designed to be a potent 5-HT2A inverse agonist, with more than 5-fold higher selectivity over 5-HT2C receptors, and no appreciable affinity for other serotonergic, adrenergic, dopaminergic, muscarinic, or histaminergic receptors8 (Table 2). The concept that 5-HT2A receptor stimulation can cause psychosis with prominent visual hallucinations is known from studies of LSD and other hallucinogenic compounds whose activity is blocked by 5-HT2A antagonists.
As an agent devoid of dopamine D2 antagonism, pimavanserin carries no risk of exacerbating motor symptoms, which was commonly seen with most atypical antipsychotics studied for psychosis in PD patients, except for clozapine and quetiapine.3 Although quetiapine did not cause motor effects, it proved ineffective in multiple studies (n = 153), likely because of the near absence of potent 5-HT2A binding.4
Pimavanserin also lacks:
- the hematologic monitoring requirement of clozapine
- clozapine’s risks of sedation, orthostasis, and anticholinergic and metabolic adverse effects.
Pimavanserin is significantly more potent than other non-antipsychotic psychotropics at the 5-HT2Areceptor, including doxepin (26 nM), trazodone (36 nM), and mirtazapine (60 nM).
Use in psychosis associated with PD. Recommended dosage is 34 mg once daily without titration (with or without food), based on results from a phase III clinical trial2 (because of the FDA breakthrough therapy designation for this compound, only 1 phase III trial was required). Pimavanserin produced significant improvement on the PD-adapted Scale for the Assessment of Positive Symptoms (SAPS-PD), a 9-item instrument extracted from the larger SAPS used in schizophrenia research. Specifically, pimavanserin was effective for both the hallucinations and delusions components of the SAPS-PD.
Pharmacologic profile, adverse effects. Pimavanserin lacks affinity for receptors other than 5-HT2A and 5-HT2C, leading to an absence of significant anticholinergic effects, orthostasis, or sedation in clinical trials.2 In all short-term clinical trials, the only common adverse reactions (incidence ≥5% and at least twice the rate of placebo) were peripheral edema (7% vs 2% placebo) and confusional state (6% vs 3% placebo).2 More than 300 patients have been treated for >6 months, >270 have been treated for at least 12 months, and >150 have been treated for at least 24 months with no adverse effects other than those seen in the short-term trials.1
There is a measurable impact on cardiac conduction seen in phase III data and in the thorough QT study. In the thorough QT study, 252 healthy participants received multiple dosages in a randomized, double-blind manner with positive controls.1 The maximum mean change from baseline was 13.5 milliseconds at dosages twice the recommended dosage, and the upper limit of the 90% CI was only slightly greater at 16.6 milliseconds. Subsequent kinetic analyses suggested concentration-dependent QTc interval prolongation in the therapeutic range, with a recommendation to halve the daily dosage in patients taking potent cytochrome P450 (CYP) 3A4 inhibitors.
In the 6-week, placebo-controlled effectiveness studies, mean increases in QTc interval were in the range of 5 to 8 milliseconds. There were sporadic reports of QTcF values ≥500 milliseconds, or changes from baseline QTc values ≥60 milliseconds in pimavanserin-treated participants, although the incidence generally was the same for pimavanserin and placebo groups. There were no reports of torsades de pointes or any differences from placebo in the incidence of adverse reactions associated with delayed ventricular repolarization.