Paroxetine specificity

TCAs, noradrenaline and specific SSRIs SNRIs, ECT, TMS combination SSRIs, SNRIs, mirtazapine nefazodone, TCAs Paroxetine... 

A breakthrough in the treatment of major depression was the discovery of fluoxetine, marketed as Prozac. Fluoxetine has a mechanism of action similar to that of imipramine with an important exception. It is a selective serotonin reuptake inhibitor, an SSRI. This strongly suggests that, in some sense, the symptoms of major depression result from a deficit in serotonin specifically. By inhibiting its reuptake from the synapse, the activity of serotonin is enhanced. Two other important drugs for major depression, sertraline (Zoloft) and paroxetine (Paxil), among several others,... 

Serotonin-specific inhibitors (SSRI) include fluoxetine, paroxetine, sertraline, citalopram and others. They are not more effective than the tricyclic antidepressants but may suit some patients better and are generally safer in overdose (see Geddes et al., 1999). While the SSRIs are devoid of the cardiac effects (membrane stabilisation, inhibition of conduction) of the tricyclics in overdose, they increase the risk of hemorrhage into the gut or brain. 

Several SSRls have been PDA approved for one or more specific anxiety diagnoses [e.g., paroxetine for social anxiety, generalized anxiety disorder (GAD)... 

Benjamin J, Ben-Zion IZ, Karbofsky E, Dannon P (2000) Double-blind placebo-controlled pilot study of paroxetine for specific phobia. Psychopharmacology (Berl) 149 194-196 Berlant J, van Kammen DP (2002) Open-label topiramate as primary or adjunctive therapy in chronic civilian posttraumatic stress disorder a preliminary report. J Clin Psychiatry 63 15-20... 

Pare C, Rees L, Saisbmy M (1962) Differentiation oftwo genetically specific types of depression by the response to antidepressants. Lancet 29 1340-1343 Pollock BG, Ferrell RE, Mulsant BH, et al (2000) Allelic variation in the serotonin transporter promoter affects onset of paroxetine treatment response in late-life depression. Neuropsychopharmacology 23 587-590... 

In general, the SSRIs have a more tolerable side effect profile than the tricyclic antidepressants with their anticholinergic effects. Review of the rate that subjects in the controlled studies discontinued a SSRI because of adverse effects provides some perspective on how well tolerated the medications are, although the specifics may vary according to dosage and design (e.g., forced titration) and are not directly comparable. The rate of discontinuation was reported to be 12% (4/48) for fluoxetine (Emslie et ah, 1997), 9.7% for paroxetine (Keller et ah, 2001), 13% (12/92) for sertraline (March et ah, 1998), and 33% (19/57) (Riddle et ah, 2001) and 7.9% (5/63) for fluvoxamine (Walkup et ah, 2001). 

All SSRIs (e.g., Feonard et ah, 1997) and in particular fluoxetine, Fluvosamine and paroxetine are metabolized by hepatic cytochrome P450 enzymes. Therefore, it is important to be aware of the possibility that the therapeutic or toxic effects of other medications metabolized by the cytochrome P450 isoenzyme system can be increased. Substantial inhibition of these isoenzymes converts a normal metabolizer into a slow metabolizer with regard to this specific pathway. Inhibition of the hepatic oxidative isoenzymes has been associated with a reduction, to a varying extent, in the clearance of many therapeutic agents, including the TCAs, several neuroleptics, antiarrhythmics, theophy-lene, terfenadine, benzodiazepines, carbamazepine, and warfarin (for a complete list, see Nemeroff et ak, 1996). 

Another approach to correct neurotransmission is to inhibit the reuptake of the neurotransmitters into their presvnaptic endings. If the presynaptic reuptake mechanism of a neurotransmitter is blocked then more of the neurotransmitter will stay in the synaptic cleft and be functionally available. Many antidepressant drugs, called reuptake inhibitors , are thought to act via this mechanism. If selective for serotonin they are called selective serotonin reuptake inhibitors (SSRIs, Chapter 1), but if selective for both serotonin and noradrenaline they are called serotonin noradrenaline reuptake inhibitors (SNRIs). Most older antidepressants, such as the tricyclic compounds amitriptyline, imipramine and clomipramine, have little specificity for any of the neurotransmitters fluoxetine, paroxetine, citalopram and a few others are specific for serotonin venlafaxine is a representative of the SNRIs. A more recent mixed-uptake inhibitor is mirtazepine, and some similar compounds are about to be launched. 

A larger set of placebo-controlled studies show conclusively that imipramine is also effective for the treatment of panic disorders. Other agents shown to be effective in panic disorders include the SSRIs paroxetine, sertraline, fluvoxamine, fluoxetine and citalopram. Generally, initial treatment of moderate to severe panic disorders may require the initiation of a short course of benzodiazepines e.g. clonazepam (0.5 1 mg twice daily), and an SSRI. The patient will obtain immediate relief from panic attacks with the benzodiazepine whereas the SSRI may take 1 6 weeks to become effective. Once a patient is relieved of initial panic attacks, clonazepam should be tapered and discontinued over several weeks and SSRI therapy continued thereafter. There are no pharmacological treatments available for specific phobias, however controlled trials have shown efficacy for several agents, e.g. phenelzine, moclobemide. clonazepam, alprazolam, fluvoxamine. sertraline and paroxetine in the treatment of social phobia (Roy-Byrne and Cowlev, 2002). 

