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|Clinical Pharmacology of SSRI's
4 - How SSRIs as a Group Differ From TCAs
The goal behind the rational development of selective serotonin reuptake inhibitors (SSRIs) is to:
This development plan has succeeded relative to tricyclic antidepressants (TCAs). Hence, SSRIs, for many physicians, have replaced TCAs as antidepressants of first choice in the treatment of patients with major depression. They also have increased the likelihood that a patient with symptoms of major depression will receive a trial of an antidepressant at an optimal dose. The reason for the increased likelihood of a trial of antidepressant is that physicians are more comfortable giving an empirical trial of SSRIs than a trial of TCAs due to their wider safety margin and better tolerability. The likelihood that an optimal dose will be used is that there is no need to titrate the dose of SSRIs for most patients. The starting dose is the optimal for most patients. Dose increases, in general, serve little purpose.
The differences in the clinical pharmacology of TCAs versus SSRIs will be reviewed in this chapter in terms of the STEPS analysis developed by the author (Table 4.1).
Table 4.2 summarizes the clinically meaningful differences between TCAs and SSRIs in terms of safety and tolerability. Due to the fact that SSRIs do not inhibit fast sodium channels, there is essentially no risk of lethality, even with a substantial overdose of these medications. In contrast, overdoses of a TCA of as little as 5 times the daily dose can be lethal.225 This issue is important because of the delay between starting an antidepressant of any class and achieving a meaningful improvement in the depressive syndrome.
The risk of a suicide attempt is a serious concern during this delayed onset interval. For many years, TCAs have been a leading cause of death due to drug overdose because of their serious toxicity profile, coupled with the fact that they are given to patients at risk for making serious suicide attempts.52,97,165 Because SSRIs avoid this problem, the prognosis of patients suffering from this condition has significantly improved.
It used to be axiomatic to warn patients about the dangers of drinking alcohol while taking antidepressants. While it is still wise to advise patients against drinking alcohol when clinically depressed, the reason is no longer because the antidepressant will necessarily cause serious potentiation of the central nervous system (CNS) depressant effects of alcohol and other CNS depressants such as benzodiazepines. Such potentiation occurs when TCAs and alcohol are taken together due to the antihistaminic effects of TCAs, particularly tertiary amine TCAs. Since SSRIs have been designed to avoid blocking the histamine receptor, they do not pharmacodynamically potentiate the effect of alcohol or other CNS depressants.76,123,155,251 Instead, they either do nothing or may mildly antagonize the depressant effects of alcohol.76,123,155,251 This change is just one example of the reduced risk of serious pharmacodynamic drug interactions due to the more focused pharmacology of SSRIs. Parenthetically, fluvoxamine and fluoxetine can pharmacokinetically potentiate the effects of 2-keto and triazolo benzodiazepines, such as diazepam and alprazolam respectively, by inhibiting the cytochrome P450 (CYP) enzymes responsible for their clearance (see Section 8).
The avoidance of pharmacodynamically mediated drug-drug interactions is important because of the length of time that patients may be on antidepressants to prevent relapse or recurrence of depressive illness. As discussed in Section 8, antidepressants are frequently used in combination with other drugs for a variety of reasons:
In addition, patients may drink alcohol socially while taking an antidepressant and then try to drive home. Alternatively, they may take "over-the-counter" (OTC) preparations and have an interaction (ie, taking an OTC drug that has sedative properties). If they are taking a tertiary amine TCA and drink alcohol or take such an OTC product, they may experience serious potentiation of sedative effects that may be dangerous, particularly if they are in a situation where they need to be mentally alert with good reaction time and coordination.
Due to the multiple pharmacodynamic effects of TCAs, there are multiple ways that they can interact with other types of drugs beyond simply the potentiation of CNS depressants. Since TCAs block a1-adrenergic receptors, they lower peripheral resistance.103 If the patient is also taking other medications that lower blood pressure (eg, diuretics, ß-blockers), they may experience a marked potentiation of the orthostatic hypotension that can occur with drugs that block a1-adrenergic receptors, that can have serious consequences in terms of falls and resultant trauma, particularly in the elderly.236 Since SSRIs have been also designed to avoid blocking the a1-adrenergic receptor, they do not potentiate the effects of concomitantly prescribed antihypertensive medications, in contrast to TCAs. This is another example of the reduced risk of pharmacodynamically mediated drug interactions as a result of the more focused pharmacology of SSRIs.
Since TCAs block the muscarinic acetylcholine receptor, they can have additive effects with other drugs that also block this receptor or which affect gastrointestinal tract motility through other actions. Since TCAs inhibit fast sodium channels, they can potentiate the effects of other drugs which affect intracardiac conduction. Such drugs include a variety of antiarrhythmics, calcium channel blockers, and B-blockers. SSRIs, because of their more focused pharmacology, have a much more limited range of pharmacodynamically mediated drug-drug interactions than do TCAs.
|TABLE 4.3 — Placebo-adjusted Incidence Rate (%) of Frequent Adverse Effects on Imipramine*
|* n = 367, placebo = 672
|Placebo-adjusted = incidence on drug minus incidence on parallel, placebo group in a double-blind, randomly assigned clinical trial.
|Data from reference: 211
Another important difference between SSRIs and TCAs is a better overall tolerability profile in terms of a lower incidence of both nuisance and serious adverse effects. SSRIs affect fewer sites of action (SOAs) and hence cause fewer types of adverse effects. Table 4.3 shows the relative incidence of adverse effects on imipramine as a representative TCA. Imipramine was chosen because it has historically been and still is one of the most commonly used TCAs. Table 5.2 shows the relative incidence of the same adverse effects on 4 of the 5 SSRIs. A comparison of these 2 tables reveals that imipramine in comparison to the SSRIs is associated with a considerably higher incidence of adverse effects mediated by the blockade of specific neuroreceptors such as muscarinic acetylcholine receptors (eg, dry mouth, constipation) and a1-adrenergic receptors (eg, dizziness). In contrast, SSRIs as a group have a higher incidence of adverse effects mediated by the indirect potentiation of serotonin via the inhibition of its uptake pump (eg, nausea). Figure 4.1 provides a visual representation of the same phenomena using amitriptyline as another representative tertiary amine TCA and sertraline as a representative SSRI.
