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Although several antimanic agents are available to treat individuals with bipolar disorder (BD), many patients have a less than satisfactory response or experience adverse effects.1 With the exception of lithium, all of the current antimanic agents are either anticonvulsant or antipsychotic drugs. It is remarkable that no drug has been developed specifically for BD, especially because this illness was conceptualized more than a century ago.
Although several antimanic agents are available to treat individuals with bipolar disorder (BD), many patients have a less than satisfactory response or experience adverse effects.1 With the exception of lithium, all of the current antimanic agents are either anticonvulsant or antipsychotic drugs. It is remarkable that no drug has been developed specifically for BD, especially because this illness was conceptualized more than a century ago. The lack of available therapeutics with a novel mechanism of action results in large part from our modest understanding of the relevant molecular and cellular substrates of this complex emotional, behavioral, and activity disorder.
Different strategies have been proposed for the development of better compounds for patients with psychiatric disorders. It has been argued that the ultimate goal of drug development should be to reduce the prevalence of psychiatric disorders through cure therapeutics and strategic preventive measures. Insel and Scolnick2 have suggested that drug development for mental disorders over the past 50 years has been significantly stalled compared with other areas of medicine and that drastically different approaches are in order.
It is our firm opinion that we are entering an era in which we will be able to develop markedly improved treatments for severe mood disorders. A growing body of data suggests that mood disorders arise from abnormalities in synaptic and neuronal plasticity cascades, leading to aberrant information processing in critical synapses and circuits.
Thus, these illnesses can best be conceptualized as genetically influenced disorders of synapses and circuits rather than simply as deficits or excesses in individual neurotransmitters. In this article, we discuss current approaches to drug development in mood disorders at the Mood and Anxiety Disorders Program of the Intramural Research Program at the National Institute of Mental Health and provide an example of one such approach.
Drug development models
At the Mood and Anxiety Disorders Program, the model of drug development that we are pursuing is based on 2 highly integrated preclinical-clinical pathways:
1. Understanding the long-term, therapeutically relevant targets of the medications currently in use. For example, in the case of SSRIs, instead of studying the initial increases in intrasynaptic serotonin, the goal would be to study the long-term changes in synaptic and neural plasticity that likely underlie their delayed therapeutic effects. Such knowledge can then be used to design new drugs directed at the target(s).
2. Understanding the pathophysiology of the illness and using that knowledge to design therapeutics to attenuate or prevent pathological processes. These may be envisioned as true disease-modifying strategies rather than simply as symptom control.
Because of space limitations, this article focuses on the first approach. For there to be substantial progress in generating novel compounds, susceptibility genes involved in illness need to be identified. However, this alone is insufficient to provide the knowledge necessary to expand our current therapeutic armamentarium. Fundamentally, we need information about the intracellular signaling cascades that are disrupted within the complex set of interacting neuronal networks and the specific changes that occur within these systems as a result of the effective treatments administered in a therapeutically relevant paradigm and in a time sensitive manner.
For example, much could be learned by gathering information at the precise point when there is a change in mood episode poles (switch from depression to mania) in BD or when an effective treatment intervention takes place. The advent of methodologies such as subtractive hybridization, messenger RNA differential display, and microarrays has illustrated the importance of hypothesis-generating by mimicking the conditions of illness or examining when effective treatments act (eg, in an animal model of mania and lithium treatment); this is particularly true when dealing with disorders whose pathophysiology remains elusive.3
Thus, it may be very useful to develop drugs that are based on pertinent intracellular signaling molecular targets of current mood stabilizers; these later could be adapted to optimize efficacy, specificity, and/or adverse-effect profiles. In addition, all current medications have a considerable onset lag before their full therapeutic properties are activated (implying changes in gene expression, protein function, and-more generally-plasticity). Thus, identification of targets after prolonged treatments in cell- and animal-based models may be a useful approach in the development of novel therapeutics.
This approach may increase the likelihood of identifying relevant downstream targets with potentially more potent and rapid actions. Ultimately, novel agents identified in this manner also may benefit patients who are treatment-resistant, because alterations in the intracellular pathways (ie, defects) could simply be bypassed to the end-stage molecular targets that are ultimately relevant to the therapeutic effects. The use of this strategy could provide clues on how to bypass the defect in a number of ways. There is no shortage of targets, and the task of determining the ones that are most therapeutically relevant is challenging.
Our work attempts to focus on those target proteins and pathways with the most evidence for involvement in mood disorders, the greatest relevance to current disease models, or where other medications with the same or similar actions have been developed and could be used for proof-of-concept trials.
