Neuropsychiatric Drug Development Research Articles

RNA m6A methyltransferase activator affects anxiety-related behaviours, monoamines and striatal gene expression in the rat.

Modification of mRNA by methylation is involved in post-transcriptional regulation of gene expression by affecting the splicing, transport, stability and translation of mRNA. Methylation of adenosine at N6 (m6A) is one of the most common and important cellular modification occurring in the mRNA of eukaryotes. Evidence that m6A mRNA methylation is involved in regulation of stress response and that its dysregulation may contribute to the pathogenesis of neuropsychiatric disorders is accumulating. We have examined the acute and subchronic (up to 18 days once per day intraperitoneally) effect of the first METTL3/METTL14 activator compound CHMA1004 (methyl-piperazine-2-carboxylate) at two doses (1 and 5 mg/kg) in male and female rats. CHMA1004 had a locomotor activating and anxiolytic-like profile in open field and elevated zero-maze tests. In female rats sucrose consumption and swimming in Porsolt's test were increased. Nevertheless, CHMA1004 did not exhibit strong psychostimulant-like properties: CHMA1004 had no effect on 50-kHz ultrasonic vocalizations except that it reduced the baseline difference between male and female animals, and acute drug treatment had no effect on extracellular dopamine levels in striatum. Subchronic CHMA1004 altered ex vivo catecholamine levels in several brain regions. RNA sequencing of female rat striata after subchronic CHMA1004 treatment revealed changes in the expression of a number of genes linked to dopamine neuron viability, neurodegeneration, depression, anxiety and stress response. Conclusively, the first-in-class METTL3/METTL14 activator compound CHMA1004 increased locomotor activity and elicited anxiolytic-like effects after systemic administration, demonstrating that pharmacological activation of RNA m6A methylation has potential for neuropsychiatric drug development.

Human iPSC-Derived Glia as a Tool for Neuropsychiatric Research and Drug Development.

Neuropsychiatric disorders such as schizophrenia or autism spectrum disorder represent a leading and growing burden on worldwide mental health. Fundamental lack in understanding the underlying pathobiology compromises efficient drug development despite the immense medical need. So far, antipsychotic drugs reduce symptom severity and enhance quality of life, but there is no cure available. On the molecular level, schizophrenia and autism spectrum disorders correlate with compromised neuronal phenotypes. There is increasing evidence that aberrant neuroinflammatory responses of glial cells account for synaptic pathologies through deregulated communication and reciprocal modulation. Consequently, microglia and astrocytes emerge as central targets for anti-inflammatory treatment to preserve organization and homeostasis of the central nervous system. Studying the impact of neuroinflammation in the context of neuropsychiatric disorders is, however, limited by the lack of relevant human cellular test systems that are able to represent the dynamic cellular processes and molecular changes observed in human tissue. Today, patient-derived induced pluripotent stem cells offer the opportunity to study neuroinflammatory mechanisms in vitro that comprise the genetic background of affected patients. In this review, we summarize the major findings of iPSC-based microglia and astrocyte research in the context of neuropsychiatric diseases and highlight the benefit of 2D and 3D co-culture models for the generation of efficient in vitro models for target screening and drug development.

Aged rhesus monkeys: Cognitive performance categorizations and preclinical drug testing

Rodent models have facilitated major discoveries in neurobiology, however, the low success rate of novel medications in clinical trials have led to questions about their translational value in neuropsychiatric drug development research. For age-related disorders of cognition such as Alzheimer’ disease (AD) there is interest in moving beyond transgenic amyloid-β and/or tau-expressing rodent models and focusing more on natural aging and dissociating “healthy” from “pathological” aging to identify new therapeutic targets and treatments. In complex disorders such as AD, it can also be argued that animals with closer neurobiology to humans (e.g., nonhuman primates) should be employed more often particularly in the later phases of drug development. The purpose of the work described here was to evaluate the cognitive capabilities of rhesus monkeys across a wide range of ages in different delayed response tasks, a computerized delayed match to sample (DMTS) task and a manual delayed match to position (DMTP) task. Based on specific performance criteria and comparisons to younger subjects, the older subjects were generally less proficient, however, some performed as well as young subjects, while other aged subjects were markedly impaired. Accordingly, the older subjects could be categorized as aged “cognitively-unimpaired” or aged “cognitively-impaired” with a third group (aged-other) falling in between. Finally, as a proof of principle, we demonstrated using the DMTP task that aged cognitively-impaired monkeys are sensitive to the pro-cognitive effects of a nicotinic acetylcholine receptor (nAChR) partial agonist, encenicline, suggesting that nAChR ligands remain viable as potential treatments for age-related disorders of cognition.

