Evidence-Based Reviews

Antipsychotics, dopamine, and pain

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Risperidone was the second FDA-approved SGA. Pain practitioners primarily prescribe it for treatmeant-resistant headaches, but patients with fibromyalgia and those with phantom and thalamic pain also may respond. Because risperidone’s properties are similar to that of many FGAs, it may potently cause EPS, TD, and prolactinemia. Neuroleptic malignant syndrome also has been reported.23

Ziprasidone is frequently overlooked by clinicians who treat pain. Although ziprasidone may be sedating, it is powerful as both a preventive and abortive (in an IM formulation) agent for treatment-resistant headaches. This might be attributed to its effects on the 5HT9 receptor. It is approved for treating bipolar depression and has been prescribed to effectively treat anxiety. For patients receiving ziprasidone, QT prolongation needs to be monitored closely.24

Olanzapine was modeled after clozapine and is effective as a mood stabilizer and an antianxiety, antipsychotic, and sleep-promoting medication. It has a useful “mellowing” effect and helps with central pain syndrome management. Patients with fibromyalgia respond well; in some cases, patients with phantom and thalamic pain also respond. Among SGAs prescribed to treat chronic pain, olanzapine has the most published studies. However, the downside is the risk of severe weight gain and diabetes. Usually, if a patient is already overweight, they gain less, but these patients typically are concerned about any additional weight gain.25

Aripiprazole is a partial dopamine agonist. It increases dopamine function in the prefrontal cortex, and by doing so it possibly improves cognition, mental acuity, goal-oriented activity, and attention. At the same time, it decreases dopamine activity in the basal ganglia and limbic system, improving catastrophizing, paranoia, abnormal pain perception, and multiple homeostasis functions. This combination of effects can be invaluable for some patients, but depending on individual susceptibility, aripiprazole might be too activating (causing agitation and akathisia) or too sedating.26

Brexpiprazole is a relative of aripiprazole, but for some patients it is better tolerated, and compliance with this medication usually is good. It partially antagonizes the D2 and 5HT1A receptors while antagonizing the 5HT2A receptors (which decreases the dopamine release in the striatum) and mimics the mechanism of action of an antidepressant. Through alpha-1-adrenergic receptor antagonism, it reduces EPS. All these effects are also part of the mechanisms of action of quetiapine, clozapine, and iloperidone, but brexpiprazole is considered to be the most alpha-1 antagonistic, which is a mechanism of action of other potential pain-controlling medications such as clonidine and tizanidine. In patients with pain who have an overactive noradrenergic system, this property may be beneficial. Its major problem stems from cytochrome P450 2D6 (CYP2D6) enzyme-dependent metabolism, which causes an approximately 5-fold increase in brexpiprazole blood level in poor CYP2D6 metabolizers. Therefore, combining brexpiprazole with CYP2D6 inhibitors such as fluoxetine, paroxetine, and duloxetine would be unwise. Aripiprazole and brexpiprazole are less associated with diabetes and sexual adverse effects than many other SGAs.27

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