An example of drug development for BD: identification of a novel molecular target
The recent identification of biochemical targets of effective treatments has been facilitated by gene and protein expression profiling, which has led to the identification of several hitherto unexpected targets. It is important to note that these methodologies are only a screening technique, and the results need independent validation. More information on understanding the molecular mechanisms presumably involved in mood stabilization through genome-wide gene expression profiling is reviewed elsewhere.3
Because target validation for complex psychiatric disorders is so challenging, the following criteria for selecting mood stabilizers for further study were put forth: (1) corroboration of a target at the protein and functional level; (2) observation with chemically dissimilar but clinically effective agents; (3) occurrence at a dose/plasma level and time frame consistent with clinical therapeutic effect; (4) localization to brain regions implicated in the neurobiology of the disorder under consideration; (5) when known, relevance to pathophysiology; and (6) when possible, tethered to human genetic findings. Application of these stringent criteria led a number of independent groups4,5 to recognize protein kinase C (PKC) as a promising direct biochemical target for developing therapeutics to treat BD.
The mood stabilizers that meet the aforementioned criteria for further testing are lithium (a monovalent cation) and valproate (an 8-carbon, branched fatty acid). Lithium and valproate are structurally dissimilar, but researchers have postulated that their similar therapeutic effects take place in the critical circuits of brain regions implicated in the pathophysiology of BD. The therapeutic effects are specific to these agents and occur at drug concentrations in vivo. The resultant biochemical changes occur only after long-term-but not short-term-administration in a time frame consistent with improvement of symptoms of the illness.
Both lithium and valproate were found to bring about markedly analogous effects on the PKC signaling cascade, actions that appear to be most pertinent to their antimanic profile. The PKC example we will now describe is one of the few instances in which a drug is being developed specifically to treat BD based on an identified molecular target. Indeed, such development has gone from identifying a direct molecular target in 1990 to a positive, proof-of-concept clinical study with a modulator of the relevant target in humans in 2007.
What is PKC?
PKC is a family of 12 closely and structurally related isozyme subspecies with a heterogeneous distribution throughout the body that depends on isoforms.6,7 In the brain, PKC has a varied distribution and plays a significant role in regulating synaptic facets of neurotransmission. A number of additional functions for PKC have been described, including regulation of neuronal excitability, neurotransmitter release, long-term alterations in gene expression and plasticity, and mediation of intracellular signaling pathways.8PKC in mania
Biochemical data support the possible connection between PKC and the pathophysiology and treatment of BD. Friedman and colleagues9 studied PKC activity and PKC translocation in response to serotonin in platelets collected from patients with BD before and during lithium treatment. They found that the ratios of platelets membrane-bound to cyto- solic PKC activity were increased in participants during a manic episode.9
In a postmortem study, the same group10 found increased PKC activity and translocation in the brains of patients with BD compared with controls.
Studies have also shown that PKC signaling pathways are altered after treatment with lithium or valproate.4,9,11-13 Long-term treatment with lithium in rats resulted in a significantly decreased PKC stimulation-induced release with phorbol esters in the cortex, hypothalamus, and hippocampus. PKC isozymes a and e were reduced in the frontal cortex and hippocampus following lithium administration.13 Similarly, valproate was also found to cause an isozyme-specific decrease in PKC a and e.4
Furthermore, studies have shown that stimulants, such as amphetamine, which are capable of triggering manic episodes in susceptible individuals and which induce manic-like behaviors in rodents,14 activate PKC and growth-associated protein (GAP)-43 phosphorylation (implicated in neurotransmitter release).15-18 Given the compelling biochemical data, a number of behavioral studies have been undertaken to validate the role of PKC in mania-related behaviors.
In rodents, the PKC inhibitor tamoxifen significantly reduced amphetamine-induced hyperactivity in a large open field without affecting spontaneous activity, and it normalized amphetamine-induced increases in visits to the center of an open field (representing risk-taking behavior); tamoxifen also attenuated amphetamine-induced phosphorylation of GAP-43.14 PKC activity is increased in the prefrontal cortex following exposure, and the ability of the prefrontal cortex to regulate emotion, thought, and action is markedly impaired by overactivity of PKC sig- naling.19 In toto, the evidence suggests that pharmacological or stress-induced activation of PKC in animals results in many of the behavioral changes seen in mania, such as hyperactivity, risk-taking behavior, and increased hedonic drive. Its inhibition attenuates these same behavioral changes in a manner similar to that of mood stabilizers on acute mania.