Cross-species neuroscience: closing the explanatory gap.

Neuroscience has seen substantial development in non-invasive methods available for investigating the living human brain. However, these tools are limited to coarse macroscopic measures of neural activity that aggregate the diverse responses of thousands of cells. To access neural activity at the cellular and circuit level, researchers instead rely on invasive recordings in animals. Recent advances in invasive methods now permit large-scale recording and circuit-level manipulations with exquisite spatio-temporal precision. Yet, there has been limited progress in relating these microcircuit measures to complex cognition and behaviour observed in humans. Contemporary neuroscience thus faces an explanatory gap between macroscopic descriptions of the human brain and microscopic descriptions in animal models. To close the explanatory gap, we propose adopting a cross-species approach. Despite dramatic differences in the size of mammalian brains, this approach is broadly justified by preserved homology. Here, we outline a three-armed approach for effective cross-species investigation that highlights the need to translate different measures of neural activity into a common space. We discuss how a cross-species approach has the potential to transform basic neuroscience while also benefiting neuropsychiatric drug development where clinical translation has, to date, seen minimal success.This article is part of the theme issue ‘Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity’.

The mouse as a model for neuropsychiatric drug development

Much has been written about the validity of mice as a preclinical model for brain disorders. Critics cite numerous examples of apparently effective treatments in mouse models that failed in human clinical trials, raising the possibility that the two species' neurobiological differences could explain the high translational failure rate in psychiatry and neurology (neuropsychiatry). However, every stage of translation is plagued by complex problems unrelated to neurobiological conservation. Therefore, although these case studies are intriguing, they cannot alone determine whether these differences observed account for translation failures. Our analysis of the literature indicates that most neuropsychiatric treatments used in humans are at least partially effective in mouse models, suggesting that neurobiological differences are unlikely to be the main cause of neuropsychiatric translation failures.

The Genetics-Driven Revival in Neuropsychiatric Drug Development

Drug development for central nervous system (CNS) disorders has dramatically slowed over the past decade, despite the growing burden attributable to neuropsychiatric disease worldwide (1). The fundamental challenges in developing treatments to target brain-based illnesses include limited understanding of underlying disease pathogenesis and pathophysiology, lack of quantitative disease biomarkers, inherent challenges in developing predictive preclinical models, and the inaccessibility of the brain to direct investigation. Most currently approved classes of psychiatric medications were discovered by serendipity and continue to target the same molecular mechanisms as their prototypes, developed .50 years ago (1). Candidate drugs entering clinical trials for CNS disorders have some of the lowest rates of ultimate regulatory approval, often as a result of lack of clinical efficacy. These candidates have historically been advanced based on performance in rodent models, which may be inherently limited in their ability to represent the complex human cognitive and behavioral processes that are affected by neuropsychiatric disease. Given this challenging atmosphere, a critical question is how the field can move forward to tackle such an enormous public health need. In this issue of Biological Psychiatry, Wendland and Ehlers (2) provide a sagacious framework championing the promise, while emphasizing the challenges, of using genetics to revitalize drug discovery programs in neuroscience. Melding this industry perspective with academic culture is likely necessary to create a common ground for accelerating discovery. The substantial heritability of nearly all neuropsychiatric disorders has been a tantalizing clue that genetics could ultimately resolve disease biology. However, because of a highly complex and generally non-Mendelian genetic architecture, investigators have only recently begun to identify high confidence genetic variants associated with disease risk (3–6). Genetic risk factors take the form of common variants tagged by single nucleotide polymorphisms (SNPs) identified in genome-wide association studies (GWAS), rare coding variants identified with whole-exome or whole-genome sequencing, and rare chromosomal structural variation such as copy number variants. Several recent, high-profile successes identifying genetic risk factors for CNS disorders have instilled much optimism that neurogenetic approaches will ultimately identify novel targets for therapeutic intervention (3–6). Genetics-based approaches help circumvent many of the limitations that may have contributed to previous drug development failures. First, heritability estimates indicate that additive genetic factors contribute substantially to neuropsychiatric disease risk, generally more than shared or nonshared environmental factors. Second, genetics establishes biological

Genetic Pharmacotherapy as an Early CNS Drug Development Strategy: Testing Glutaminase Inhibition for Schizophrenia Treatment in Adult Mice.