While these adverse effects are less dramatic than the potentially life-threatening toxicity problems that can occur while taking TCAs, they can have serious consequences. The orthostatic hypotension that can occur on TCAs may cause falls with resultant trauma.236 The chronic anticholinergic effects can lead the patient to discontinue treatment during the maintenance phase of treatment and thus increase the risk of relapse. In clinical studies, the discontinuation of tertiary amine TCAs such as imipramine can be 3 times higher than the discontinuation rate on an SSRI (eg, 22% versus 7% respectively).211
While rational drug development reduces the risk of safety and tolerability problems with SSRIs versus TCAs, it does not reduce the relative antidepressant efficacy of the SSRIs. As a group, they are as effective as TCAs in the treatment of outpatients with major depression. This conclusion is supported by a meta-analysis of the double-blind clinical trials of TCAs and SSRIs versus placebo and the double-blind trials directly comparing the antidepressant efficacy of SSRIs and TCAs (Table 4.4). In this table, efficacy is presented as response rate on each treatment after a 6-week treatment trial. A responder is defined as a patient who experienced at least a 50% reduction in the depressive symptomatology as assessed by a standardized assessment instrument, such as the Hamilton Depression Rating Scale (HDRS).
The conclusion that TCAs and SSRIs have comparable antidepressant efficacy in this group of patients is based on the fact that they both produce overall response rates of 60% to 65% and that these rates are comparably superior to those achieved in a double-blind, parallel, placebo-treated, control group. Both the SSRIs and the TCAs produce a 25% to 30% higher response rate than placebo. This difference between drug and placebo is also statistically superior to a comparable degree.
Nonetheless, these results do not necessarily mean that exactly the same patients respond to SSRIs and TCAs. Some investigators have suggested that TCAs may work better in patients who are hospitalized for major depression.69,70,244 That opinion is based on a few double-blind, active, controlled studies. However, this issue remains controversial and is not presently resolved. There are also studies showing that up to 50% patients who fail to respond to desipramine or a similar TCA will respond to an SSRI and vice versa.1,2,81,124,164,190,191
That finding further suggests that the spectrum of antidepressant activity by these 2 classes of drugs is not identical. However, considerable work needs to be done to confirm or reject these hypotheses. Nonetheless, current data do suggest that it is clinically reasonable to give a patient who has failed on an SSRI a trial of a TCA or related drug as well as vice versa.
This topic is clearly important and is part of the STEPS analysis.49 However, a full discussion is beyond the scope of this book, given its focus on clinical pharmacology. Nonetheless, the clinical pharmacology of SSRIs versus TCAs must be considered when analyzing the issue of payment or cost-effectiveness of these different classes of antidepressants. The acquisition cost of SSRIs is higher than that of TCAs but that is only one part of the cost-effectiveness equation. To be meaningful, a cost-effectiveness analysis must be much more comprehensive and consider such factors as:
Simplicity refers to how easy it is for the physician to prescribe the optimal dose and for the patient to take it. One advantage shared by both TCAs and SSRIs is that they can generally be taken once a day and be effective. Other than this shared feature, optimal dosing with TCAs is often more problematic than with SSRIs.
Traditionally, treatment with TCAs is begun at what is usually a subtherapeutic dose and gradually titrated upward to an effective antidepressant dose. This approach is taken so that the patient can develop some tolerance to the adverse effects caused by these drugs due to their ability to block specific neuroreceptors as discussed earlier in this section. In contrast, SSRIs as a class can usually be started at the effective dose from the beginning. As discussed in the next section, there is generally no need to titrate the dose of the SSRIs upward in most patients.
With TCAs, the physician can use therapeutic drug monitoring (TDM) to ensure that the patient is achieving plasma drug concentration within a range associated with the optimal antidepressant response with the minimum risk of adverse effects in most patients.221 This is an advantage from a clinical pharmacology perspective, but is often viewed as a cumbersome disadvantage from a clinical practice standpoint. As discussed in the next section, TDM can also be used with SSRIs for the same purpose, although it is rarely done.
The difference between TCAs and SSRIs with regard to TDM is that it is a standard of care issue with TCAs, but not with SSRIs, due to the difference in their therapeutic indexes (ie, the difference between a dose that is therapeutic and one that is toxic). Patients who are slow metabolizers of TCAs can develop potentially toxic concentrations on conventional antidepressant doses of these medications because of the multiple SOAs affected by TCAs over their clinically relevant pha rmacological range. If the patient slowly clears these drugs either due to a genetically- or environmentally-induced deficiency in the cytochrome P450 enzymes responsible for their biotransformation and eventual elimination, they can develop concentrations that affect fast sodium channels and, hence, delay intracardiac conduction. Sufficient slowing can lead to conduction delays and set the stage for potentially life-threatening cardiac arrhythmias even though the patient is taking what is normally a therapeutic dose. The avoidance of potentially toxic concentrations is the primary reason for using TDM with TCAs.221