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Mechanism of Action
Guanfacine is a selective central alpha2A-adrenergic receptor agonist in that it has a 1520 times higher affinity for this receptor subtype than for the alpha2B or alpha2C subtypes.
Guanfacine is a known antihypertensive agent. By stimulating central alpha2A-adrenergic receptors, guanfacine reduces sympathetic nerve impulses from the vasomotor center to the heart and blood vessels. This results in a decrease in peripheral vascular resistance and a reduction in heart rate.
Effects on Height, Weight, and Body Mass Index (BMI)
Patients taking INTUNIV® demonstrated similar growth compared to normative data. Patients taking INTUNIV® had a mean increase in weight of 0.5 kg (1 lb) compared to those receiving placebo over a comparative treatment period. Patients receiving INTUNIV® for at least 12 months in open-label studies gained an average of 8 kg (17 lbs) in weight and 8 cm (3 in) in height. The height, weight, and BMI percentile remained stable in patients at 12 months in the long-term studies compared to when they began receiving INTUNIV®.
Effect on ECG
The effect of two dose levels of immediate-release guanfacine (4 mg and 8 mg) on the QT interval was evaluated in a double-blind, randomized, placebo- and active-controlled, cross-over study in healthy adults. A dose-dependent decrease in heart rate was observed during the first 12 hours, at time of maximal concentrations. The mean change in heart rate was -13 bpm at 4 mg and -22 bpm at 8 mg. An apparent increase in mean QTc was observed for both doses. However, guanfacine does not appear to interfere with cardiac repolarization of the form associated with pro-arrhythmic drugs. This finding has no known clinical relevance.
Absorption and Distribution
Guanfacine is readily absorbed and approximately 70% bound to plasma proteins independent of drug concentration. After oral administration of INTUNIV® the time to peak plasma concentration is approximately 5 hours in children and adolescents with ADHD.
Immediate-release guanfacine and INTUNIV® have different pharmacokinetic characteristics; dose substitution on a milligram for milligram basis will result in differences in exposure.
A comparison across studies suggests that the Cmax is 60% lower and AUC0-∞ 43% lower, respectively, for INTUNIV® compared to immediate-release guanfacine. Therefore, the relative bioavailability of INTUNIV® to immediate-release guanfacine is 58%. The mean pharmacokinetic parameters in adults following the administration of INTUNIV®1 mg once daily and immediate-release guanfacine 1mg once daily are summarized in Table 4.
Table 4: Pharmacokinetic Parameters in Adults
|Parameter||INTUNIV® 1 mg once daily
|Immediate-release guanfacine 1 mg once daily
|Cmax (ng/mL)||1.0 ± 0.3||2.5 ± 0.6|
|AUC0-∞ (ng.h/mL)||32 ± 9||56 ± 15|
|tmax (h)||6.0 (4.0 – 8.0)||3.0 (1.5-4.0)|
|t½ (h)||18 ± 4||16 ± 3|
|Note: Values are mean +/- SD, except for tmax which is median (range)|
Exposure to guanfacine was higher in children (ages 6-12) compared to adolescents (ages 13-17) and adults. After oral administration of multiple doses of INTUNIV® 4 mg, the Cmax was 10 ng/mL compared to 7 ng/mL and the AUC was 162 ng h/mL compared to 116 ng h/mL in children (ages 6-12) and adolescents (ages 13-17), respectively. These differences are probably attributable to the lower body weight of children compared to adolescents and adults.
The pharmacokinetics were affected by intake of food when a single dose of INTUNIV® 4 mg was administered with a high-fat breakfast. The mean exposure increased (Cmax ~75% and AUC ~40%) compared to dosing in a fasted state.
Following administration of INTUNIV® in single doses of 1 mg, 2 mg, 3 mg, and 4 mg to adults, Cmax and AUC0-∞ of guanfacine were proportional to dose.
Metabolism and Elimination
In vitro studies with human liver microsomes and recombinant CYP's demonstrated that guanfacine was primarily metabolized by CYP3A4. In pooled human hepatic microsomes, guanfacine did not inhibit the activities of the major cytochrome P450 isoenzymes (CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6 or CYP3A4/5). Guanfacine is a substrate of CYP3A4/5 and exposure is affected by CYP3A4/5 inducers/inhibitors.
Renal and Hepatic Impairment
The impact of renal impairment on PK of guanfacine in children was not assessed [see Use in Specific Populations].