Genetic pharmacotherapy is an early drug development strategy for the identification of novel CNS targets in mouse models prior to the development of specific ligands. Here for the first time, we have implemented this strategy to address the potential therapeutic value of a glutamate-based pharmacotherapy for schizophrenia involving inhibition of the glutamate recycling enzyme phosphate-activated glutaminase. Mice constitutively heterozygous for GLS1, the gene encoding glutaminase, manifest a schizophrenia resilience phenotype, a key dimension of which is an attenuated locomotor response to propsychotic amphetamine challenge. If resilience is due to glutaminase deficiency in adulthood, then glutaminase inhibitors should have therapeutic potential. However, this has been difficult to test given the dearth of neuroactive glutaminase inhibitors. So, we used genetic pharmacotherapy to ask whether adult induction of GLS1 heterozygosity would attenuate amphetamine responsiveness. We generated conditional floxGLS1 mice and crossed them with global CAGERT2cre∕+ mice to produce GLS1 iHET mice, susceptible to tamoxifen induction of GLS1 heterozygosity. One month after tamoxifen treatment of adult GLS1 iHET mice, we found a 50% reduction in GLS1 allelic abundance and glutaminase mRNA levels in the brain. While GLS1 iHET mice showed some recombination prior to tamoxifen, there was no impact on mRNA levels. We then asked whether induction of GLS heterozygosity would attenuate the locomotor response to propsychotic amphetamine challenge. Before tamoxifen, control and GLS1 iHET mice did not differ in their response to amphetamine. One month after tamoxifen treatment, amphetamine-induced hyperlocomotion was blocked in GLS1 iHET mice. The block was largely maintained after 5 months. Thus, a genetically induced glutaminase reduction—mimicking pharmacological inhibition—strongly attenuated the response to a propsychotic challenge, suggesting that glutaminase may be a novel target for the pharmacotherapy of schizophrenia. These results demonstrate how genetic pharmacotherapy can be implemented to test a CNS target in advance of the development of specific neuroactive inhibitors. We discuss further the advantages, limitations, and feasibility of the wider application of genetic pharmacotherapy for neuropsychiatric drug development.

Glo1 inhibitors for neuropsychiatric and anti-epileptic drug development

Many current pharmacological treatments for neuropsychiatric disorders, such as anxiety and depression, are limited by a delayed onset of therapeutic effect, adverse side effects, abuse potential or lack of efficacy in many patients. These off-target effects highlight the need to identify novel mechanisms and targets for treatment. Recently, modulation of Glo1 (glyoxalase I) activity was shown to regulate anxiety-like behaviour and seizure-susceptibility in mice. These effects are likely to be mediated through the regulation of MG (methylglyoxal) by Glo1, as MG acts as a competitive partial agonist at GABA(A) (γ-aminobutyric acid A) receptors. Thus modulation of MG by Glo1 represents a novel target for treatment. In the present article, we evaluate the therapeutic potential of indirectly modulating MG concentrations through Glo1 inhibitors for the treatment of neuropsychiatric disorders.

Moving psychopharmacological drug development to the developing world.

