Depression is one of the world's great public health problems. As there are no perfect or uniformly effective treatments for depression, it is not surprising that treatment-resistant depression (TRD) is likewise an important public health problem. Although the potential benefits of antidepressants are now well documented, no widely used antidepressant can be expected to be effective in more than half the patients who begin to take it. Even under optimal circumstances (ie, a patient who is fully adherent to 12 weeks of treatment), there is only a 60% to 70% chance that the first choice of medication will be effective. The need for effective alternate strategies for TRD, as well as the need for innovations in service delivery systems to ensure those strategies are implemented in a timely manner, are foremost to fully realizing the potential benefits of antidepressant therapies.

Over the years, hierarchies of treatment strategies for TRD have been based on the widespread use of particular treatments, their ease of use, and their safety or complexity. One of the strategies consistently used since its introduction 20 years ago has been augmentation of the ineffective antidepressant by a second medication. The second agent may or may not have antidepressant effects of its own, but when used in combination with a primary antidepressant the agent reliably increases a patient's likelihood of response and symptom remission.

Augmentation strategies have become popular in patients with a partial response or residual symptoms. In a survey conducted by the American Society of Consultant Pharmacists, 65% of 169 psychiatrists indicated that they would augment treatment as the next step in a partial responder in contrast to non-responders in whom they would switch to another drug (J.C. Nelson, unpublished data). This decision is based on common sense and practicality rather than empirical evidence. Many clinicians and patients think that if a patient achieves at least a partial response to a medication, it makes sense to continue that drug and add a second. To some extent, the patient preferences for treatment shown in the Systematic Treatment Alternatives to Relieve Depression (STAR*D) study reflect this approach. This discussion will update clinicians on the use of augmentation agents for the treatment of major depressive disorder (MDD).

Lithium augmentation has been used since it was first described by de Montigny and colleagues in 1981. This strategy is based on a rational neurochemical hypothesis that lithium has a synergistic effect when added to a tricyclic antidepressant (TCA). Lithium increases serotonin turnover and the TCA increases postsynaptic serotonin receptor sensitivity. Recent debates have focused on whether lithium is as effective combined with selective serotonin reuptake inhibitors (SSRIs) as it is combined with a TCA. The rapid effects sometimes observed with lithium augmentation of TCAs does not seem to occur with SSRIs, conceivably because SSRIs do not increase postsynaptic serotonin receptor sensitivity.

There is mounting evidence to suggest that the efficacy of all available antidepressants when used as monotherapy to treat major depressive disorder (MDD) is, at best, modest. For example, a meta-analysis of all double-blind placebo-controlled studies of antidepressants published since 1980 revealed response rates of 53% for antidepressants versus 36% for placebo (difference in response rate of 16.8%) (Slide 1). To make matters worse, if one is to assume that “negative trials” (ie, trials which do not demonstrate the superiority of a drug over placebo) are less likely to be published than “positive trials” (trials which demonstrate the superiority of a drug versus placebo), it is quite possible that the margin of efficacy of antidepressants when compared to placebo is ≤16.8%. Thus, if one were to include all unpublished along with published double-blind, placebo-controlled trials of antidepressants for MDD, this efficacy margin could be ≤10%.

In spite of the tremendous advances made in developing antidepressant treatments and exploring augmentation with second-generation antipsychotics (SGAs), significant obstacles remain for psychiatrists: How should clinicians make use of cutting-edge augmentation studies? How should they use the antipsychotics in their treatment algorithm? How should they think about dosing and side effects?

The first major problem is there have been no great algorithmic studies on adjunctive strategies. In fact, the most useful algorithmic study was the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) study, from which SGAs were entirely absent. The second problem is that STAR*D showed a clear pattern in which the efficacy of an augmentation agent depended on its placement in the algorithm. Those agents occurring later in the algorithm, such as monoamine oxidase inhibitors (MAOIs), adjunctive triiodothyronine (T3), and adjunctive lithium, appeared to be less effective than adjunctive buspirone, which occurred earlier in the algorithm. This was not the result of a direct comparison. Because of these two problems, it is therefore difficult to determine how to rank the efficacy of SGAs among the agents investigated by the STAR*D study. Among clinical treatment strategies, SGAs might appear toward the end of first-line strategies, perhaps after combinations and stimulants but before buspirone.