Trends in the development of new antidepressants. Is there a light at the end of the tunnel?

Since the introduction of tricyclic antidepressants (TCAs) and monoamine oxidase inhibitors (MAOIs) in mid-1950's, treatment of depression has been dominated by monoamine hypotheses. The well-established clinical efficacy of TCAs and MAOIs is due, at least in part, to the enhancement of noradrenergic or serotonergic mechanisms, or to both. Unfortunately, their very broad mechanisms of action also include many unwanted effects related to their potent activity on cholinergic, adrenergic and histaminergic receptors. The introduction of selective serotonin reuptake inhibitors (SSRIs) over twenty years ago had been the next major step in the evolution of antidepressants to develop drugs as effective as the TCAs but of higher safety and tolerability profile. During the past two decades SSRIs (fluoxetine, fluvoxamine, paroxetine, sertraline, citalopram) gained incredible popularity and have become the most widely prescribed medication in the psychiatric practice. The evolution of antidepressants continued resulting in introduction of selective and reversible monoamine oxidase inhibitors (eg. moclobemid), selective noradrenaline (eg. reboxetine), dual noradrenaline and serotonin reuptake inhibitors (milnacipram, venlafaxin, duloxetin) and drugs with distinct neurochemical profiles such as mirtazapine, nefazadone and tianeptine. Different novel serotonin receptor ligands have also been intensively investigated. In spite of the remarkable structural diversity, most currently introduced antidepressants are 'monoamine based'. Furthermore, these newer agents are neither more efficacious nor rapid acting than their predecessors and approximately 30% of the population do not respond to current therapies. By the turn of the new millennium, we are all witnessing a result of innovative developmental strategies based on the better understanding of pathophysiology of depressive disorder. Several truly novel concepts have emerged suggesting that the modulation of neuropeptide (substance P, corticotrophin-releasing factor, neuropeptide Y, vasopressin V1b, melanin-concentrating hormone-1), N-methyl-D-aspartate, nicotinic acetylcholine, dopaminergic, glucocorticoid, delta-opioid, cannabinoid and cytokine receptors, gamma-amino butyric acid (GABA) and intracellular messenger systems, transcription, neuroprotective and neurogenic factors, may provide an entirely new set of potential therapeutic targets, giving hope that further major advances might be anticipated in the treatment of depressive disorder soon. The goal of this review is to give a brief overview of the major advances from monoamine-based treatment strategies, and particularly focus on the new emerging approaches in the treatment of depression.