Doxepin 10 mg · Tablets

The mechanism of action of doxepin in sleep maintenance is unclear; however, doxepin’s effect could be mediated through antagonism of the H1 receptor.

Absorption — The median time to peak concentrations (Tmax) of doxepin occurred at 3.5 hours postdose after oral administration of a 6 mg dose to fasted healthy subjects. Peak plasma concentrations (Cmax) of doxepin increased in approximately a dose-proportional manner for 3 mg and 6 mg doses. The AUC was increased by 41% and Cmax by 15% when 6 mg doxepin was administered with a high fat meal. Additionally, compared to the fasted state, Tmax was delayed by approximately 3 hours. Therefore, for faster onset and to minimize the potential for next day effects, it is recommended that doxepin not be taken within 3 hours of a meal [ see Dosage and Administration (2.3)].

Distribution — Doxepin is widely distributed throughout the body tissues. The mean apparent volume of distribution following a single 6 mg oral dose of doxepin to healthy subjects was 11,930 liters. Doxepin is approximately 80% bound to plasma proteins.

Metabolism — Following oral administration, doxepin is extensively metabolized by oxidation and demethylation.The primary metabolite is N-desmethyldoxepin (nordoxepin).

The primary metabolite undergoes further biotransformation to glucuronide conjugates.

In vitro studies have shown that CYP2C19 and CYP2D6 are the major enzymes involved in doxepin metabolism, and that CYP1A2 and CYP2C9 are involved to a lesser extent.

Doxepin appears not to have inhibitory effects on human CYP enzymes at therapeutic concentrations. The potential of doxepin to induce metabolizing enzymes is not known. Doxepin is not a Pgp substrate.

Excretion — Doxepin is excreted in the urine mainly in the form of glucuronide conjugates.

Less than 3% of a doxepin dose is excreted in the urine as parent compound or nordoxepin. The apparent terminal half-life (t ½) of doxepin was 15.3 hours and for nordoxepin was 31 hours.

Drug Interactions — Since doxepin is metabolized by CYP2C19 and CYP2D6, inhibitors of these CYP isozymes may increase the exposure of doxepin.

Cimetidine — The effect of cimetidine, a non-specific inhibitor of CYP1A2, 2C19, 2D6, and 3A4, on doxepin plasma concentrations was evaluated in healthy subjects. When cimetidine 300 mg BID was co-administered with a single dose of doxepin 6 mg, there was approximately a 2­ fold increase in doxepin Cmax and AUC compared to doxepin given alone. A maximum dose of doxepin in adults and elderly should be 3 mg, when doxepin is co-administered with cimetidine.

Sertraline — The effect of sertraline HCl, a selective serotonin reuptake inhibitor, on doxepin plasma concentrations was evaluated in a daytime study conducted with 24 healthy subjects. Following co-administration of doxepin 6 mg with sertraline 50 mg (at steady-state), the doxepin mean AUC and Cmax estimates were approximately 21% and 32% higher, respectively, than those obtained following administration of doxepin alone. Psychomotor function as measured by the digit symbol substitution test and symbol copy test performance was decreased more at 2-4 hours post dosing for the combination of sertraline and doxepin as compared to doxepin alone, but subjective measures of alertness were comparable for the two treatments.

Renal Impairment — The effects of renal impairment on doxepin pharmacokinetics have not been studied. Because only small amounts of doxepin and nordoxepin are eliminated in the urine, renal impairment would not be expected to result in significantly altered doxepin concentrations.

Hepatic Impairment — The effects of doxepin in patients with hepatic impairment have not been studied. Because doxepin is extensively metabolized by hepatic enzymes, patients with hepatic impairment may display higher doxepin concentrations than healthy individuals.

Poor Metabolizers of CYPs — Poor metabolizers of CYP2C19 and CYP2D6 may have higher doxepin plasma levels than normal subjects.