Safety and Efficacy Studies
The efficacy of INTUNIV® in the treatment of ADHD was established in 3 placebo-controlled monotherapy trials (Studies 1, 2 and 4) and in 1 placebo-controlled adjunctive trial with psychostimulants (Study 3) in pediatric population. Studies 1, 2, and 3 were conducted in children and adolescents ages 6-17 and Study 4 was conducted in children ages 6-12 years.
Studies 1 and 2: Fixed-dose INTUNIV® Monotherapy
Study 1 was a double-blind, placebo-controlled, parallel-group, fixed dose study, in which efficacy of once daily dosing with INTUNIV® (2 mg, 3 mg and 4 mg) was evaluated for 5 weeks (n=345). Study 2 was a double-blind, placebo-controlled, parallel-group, fixed-dose study, in which efficacy of once daily dosing with INTUNIV® (1 mg, 2 mg, 3 mg and 4 mg) was evaluated for 6 weeks (n=324). In both studies, randomized subjects in 2 mg, 3 mg and 4 mg dose groups were titrated to their target fixed dose, and continued on the same dose until a dose tapering phase started The lowest dose of 1 mg used in Study 2 was assigned only to patients less than 50 kg (110 lbs). Patients who weighed less than 25 kg (55 lbs) were not included in either study.
Signs and symptoms of ADHD were evaluated on a once weekly basis using the clinician administered and scored ADHD Rating Scale (ADHD-RS-IV), which includes both hyperactive/impulsive and inattentive subscales. The primary efficacy outcome was the change from baseline to endpoint in ADHD-RS-IV total scores. Endpoint was defined as the last post-randomization treatment week for which a valid score was obtained prior to dose tapering (up to Week 5 in Study 1 and up to Week 6 in Study 2).
The mean reductions in ADHD-RS-IV total scores at endpoint were statistically significantly greater for INTUNIV® compared to placebo for Studies 1 and 2. Placebo-adjusted changes from baseline were statistically significant for each of the 2 mg, 3 mg, and 4 mg INTUNIV® randomized treatment groups in both studies, as well as the 1 mg INTUNIV® treatment group (for patients 55-110 lbs) that was included only in Study 2 (see Table 5).
Dose-responsive efficacy was evident, particularly when data were examined on a weight-adjusted (mg/kg) basis. When evaluated over the dose range of 0.01-0.17 mg/kg/day, clinically relevant improvements were observed beginning at doses in the range 0.05-0.08 mg/kg/day. Doses up to 0.12 mg/kg/day were shown to provide additional benefit.
Controlled, monotherapy long-term efficacy studies ( > 9 weeks) have not been conducted.
In the monotherapy trials (Studies 1 and 2), subgroup analyses were performed to identify any differences in response based on gender or age (6-12 vs. 13-17). Analyses of the primary outcome did not suggest any differential responsiveness on the basis of gender. Analyses by age revealed a statistically significant treatment effect only in the 6-12 age subgroup. Due to the relatively small proportion of adolescent patients (ages 13-17) enrolled into these studies (approximately 25%), these data may not be sufficient to demonstrate efficacy in the adolescent patients. In these studies, patients were randomized to a fixed dose of INTUNIV® rather than optimized by body weight. Therefore, some adolescent patients were randomized to a dose that might have resulted in relatively lower plasma guanfacine concentrations compared to the younger patients. Over half (55%) of the adolescent patients received doses of 0.01-0.04mg/kg. In studies in which systematic pharmacokinetic data were obtained, there was a strong inverse correlation between body weight and plasma guanfacine concentrations.
Table 5: Fixed dose Studies
|Study (Age Range)||Primary Efficacy Measure||Treatment Group|
|Placebo||Intuniv® 1mg||Intuniv® 2mg||Intuniv® 3mg||Intuniv® 4mg|
|1 (6 - 17 years)||Mean Baseline (SD)||38.1 (9.34)||--||36.1 (9.99)||36.8 (8.72)||38.4 (9.21)|
|LS Mean Change from Baseline (SE)||-8.5 (1.42)||--||-15.9 (1.37)||-16.0 (1.38)||-18.5 (1.39)|
|LS Mean Difference from Placebo (95% CI)||--||--||-7.4a (11.3,-3.5)||-7.5a (11.4, -3.6)||-10.0a (-13.9, -6.1)|
|2 (6 - 17 years)||Mean Baseline (SD)||39.3 (8.85)||41.7 (7.81)||39.9 (8.74)||39.1 (9.22)||40.6 (8.57)|
|LS Mean Change from Baseline (SE) LS Mean||-12.7 (1.60)||-19.4 (1.69)||-18.1 (1.60)||-20.0 (1.64)||-20.6 (1.60)|
|Difference from Placebo (95% CI)||--||-6.8a (11.3,-2.2)||-5.4a (9.9, -0.9)||-7.3a (11.8,-2.8)||-7.9a (12.3, -3.4)|
|LS Mean: least-square mean; SD:
standard deviation; SE: standard error; 95% CI (unadjusted)
aDoses were shown to be statistically significantly superior to placebo.
