Guanfacine Hydrochloride

INDICATIONS AND USAGE Guanfacine tablets are indicated in the management of hypertension. Guanfacine tablets may be given alone or in combination with other antihypertensive agents, especially thiazide-type diuretics.

DOSAGE AND ADMINISTRATION The recommended initial dose of guanfacine tablets when given alone or in combination with another antihypertensive drug is 1 mg daily given at bedtime to minimize somnolence. If after 3 to 4 weeks of therapy 1 mg does not give a satisfactory result, a dose of 2 mg may be given, although most of the effect of guanfacine tablets are seen at 1 mg (see CLINICAL PHARMACOLOGY ). Higher daily doses have been used, but adverse reactions increase significantly with doses above 3 mg/day. The frequency of rebound hypertension is low, but it can occur. When rebound occurs, it does so after 2 to 4 days, which is delayed compared with clonidine hydrochloride. This is consistent with the longer half-life of guanfacine. In most cases, after abrupt withdrawal of guanfacine, blood pressure returns to pretreatment levels slowly (within 2 to 4 days) without ill effects.

ADVERSE REACTIONS Adverse reactions noted with guanfacine hydrochloride are similar to those of other drugs of the central α 2 -adrenoreceptor agonist class: dry mouth, sedation (somnolence), weakness (asthenia), dizziness, constipation, and impotence. While the reactions are common, most are mild and tend to disappear on continued dosing. Skin rash with exfoliation has been reported in a few cases; although clear cause and effect relationships to guanfacine hydrochloride could not be established, should a rash occur, guanfacine hydrochloride should be discontinued and the patient monitored appropriately. In the dose-response monotherapy study described under CLINICAL PHARMACOLOGY , the frequency of the most commonly observed adverse reactions showed a dose relationship from 0.5 to 3 mg as follows: Adverse Reaction Placebo n=59 0.5 mg n=60 1 mg n=61 2 mg n=60 3 mg n=59 Dry Mouth 0% 10% 10% 42% 54% Somnolence 8% 5% 10% 13% 39% Asthenia 0% 2% 3% 7% 3% Dizziness 8% 12% 2% 8% 15% Headache 8% 13% 7% 5% 3% Impotence 0% 0% 0% 7% 3% Constipation 0% 2% 0% 5% 15% Fatigue 2% 2% 5% 8% 10% The percent of patients who dropped out because of adverse reactions are shown below for each dosage group. Placebo 0.5 mg 1 mg 2 mg 3 mg Percent dropouts 0% 2.0% 5.0% 13% 32% The most common reasons for dropouts among patients who received guanfacine were dry mouth, somnolence, dizziness, fatigue, weakness, and constipation. In the 12-week, placebo-controlled, dose-response study of guanfacine administered with 25 mg chlorthalidone at bedtime, the frequency of the most commonly observed adverse reactions showed a clear dose relationship from 0.5 to 3 mg as follows: Adverse Reaction Placebo n=73 0.5 mg n=72 1 mg n=72 2 mg n=72 3 mg n=72 Dry Mouth 5 (7%) 4 (5%) 6 (8%) 8 (11%) 20 (28%) Somnolence 1 (1%) 3 (4%) 0 (0%) 1 (1%) 10 (14%) Asthenia 0 (0%) 2 (3%) 0 (0%) 2 (2%) 7 (10%) Dizziness 2 (2%) 1 (1%) 3 (4%) 6 (8%) 3 (4%) Headache 3 (4%) 4 (3%) 3 (4%) 1 (1%) 2 (2%) Impotence 1 (1%) 1 (0%) 0 (0%) 1 (1%) 3 (4%) Constipation 0 (0%) 0 (0%) 0 (0%) 1 (1%) 1 (1%) Fatigue 3 (3%) 2 (3%) 2 (3%) 5 (6%) 3 (4%) There were 41 premature terminations because of adverse reactions in this study. The percent of patients who dropped out and the dose at which the dropout occurred were as follows: Dose Placebo 0.5 mg 1 mg 2 mg 3 mg Percent dropouts 6.9% 4.2% 3.2% 6.9% 8.3% Reasons for dropouts among patients who received guanfacine were: somnolence, headache, weakness, dry mouth, dizziness, impotence, insomnia, constipation, syncope, urinary incontinence, conjunctivitis, paresthesia, and dermatitis. In a second 12-week placebo-controlled combination therapy study in which the dose could be adjusted upward to 3 mg per day in 1-mg increments at 3-week intervals, i.e., a setting more similar to ordinary clinical use, the most commonly recorded reactions were: dry mouth, 47%; constipation, 16%; fatigue, 12%; somnolence, 10%; asthenia, 6%; dizziness, 6%; headache, 4%; and insomnia, 4%. Reasons for dropouts among patients who received guanfacine were: somnolence, dry mouth, dizziness, impotence, constipation, confusion, depression, and palpitations. In the clonidine/guanfacine comparison described in CLINICAL PHARMACOLOGY , the most common adverse reactions noted were as follows: Adverse Reactions Guanfacine (n=279) Clonidine (n=278) Dry Mouth 30% 37% Somnolence 21% 35% Dizziness 11% 8% Constipation 10% 5% Fatigue 9% 8% Headache 4% 4% Insomnia 4% 3% Adverse reactions occurring in 3% or less of patients in the three controlled trials of guanfacine hydrochloride with a diuretic were: Cardiovascular - bradycardia, palpitations, substernal pain Gastrointestinal - abdominal pain, diarrhea, dyspepsia, dysphagia, nausea CNS - amnesia, confusion, depression, insomnia, libido decrease ENT disorders - rhinitis, taste perversion, tinnitus Eye disorders - conjunctivitis, iritis, vision disturbance Musculoskeletal - leg cramps, hypokinesia Respiratory - dyspnea Dermatologic - dermatitis, pruritus, purpura, sweating Urogenital - testicular disorder, urinary incontinence Other - malaise, paresthesia, paresis Adverse reaction reports tend to decrease over time. In an open-label trial of one year's duration, 580 hypertensive subjects were given guanfacine, titrated to achieve goal blood pressure, alone (51%), with diuretic (38%), with beta blocker (3%), with diuretic plus beta blocker (6%), or with diuretic plus vasodilator (2%). The mean daily dose of guanfacine reached was 4.7 mg. Adverse Reaction Incidence of adverse reactions at any time during the study n = 580 Incidence of adverse reactions at end of one year n = 580 Dry Mouth 60% 15% Drowsiness 33% 6% Dizziness 15% 1% Constipation 14% 3% Weakness 5% 1% Headache 4% 0.2% Insomnia 5% 0% There were 52 (8.9%) dropouts due to adverse effects in this 1-year trial. The causes were: dry mouth (n = 20), weakness (n = 12), constipation (n = 7), somnolence (n = 3), nausea (n = 3), orthostatic hypotension (n = 2), insomnia (n = 1), rash (n = 1), nightmares (n = 1), headache (n = 1), and depression (n = 1). Postmarketing Experience An open-label postmarketing study involving 21,718 patients was conducted to assess the safety of guanfacine hydrochloride 1 mg/day given at bedtime for 28 days. Guanfacine hydrochloride was administered with or without other antihypertensive agents. Adverse events reported in the postmarketing study at an incidence greater than 1% included dry mouth, dizziness, somnolence, fatigue, headache and nausea. The most commonly reported adverse events in this study were the same as those observed in controlled clinical trials. Less frequent, possibly guanfacine hydrochloride-related events observed in the postmarketing study and/or reported spontaneously include: BODY AS A WHOLE - asthenia, chest pain, edema, malaise, tremor CARDIOVASCULAR - bradycardia, palpitations, syncope, tachycardia CENTRAL NERVOUS SYSTEM - paresthesias, vertigo EYE DISORDERS - blurred vision GASTROINTESTINAL SYSTEM - abdominal pain, constipation, diarrhea, dyspepsia LIVER AND BILLIARY SYSTEM - abnormal liver function tests MUSCULOSKELETAL SYSTEM - arthralgia, leg cramps, leg pain, myalgia PSYCHIATRIC - agitation, anxiety, confusion, depression, insomnia, nervousness RREPRODUCTIVE SYSTEM, Male - impotence RESPIRATORY SYSTEM - dyspnea SKIN AND APPENDAGES - alopecia, dermatitis, exfoliative dermatitis, pruritus, rash SPECIAL SENSES - alterations in taste URINARY SYSTEM - nocturia, urinary frequency Rare, serious disorders with no definitive cause and effect relationship to guanfacine hydrochloride have been reported spontaneously and/or in the postmarketing study. These events include acute renal failure, cardiac fibrillation, cerebrovascular accident, congestive heart failure, heart block, and myocardial infarction.

