CNS drug development: lessons from the development of ondansetron, aprepitant, ramelteon, varenicline, lorcaserin, and suvorexant. Part I.

This column is the first in a two-part series exploring lessons for psychiatric drug development that can be learned from the development of six central nervous system drugs with novel mechanisms of action over the past 25 years. Part 1 presents a brief overview of the neuroscience that supported the development of each drug, including the rationale for selecting a) the target, which in each case was a receptor for a specific neurotransmitter system, and b) the indication, which was based on an understanding of the role that target played in a specific neural circuit in the brain. The neurotransmitter systems on which the development of these agents were based included serotonin for ondansetron and lorcaserin, dopamine for varenicline, substance P (or neurokinin) for aprepitant, melatonin for ramelteon, and orexin for suvorexant. The indications were chemotherapy-induced nausea and vomiting for ondansetron and aprepitant, smoking cessation for varenicline, weight loss for lorcaserin, and insomnia for suvorexant and ramelteon.

History and evolution of the monoamine hypothesis of depression.

The symptoms of depression can be improved by agents that act by various mechanisms to increase synaptic concentrations of monoamines. This finding led to the adoption of the monoamine hypothesis of depression, first put forward over 30 years ago, which proposes that the underlying biological or neuroanatomical basis for depression is a deficiency of central noradrenergic and/or serotonergic systems and that targeting this neuronal lesion with an antidepressant would tend to restore normal function in depressed patients. The hypothesis has enjoyed considerable support, since it attempts to provide a pathophysiologic explanation of the actions of antidepressants. However, in its original form it is clearly inadequate, as it does not provide a complete explanation for the actions of antidepressants, and the pathophysiology of depression itself remains unknown. The hypothesis has evolved over the years to include, for example, adaptive changes in receptors to explain why there should be only a gradual clinical response to antidepressant treatment when the increase in availability of monoamines is rapid. Still, the monoamine hypothesis does not address key issues such as why antidepressants are also effective in other disorders such as panic disorder, obsessive-compulsive disorder, and bulimia, or why all drugs that enhance serotonergic or noradrenergic transmission are not necessarily effective in depression. Despite these limitations, however, it is clear that the development of the monoamine hypothesis has been of great importance in understanding depression and in the development of safe and effective pharmacologic agents for its treatment.