Study 3: Flexible-dose INTUNIV®as Adjunctive Therapy to Psychostimulants
Study 3 was a double-blind, randomized, placebo-controlled, dose-optimization study, in which efficacy of once daily optimized dosing (morning or evening) with INTUNIV® (1mg, 2mg, 3mg and 4mg), when co-administered with psychostimulants, was evaluated for 8 weeks, in children and adolescents aged 6-17 years with a diagnosis of ADHD, with a sub-optimal response to stimulants (n=455). Subjects were started at the 1 mg INTUNIV® dose level and were titrated weekly over a 5-week dose-optimization period to an optimal INTUNIV® dose not to exceed 4 mg/day based on tolerability and clinical response. The dose was then maintained for a 3-week dose maintenance period before entry to 1 week of dose tapering. Subjects took INTUNIV® either in the morning or the evening while maintaining their current dose of psychostimulant treatment given each morning. Allowable psychostimulants in the study were ADDERALL XR®, VYVANSE®, CONCERTA®, FOCALIN XR®, RITALIN LA®, METADATE CD® or FDA-approved generic equivalents.
Symptoms of ADHD were evaluated on a weekly basis by clinicians using the ADHD Rating Scale (ADHD-RS-IV), which includes both hyperactive/impulsive and inattentive subscales. The primary efficacy outcome was the change from baseline to endpoint in ADHD-RS-IV total scores. Endpoint was defined as the last post-randomization treatment week prior to dose tapering for which a valid score was obtained (up to Week 8).
Mean reductions in ADHD-RS-IV total scores at endpoint were statistically significantly greater for INTUNIV®given in combination with a psychostimulant compared to placebo given with a psychostimulant for Study 3, for both morning and evening INTUNIV® dosing (see Table 6). Nearly two-thirds (64.2%) of subjects reached optimal doses in the 0.05-0.12 mg/kg/day range.
Study 4: Flexible-dose INTUNIV® Monotherapy
Study 4 was a double-blind, randomized, placebo-controlled, dose-optimization study, in which efficacy of once daily dosing (morning or evening) with INTUNIV® (1mg, 2mg, 3mg, and 4mg) was evaluated for 8 weeks in children aged 6-12 years (n=340).
Signs and symptoms of ADHD were evaluated on a once weekly basis using the clinician administered and scored ADHD Rating Scale (ADHD-RS-IV), which includes both hyperactive/impulsive and inattentive subscales. The primary efficacy outcome was the change from baseline score at endpoint on the ADHD-RS-IV total scores. Endpoint was defined as the last post-randomization treatment week for which a valid score was obtained prior to dose tapering (up to Week 8).
Mean reductions in ADHD-RS-IV total scores at endpoint were statistically significantly greater for INTUNIV® compared to placebo in both AM and PM dosing groups of INTUNIV® (see Table 6).
Table 6: Flexible-Dose studies
|Study (Age Range)||Treatment Group|
|Placebo||Intuniv® 1mg - 4mg|
|3a (6 - 17 years)||Mean Baseline (SD)||37.7 (7.75)||37.6 (8.13)||37.0 (7.65)|
|LS Mean Change from Baseline (SE)||-15.9 (0.96)||-20.3 (0.97)||-21.2 (0.97) -5.3b|
|LS Mean Difference from Placebo (95% CI)||-||-4.5b (-7.5, -1.4)||(-8.3, -2.3)|
|4 (6 - 12 years)||Mean Baseline (SD)||42.9 (6.21)||41.7 (6.39)||41.6 (6.66)|
|LS Mean Change from Baseline (SE)||-10.6 (1.20)||-20.0 (1.23)||-20.4 (1.19)|
|LS Mean Difference from Placebo||--||-9.4b (-12.8, -6.0)||-9.8b (-13.1, -6.4)|
|LS Mean: least-square mean; SD:
standard deviation; SE: standard error; 95% CI (unadjusted)
aTreatment was given in combination with a psychostimulant.
bDoses were shown to be statistically significantly superior to placebo.
Last reviewed on RxList: 9/9/2013
This monograph has been modified to include the generic and brand name in many instances.
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