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Aripiprazole’s Receptor Pharmacology and Extrapyramidal Side Effects

To The Editor: In the September 2007 issue of the Journal, David Mamo, M.D., M.Sc., et al. (1) reported that at clinically effective doses (10–30 mg/day) aripiprazole exerts more than 80% striatal dopamine D₂ receptor occupancy in patients with schizophrenia. Correspondingly, higher striatal D₂ occupancy (>90%) was associated with the development of extrapyramidal side effects (dystonia, parkinsonism). Aripiprazole was distinguished by a low serotonin 5-HT₂:D₂ affinity ratio (52% [SD=18%] versus 87% [SD=4%], respectively) and a low 5-HT_1A receptor occupancy (mean=16%) (1). The authors implied that aripiprazole’s partial D₂ agonist properties accounted for a low propensity for extrapyramidal side effects.

In several controlled trials, the rate of aripiprazole-induced extrapyramidal side effects was reported similar to placebo; however, it was the rate of aripiprazole-induced akathisia reported as being significantly higher (approximately 20%) in patients with schizophrenia and bipolar mania (2, 3). The estimated rate of aripiprazole-induced akathisia in naturalistic settings has been shown to be even higher (4). The pathophysiological mechanisms and treatment of akathisia differ from that of other extrapyramidal side effects. Anticholinergic agents that are efficacious in treating dystonia and parkinsonism are apparently ineffective in treating akathisia. In contrast, a robust anti-akathisia effect of mianserin and mirtazapine (both with marked 5-HT_2A antagonism), comparable with propranolol (the drug of choice for treating akathisia), has consistently been shown in patients treated with first-generation antipsychotic agents, underscoring the role of 5-HT_2A receptor antagonism in the pathophysiology and treatment of akathisia (5). Preliminary evidence indicating an anti-akathisia effect of the selective 5-HT_2A inverse agonist ACP-3 (6) supports this assumption. Notably, the 5-HT_1A agonist buspirone was ineffective as an anti-akathisia agent (7). We suggest that aripiprazole’s partial agonism at the D₂ and 5-HT_1A receptors accounts for the low incidence of extrapyramidal side effects but does not "protect" against akathisia. Noteworthy, five (42%) of 12 participants in the study developed restlessness (1), which may well have been an expression of akathisia. A sufficient degree of the 5-HT_2A antagonism and a high 5-HT_2A:D₂ occupancy ratio are essential to prevent or ameliorate drug-induced akathisia. Elucidation of pathophysiological mechanisms and efficacious treatment of akathisia are imperative because of akathisia’s association with non-compliance and suicidal and violent behavior.

Footnotes

The authors report no competing interests.

This letter (doi: 10.1176/appi.ajp.2007.07091513) was accepted for publication in November 2007.

References

1. Mamo D, Graff A, Mizrahi R, Shammi CM, Romeyer F, Kapur S: Differential effects of aripiprazole on D₂, 5-HT₂, and 5-HT_1A receptor occupancy in patients with schizophrenia: a triple tracer PET study. Am J Psychiatry 2007; 164:1411–1417[Abstract/Free Full Text]
2. Fleischhacker W: Aripiprazole. Expert Opin Pharmacother 2005; 6:2091–2101[CrossRef][Medline]
3. Kinghorn WA, McEvoy JP: Aripiprazole: pharmacology, efficacy, safety and tolerability. Expert Rev Neurother 2005; 5:297–307[CrossRef][Medline]
4. Ghaemi SN, Hsu DJ, Rosenquist KJ, Pardo TB, Goodwin FK: Extrapyramidal side effects with atypical neuroleptics in bipolar disorder. Prog Neuropsychopharmacol Biol Psychiatry 2006; 30:209–213[CrossRef][Medline]
5. Poyurovsky M, Pashinian A, Weizman R, Fuchs C, Weizman A: Low-dose mirtazapine: a new option in the treatment of antipsychotic-induced akathisia: a randomized, double-blind, placebo- and propranolol-controlled trial. Biol Psychiatry 2006; 59:1071-1077[CrossRef][Medline]
6. Grahnen AE, Vanover KE, Weiner M, Nilsson L, Tolf B, Hacksel U, Davis MR: Reduction of haloperidol-induced side effects by ACP-103 in healthy volunteers. Clin Pharmacol Ther 2005; 77:P98–P99
7. Poyurovsky M, Weizman A: Serotonergic agents in the treatment of acute neuroleptic-induced akathisia: open-label study of buspirone and mianserin. Int Clin Psychopharmacol 1997; 12:263–268[Medline]

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