The mechanism of action of nefazodone, as with other antidepressants, is unknown.
Preclinical studies have shown that nefazodone inhibits neuronal uptake of serotonin and norepinephrine.
Nefazodone occupies central 5-HT2 receptors at nanomolar concentrations, and acts as an antagonist at this receptor. Nefazodone was shown to antagonize alpha1-adrenergic receptors, a property which may be associated with postural hypotension. In vitro binding studies showed that nefazodone had no significant affinity for the following receptors: alpha2 and beta adrenergic, 5-HT1A, cholinergic, dopaminergic, or benzodiazepine.
Nefazodone is rapidly and completely absorbed but is subject to extensive metabolism, so that its absolute bioavailability is low, about 20%, and variable. Peak plasma concentrations occur at about one hour and the half-life of nefazodone is 2 to 4 hours.
Both nefazodone and its pharmacologically similar metabolite, hydroxynefazodone, exhibit nonlinear kinetics for both dose and time, with AUC and Cmax increasing more than proportionally with dose increases and more than expected upon multiple dosing over time, compared to single dosing. For example, in a multiple-dose study involving BID dosing with 50, 100, and 200 mg, the AUC for nefazodone and hydroxynefazodone increased by about 4 fold with an increase in dose from 200 to 400 mg per day; Cmax increased by about 3 fold with the same dose increase. In a multiple-dose study involving BID dosing with 25, 50, 100, and 150 mg, the accumulation ratios for nefazodone and hydroxynefazodone AUC, after 5 days of BID dosing relative to the first dose, ranged from approximately 3 to 4 at the lower doses (50 to 100 mg/day) and from 5 to 7 at the higher doses (200 to 300 mg/day); there were also approximately 2 to 4 fold increases in Cmax after 5 days of BID dosing relative to the first dose, suggesting extensive and greater than predicted accumulation of nefazodone and its hydroxy metabolite with multiple dosing. Steady-state plasma nefazodone and metabolite concentrations are attained within 4 to 5 days of initiation of BID dosing or upon dose increase or decrease.
Nefazodone is extensively metabolized after oral administration by n-dealkylation and aliphatic and aromatic hydroxylation, and less than 1% of administered nefazodone is excreted unchanged in urine. Attempts to characterize three metabolites identified in plasma, hydroxynefazodone (HO-NEF), meta-chlorophenylpiperazine (mCPP), and a triazole-dione metabolite, have been carried out. The AUC (expressed as a multiple of the AUC for nefazodone dosed at 100 mg BID) and elimination half-lives for these three metabolites were as follows:
AUC Multiples and T1/2 for Three Metabolites of Nefazodone (100 mg BID)
| Metabolite || AUC Multiple || T1/2 |
|HO-NEF||0.4||1.5 to 4 h|
|mCPP||0.07||4 to 8 h|
HO-NEF possesses a pharmacological profile qualitatively and quantitatively similar to that of nefazodone. mCPP has some similarities to nefazodone, but also has agonist activity at some serotonergic receptor subtypes. The pharmacological profile of the triazole-dione metabolite has not yet been well characterized. In addition to the above compounds, several other metabolites were present in plasma but have not been tested for pharmacological activity.
After oral administration of radiolabeled nefazodone, the mean half-life of total label ranged between 11 and 24 hours. Approximately 55% of the administered radioactivity was detected in urine and about 20 to 30% in feces.
Nefazodone is widely distributed in body tissues, including the central nervous system (CNS). In humans the volume of distribution of nefazodone ranges from 0.22 to 0.87 L/kg.
At concentrations of 25 to 2500 ng/mL nefazodone is extensively (> 99%) bound to human plasma proteins in vitro. The administration of 200 mg BID of nefazodone for 1 week did not increase the fraction of unbound warfarin in subjects whose prothrombin times had been prolonged by warfarin therapy to 120 to 150% of the laboratory control (see PRECAUTIONS, Drug Interactions). While nefazodone did not alter the in vitro protein binding of chlorpromazine, desipramine, diazepam, diphenylhydantoin, lidocaine, prazosin, propranolol, or verapamil, it is unknown whether displacement of either nefazodone or these drugs occurs in vivo. There was a 5% decrease in the protein binding of haloperidol; this is probably of no clinical significance.
Effect of Food
Food delays the absorption of nefazodone and decreases the bioavailability of nefazodone by approximately 20%.
In studies involving 29 renally impaired patients, renal impairment (creatinine clearances ranging from 7 to 60 mL/min/1.73 m2) had no effect on steady-state nefazodone plasma concentrations.
In a multiple-dose study of patients with liver cirrhosis, the AUC values for nefazodone and HO-NEF at steady state were approximately 25% greater than those observed in normal volunteers.
