Dual action antidepressants and some important considerations

Emerging treatments for major depression

Antidepressant drugs were introduced into clinical practice in the mid-20th Century. While for the most part they have proven effective for the amelioration of depressive symptoms, they are associated with significant deficiencies. These well-recognized shortcomings have given impetus to the pursuit of new molecules that seek to improve on the efficacy, tolerability and safety of existing medications. The following article reviews several new compounds that may have antidepressant potential. Some are more advanced in development, having undergone clinical trials, whereas the clinical potential of others is yet to be explored. For this latter group of compounds, the antidepressant potential relies on their activity in validated animal models. Agomelatine and duloxetine are in the first category, having shown antidepressant efficacy in clinical trials. The blockade of cortisol secretion continues to be a focus of attention for the development of new antidepressants. Thus, synthesis inhibitors, nonpeptide antagonists of corticotropin-releasing factor and glucocorticoid receptor antagonists show some promise in clinical and preclinical tests. Antagonists of the neuropeptide substance P, vasopressin and neuropeptide Y represent a departure of approach from traditional monoamine receptor-based mechanisms. While the clinical results with one substance P antagonist have led to the cessation of further trials, other molecules are in development. Approaches to treatment based on glutamatergic transmission arose from observations in animal models. The clinical evaluation of such compounds awaits further development. The extent to which new agents can be judged to have met the goals of efficacy, tolerability and safety rely not only acute treatment trials but also on longer-term outcomes and postmarketing surveillance. Whether any of the new agents canvassed here prove to be significantly better than existing agents is clearly a judgement for the future.

Megestrol acetate and mirtazapine for the treatment of unplanned weight loss in the elderly

A rising concern among clinicians is treatment of unplanned weight loss in the elderly, especially given the predicted growth of this population over the next few decades. Unexpected weight loss in the geriatric patient worsens overall health outcomes. A variety of pharmacotherapeutic options are available for treatment; however, evidence underlying their use is limited, and none has gained approval from the United States Food and Drug Administration for this indication. At present, no guidelines support the choice of one agent over another. Although several drug interventions have been employed for this problem, megestrol acetate and mirtazapine are becoming increasingly used for appetite stimulation. These drugs represent two feasible options for geriatric patients because of their generally favorable adverse-effect profiles and few drug interactions, but they are often misused. In a comprehensive search of the MEDLINE and International Pharmaceutical Abstracts databases, we identified all published reports on the use of megestrol acetate or mirtazapine for the treatment of weight loss and on any adverse events associated with these drugs. Special emphasis was placed on trials performed in an elderly population. Results were conflicting, most likely because of differing study designs and small numbers of patients. Megestrol acetate and mirtazapine appear to be effective for appetite stimulation and weight gain in some settings. However, applicability of the data to elderly individuals is unclear, and adverse events reported in a few of the trials and in case reports were not benign. Therefore, the use of megestrol acetate or mirtazapine for weight loss should be thoroughly evaluated on an individual basis. Pharmacotherapy should be used only after all underlying causes of weight loss are assessed and treated.

Tricyclic antidepressant pharmacology and therapeutic drug interactions updated

New data on the pharmacology of tricyclic antidepressants (TCAs), their affinities for human cloned CNS receptors and their cytochrome P450 enzyme inhibition profiles, allow improved deductions concerning their effects and interactions and indicate which of the TCAs are the most useful. The relative toxicity of TCAs continues to be more precisely defined, as do TCA interactions with selective serotonin reuptake inhibitors (SSRIs). TCA interactions with monoamine oxidase inhibitors (MAOIs) have been, historically, an uncertain and difficult question, but are now well understood, although this is not reflected in the literature. The data indicate that nortriptyline and desipramine have the most pharmacologically desirable characteristics as noradrenaline reuptake inhibitors (NRIs), and as drugs with few interactions that are also safe when coadministered with either MAOIs or SSRIs. Clomipramine is the only available antidepressant drug that has good evidence of clinically relevant serotonin and noradrenaline reuptake inhibition (SNRI). These data assist drug selection for monotherapy and combination therapy and predict reliably how and why pharmacodynamic and pharmacokinetic interactions occur. In comparison, two newer drugs proposed to have SNRI properties, duloxetine and venlafaxine, may have insufficient NRI potency to be effective SNRIs. Combinations such as sertraline and nortriptyline may therefore offer advantages over drugs like venlafaxine that have fixed ratios of SRI/NRI effects that are not ideal. However, no TCA/SSRI combination is sufficiently safe to be universally applicable without expert knowledge. Standard texts (e.g. the British National Formulary) and treatment guidelines would benefit by taking account of these new data and understandings.

A review of serotonin toxicity data: implications for the mechanisms of antidepressant drug action

Data now exist from which an accurate definition for serotonin toxicity (ST), or serotonin syndrome, has been developed; this has also lead to precise, validated decision rules for diagnosis. The spectrum concept formulates ST as a continuum of serotonergic effects, mediated by the degree of elevation of intrasynaptic serotonin. This progresses from side effects through to toxicity; the concept emphasizes that it is a form of poisoning, not an idiosyncratic reaction. Observations of the degree of ST precipitated by overdoses of different classes of drugs can elucidate mechanisms and potency of drug actions. There is now sufficient pharmacological data on some drugs to enable a prediction of which ones will be at risk of precipitating ST, either by themselves or in combinations with other drugs. This indicates that some antidepressant drugs, presently thought to have serotonergic effects in animals, do not exhibit such effects in humans. Mirtazapine is unable to precipitate serotonin toxicity in overdose or to cause serotonin toxicity when mixed with monoamine oxidase inhibitors, and moclobemide is unable to precipitate serotonin toxicity in overdose. Tricyclic antidepressants (other than clomipramine and imipramine) do not precipitate serotonin toxicity and might not elevate serotonin or have a dual action, as has been assumed.

A systematic review of the serotonergic effects of mirtazapine in humans: implications for its dual action status

A systematic review of published work concerning mirtazapine was undertaken to assess possible evidence of serotonergic effects or serotonin toxicity (ST) in humans, because drug toxicity and interaction data from human over-doses is an useful source of information about the nature and potency of drug effects. There is a paucity of evidence for mirtazapine having effects on any indicator of serotonin elevation, which leads to an emphasis on ST as an important line of evidence. Mirtazapine is compared with its analogue mianserin, and other serotonergic drugs. Although mirtazapine is referred to as a dual-action 'noradrenergic and specific serotonergic drug' (NaSSA) little evidence to support that idea exists, except from initial microdialysis studies in animals showing small effects; those have not subsequently been replicated or substantiated by independent researchers. Also, new data indicate its affinity for Alpha 2 adrenoceptors is not different to mianserin. It appears to exhibit no serotonergic symptoms or toxicity in over-dose by itself, nor is there evidence that it precipitates ST in combination with monoamine oxidase inhibitors, as would be expected if it raises intra-synaptic serotonin levels. Mirtazapine has no demonstrable serotonergic effects in humans and there is insufficient evidence to designate it as a dual-action drug.