Mirtazapine (Page 6 of 9)

11 DESCRIPTION

Mirtazapine tablets, USP contain mirtazapine. Mirtazapine has a tetracyclic chemical structure and belongs to the piperazino-azepine group of compounds. It is designated 1,2,3,4,10,14b-Hexahydro-2-methylpyrazino[2,1-a]pyrido[2,3-c]benzazepine and has the molecular formula of C17 H19 N3 . Its molecular weight is 265.36. The structural formula is the following and it is the racemic mixture:

Mirtazapine Structural Formula

Mirtazapine, USP is a white to creamy white crystalline powder which is slightly soluble in water.

Mirtazapine tablets are available for oral administration as scored film-coated tablets containing 15 or 30 mg of mirtazapine, and unscored film-coated tablets containing 45 mg of mirtazapine. Each tablet contains the following inactive ingredients: anhydrous lactose, colloidal silicon dioxide, croscarmellose sodium, FD&C Blue No. 2 Aluminum Lake, FD&C Yellow No. 6 Aluminum Lake, hypromellose, magnesium stearate, microcrystalline cellulose, polydextrose, polyethylene glycol, pregelatinized starch (corn), sodium lauryl sulfate, titanium dioxide and triacetin.

12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

The mechanism of action of mirtazapine for the treatment of major depressive disorder, is unclear. However, its efficacy could be mediated through its activity as an antagonist at central presynaptic α2 -adrenergic inhibitory autoreceptors and heteroreceptors and enhancing central noradrenergic and serotonergic activity.

12.2 Pharmacodynamics

In preclinical studies, mirtazapine acts as an antagonist at α2 -adrenergic inhibitory autoreceptors and heteroreceptors and as an antagonist at serotonin 5-HT2 and 5-HT3 receptors. Mirtazapine has no significant affinity for the 5-HT1A and 5-HT1B receptors.

Mirtazapine also acts as an antagonist of histamine (H1 ) receptors, peripheral α1-adrenergic receptors, and muscarinic receptors. Actions at these receptors may explain some of the other clinical effects of mirtazapine (e.g., its prominent somnolent effects and orthostatic hypotension may be explained by its inhibition of histamine (H1 ) receptors and peripheral α1-adrenergic receptors, respectively).

Cardiac Electrophysiology

The effect of mirtazapine on QTc interval was assessed in healthy subjects. At a dose of 75 mg (1.67 times the maximum recommended dosage), mirtazapine does not prolong the QTc interval to a clinically meaningful extent.

12.3 Pharmacokinetics

Plasma levels of mirtazapine are linearly related to dose over a dose range of 15 to 80 mg (1.78 times the maximum recommended dose). Steady state plasma levels of mirtazapine are attained within 5 days, with about 50% accumulation (accumulation ratio = 1.5). The (–) enantiomer has an elimination half-life that is approximately twice as long as the (+) enantiomer and therefore achieves plasma levels that are about 3 times as high as that of the (+) enantiomer.

Absorption

Mirtazapine has an absolute bioavailability of about 50% following oral administration. Peak plasma concentrations of mirtazapine are reached within about 2 hours post dose.

Food Effect

The presence of food in the stomach has a minimal effect on both the rate and extent of absorption.

Distribution

Mirtazapine is approximately 85% bound to plasma proteins over a concentration range of 0.01 to 10 mcg/mL.

Elimination

Mirtazapine has a half-life of about 20 to 40 hours following oral administration of mirtazapine tablets.

Metabolism

Mirtazapine is extensively metabolized after oral administration. Major pathways of bio-transformation are demethylation and hydroxylation followed by glucuronide conjugation. In vitro data from human liver microsomes indicate that CYP2D6 and CYP1A2 are involved in the formation of the 8-hydroxy metabolite of mirtazapine, whereas CYP3A is considered to be responsible for the formation of the N-desmethyl and N-oxide metabolite. Several unconjugated metabolites possess pharmacological activity but are present in the plasma at very low levels.

Excretion

Mirtazapine and its metabolites are eliminated predominantly (75%) via urine with 15% in feces.

Specific Populations

Geriatric Patients

Following oral administration of mirtazapine tablets 20 mg/day for 7 days to subjects of varying ages (range 25 to 74 years old), oral clearance of mirtazapine was reduced in the elderly compared to the younger subjects. The clearance in elderly males was 40% lower compared to younger males, while the clearance was 10% lower in elderly females compared to younger females [see Warnings and Precautions (5.15), Use in Specific Populations (8.5)].

Male and Female Patients

The mean elimination half-life of mirtazapine after oral administration ranges from approximately 20 to 40 hours across age and gender subgroups, with females of all ages exhibiting significantly longer elimination half-lives than males (mean half-life of 37 hours for females vs. 26 hours for males).

Race

There have been no clinical studies to evaluate the effect of race on the pharmacokinetics of mirtazapine.

Patients with Renal Impairment

When compared to subjects with normal renal function, total body clearance of mirtazapine was reduced approximately 30% in renal impaired patients with GFR = 11–39 mL/min/1.73 m2 and approximately 50% in renal impaired patients with GFR = < 10 mL/min/1.73 m2) [see Warnings and Precautions (5.15), Use in Specific Populations (8.6)].

Patients with Hepatic Impairment

Following a single 15-mg oral dose of mirtazapine, the oral clearance of mirtazapine in patients with hepatic impairment was decreased by approximately 30%, compared to subjects with normal hepatic function [see Warnings and Precautions (5.13, 5.15), Use in Specific Populations (8.6)].

Drug Interactions Studies

Warfarin

Mirtazapine (30 mg daily) at steady state caused a statistically significant increase (0.2) in the International Normalized Ratio (INR) in subjects treated with warfarin [see Drug Interactions (7)].

QTc-Prolonging Drugs

The risk of QT prolongation and/or ventricular arrhythmias (e.g., Torsades de Pointes) may be increased with concomitant use of medicines which prolong the QTc interval (e.g., some antipsychotics and antibiotics) and in mirtazapine overdose [see Warnings and Precautions (5.5), Adverse Reactions (6.1, 6.2), Drug Interactions (7), and Overdosage (10)].

Phenytoin

In healthy male subjects (n = 18), phenytoin (200 mg daily, at steady state) increased mirtazapine (30 mg daily, at steady state) clearance about 2-fold, resulting in a decrease in average plasma mirtazapine concentrations of 45% [see Drug Interactions (7)]. Mirtazapine did not significantly affect the pharmacokinetics of phenytoin.

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