Paroxetine (Page 5 of 8)

8.3 Nursing Mothers

Like many other drugs, paroxetine is secreted in human milk. Because of the potential for serious adverse reactions in nursing infants from paroxetine extended-release tablets, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.

8.4 Pediatric Use

The safety and effectiveness of paroxetine extended-release tablets in pediatric patients have not been established [see Boxed Warning, Warnings and Precautions (5.1)].

Three placebo-controlled trials in 752 pediatric patients with MDD have been conducted with immediate-release paroxetine, and effectiveness was not established in pediatric patients.

Decreased appetite and weight loss have been observed in association with the use of SSRIs.

In placebo-controlled clinical trials conducted with pediatric patients, the following adverse reactions were reported in at least 2% of pediatric patients treated with immediate-release paroxetine hydrochloride and at a rate at least twice that for pediatric patients receiving placebo: emotional lability (including self-harm, suicidal thoughts, attempted suicide, crying, and mood fluctuations), hostility, decreased appetite, tremor, sweating, hyperkinesia, and agitation.

Adverse reactions upon discontinuation of treatment with immediate-release paroxetine hydrochloride in the pediatric clinical trials that included a taper phase regimen, which occurred in at least 2% of patients and at a rate at least twice that of placebo, were: emotional lability (including suicidal ideation, suicide attempt, mood changes, and tearfulness), nervousness, dizziness, nausea, and abdominal pain.

8.5 Geriatric Use

SSRIs and SNRIs, including paroxetine extended-release tablets, have been associated with cases of clinically significant hyponatremia in elderly patients, who may be at greater risk for this adverse reaction [see Warnings and Precautions (5.9)].

In premarketing clinical trials with immediate-release paroxetine hydrochloride, 17% of paroxetine treated patients (approximately 700) were 65 years or older. Pharmacokinetic studies revealed a decreased clearance in the elderly, and a lower starting dose is recommended; however, no overall differences in safety or effectiveness were observed between these subjects and younger subjects [see Dosage and Administration (2.5), Clinical Pharmacology (12.3)].

8.6 Renal and/or Hepatic Impairment

Increased plasma concentrations of paroxetine occur in patients with renal and hepatic impairment. The initial dosage should be reduced in patients with severe renal impairment and patients with severe hepatic impairment [see Dosage and Administration (2.5) and Clinical Pharmacology (12.3)].

10 OVERDOSAGE

Human Experience

Since the introduction of immediate-release paroxetine hydrochloride in the United States, spontaneous cases of deliberate or accidental overdosage during paroxetine treatment have been reported worldwide. These include overdoses with paroxetine alone and in combination with other substances. There are reports of fatalities that appear to involve paroxetine alone.

Commonly reported adverse reactions associated with paroxetine overdosage include somnolence, coma, nausea, tremor, tachycardia, confusion, vomiting, and dizziness. Other notable signs and symptoms observed with overdoses involving paroxetine (alone or with other substances) include mydriasis, convulsions (including status epilepticus), ventricular dysrhythmias (including torsade de pointes), hypertension, aggressive reactions, syncope, hypotension, stupor, bradycardia, dystonia, rhabdomyolysis, symptoms of hepatic dysfunction (including hepatic failure, hepatic necrosis, jaundice, hepatitis, and hepatic steatosis), serotonin syndrome, manic reactions, myoclonus, acute renal failure, and urinary retention.

Overdose Management

No specific antidotes for paroxetine are known. If over-exposure occurs, call your poison control center at 1-800-222-1222 for latest recommendations.

