Prescription Drug Information: Venlafaxine Hydrochloride (Page 8 of 12)

12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

The exact mechanism of the antidepressant action of venlafaxine in humans is unknown, but is thought to be related to the potentiation of serotonin and norepinephrine in the central nervous system, through inhibition of their reuptake. Non- clinical studies have demonstrated that venlafaxine and its active metabolite, ODV, are potent and selective inhibitors of neuronal serotonin and norepinephrine reuptake and weak inhibitors of dopamine reuptake.

12.2 Pharmacodynamics

Venlafaxine and ODV have no significant affinity for muscarinic-cholinergic, H 1 -histaminergic, or α 1 -adrenergic receptors in vitro. Pharmacologic activity at these receptors is hypothesized to be associated with the various anticholinergic, sedative, and cardiovascular effects seen with other psychotropic drugs. Venlafaxine and ODV do not possess monoamine oxidase (MAO) inhibitory activity.

Cardiac Electrophysiology

The effect of venlafaxine on the QT interval was evaluated in a randomized, double-blind, placebo-and positive-controlled three-period crossover thorough QT study in 54 healthy adult subjects. No significant QT prolongation effect of venlafaxine 450 mg was detected.

12.3 Pharmacokinetics

Steady-state concentrations of venlafaxine and ODV in plasma are attained within 3 days of oral multiple-dose therapy. Venlafaxine and ODV exhibited linear kinetics over the dose range of 75 to 450 mg per day. Mean±SD steady-state plasma clearance of venlafaxine and ODV is 1.3±0.6 and 0.4±0.2 L/h/kg, respectively; apparent elimination half-life is 5±2 and 11±2 hours, respectively; and apparent (steady-state) volume of distribution is 7.5±3.7 and 5.7±1.8 L/kg, respectively. Venlafaxine and ODV are minimally bound at therapeutic concentrations to plasma proteins (27% and 30%, respectively).

Absorption and Distribution

Venlafaxine is well absorbed and extensively metabolized in the liver. ODV is the major active metabolite. On the basis of mass balance studies, at least 92% of a single oral dose of venlafaxine is absorbed. The absolute bioavailability of venlafaxine is approximately 45%.
Administration of venlafaxine hydrochloride extended-release capsules (150 mg once daily) generally resulted in lower C max and later T max values than for venlafaxine hydrochloride (immediate release) administered twice daily (Table 16). When equal daily doses of venlafaxine were administered as either an immediate-release tablet or the extended-release capsule, the exposure to both venlafaxine and ODV was similar for the two treatments, and the fluctuation in plasma concentrations was slightly lower with the venlafaxine hydrochloride extended-release capsules. Therefore, venlafaxine hydrochloride extended-release capsules provide a slower rate of absorption, but the same extent of absorption compared with the immediate-release tablet.

Table 16: Comparison of C max and T max Values for Venlafaxine and ODV Following Oral Administration of Venlafaxine Hydrochloride Extended-Release Capsules and Venlafaxine Hydrochloride (Immediate Release)
Venlafaxine ODV
C max (ng/mL) T max (h) C max (ng/mL) T max (h)
Venlafaxine Hydrochloride Extended-Release Capsules (150 mg once daily) 150 5.5 260 9
Venlafaxine Hydrochloride (75 mg twice daily) 225 2 290 3

Food did not affect the bioavailability of venlafaxine or its active metabolite, ODV. Time of administration (AM versus PM) did not affect the pharmacokinetics of venlafaxine and ODV from the 75 mg venlafaxine hydrochloride extended-release capsules.
Venlafaxine is not highly bound to plasma proteins; therefore, administration of venlafaxine hydrochloride extended-release capsules to a patient taking another drug that is highly protein-bound should not cause increased free concentrations of the other drug.

Metabolism and Elimination

Following absorption, venlafaxine undergoes extensive presystemic metabolism in the liver, primarily to ODV, but also to N-desmethylvenlafaxine, N,O-didesmethylvenlafaxine, and other minor metabolites. In vitro studies indicate that the formation of ODV is catalyzed by CYP2D6; this has been confirmed in a clinical study showing that patients with low CYP2D6 levels (poor metabolizers) had increased levels of venlafaxine and reduced levels of ODV compared to people with normal CYP2D6 levels (extensive metabolizers) [see Use in Specific Populations (8.7)] .
Approximately 87% of a venlafaxine dose is recovered in the urine within 48 hours as unchanged venlafaxine (5%), unconjugated ODV (29%), conjugated ODV (26%), or other minor inactive metabolites (27%). Renal elimination of venlafaxine and its metabolites is thus the primary route of excretion.

13 NONCLINICAL TOXICOLOGY

13.1 Carcinogenesis, Mutagenesis, Impairment of Fertility

Carcinogenesis

Tumors were not increased by venlafaxine treatment in mice or rats. Venlafaxine was given by oral gavage to mice for 18 months at doses up to 120 mg/kg per day, which was 1.7 times the maximum recommended human dose on a mg/m 2 basis. Venlafaxine was also given to rats by oral gavage for 24 months at doses up to 120 mg/kg per day. In rats receiving the 120 mg/kg dose, plasma concentrations of venlafaxine at necropsy were 1 times (male rats) and 6 times (female rats) the plasma concentrations of patients receiving the maximum recommended human dose. Plasma levels of the O-desmethyl metabolite (ODV) were lower in rats than in patients receiving the maximum recommended dose. O-desmethylvenlafaxine (ODV), the major human metabolite of venlafaxine, administered by oral gavage to mice and rats for 2 years did not increase the incidence of tumors in either study. Mice received ODV at dosages up to 500/300 mg/kg/day (dosage lowered after 45 weeks of dosing). The exposure at the 300 mg/kg/day dose is 9 times that of a human dose of 225 mg/day. Rats received ODV at dosages up to 300 mg/kg/day (males) or 500 mg/kg/day (females). The exposure at the highest dose is approximately 8 (males) or 11 (females) times that of a human dose of 225 mg/day.

