Mismatch negativity as an index of target engagement for excitation/inhibition-based treatment development: a double-blind, placebo-controlled, randomized, single-dose cross-over study of the serotonin type-3 receptor antagonist CVN058

Current emerging therapeutic targets and clinical investigational agents for schizophrenia: Challenges and opportunities

Since the first discovery of antipsychotics in the 1950s, targeting dopaminergic drugs has manifested to well manage the positive symptoms of schizophrenia with limited efficacy for the negative and cognitive symptoms. In past decades, extensive efforts have been undertaken towards the development of innovative agents that can effectively stabilize the dopamine and serotonin systems or target to nondopaminergic pathways, leading to various promising drug candidates entering into clinical trials. Notably, the sigma-2, 5-HT2A, and α1A receptor antagonist roluperidone, as well as a fixed-dose combination of the M1/4 receptor agonist KarXT, have been submitted for NDA applications. The dual agonist ulotaront, which targets TAAR1 and 5-HT1A receptors, and the GlyT1 inhibitor iclepertin have advanced into phase 3 clinical trials. Nevertheless, satisfactory therapeutic strategies for schizophrenia remain elusive. This review highlights current clinical endeavors in developing novel chemical small-molecule entities and fixed-dose combinations for the treatment of schizophrenia since 2017, thus facilitating the efficient development of the next generation of antipsychotics.

Negative Symptoms in Schizophrenia: An Update on Research Assessment and the Current and Upcoming Treatment Landscape

The negative symptoms of schizophrenia include diminished emotional expression, avolition, alogia, anhedonia, and asociality, and due to their low responsiveness to available treatments, are a primary driver of functional disability in schizophrenia. This narrative review has the aim of providing a comprehensive overview of the current research developments in the treatment of negative symptoms in schizophrenia, and begins by introducing the concepts of primary, secondary, prominent, predominant, and broadly defined negative symptoms. We then compare and contrast commonly used research assessment scales for negative symptoms and review the evidence for the specific utility of widely available off-label and investigational treatments that have been studied for negative symptoms. Mechanism of action/putative treatments included are antipsychotics (D2R antagonists), N-methyl-D-aspartate receptor (NMDAR) and other glutamatergic modulators, serotonin receptor (5-HTR) modulators, anti-inflammatory agents, antidepressants, pro-dopaminergic modulators (non-D2R antagonists), acetylcholine modulators, oxytocin, and phosphodiesterase (PDE) inhibitors. With the caveat that no compounds are definitively proven as gold-standard treatments for broadly defined negative symptoms, the evidence base supports several potentially beneficial off-label and investigational medications for treating negative symptoms in schizophrenia, such as monotherapy with cariprazine, olanzapine, clozapine, and amisulpride, or adjunctive use of memantine, setrons such as ondansetron, minocycline, and antidepressants. These medications are widely available worldwide, generally tolerable and could be considered for an off-label, time-limited trial for a predesignated period of time, after which a decision to switch or stay can be made based on clinical response. Among investigational medications, NMDAR agonists, muscarinic agonists, and LB-102 remain under study. Suggestions for future research include reducing placebo effects by designing studies with a smaller number of high-quality study sites, potentially increasing the use of more precise rating scales for negative symptoms, and focused studies in people with predominant negative symptoms.

Finding the Right Dose: NMDA Receptor-Modulating Treatments for Cognitive and Plasticity Deficits in Schizophrenia and the Role of Pharmacodynamic Target Engagement

Cognitive impairment associated with schizophrenia (CIAS) and related deficits in learning (plasticity) are among the leading causes of disability in schizophrenia. Despite this, there are no Food and Drug Administration-approved treatments for CIAS, and the development of treatments has been limited by numerous phase 2/3 failures of compounds that showed initial promise in small-scale studies. NMDA-type glutamate receptors (NMDARs) have been proposed to play an important role in schizophrenia; moreover, the NMDAR has a well-characterized role in cognition, learning, and neuroplasticity. We review previously published clinical trials in CIAS that focused on NMDAR modulator treatments, focusing on published and recent developments of the use of novel NMDAR-modulating treatments for CIAS both alone and combined with plasticity/learning paradigms to enhance learning. We use this discussion of previous studies to highlight the importance of incorporating pharmacodynamic target engagement biomarkers early in treatment development, which can help predict which compounds will succeed or fail in phase 3. A range of direct and indirect NMDAR modulators are covered, including D-serine, D-cycloserine, memantine, and glycine and first-generation glycine transport inhibitors (e.g., sarcosine and bitopertin), as well as recent positive studies of iclepertin, a novel glycine transport inhibitor, and luvadaxistat, a D-amino acid oxidase inhibitor that increases brain D-serine levels, and indirect noninvasive brain stimulation NMDAR-modulating treatments. Several examples of successful use of pharmacodynamic target engagement biomarkers for dose/drug discovery are emphasized, including the mismatch negativity, auditory steady state, and time-frequency event-related potential approaches.

Knowing is Half the Battle: The Factors Leading to Efficient Recruitment of Representative Samples in Schizophrenia Research

BACKGROUND AND OBJECTIVES

Drug development in schizophrenia is limited by the differential scaling of the active treatment and placebo arms of a study, such that, as the number of sites increases, the magnitude of placebo response disproportionately increases. The objective of this article was to identify factors conducive to efficient recruitment as a step towards trial designs allowing recruitment of more participants per site, leading to reduced variability, and potentially a smaller placebo effect.