In contrast to anticonvulsants and alcohol, drugs such as bupropion, fluoxetine, fluvoxamine, nefazodone, quinidine, paroxetine, and some antipsychotics can inhibit specific CYP enzymes (7, 11, 36, 37, 41, 42, 43 and 44). Thus, TCAs, certain BZDs, bupropion, some steroids, and antipsychotics can all have their metabolism inhibited by drugs such as fluoxetine. For example, fluoxetine at 20 mg/day produces on average a 500% increase in the levels of coprescribed drugs which are principally dependent on CYP 2D6 for their clearance. That can lead to serious or even life-threatening toxicity if the drug has a narrow therapeutic index and the dose is not adjusted for the change in clearance caused by the coadministration of fluoxetine. 

A higher percentage of patients would be expected to relapse in the crossover design for two reasons. First, a significant portion of the patients in the crossover study had, in fact, responded specifically to the drug treatment. After a period of stabilization, these patients were randomly reassigned to placebo and thus would be expected to relapse. Second, a basic problem in the crossover design is that withdrawal symptoms can mimic the recurrence of depressive symptoms. That is true for the SSRIs, particularly fluvoxamine and paroxetine, because of their relatively short half-lives. 

Fluvoxamine, fluoxetine, and paroxetine have nonlinear pharmacokinetics, which means that dose increases lead to disproportionately greater increases in plasma drug levels (25). In contrast, citalopram and sertraline have linear pharmacokinetics. For these reasons, dose increases with fluvoxamine, fluoxetine, and paroxetine can lead to greater than proportional increases in concentration-dependent effects such as serotonin-mediated adverse effects (e.g., nausea) and inhibition of specific CYP enzymes. 

Citalopram and paroxetine have the largest age-related change in plasma drug levels in comparison with other SSRIs. Their levels can be up to 100% greater in physically healthy individuals older than 65 years of age versus younger individuals. For this reason, the recommendation is to start these two SSRIs at half the usual dose and adjust upward more slowly in elderly patients. Age-related changes in SSRI plasma levels are important because elderly patients are likely to be on concomitant medications and effects on specific CYP enzymes are concentration dependent. 

As mentioned previously, beyond the unusually long half-life of fluoxetine and norfluoxetine, the other clinically meaningful distinction between the SSRIs is whether they produce substantial inhibition of specific CYP enzyme (Table 7-29). Fluvoxamine, fluoxetine, and paroxetine do, whereas citalopram and sertraline do not. As mentioned earlier, it is doubtful that the first three would be developed or approved for today s market because of their effects on CYP enzymes, which can cause serious and even fatal drug-drug interactions. 

The earliest and unfortunately still one of the commonest treatments of social phobia is self-medication with alcohol. The behaviorally disinhibiting actions of alcohol allow many social phobics to engage in social contacts that would otherwise be impossible. Legitimate therapeutic drugs for social phobia are now being discovered at a fast pace (Fig. 9—7). In fact, one of the SSRIs (paroxetine) already has been formally approved for use in the treatment of social phobia, and several other SSRIs and antidepressants are rapidly accumulating evidence of their efficacies in this condition as well. Specifically, studies of all five SSRIs (paroxetine, fluvoxamine, fluoxetine, sertraline, and citalopram) have indicated their efficacy in social phobia. Currently, SSRIs are considered first-line treatments for social phobia. 

Problems and side effects associated with buspirone include dizziness, headache, nausea, and restlessness. Antidepressants such as paroxetine and venlafaxine also produce a number of side effects (described in Chapter 7) depending on the specific agent. Nonetheless, these newer, nonbenzodiazepine... 

Hence, there is little doubt that antidepressants may be useful as an adjunct in the treatment of patients with chronic pain. Traditional tricyclic medications such as amitriptyline and nortriptyline are often considered the drugs of choice for chronic pain.52 Newer drugs such as the SSRIs (e.g., paroxetine) and SNRIs (e.g., venlafaxine) might also be considered for some patients with fibromyalgia, neuropathies, and other forms of chronic pain.29 Future research should help clarify how specific antidepressants can be used most effectively as part of a comprehensive regimen for treating various types of chronic pain. 

Fluoxetine was the first SSRI to reach general clinical use. Paroxetine and sertraline differ mainly in having shorter half-lives and different potencies as inhibitors of specific P450 isoenzymes. While the SSRIs have not been shown to be more effective overall than prior drugs, they lack many of the toxicities of the tricyclic and heterocyclic antidepressants. Thus, patient acceptance has been high despite adverse effects such as nausea, decreased libido, and even decreased sexual function. 

Inhibitors obtained from in vitro data include a number of compounds with different selectivities and specificity toward this enzyme. Most of them have not been tested for their in vivo effects and some may also inhibit other enzymes (ticlopidine, fluvoxamine, miconazole, nefazodone, paroxetine, etc.). This query did not yield any additional inhibitors. A summary of substrates (including partial ones) and inhibitors is shown in Table 1 and indicates that clopidogrel and ticlopidine, whose effects have been well documented in vivo and in vitro, are effective inhibitors of CYP2B6 and can be used in an in vivo DI program. 

The adverse sexual effects of SSRIs have been reviewed (58). The use of SSRIs is most often associated with delayed ejaculation and absent or delayed orgasm, but reduced desire and arousal have also been reported. Estimates of the prevalence of sexual dysfunction with SSRIs vary from a small percentage to over 80%. Prospective studies that enquire specifically about sexual function have reported the highest figures. Similar sexual disturbances are seen in patients taking SSRIs for the treatment of anxiety disorders (59), showing that SSRI-induced sexual dysfunction is not limited to patients with depression. It is not clear whether the relative incidence of sexual dysfunction differs between the SSRIs, but it is possible that paroxetine carries the highest risk (58). 

A 53-year-old man took paroxetine 40 mg/day and risperidone 6 mg/day, having previously taken lower doses of both. Within 2 hours he developed ataxia, shivering, and tremor. He had profound sweating but was apyrexial, and was confused, with involuntary jerking movements of his limbs. He recovered without specific treatment over the next 2 days. 

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