Byline: T. Sathyanarayana Rao, Chittaranjan. Andrade For a medical discipline to progress, there should be advances not only in the understanding of disease, but also in the treatment thereof. There were spectacular breakthroughs in the treatment of psychiatric disorders in the middle decades of the 20 [sup]th century; these were the introduction of electroconvulsive therapy, mood stabilizers, neuroleptics, monoamine oxidase inhibitors, tricyclic antidepressants and benzodiazepines. [sup][1],[2] With the exception of the rediscovery of clozapine, [sup][3] subsequent advances heralded improvements in acceptability and tolerability but not necessarily overall efficacy. [sup][4] Examples of such advances were the introduction of the selective serotonin reuptake inhibitors and other newer antidepressants, and the introduction of the atypical antipsychotics. Whereas there has been good news on some fronts, such as the introduction of phosphodiesterase 5 inhibitors for erectile dysfunction and the introduction of cholinesterase inhibitors and memantine for dementia, the psychopharmacological management of major mental illness has hit a brick wall. [sup][5],[6] There have been unimpressive or disappointing results with innovative treatments such as S-adenosyl methionine, L-methylfolate, omega-3 fatty acids, glutamatergic agents, neuropeptides and many others. This has even resulted in opinion being expressed that the future of psychopharmacology in schizophrenia appears bleak [sup][5] and that the pipeline for psychiatric drugs has run dry. [sup][7] Without a proper understanding of the pathophysiology underlying major mental illness, treatment breakthroughs must rely on serendipity. Subsequent drug development would likely target incremental advances, based on me-too approaches. However, me-too approaches cannot be indefinitely pursued and there is a limit to how commercially successful such me-too drugs will be. There is a long list of other reasons, as well, why research and development of neuropsychiatric drugs is difficult and economically challenging. [sup][8] Many leading multinational pharmaceutical companies are therefore downsizing or abandoning their neuropsychiatric drug development program because the enormous cost of researching a drug and bringing it to the market is economically unviable when there is no assurance that the drug will be approved by the regulatory authorities, or that it will subsequently succeed. [sup][8] Cariprazine is a recent example; the USA Food and Drug Administration acknowledged that the drug demonstrated efficacy but asked for more data on issues related to dosing and adverse effects. Generating the necessary data would be an additional financial burden with no certainty about future profitability. With this background, we suggest an audacious idea. So far, the vast majority of recent drugs have been discovered, developed, or both discovered and developed by pharmaceutical companies in the Western world. The costs have been very high due to the higher costs of living, higher manpower and infrastructural costs and higher legal risks in the West. All these costs are far lower in developing countries. Developing countries also have a rich history of traditional medicine that can be explored for new leads. It may therefore make sense for the drug development industry to move to developing countries, lock, stock and barrel. At present, drugs are developed in the West at formidable cost and afterwards marketed in developing countries. Under the new patent regime, generics of these cannot be marketed in developing countries until the patents expire; therefore, new drugs will be inaccessible or unaffordable to the poor across the world until the patents expire. [sup][9] Consider the situation where the entire drug development process is shifted to developing countries. This would mean that the laboratory research as well as the Phases I, II and III clinical trials (and later, post marketing surveillance) could be completed at low cost for specific marketing in the countries of origin and in other participating countries. …

Innovation in Therapeutics Development at the NCATS

Never before has there been such a sense of urgency to rethink the way in which therapeutics are discovered and developed. Drug development has grown less efficient with every passing year (http://www.phrma.org; Scannell et al, 2012), at the same time as fundamental understanding of human biology in health and disease has exploded. For example, the number of diseases with an identified molecular basis is fast approaching 5000, but the vast majority of these have no Food and Drug Administration (FDA)-approved therapy (http://www.omim.org/statistics/geneMap). Neuroscience has been particularly affected by these trends, such that, although advances in our understanding of neural development, plasticity, and circuitry were being celebrated at the end of the Decade of the Brain in 2010, many large and small pharmaceutical companies were abandoning neuroscience because of the high rate of failure in neuropsychiatric drug development. In part to address these problems in therapeutic development, the NIH commissioned its Science Management Review Board (SMRB) to study the problems in translation and recommend changes to the NIH structure to address them. In response to the SMRB's report (http://smrb.od.nih.gov/documents/reports/TMAT_122010.pdf), NIH proposed the creation of a new Center at NIH that would address the scientific and operational roadblocks to translational efficiency. On 23 December 2011, the National Center for Advancing Translational Sciences (NCATS) was created through the Consolidated Appropriations Act, 2012, (P.L. 112-74), which amended the Public Health Service (PHS) Act to include the NCATS. The mission of the Center is to create and test innovative methods, strategies, and technologies that enhance the development, testing, and implementation of interventions that improve human health. The NCATS is focused on what is common among diseases and the translational process, to identify and disseminate general principles that will accelerate translation of fundamental discoveries into tangible improvements in human health.

A strategy for developing new treatment paradigms for neuropsychiatric and neurocognitive symptoms in Alzheimer’s disease

