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Mechanism of Action
The exact mechanism of action of alprazolam is unknown. Benzodiazepines bind to gamma aminobutyric acid (GABA) receptors in the brain and enhance GABA-mediated synaptic inhibition; such actions may be responsible for the efficacy of alprazolam in anxiety disorder and panic disorder.
Following oral administration, alprazolam is readily absorbed. The peak plasma concentration is reached about 1.5 to 2 hours after administration of NIRAVAM given with or without water. When taken with water, mean Tmax occurs about 15 minutes earlier than when taken without water with no change in Cmax or AUC. Plasma levels are proportional to the dose given; over the dose range of 0.5 mg to 3.0 mg, peak levels of 8.0 to 37 ng/mL are observed. The elimination half-life of alprazolam is approximately 12.5 hours (range 7.9 - 19.2 hours) after administration of NIRAVAM in healthy adults.
Food decreased the mean Cmax by about 25% and increased the mean Tmax by 2 hours from 2.2 hours to 4.4 hours after the ingestion of a high-fat meal. Food did not affect the extent of absorption (AUC) or the elimination half-life.
In vitro, alprazolam is bound (80 percent) to human serum protein. Serum albumin accounts for the majority of the binding.
Alprazolam is extensively metabolized in humans, primarily by cytochrome P450 3A4 (CYP3A4), to two major metabolites in the plasma: 4-hydroxyalprazolam and α-hydroxyalprazolam. A benzophenone derived from alprazolam is also found in humans. Their half-lives appear to be similar to that of alprazolam. The plasma concentrations of 4-hydroxyalprazolam and α-hydroxyalprazolam relative to unchanged alprazolam concentration were always less than 4%. The reported relative potencies in benzodiazepine receptor binding experiments and in animal models of induced seizure inhibition are 0.20 and 0.66, respectively, for 4hydroxyalprazolam and α-hydroxyalprazolam. Such low concentrations and the lesser potencies of 4hydroxyalprazolam and α-hydroxyalprazolam suggest that they are unlikely to contribute much to the pharmacological effects of alprazolam. The benzophenone metabolite is essentially inactive.
Alprazolam and its metabolites are excreted primarily in the urine.
Changes in the absorption, distribution, metabolism and excretion of benzodiazepines have been reported in a variety of disease states including alcoholism, impaired hepatic function and impaired renal function. Changes have also been demonstrated in geriatric patients. A mean half-life of alprazolam of 16.3 hours has been observed in healthy elderly subjects (range: 9.0 - 26.9 hours, n=16) compared to 11.0 hours (range: 6.3 - 15.8 hours, n=16) in healthy adult subjects. In patients with alcoholic liver disease, the half-life of alprazolam ranged between 5.8 and 65.3 hours (mean: 19.7 hours, n=17) as compared to between 6.3 and 26.9 hours (mean=11.4 hours, n=17) in healthy subjects. In an obese group of subjects, the half-life of alprazolam ranged between 9.9 and 40.4 hours (mean=21.8 hours, n=12) as compared to between 6.3 and 15.8 hours (mean=10.6 hours, n=12) in healthy subjects.
Because of its similarity to other benzodiazepines, it is assumed that alprazolam undergoes transplacental passage and that it is excreted in human milk.
Race — Maximal concentrations (Cmax) and half-life of alprazolam are approximately 15% and 25% higher in Asians compared to Caucasians.
Pediatrics — The pharmacokinetics of alprazolam in pediatric patients have not been studied.
Gender — Gender has no effect on the pharmacokinetics of alprazolam.
Cigarette Smoking — Alprazolam concentrations may be reduced by up to 50% in smokers compared to nonsmokers.
Alprazolam is primarily eliminated by metabolism via cytochrome P450 3A (CYP3A). Most of the interactions that have been documented with alprazolam are with drugs that inhibit or induce CYP3A.
Compounds that are potent inhibitors of CYP3A would be expected to increase plasma alprazolam concentrations. Drug products that have been studied in vivo, along with their effect on increasing alprazolam AUC, are as follows: ketoconazole, 3.98 fold; itraconazole, 2.70 fold; nefazodone, 1.98 fold; fluvoxamine, 1.96 fold; and erythromycin, 1.61 fold [see CONTRAINDICATIONS, WARNINGS AND PRECAUTIONS, and DRUG INTERACTIONS].
CYP3A inducers would be expected to decrease alprazolam concentrations and this has been observed in vivo. The oral clearance of alprazolam (given in a 0.8 mg single dose) was increased from 0.90 RMG 0.21 mL/min/kg to 2.13 RMG 0.54 mL/min/kg and the elimination t½ was shortened (from 17.1 RMG 4.9 to 7.7 RMG 1.7 h) following administration of 300 mg/day carbamazepine for 10 days [see DRUG INTERACTIONS]. However, the carbamazepine dose used in this study was fairly low compared to the recommended doses (1000 mg - 1200 mg/day); the effect at usual carbamazepine doses is unknown.
The ability of alprazolam to induce or inhibit human hepatic enzyme systems has not been determined. However, this is not a property of benzodiazepines in general. Further, alprazolam did not affect the prothrombin or plasma warfarin levels in male volunteers administered sodium warfarin orally.
Animal Toxicology and/or Pharmacology
When rats were treated with oral alprazolam doses of 3, 10, and 30 mg/kg per day (3 to 30 times the maximum recommended human dose of 10 mg per day on a mg/m² basis) for 2 years, a tendency for a dose related increase in the number of cataracts was observed in females, and a tendency for a dose related increase in corneal vascularization was observed in males. These lesions did not appear until after 11 months of treatment.
The efficacy of alprazolam in the treatment of anxiety symptoms was demonstrated in five short-term (4 weeks), randomized, double-blind, placebo-controlled studies. The studies included patients with a diagnosis of anxiety or anxiety with associated depressive symptomatology. Alprazolam doses ranged from 0.5 to 4 mg per day. The mean daily doses ranged from 1.6 to 2.4 mg. Treatment with alprazolam was statistically significantly superior to placebo treatment, as measured by the following psychometric instruments: Hamilton Anxiety Rating Scale, Physician's Global Impressions, Target Symptoms, Patient's Global Impressions and Self-Rating Symptom Scale.
The efficacy of alprazolam in the treatment of panic disorder was demonstrated in three short-term (up to 10 weeks), randomized, double-blind, placebo-controlled studies. Patients in the studies had diagnoses corresponding closely to DSM-III-R criteria for panic disorder (with or without agoraphobia).
The average dose of alprazolam was 5 mg to 6 mg per day in two of the studies, and the doses of alprazolam were fixed at 2 mg and 6 mg per day in the third study. In all three studies, alprazolam was superior to placebo on a variable defined as “the number of patients with zero panic attacks” (range, 37 - 83% met this criterion), as well as on a global improvement score. In two of the three studies, alprazolam was superior to placebo on a variable defined as “change from baseline on the number of panic attacks per week” (range, 3.3 - 5.2), and also on a phobia rating scale. A subgroup of patients who were improved on alprazolam during short-term treatment in one of these trials was continued on an open basis up to 8 months, without apparent loss of benefit.
Last reviewed on RxList: 6/24/2011
This monograph has been modified to include the generic and brand name in many instances.
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