Venlafaxine action

Atypical Antidepressants. StmcturaHy diverse dmgs such as the tetracyclic mianserin (46) and various bicyclic and tricyclic compounds such as trazodone (47), venlafaxine (48), nefazodone (49), and amfebutamone (50) are atypical antidepressants. The exact mechanism of action is unclear but probably... 

One of these compounds, venlafaxine (licensed in the UK in 1996), is regarded as an inhibitor of both 5-HT and noradrenaline reuptake but this is based on its actions in vitro. At low doses in vivo, it is a more potent inhibitor of 5-HT (Ki 39 nM) than noradrenaline reuptake (K 210 nM). Moreover, its active metabolite, O-demethylven-lafaxine, is a weaker inhibitor of NA reuptake, and has a longer half-life, than its parent compound. However, at high doses, venlafaxine inhibits reuptake of both these monoamines but has negligible activity at muscarinic, Hi-receptors or ai-adrenoceptors and... 

Tricyclic drugs have, as the name implies, a three-ring structure, and interfere with reuptake of norepinephrine and/or serotonin into axon terminals. Tricyclic drugs include imipramine (Tofranil), amitriptyline (Elavil), clomipramine (Anafranil), and nortriptyline (Pamelor, Aventil). Tricyclics have the occasional but unfortunate cardiovascular side effects of arrhythmia and postural hypotension. Newer, nontricyclic antidepressants have been developed that are collectively referred to as SSRIs. These have a potent and selective action on serotonin, and lack the cardiovascular side effects of the tricyclics. These include fluoxetine (Prozac), paroxetine (Paxil), sertraline (Zoloft), and fluvoxamine (Luvox). A fifth SSRI, citalopram (Celexa) has been used in Europe and has recently been approved in the United States. Venlafaxine (Effexor) blocks reuptake of norepinephrine and serotonin, while bupropion (Wellbutrin) acts on both dopamine and norepinephrine. 

The traditional scheme is complicated by the fact that some antidepressants exhibit characteristics of more than one class. For example, clomipramine, a tricyclic antidepressant (TCA) with side effects and toxicity similar to other TCAs, works more like the selective serotonin reuptake inhibitors (SSRls). Similarly, venlafaxine and duloxetine, which are usually grouped with the atypical antidepressants, have a side effect and safety profile comparable to the SSRls. Although a classihcation system based on mechanism of action offers some advantage (see Table 3.7), even this scheme is limited by the fact that antidepressants that work in the same way may have widely divergent side effect and safety profiles. In the following discussion, the traditional classification system is adopted. Although fraught with problems and inconsistencies. 

Venlafaxine (Effexor, Effexor XR). Venlafaxine works by blocking the reuptake of both serotonin and norepinephrine. Because of this dual action, some believe that venlafaxine may be more effective than the SSRIs when treating severe depression. Its side effects and toxicity are similar to the SSRIs with abdominal discomfort, sexual dysfunction, and anxiety being commonly reported. At higher doses, it may mildly elevate blood pressure therefore, blood pressure should be checked periodically. When stopping venlafaxine, serotonin discontinuation symptoms may be especially problematic. Therefore, gradually tapering of the dose every 2-4 weeks is recommended. 

Milnacipran is also a dual-action antidepressant which, like venlafaxine, has been shown to be more effective than the SSRIs in the treatment of severe, hospitalized and suicidally depressed patients. At lower therapeutic doses, milnacipran blocks the noradrenaline transporters and therefore resembles the NRI antidepressants. Higher doses result in the serotonergic component becoming apparent (i.e. an SSRI-like action). The main problem with milnacipram appears to be its lack of linear kinetics with some evidence that it has a U-shaped dose-response curve (Figure 7.3). 

Administer a high therapeutic dose of a "dual action" antidepressant such as venlafaxine or possibly mirtazepine. A discontinuation syndrome may occur if venlafaxine is abruptly withdrawn. The... 

Beyond their action upon SERT and NET, venlafaxine (1), milnacipran (2) and duloxetine (3) are remarkably selective molecules. All three of them have displayed very low in vitro affinity Ki > 3(X)0 nM) for ai- and a2-adrenergic, histamine Hj, muscarinic, and DA D2 receptors, consistent with favorable side-effect profiles. Venlafaxine (1) and duloxetine (3) also have low affinity for a number of serotonergic receptors, and do not inhibit monoamine oxidase A or B. An expanded in vitro receptor profile of >50 receptors and binding sites... 

Venlafaxine and milnacipran are two members of a new class of antidepressants that have selective effects on the reuptake of both serotonin and noradrenaline—serotonin noradrenaline reuptake inhibitors (SNRIs). In theory, based on the findings of B. M. Baron and colleagues [1988 and of J. C. Nelson and colleagues (1991), the combination of these two pharmacological actions should be associated with superior efficacy either in terms of rapid onset of action or extra efficacy at the end of treatment. 

The advantage of venlafaxine in terms of more rapid onset of action is apparently only seen when the drug is escalated rapidly to the high dose. In a recent study comparing venlafaxine with imipramine in inpatients with severe depression, a significant difference was observed between the two active treatments by the second week [Benkert et al. 1996. ... 