Mutagenesis

Venlafaxine and the major human metabolite, ODV, were not mutagenic in the Ames reverse mutation assay in Salmonella bacteria or the Chinese hamster ovary/HGPRT mammalian cell forward gene mutation assay. Venlafaxine was also not mutagenic or clastogenic in the in vitro BALB/c-3T3 mouse cell transformation assay, the sister chromatid exchange assay in cultured Chinese hamster ovary cells, or in the in vivo chromosomal aberration assay in rat bone marrow. ODV was not clastogenic in the in vitro Chinese hamster ovary cell chromosomal aberration assay or in the in vivo chromosomal aberration assay in rats.

Impairment of Fertility
Reproduction and fertility studies of venlafaxine in rats showed no adverse effects of venlafaxine on male or female fertility at oral doses of up to 2 times the maximum recommended human dose of 225 mg/day on a mg/m 2 basis. However, reduced fertility was observed in a study in which male and female rats were treated with O-desmethylvenlafaxine (ODV), the major human metabolite of venlafaxine, prior to and during mating and gestation. This occurred at an ODV exposure (AUC) approximately 2 to 3 times that associated with a human venlafaxine dose of 225 mg/day.

14 CLINICAL STUDIES

14.1 Major Depressive Disorder

The efficacy of venlafaxine hydrochloride extended-release capsules as a treatment for Major Depressive Disorder (MDD) was established in two placebo-controlled, short-term (8 weeks for study 1; 12 weeks for study 2), flexible-dose studies, with doses starting at 75 mg per day and ranging to 225 mg per day in adult outpatients meeting DSM-III-R or DSM-IV criteria for MDD. In moderately depressed outpatients, the initial dose of venlafaxine was 75 mg per day. In both studies, venlafaxine hydrochloride extended-release capsules demonstrated superiority over placebo on the primary efficacy measure defined as change from baseline in the HAM-D-21 total score to the endpoint visit, venlafaxine hydrochloride extended-release capsules also demonstrated superiority over placebo on the key secondary efficacy endpoint, the Clinical Global Impressions (CGI) Severity of Illness scale. Examination of gender subsets of the population studied did not reveal any differential responsiveness on the basis of gender.

A 4-week study of inpatients meeting DSM-III-R criteria for MDD with melancholia utilizing venlafaxine hydrochloride in a range of 150 to 375 mg per day (divided in a three-times-a-day schedule) demonstrated superiority of venlafaxine hydrochloride over placebo based on the HAM-D-21 total score. The mean dose in completers was 350 mg per day (study 3).

In a longer-term study, adult outpatients with MDD who had responded during an 8-week open-label study on venlafaxine hydrochloride extended-release capsules (75, 150, or 225 mg, once daily every morning) were randomized to continuation of their same venlafaxine hydrochloride extended-release capsules dose or to placebo, for up to 26 weeks of observation for relapse. Response during the open-label phase was defined as a CGI Severity of Illness item score of ≤3 and a HAM-D-21 total score of ≤10 at the day 56 evaluation. Relapse during the double-blind phase was defined as follows: (1) a reappearance of major depressive disorder as defined by DSM-IV criteria and a CGI Severity of Illness item score of ≥4 (moderately ill), (2) 2 consecutive CGI Severity of Illness item scores of ≥4, or (3) a final CGI Severity of Illness item score of ≥4 for any patient who withdrew from the study for any reason.

Patients receiving continued venlafaxine hydrochloride extended-release capsules treatment experienced statistically significantly lower relapse rates over the subsequent 26 weeks compared with those receiving placebo (study 4).

In a second longer term trial, adult outpatients with MDD, recurrent type, who had responded (HAM-D-21 total score ≤ 12 at the day 56 evaluation) and continued to be improved [defined as the following criteria being met for days 56 through 180: (1) no HAM-D-21 total score ≥ 20; (2) no more than 2 HAM-D-21 total scores > 10, and (3) no single CGI Severity of Illness item score ≥ 4 (moderately ill)] during an initial 26 weeks of treatment on venlafaxine hydrochloride [100 to 200 mg per day, on a twice daily schedule] were randomized to continuation of their same venlafaxine hydrochloride dose or to placebo. The follow-up period to observe patients for relapse, defined as a CGI Severity of Illness item score ≥ 4, was for up to 52 weeks. Patients receiving continued venlafaxine hydrochloride treatment experienced statistically significantly lower relapse rates over the subsequent 52 weeks compared with those receiving placebo (study 5).

Table 17: Major Depressive Disorder Studies:
SD: standard deviation; LS Mean: least-squares mean; CI: confidence interval. a Difference (drug minus placebo) in least-squares mean change from baseline * Doses statistically significantly superior to placebo.
Study number Treatment Group Primary Efficacy Measure: HAM-D Score
Mean Baseline Score (SD) LS Mean Change from Baseline Placebo Subtracted Difference a (95%CI)
Study 1 Venlafaxine Hydrochloride Extended-Release Capsules (75 to 225 mg/day)* 24.5 -11.7 -4.45(-6.66, -2.25)
Placebo 23.6 -7.24
Study 2 Venlafaxine Hydrochloride Extended-Release Capsules (75 to 225 mg/day)* 24.5 -15.11 -6.4(-8.45,-4.34)
Placebo 24.9 -8.71
Study 3 Venlafaxine Hydrochloride (immediate release)(150 to 375 mg/day)* 28.2 (0.5) -14.9 -10.2 (-14.4,-6)
Placebo 28.6 (0.6) -4.7

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