Guanfacine, sold under the brand name Tenex (immediate-release) and Intuniv (extended-release) among others, is an oral alpha-2a agonist medication used to treat attention deficit hyperactivity disorder (ADHD) and high blood pressure. Guanfacine is FDA-approved for monotherapy treatment of ADHD, as well as being used for augmentation of other treatments, such as stimulants. Guanfacine is also used off-label to treat tic disorders, anxiety disorders and PTSD.
Common side effects include sleepiness, constipation, and dry mouth. Other side effects may include low blood pressure and urinary problems. The FDA has categorized Guanfacine as "Category B" in pregnancy, which means animal-reproduction studies have not demonstrated a fetal risk or an adverse effect during pregnancy or breastfeeding. It appears to work by activating the α2A receptors in the brain, thereby decreasing sympathetic nervous system activity.
Guanfacine was approved for medical use in the United States in 1986. It is available as a generic medication. In 2020, it was the 300th most commonly prescribed medication in the United States, with more than 1million prescriptions.
Medical uses
Guanfacine is FDA-approved as monotherapy or augmentation with stimulants to treat attention deficit hyperactivity disorder. It is also FDA approved to treat high blood pressure. Guanfacine can offer a synergistic enhancement of stimulants such as amphetamines and methylphenidate for treating ADHD, and in many cases can also help control the side effect profile of stimulant medications. Guanfacine is also used off-label to treat tic disorders, anxiety disorders, and PTSD.
An off-label use of guanfacine is for the treatment of anxiety, such as generalized anxiety disorder and post-traumatic stress disorder symptoms. Guanfacine and other α2A agonists have anxiolytic-like action, thereby reducing the emotional responses of the amygdala, and strengthening prefrontal cortical regulation of emotion, action and thought. These actions arise from both inhibition of stress-induced catecholamine release, and from prominent, post-synaptic actions in the prefrontal cortex. Due to its prolonged half-life, it also has been seen to improve sleep interrupted by nightmares in PTSD patients. All of these actions likely contribute to the relief of the hyperarousal, re-experiencing of memory, and impulsivity associated with PTSD. Guanfacine appears to be especially helpful in treating children who have been traumatized or abused.
Adverse effects
Side effects of guanfacine are dose-dependent.
Very common (>10% incidence) adverse effects include sleepiness, tiredness, headache, and stomach ache.
Common (1–10% incidence) adverse effects include decreased appetite, nausea, dry mouth, urinary incontinence, and rashes.
Interactions
Guanfacine availability is significantly affected by the CYP3A4 and CYP3A5 enzymes. Medications that inhibit or induce those enzymes change the amount of guanfacine in circulation and thus its efficacy and rate of adverse effects. Because of its impact on the heart, it should be used with caution with other cardioactive drugs. A similar concern is appropriate when it is used with sedating medications.
Pharmacology
Guanfacine is a highly selective agonist of the α2A adrenergic receptor, with low affinity for other receptors. However it may also be a potent 5-HT2B receptor agonist, which can be associated with valvulopathy, although not all 5-HT2B agonists have this effect.
Mechanism of action
Guanfacine works by activating α2A adrenoceptors within the central nervous system. This leads to reduced peripheral sympathetic outflow and thus a reduction in peripheral sympathetic tone, which lowers both systolic and diastolic blood pressure.
In ADHD, guanfacine works by strengthening the regulation of attention and behavior by the prefrontal cortex. These enhancing effects on prefrontal cortical functions are believed to be due to drug stimulation of post-synaptic α2A adrenoceptors on dendritic spines. cAMP-mediated opening of HCN and KCNQ channels is inhibited, which enhances prefrontal cortical synaptic connectivity and neuronal firing. The use of guanfacine for treating prefrontal disorders was developed by the Arnsten Lab at Yale University.