After single doses of 300 mg to younger (18 to 45 years) and older patients (> 65 years), Cmax and AUC for nefazodone and hydroxynefazodone were up to twice as high in the older patients. With multiple doses, however, differences were much smaller, 10 to 20%. A similar result was seen for gender, with a higher Cmax and AUC in women after single doses but no difference after multiple doses.
Treatment with nefazodone should be initiated at half the usual dose in elderly patients, especially women (see DOSAGE AND ADMINISTRATION), but the therapeutic dose range is similar in younger and older patients.
Clinical Efficacy Trial Results
Studies in Outpatients with Depression
During its premarketing development, the efficacy of nefazodone was evaluated at doses within the therapeutic range in five well-controlled, short-term (6 to 8 weeks) clinical investigations. These trials enrolled outpatients meeting DSM-III or DSM-IIIR criteria for major depression. Among these trials, two demonstrated the effectiveness of nefazodone, and two provided additional support for that conclusion.
One trial was a 6 week dose-titration study comparing nefazodone in two dose ranges (up to 300 mg/day and up to 600 mg/day [mean modal dose for this group was about 400 mg/day], on a BID schedule) and placebo. The second trial was an 8 week dose-titration study comparing nefazodone (up to 600 mg/day; mean modal dose was 375 mg/day), imipramine (up to 300 mg/day), and placebo, all on a BID schedule. Both studies demonstrated nefazodone, at doses titrated between 300 mg to 600 mg/day (therapeutic dose range), to be superior to placebo on at least three of the following four measures: 17 Item Hamilton Depression Rating Scale or HDRS (total score), Hamilton Depressed Mood item, Clinical Global Impressions (CGI) Severity score, and CGI Improvement score. Significant differences were also found for certain factors of the HDRS (e.g., anxiety factor, sleep disturbance factor, and retardation factor). In the two supportive studies, nefazodone was titrated up to 500 or 600 mg/day (mean modal doses of 462 mg/day and 363 mg/day). In the fifth study, the differentiation in response rates between nefazodone and placebo was not statistically significant. Three additional trials were conducted using subtherapeutic doses of nefazodone.
Overall, approximately two thirds of patients in these trials were women, and an analysis of the effects of gender on outcome did not suggest any differential responsiveness on the basis of sex. There were too few elderly patients in these trials to reveal possible age-related differences in response.
Since its initial marketing as an antidepressant drug product, additional clinical investigations of nefazodone have been conducted. These studies explored nefazodone’s use under conditions not evaluated fully at the time initial marketing approval was granted.
Studies in “Inpatients”
Two studies were conducted to evaluate nefazodone’s effectiveness in hospitalized depressed patients. These were 6 week, dose-titration trials comparing nefazodone (up to 600 mg/day) and placebo, on a BID schedule. In one study, nefazodone was superior to placebo. In this study, the mean modal dose of nefazodone was 503 mg/day, and 85% of these inpatients were melancholic; at baseline, patients were distributed at the higher end of the 7 point CGI Severity scale, as follows: 4 = moderately ill (17%); 5 = markedly ill (48%); 6 = severely ill (32%). In the other study, the differentiation in response rates between nefazodone and placebo was not statistically significant. This result may be explained by the “high” rate of spontaneous improvement among the patients randomized to placebo.
Studies of “Relapse Prevention in Patients Recently Recovered (Clinically) from Depression”
Two studies were conducted to assess nefazodone’s capacity to maintain a clinical remission in acutely depressed patients who were judged to have responded adequately (HDRS total score ≤ 10) after a 16 week period of open treatment with nefazodone (titration up to 600 mg/day). In one study, nefazodone was superior to placebo. In this study, patients (n = 131) were randomized to continuation on nefazodone or placebo for an additional 36 weeks (1 year total). This study demonstrated a significantly lower relapse rate (HDRS total score ≥ 18) for patients taking nefazodone compared to those on placebo. The second study was of appropriate design and power, but the sample of patients admitted for evaluation did not suffer relapses at a high enough incidence to provide a meaningful test of nefazodone’s efficacy for this use.
Comparisons of Clinical Trial Results
Highly variable results have been seen in the clinical development of all antidepressant drugs. Furthermore, in those circumstances when the drugs have not been studied in the same controlled clinical trial(s), comparisons among the findings of studies evaluating the effectiveness of different antidepressant drug products are inherently unreliable. Because conditions of testing (e.g., patient samples, investigators, doses of the treatments administered and compared, outcome measures, etc.) vary among trials, it is virtually impossible to distinguish a difference in drug effect from a difference due to one or more of the confounding factors just enumerated.