Major depression has become a health crisis of epidemic proportions in the modern world. The prevalence of major depression has risen over the last several generations in every country examined, and age of symptom onset has decreased. Currently the fourth leading health burden worldwide, major depression will rank second after cardiac disease as a cause of international medical morbidity by the year 2020. One in six individuals in the United States will experience an episode of major depression in his or her lifetime, and the risk of subsequent episodes rises dramatically once a person has been depressed. Indeed, depression is now recognized to be a highly chronic and recurrent illness. On average, patients with major depression are symptomatic 60% of the time, even when receiving community-standard antidepressant treatment. Recent estimates place the economic burden of depression in the United States at 83 billion dollars a year.

Depression is associated with greater disability than are most other chronic illnesses and is a risk factor for mortality. Suicide ranks among the top ten causes of death in the United States, and best estimates suggest that 60-70% of people who kill themselves are clinically depressed. Between 10-15% of severely depressed people eventually commit suicide. In addition, many studies indicate that depression significantly increases all-cause mortality independently of suicide. Depression predicts the later development of a number of medical conditions, including cardiac and cerebrovascular disease, hypertension,diabetes,obesity and the metabolic syndrome,dementia, and cancer. Depression also markedly increases mortality in patients who are medically ill and has been associated with decreased responses to pharmacological treatments for cancer and hepatitis C.

Unfortunately, most patients with depression do not experience a complete resolution of symptoms with antidepressant treatment and 10-20% of patients are refractory to all currently available modalities, including electroconvulsive shock (ECT) therapy. ECT is often effective in patients who have failed adequate trials of multiple antidepressants, but is associated with the risk of anesthesia and with significant short term memory impairment. Responses to ECT are short-lived, and many patients who respond subsequently relapse, even when on maintenance antidepressants. In addition to efficacy issues, many patients are unable to tolerate side effects associated with antidepressants or ECT. The risks of not responding to (or tolerating) treatment have been highlighted by recent studies documenting that partial-but incomplete-response is associated with an increased risk of full symptomatic relapse (even when on therapy) and a worse long term disease course, as well as with significantly impaired quality of life. Treatment resistance also results in a six times increase in direct health care costs. These factors highlight the tremendous need to identify novel treatment strategies, especially for depressed patients who are unresponsive to conventional therapies.

One possible mechanism that may contribute to treatment resistance is increased proinflammatory cytokine production and release. Several lines of evidence indicate that proinflammatory cytokines participate in the pathophysiology of major depression and may thus represent a novel target for the pharmacological treatment of the disorder. First, a high percentage of patients who receive cytokine therapies (such as interferon-alpha for malignant melanoma or hepatitis C infection) develop depressive symptoms, and many patients meet full criteria for major depression. Interferon-alpha-induced depressive symptoms can be ameliorated by pre-treatment with an antidepressant and respond to antidepressants once they have emerged. Second, many studies report that, as a group, medically healthy patients with depression exhibit elevated measures of proinflammatory cytokines, including tumor necrosis factor (TNF)-alpha, interleukin (IL)-1 and IL-6. Moreover, a positive relationship between serum concentrations of proinflammatory cytokines and severity of depressive symptoms has been recently reported. Third, antidepressants have been shown to have anti-inflammatory activity and may work-at least in part-by reducing inflammatory activity, given evidence that clinical response is associated with reductions in cytokine levels. These data raise the possibility that cytokine antagonists, such as the chimeric anti-TNF-alpha antibody infliximab, might have antidepressant efficacy. Of special relevance to this proposal, patients who are treatment resistant have been shown to exhibit increased inflammatory activity (as reflected by increased plasma concentrations of interleukin \[IL\]-6 and the soluble IL-6 receptor \[sIL-6R\]), suggesting that cytokine antagonists might be especially effective in these patients.

Providing care to patients with inflammatory bowel disease has given us the clear clinical impression that infliximab rapidly improves mood and energy levels in many patients prior to any demonstrable changes in bowel pathology. This impression is in line with a growing body of evidence suggesting that TNF-alpha antagonists improve emotional functioning and fatigue in patients receiving these agents for rheumatoid arthritis and inflammatory bowel disease. These findings in patients with inflammatory diseases are consistent with the notion that TNF-alpha antagonists such as infliximab might provide acute symptomatic relief for medically healthy patients with treatment-resistant major depression and that symptom improvement might result from decreased inflammatory activity. Moreover, medically healthy depressed patients with increased inflammatory activity may be most likely to benefit from anti-TNF-alpha therapy.