Pharmacokinetics Relative to an intravenous dose of 3 mg, the absolute oral bioavailability of guanfacine is about 80%. Peak plasma concentrations occur from 1 to 4 hours with an average of 2.6 hours after single oral doses or at steady state. The area under the concentration-time curve (AUC) increases linearly with the dose. In individuals with normal renal function, the average elimination half-life is approximately 17 hr (range 10 to 30 hr). Younger patients tend to have shorter elimination half-lives (13 to 14 hr) while older patients tend to have half-lives at the upper end of the range. Steady state blood levels were attained within 4 days in most subjects. In individuals with normal renal function, guanfacine and its metabolites are excreted primarily in the urine. Approximately 50% (40 to 75%) of the dose is eliminated in the urine as unchanged drug; the remainder is eliminated mostly as conjugates of metabolites produced by oxidative metabolism of the aromatic ring. The guanfacine-to-creatinine clearance ratio is greater than 1.0, which would suggest that tubular secretion of drug occurs. The drug is approximately 70% bound to plasma proteins, independent of drug concentration. The whole body volume of distribution is high (a mean of 6.3 L/kg), which suggests a high distribution of drug to the tissues. The clearance of guanfacine in patients with varying degrees of renal insufficiency is reduced, but plasma levels of drug are only slightly increased compared to patients with normal renal function. When prescribing for patients with renal impairment, the low end of the dosing range should be used. Patients on dialysis also can be given usual doses of guanfacine hydrochloride as the drug is poorly dialyzed.