Clinical Efficacy

Clinical studies with antidepressants invariably involve self-reporting of symptoms using standardized questionnaires including the Hamilton Depression Rating (HAM-D) scale for depression (17 or 21 items of a 23-item scale). A positive NCE treatment usually registers at greater than 50% in the baseline HAM-D score on a 17- or 21-item scale. The Montgomery-Asberg Depression Rating Scale (MADRAS) is a five-item scale that is used to identify anxiolytic-like activity, and is gaining prominence in the US and Europe. Whichever rating scale is used, a wealth of clinical data exist to show that all current antidepressants seem to be effective in 20-70% of those treated, with placebo responses occurring in 30-50% of treated individuals.32 Thus, the major issue with the current clinical instruments and trial design is that the overall efficacy of antidepressants may be less than 50%. Retrospective analysis of completed antidepressant trials has revealed that four out of six trials do not differentiate from placebo.32

The limitations and challenges of antidepressant clinical trials are well documented and relate to several inherent variables; these include the spontaneous remission observed in the length of the normal 6- to 8-week clinical trial and the power of placebo in these studies. The different phenotypes of depression, ranging from mild to severe forms of the illness, add to the 'noise' of the trial. To meet regulatory requirements and approval in the US, Europe, and Japan, large clinical trials are required with at least 2500 patients (at a cost of around $15 000/patient based on 2005 figures). As long as the Regulators' requirements are for a double-blind, placebo-controlled trial with a positive arm, the high cost to risk ratio of such studies is driving many pharmaceutical companies to seek alternative clinical assessment strategies, for example, the seminal work by Kahn and co-workers on 'fixed versus flexible' dose design and to engage in continuous phenotypic refinement of trial populations to determine patient subsets (stratification) that will improve efficacy scale ratings.35

Consensus documents agree on the use of DSM-IV-TR criteria (Table 1) providing guidelines to improve the management and outcome measures of antidepressant and bipolar depression trials in the future. Efficacy varies little between classes of antidepressants, and the advantages of the newer compounds such as SSRIs are based on their improved side-effect profile rather than their antidepressant efficacy (Table 7).2-5

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