11 DESCRIPTION

Paroxetine extended-release tablets, USP contain paroxetine hydrochloride, an SSRI. It is the hydrochloride salt of a phenylpiperidine compound identified chemically as (-)-trans -4R- (4′-fluorophenyl)-3S -[(3′,4’methylenedioxyphenoxy) methyl] piperidine hydrochloride hemihydrate and has the empirical formula of C19 H20 FNO3 ∙HCl∙1/2H2 O. The molecular weight is 374.8 g/mol (329.4 g/mol as free base). The structural formula of paroxetine hydrochloride is:

Chemical Structure

Paroxetine hydrochloride is an odorless, off-white powder, having a melting point range of 120°C to 138°C and a solubility of 5.4 mg/mL in water. Paroxetine extended-release tablets, USP are intended for oral administration. Each extended-release tablet contains 12.5 mg, 25 mg, or 37.5 mg paroxetine equivalent to 14.25 mg, 28.51 mg, or 42.76 mg of paroxetine hydrochloride, respectively. One layer of the tablet consists of a degradable barrier layer and the other contains the active material in a hydrophilic matrix.

Inactive ingredients consist of colloidal silicon dioxide, hypromellose, lactose monohydrate, magnesium stearate, ferric oxide yellow (12.5mg, 25 mg and 37.5 mg), glyceryl dibehenate, D&C Yellow No. 10 aluminum lake (12.5 mg), FD&C Blue No. 2 aluminum lake (12.5 mg and 37.5 mg), FD&C Red No. 40 aluminum lake (12.5 mg), FD&C Yellow No. 6 aluminum lake (12.5 mg), ferric oxide red (25 mg), ferrosoferric oxide (25 mg), methacrylic acid and ethyl acrylate copolymer dispersion, polyethylene glycol, polysorbate 80, sodium lauryl sulfate, talc, titanium dioxide and triethyl citrate.

FDA approved dissolution test specifications differ from USP.

12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

The mechanism of action of paroxetine in the treatment of major depressive disorder (MDD), panic disorder (PD), social anxiety disorder (SAD), and premenstrual dysphoric disorder (PMDD) is unknown, but is presumed to be linked to potentiation of serotonergic activity in the central nervous system resulting from inhibition of neuronal reuptake of serotonin (5-HT).

12.2 Pharmacodynamics

Studies at clinically relevant doses in humans have demonstrated that paroxetine blocks the uptake of serotonin into human platelets. In vitro studies in animals also suggest that paroxetine is a potent and highly selective inhibitor of neuronal serotonin reuptake (SSRI) and has only very weak effects on norepinephrine and dopamine neuronal reuptake.

12.3 Pharmacokinetics

Absorption

Paroxetine extended-release tablets contain a degradable polymeric matrix designed to control the dissolution rate of paroxetine over a period of approximately 4 to 5 hours. In addition to controlling the rate of drug release in vivo, an enteric coat delays the start of drug release until paroxetine extended-release tablets have left the stomach.

Paroxetine extended-release tablets are completely absorbed after oral dosing of a solution of the hydrochloride salt. In a study in which normal male and female subjects (n = 23) received single oral doses of paroxetine extended-release tablets at 4 dosage strengths (12.5 mg, 25 mg, 37.5 mg, and 50 mg), paroxetine Cmax and AUC0-inf increased disproportionately with dose (as seen also with immediate-release formulations). Mean Cmax and AUC0-inf values at these doses were 2.0, 5.5, 9.0, and 12.5 ng/mL, and 121, 261, 338, and 540 ng∙hr./mL, respectively. Tmax was observed typically between 6 and 10 hours post-dose, reflecting a reduction in absorption rate compared with immediate-release formulations. The bioavailability of 25 mg paroxetine extended-release tablets is not affected by food.

Distribution

Paroxetine distributes throughout the body, including the CNS, with only 1% remaining in the plasma.

Approximately 95% and 93% of paroxetine is bound to plasma protein at 100 ng/mL and 400 ng/mL, respectively. Under clinical conditions, paroxetine concentrations would normally be less than 400 ng/mL. Paroxetine does not alter the in vitro p rotein binding of phenytoin or warfarin.