TCAs in more serious forms of depression such as melancholic or psychotic depression. Some studies have suggested that the SSRls do not work as well as the TCAs in melancholic depression (Roose et al. 1994]. Likewise, one study has suggested that venlafaxine, a drug with a mechanism of action similar to that of the TCAs, was superior to fluoxetine in the treatment of inpatients with melancholic depression (Clerc et al. 1994]. Still, other metaanalyses have failed to find a difference in the efficacy of SSRls versus TCAs in serious forms of depression [Nierenberg 1994]. Nonetheless, given that most studies have employed TCAs, and some debate exists about the utility of SSRls in severe subtypes, it may be prudent to start with a TCA in most patients until the debate is further resolved. For patients who present a significant suicide risk or who have not been able to tolerate TCAs, the SSRls in combination with a standard antipsychotic appears an effective option. 

Montgomery SA Rapid onset of action of venlafaxine. Int Chn Psychopharmacol 10 (suppl2) 21-27, 1995b... 

Although the efficacy of tricyclic antidepressants in the treatment of unipolar depression is beyond reproach, the side-effect profile of these agents makes them less desirable as first-line therapeutic agents. Introduction of selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine, paroxetine, sertraline, citalopram and fluvoxamine in the past decade has revolutionized the treatment of depression universally. The side-effect profile of SSRIs, such as nausea, diarrhea and sexual dysfunction, is considerably more benign than that of tricyclic drugs. Multiple controlled trials have proven the efficacy of SSRIs vs. placebo (Nemeroff, 1994). Recently, a number of SNRIs (serotonin and noradrenaline reuptake inhibitors) and so-called atypical antidepressants have been marketed that may have additional advantages over SSRIs, such as more rapid onset of action (venlafaxine. mirtazapine) and low sexual side-effect potential ( bupropion, nefazodone). Additionally, it appears that venlafaxine may be more efficacious in cases of treatment-refractory depression (Clerc et al., 1994 Fatemi et al., 1999). Finally, in a recent report (Thase et al., 2001),... 

Treatment of GAD can be undertaken using a number of pharmacological agents. Benzodiazepines have been found to be superior to placebo in several studies and all benzodiazepines appear to be equally effective. However, side effects include sedation, psvchomotor impairment, amnesia and tolerance (Chapter 1). Recent clinical data indicate that SSRIs and SNRIs are effective in the treatment of acute GAD symptoms. Venlafaxine, paroxetine and imipramine have been shown to be effective antianxiety medications in placebo-controlled studies. Case studies also indicate the usefulness of clomipramine, nefazodone, mirtazapine, fluoxetine and fluvoxamine in GAD. Buspirone, a 5-HTla receptor partial agonist, has been shown to be effective in several placebo-controlled, double-blind trials (Roy-Byme and Cowley, 2002). Buspirone has a later onset of action than both benzodiazepines and SSRIs but with the advantage of being non-addictive and non-sedating. 

In contrast to the SSRIs, venlafaxine has an ascending dose-response curve consistent with its sequential, concentration (and hence dose) dependent effects on serotonin and NE uptake pumps (Table 7-9). At 225 mg per day, the magnitude of the antidepressant effect is 50% higher than that seen with the SSRIs. Also consistent with its dual mechanism of action at higher concentrations, venlafaxine at a dose of 225 mg per day produced an antidepressant response in hospitalized patients with melancholia superior to both placebo and fluoxetine (114, US, 116, H7, H8, H9, 120, 121, 122, 123, 124,125, 126,127, 128,129, 130,131, 132,... 

Nefazodone has some unusual pharmacological properties, not only inhibiting serotonin uptake, but also blocking the 5-HT 2a receptor. It may be that the antidepressant effect of serotonin agents is due to mediation of 5-HT transmission in the absence (or even the blockade) of 5-HT2a transmission. As a result of these actions, nefazodone is even more specific in terms of affecting a subtype of serotonin receptor than are the SSRIs or venlafaxine. 

Unlike venlafaxine and the SSRIs, nefazodone in adults is started at 200 mg per day and then requires at least a one time increase to 300 mg per day to achieve an effective dose. Similar to the immediate release formulation of venlafaxine, nefazodone requires twice-a-day administration and may induce a greater response rate with higher doses if the patient fails to respond initially. Thus, like venlafaxine, nefazodone appears to have a dual mechanism of action, although the specific mechanisms involved are different for these two agents. At lower doses, the most potent action of nefazodone is 5-HT 2 blockade, whereas higher concentrations produce greater inhibition of the 5-HT uptake pump ( 254, 255 and 256). 

This antidepressant can interact with other drugs via its two mechanisms of action serotonin and NE uptake inhibition. The former action means that the same pharmacodynamic interactions will occur with venlafaxine as with SSRIs, including the serotonin syndrome. At higher doses, venlafaxine is also prone to the same pharmacodynamic interactions as NSRIs such as secondary amine TCAs like desipramine and with newer NSRIs such reboxetine. Thus, the combination of high-dose venlafaxine plus an MAOl could produce a hypertensive crisis as well as the serotonin syndrome. 

Harvey AT, Rudolph RL, Preskorn SH. Evidence of the dual mechanisms of action of venlafaxine. Arch Gen Psychiatry 2000 57 503-509. 

Other Serotonergic Antidepressants In recent years, new antidepressants with serotonergic actions have also been shown to benefit patients with chronic PTSD, many of whom had been treatment refractory. These drugs include venlafaxine (278), mirtazapine (279), and nefazodone (280). 

The majority of positive studies have found that later generation antidepressants, especially those with strong serotonergic effects, are effective and may have a more rapid onset of action when used for PMDD (27). Thus, studies with SSRIs (e.g., fluoxetine, sertraline), nefazodone, venlafaxine, and clomipramine have all shown promise. In several studies, luteal phase dosing has been as effective or more effective than continuous dosing in women with PMDD. 

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