These data strongly suggested that further study of PKC inhibitors in humans with BD was indicated. When the decision was made to further validate this target in humans, tamoxifen was the only relatively selective PKC inhibitor available to test. Tamoxifen, a synthetic antiestrogen widely used in the treatment of breast cancer, has a favorable adverse-effect profile compared with other chemotherapeutic agents20; considerable data exist on its safety profile and ability to cross the blood-brain barrier.
Finally, the exciting recent identification of a bipolar susceptibility gene that is an upstream regulator of PKC has strengthened the potential role of PKC signaling pathway in the pathophysiology of BD. For example, 2 recent, independent genome-wide association studies identified diacylglycerol (DAG) kinase e (DGKH) as a gene associated with higher risk of BD.21 DGKH is a major regulator of DAG, which activates all known classic and novel isoforms of PKC.
PKC inhibitors: novel therapeutics for acute mania?
Based on the data generated since the 1990s, researchers embarked on proof-of-concept studies with tamoxifen for acute mania. The first study included 7 patients (5 male and 2 female) with acute bipolar mania.22 Participants were inpatients or outpatients aged 18 to 65 years with a diagnosis of bipolar mood disorder manic episode based on a DSM-IV structured clinical interview. Tamoxifen resulted in a significant decrease in manic symptoms as rated by the Young Mania Rating Scale (YMRS), with a mean decrease of 10 points. In addition, 71% of patients met response criteria (ie, a 50% decrease in the YMRS score from baseline).
In a double-blind, placebo-controlled monotherapy pilot trial in patients with bipolar mania, tamoxifen was found to have significant antimanic effects as early as day 5 and throughout the 3 weeks of the trial.23 Very recently, these studies were replicated in a larger placebo-controlled study.24 These studies confirm the hypothesis that directly inhibiting PKC improves manic symptoms. It is important to emphasize that while these findings are encouraging, the results are preliminary and based on fairly small sample sizes; the results need to be confirmed in larger studies involving several hundred participants and with more selective PKC inhibitors.
Nonetheless, this strategy illustrates that it is possible to develop a drug to treat a specific disorder; in this case, the illness was BD, for which no novel mechanism of action had ever been identified and proved in clinical studies.
The evidence to date on multiple levels (ie, preclinical, clinical) supports further study of PKC inhibitors in BD. Other selective PKC inhibitors are currently in phases 1 through 3 of development for treatment of a variety of conditions (eg, diabetic complications) and are possible candidates to test in BD.25 Naturally, issues regarding PKC isoform selectivity, brain penetrance, and short- and long-term tolerability will need to be examined.
Finally, a point that merits further discussion is our need to integrate current technologies with the drug evaluation process in proof-of-concept studies. The importance of identifying endophenotypes in complex neuropsychiatric disorders has been reviewed elsewhere.26 Briefly, endophenotypes can be conceptualized as quantifiable components along the pathway between phenotype of disease and distal genotype. An endophenotype may be biochemical, neuropsychological, endocrinological, neuroanatomical, cognitive, or neurophysiological (eg, seen in magnetic encephalography, polysomnography) in nature.
These new investigational tools are beyond the scope of this review. We note them, however, because they may have considerable utility in predicting phenomena such as time to response, remission, and degree of improvement. Given our current inability to predict who will respond to which medication and within what time frame, the evaluation of characteristics observed using valuable new technologies may provide a better understanding of the neurobiological basis involved in symptom improvement, and it also may allow for the identification of surrogate outcomes and molecular targets for the next generation of treatments. Indeed, such strategies are being increasingly used in our study of complex neuro-behavioral medical illnesses.
Thus, there is now strong evidence to support the view that targeting intracellular signaling cascades is a useful strategy in drug development for BD. The use of modulators of ubiquitous kinases in the CNS, however, is met with some trepidation because of possible problems in specificity, tolerability, and safety. At present, many kinases are in the early stages of drug development for diverse medical illnesses.27 It should be emphasized that while appropriate concerns regarding these compounds are in order, lithium is a drug that targets multiple signaling cascade molecules, has been found to be generally safe, and has been a primary therapeutic used for BD for more than half a century. This latter point illustrates that it may, indeed, be possible to develop safe and effective signaling system modulators for CNS illnesses.
We believe that the strategy of drug development research we have described here will, in due course, result in noticeably improved therapeutics for psychiatric illnesses.
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