Elimination

Metabolism

The mean elimination half-life of paroxetine was 15 to 20 hours throughout a range of single doses of paroxetine extended-release tablets (12.5 mg, 25 mg, 37.5 mg, and 50 mg). During repeated administration of paroxetine extended-release tablets (25 mg once daily), steady state was reached within 2 weeks (i.e., comparable to immediate-release formulations). In a repeat-dose study in which normal male and female subjects (n = 23) received paroxetine extended-release tablets (25 mg daily), mean steady state Cmax , Cmin , and AUC0-24 values were 30 ng/mL, 20 ng/mL, and 550 ng∙hr./mL, respectively.

Based on studies using immediate-release formulations, steady-state drug exposure based on AUC0-24 was several-fold greater than would have been predicted from single-dose data. The excess accumulation is a consequence of the fact that 1 of the enzymes that metabolizes paroxetine is readily saturable.

In steady-state dose proportionality studies involving elderly and nonelderly patients, at doses of the immediate-release formulation of 20 mg to 40 mg daily for the elderly and 20 mg to 50 mg daily for the nonelderly, some nonlinearity was observed in both populations, again reflecting a saturable metabolic pathway (Figure 3).

Paroxetine is extensively metabolized after oral administration. The principal metabolites are polar and conjugated products of oxidation and methylation, which are readily cleared. Conjugates with glucuronic acid and sulfate predominate, and major metabolites have been isolated and identified. Data indicate that the metabolites have no more than 1/50 the potency of the parent compound at inhibiting serotonin uptake. The metabolism of paroxetine is accomplished in part by CYP2D6. Saturation of this enzyme at clinical doses appears to account for the nonlinearity of paroxetine kinetics with increasing dose and increasing duration of treatment. The role of this enzyme in paroxetine metabolism also suggests potential drug-drug interactions [see Drug Interactions (7.3)].

Excretion

Approximately 64% of a 30-mg oral solution dose of paroxetine was excreted in the urine with 2% as the parent compound and 62% as metabolites over a 10-day post-dosing period. About 36% was excreted in the feces (probably via the bile), mostly as metabolites and less than 1% as the parent compound over the 10-day post-dosing period.

The elimination half-life is approximately 15 to 20 hours after a single dose of paroxetine extended-release tablets. Paroxetine metabolism is mediated in part by CYP2D6, and the metabolites are primarily excreted in the urine and to some extent in the feces. Pharmacokinetic behavior of paroxetine has not been evaluated in subjects who are deficient in CYP2D6 (poor metabolizers).

Drug Interaction Studies

There are clinically significant, known drug interactions between paroxetine and other drugs [see Drug Interactions (7)].

Figure 1. Impact of Paroxetine on the Pharmacokinetics of Co-Administered Drugs (log scale)
Figure 1
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Figure 2. Impact of Co-Administered Drugs on the Pharmacokinetics of Paroxetine
Figure 2
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Theophylline: Reports of elevated theophylline levels associated with immediate-release paroxetine treatment have been reported. While this interaction has not been formally studied, it is recommended that theophylline levels be monitored when these drugs are concurrently administered.

Drugs Metabolized by Cytochrome CYP3A4

An in vivo interaction study involving the coadministration under steady-state conditions of paroxetine and terfenadine, a substrate for CYP3A4, revealed no effect of paroxetine on terfenadine pharmacokinetics. In addition, in vitro studi es have shown ketoconazole, a potent inhibitor of CYP3A4 activity, to be at least 100 times more potent than paroxetine as an inhibitor of the metabolism of several substrates for this enzyme, including terfenadine, astemizole, cisapride, triazolam, and cyclosporine. Paroxetine’s extent of inhibition of CYP3A4 activity is not expected to be of clinical significance.

Specific Populations

The impact of specific populations on the pharmacokinetics of paroxetine are shown in Figure 3.

Figure 3. Impact of Specific Population on the Pharmacokinetics of Paroxetine (log scale)
Figure 3
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