Successful disease modifying drug development for Alzheimer’s disease (AD) has hit a roadblock with the recent failures of amyloid-based therapies, highlighting the translational disconnect between preclinical animal models and clinical outcome. Although disease modifying therapies are the Holy Grail to pursue, symptomatic therapies addressing cognitive and neuropsychiatric aspects of the disease are also extremely important for the quality of life of patients and caregivers. Despite the fact that neuropsychiatric problems in Alzheimer patients are the major driver for costs associated with institutionalization, no good preclinical animal models with predictive validity have been documented. We propose a combination of quantitative systems pharmacology (QSP), phenotypic screening and preclinical animal models as a novel strategy for addressing the bottleneck in both cognitive and neuropsychiatric drug discovery and development for AD. Preclinical animal models such as transgene rats documenting changes in neurotransmitters with tau and amyloid pathology will provide key information that together with human imaging, pathology and clinical data will inform the virtual patient model. In this way QSP modeling can partially overcome the translational disconnect and reduce the attrition of drug programs in the clinical setting. This approach is different from target driven drug discovery as it aims to restore emergent properties of the networks and therefore likely will identify multitarget drugs. We review examples on how this hybrid humanized QSP approach has been helpful in predicting clinical outcomes in schizophrenia treatment and cognitive impairment in AD and expand on how this strategy could be applied to neuropsychiatric symptoms in dementia. We believe such an innovative approach when used carefully could change the Research and Development paradigm for symptomatic treatment in AD.

Lost in Translation: Neuropsychiatric Drug Development

Recent studies have identified troubling method and practice lapses in neuropsychiatric drug developments. These problems have resulted in errors that are of sufficient magnitude to invalidate clinical trial data and interpretations. We identify two potential sources for these difficulties: investigators selectively choosing scientific practices for demonstrations of efficacy in human-testing phases of drug development and investigators failing to anticipate the needs of practitioners who must optimize treatment for the individual patient. When clinical investigators neglect to use clinical trials as opportunities to test hypotheses of disease mechanisms in humans, the neuropsychiatric knowledge base loses both credibility and scope. When clinical investigators do not anticipate the need to translate discoveries into applications, the practitioner cannot provide optimal care for the patient. We conclude from this evidence that clinical trials, and other aspects of neuropsychiatric drug development, must adopt more practices from basic science and show greater responsiveness to conditions of clinical practice. We feel that these changes are necessary to overcome current threats to the validity and utility of studies of neurological and psychiatric drugs.

Applications of Positron Emission Tomography in Neuropsychiatric Pharmaceutical Drug Development

Positron Emission Tomography (PET) can be used to quantify proteins of interest in the brain, assess the function of these proteins, and quantify cerebral glucose metabolism and blood flow. Its value in neuropsychiatric pharmaceutical drug development is extensive, from the identification of relevant pathophysiology in disease states, to measurement of blood-brain barrier penetration and regional cerebral occupancy of a pharmaceutical agent, to predictions of treatment outcome from a specific pharmacologic intervention in a specific patient. In this paper, we briefly review some basics of brain imaging using PET, and describe its applications to the field of neuropsychiatric pharmaceutical development, including relevant examples from the existing literature. We conclude with a discussion of future developments that will make PET increasingly available and useful for such purposes. Keywords: Radioligands, investigational new drug, major depressive disorder, Alzheimer Disease, serotonin 1A receptor

Development of Radioligands for Imaging of Brain Norepinephrine Transporters In Vivo with Positron Emission Tomography

In the central nervous system (CNS) and in the periphery, specific proteins (transporters) are responsible for the regulation of the synaptic concentrations of the major monoamine neurotransmitters, noradrenaline (NE), serotonin (5-HT) and dopamine (DA). Several reports have shown that the expression of these transporters within the CNS may be altered in patients with certain neurodegenerative or neuropsychiatric disorders. Therefore, in the CNS the monoamine transporters are major targets for existing and developmental drugs. The best known drugs targeting these transporters are the selective 5-HT reuptake inhibitors (SSRIs) (e.g. citalopram, Celexa) that are most frequently used in the treatment of clinical depression. Selective NE reuptake inhibitors (NRIs) have also found use for the treatment of depression and other conditions such as attention deficit hyperactivity (ADHD) disorder. Given that the NE transporter (NET) is also a binding site for cocaine and drugs of abuse, there is a great need for a probe to assess the densities of NET in vivo by brain imaging with either positron emission tomography (PET) or single photon emission tomography (SPET). PET in particular has the potential to measure NET densities quantitatively and with high resolution in the human brain in vivo. The quality of a PET image depends crucially on the radioligand used in the emission measurement. Commonly used radionuclides in PET radioligands are carbon 11 (t(1/2) = 20.4 min) and fluorine-18 (t(1/2) = 109.8 min). This review specifically summarizes the present status of the development of (11)C- or (18)F-labeled ligands as tools for imaging NET in brain with PET in support of neuropsychiatric clinical research and drug development.

Cryopreservation of rat hippocampal slices by vitrification

Although much interest has attended the cryopreservation of immature neurons for subsequent therapeutic intracerebral transplantation, there are no reports on the cryopreservation of organized adult cerebral tissue slices of potential interest for pharmaceutical drug development. We report here the first experiments on cryopreservation of mature rat transverse hippocampal slices. Freezing at 1.2 °C/min to −20 °C or below using 10 or 30% v/v glycerol or 20% v/v dimethyl sulfoxide yielded extremely poor results. Hippocampal slices were also rapidly inactivated by simple exposure to a temperature of 0 °C in artificial cerebrospinal fluid (aCSF). This effect was mitigated somewhat by 0.8 mM vitamin C, the use of a more “intracellular” version of aCSF having reduced sodium and calcium levels and higher potassium levels, and the presence of a 25% w/v mixture of dimethyl sulfoxide, formamide, and ethylene glycol (“V EG solutes”; Cryobiology 48, pp. 22–35, 2004). It was not mitigated by glycerol, aspirin, indomethacin, or mannitol addition to aCSF. When RPS-2 (Cryobiology 21, pp. 260–273, 1984) was used as a carrier solution for up to 50% w/v V EG solutes, 0 °C was more protective than 10 °C. Raising V EG concentration to 53% w/v allowed slice vitrification without injury from vitrification and rewarming per se, but was much more damaging than exposure to 50% w/v V EG. This problem was overcome by using the analogous 61% w/v VM3 vitrification solution (Cryobiology 48, pp. 157–178, 2004) containing polyvinylpyrrolidone and two extracellular “ice blockers.” With VM3, it was possible to attain a tissue K +/Na + ratio after vitrification ranging from 91 to 108% of that obtained with untreated control slices. Microscopic examination showed severe damage in frozen–thawed slices, but generally good to excellent ultrastructural and histological preservation after vitrification. Our results provide the first demonstration that both the viability and the structure of mature organized, complex neural networks can be well preserved by vitrification. These results may assist neuropsychiatric drug evaluation and development and the transplantation of integrated brain regions to correct brain disease or injury.

Use of PET and the radioligand [ carbonyl- 11C]WAY-100635 in psychotropic drug development

Positron-emission tomography (PET) provides potential in neuropsychiatric drug development by expanding knowledge of drug action in the living human brain and reducing time consumption and costs. The 5-hydroxytryptamine 1A (5-HT 1A) receptor is of central interest as a target for the treatment of anxiety, depression, and schizophrenia. Research on the clinical significance of the 5-HT 1A receptor now benefits from the highly selective radioligand [ carbonyl- 11C]WAY-100635 (WAY) for quantitative determination of 5-HT 1A receptors in the primate and human brain in vivo using PET. In this paper, three studies are reviewed to demonstrate the suitability of WAY as radioligand for quantification of central 5-HT 1A receptors in brain and as an applicable tool for drug development. In the first study a monkey model was used to characterize WAY binding. It was confirmed that the reference ligand 8-OH-DPAT and psychoactive drugs such as buspirone and pindolol occupies 5-HT 1A receptors in the primate brain. Pindolol is an β-adrenoreceptor antagonist with a high affinity to 5-HT 1A receptors. This drug has been suggested in combination with selective serotonin reuptake inhibitors for the treatment of depression and was given to healthy males in the second study. Pindolol induced a marked inhibition of central 5-HT 1A receptors as calculated by the ratio-analysis method and simplified reference tissue model, 2 h after administration of 10 mg as a single oral dose. This observation suggests that pindolol may have a role for the suggested potentiation of selective serotonin reuptake inhibitor treatment of depression. The third study was on robalzotan (NAD-299), a recently developed 5-HT 1A receptor antagonist and putative drug with implications for the treatment of depression. In the cynomolgus monkey brain, robalzotan in the dose range 2–100 μg/kg IV occupied 5-HT 1A receptors in a dose-dependent and saturable manner with a maximal calculated occupancy of 70–80%. The relationship between robalzotan plasma concentration and 5-HT 1A receptor occupancy could be described by a hyperbolic function that was used to guide the selection of appropriate doses in man. In a subsequent PET study of robalzotan binding to 5-HT 1A receptors in the living human brain, similar results have been replicated recently. These studies reviewed here illustrate and corroborate that quantitative neuroimaging of receptor binding has potential for the evaluation and dose finding of new central nervous system drugs.