PATIENTS AND METHODS

Using the information of 554 individuals, we calculated the percentage of individuals who were screened, consented, and retained in our research, along with rationale for nonconsent. Independent t tests and Chi-squared tests were performed to compare participant characteristics.

RESULTS

Out of the 273 individuals who were fully screened, 84 did not progress to the consented stage owing to various reasons, leading to a total of 189 individuals who were screened and consented and a total of 365 individuals who were either incompletely screened or not consented into a study. Individuals with an externally validated medical history showed the highest yield in being consented and retained in research as new participants. In particular, chart reviews from clinics were highly efficient (25.8%) in facilitating new participants' enrollment, even when these individuals were not actively/prospectively seeking research. The most common reason for nonconsent was difficulty in telephone or email contact. Consenting and nonconsenting participants were similar in demographics, and recruitment sources only differed in their reported substance use.

CONCLUSIONS

Referrals and chart reviews from known clinics were the most efficient method in retaining new participants, highlighting the importance of external validation and communication between research and clinical staff within a system. Consenting participants showed no significant differences from the database as a whole, demonstrating that the study samples were representative. Future studies should aim to confirm the present findings at other academic and commercial research centers.

Neuropsychiatric drug development: Perspectives on the current landscape, opportunities and potential future directions

Mental health represents a major challenge to our societies. One key difficulty associated with neuropsychiatric drug development is the lack of connection between the underlying biology and the disease. Nevertheless, there is growing optimism in this field with recent drug approvals (the first in decades) and renewed interest from pharmaceutical companies and investors. Here we review some of the most promising drug discovery and development endeavors currently deployed by industry. We also present elements illustrating the renewed interest from key stakeholders in neuropsychiatric drug development and provide potential future directions in this field.

The Less Things Change, the More They Remain the Same: Impaired Neural Plasticity as a Critical Target for Drug Development in Neuropsychiatry

Neuropsychiatric disability is related to reduced ability to change in response to clinical interventions, e.g., plasticity. Study of biomarkers and interventional strategies for plasticity, however, are sparse. In this chapter, we focus on the serial frequency discrimination task (SFDT), which is sensitive to impairments in early auditory processing (EAP) and auditory learning and has been most thoroughly studied in dyslexia and schizophrenia. In the SFDT, participants are presented with repeated paired tones ("reference" and "test") and indicate which tone is higher in pitch. Plasticity during the SFDT is critically dependent upon interactions between prefrontal "cognitive control" regions, and lower-level perceptual and motor regions that may be detected using both fMRI and time-frequency event-related potential (TF-ERP) approaches. Additionally, interactions between the cortex and striatum give insights into glutamate/dopamine interaction mechanisms. The SFDT task has been utilized in the development of N-methyl-D-aspartate receptor (NMDAR) targeted medications, which significantly modulate sensory and premotor neurophysiological activity. Deficits in pitch processing play a critical role in impaired neuro- and social cognitive function in schizophrenia and may contribute to similar impairments in dyslexia. Thus, the SFDT may be ideal for development of treatments aimed at amelioration of neuro- and social cognitive deficits across neuropsychiatric disorders.

Mismatch Negativity (MMN) as a Pharmacodynamic/Response Biomarker for NMDA Receptor and Excitatory/Inhibitory Imbalance-Targeted Treatments in Schizophrenia

Schizophrenia is a major mental disorder that affects approximately 0.5% of the population worldwide. Persistent negative symptoms and cognitive impairments associated with schizophrenia (CIAS) are key features of the disorder and primary predictors of long-term disability. At the neurochemical level, both CIAS and negative symptoms are potentially attributable to dysfunction or dysregulation of N-methyl-D-aspartate receptor (NMDAR)-mediated neurotransmission within cortical and subcortical brain regions. At present, there are no approved treatments for either CIAS or persistent negative symptoms. Development of novel treatments, moreover, is limited by the lack of biomarkers that can be used translationally across preclinical and early-stage clinical investigation. The present chapter describes the use of mismatch negativity (MMN) as a pharmacodynamic/response (PD/R) biomarker for early-stage clinical investigation of NMDAR targeted therapies for schizophrenia. MMN indexes dysfunction of early auditory processing (EAP) in schizophrenia. In humans, deficits in MMN generation contribute hierarchically to impaired cognition and functional outcome. Across humans, rodents, and primates, MMN has been linked to impaired NMDAR function and resultant disturbances in excitatory/inhibitory (E/I) balance involving interactions between glutamatergic (excitatory) pyramidal and GABAeric (inhibitory) local circuit neurons. In early-stage clinical trials, MMN has shown sensitivity to the acute effects of novel pharmacological treatments. These findings support use of MMN as a pharmacodynamic/response biomarker to support preclinical drug discovery and early-stage proof-of-mechanisms studies in schizophrenia and other related neuropsychiatric disorders.

Dose-Dependent Augmentation of Neuroplasticity-Based Auditory Learning in Schizophrenia: A Double-Blind, Placebo-Controlled, Randomized, Target Engagement Clinical Trial of the NMDA Glutamate Receptor Agonist d-serine

BACKGROUND

Patients with schizophrenia show reduced NMDA glutamate receptor-dependent auditory plasticity, which is rate limiting for auditory cognitive remediation (AudRem). We evaluate the utility of behavioral and neurophysiological pharmacodynamic target engagement biomarkers, using a d-serine+AudRem combination.

METHODS

Forty-five participants with schizophrenia or schizoaffective disorder were randomized to 3 once-weekly AudRem visits + double-blind d-serine (80, 100, or 120 mg/kg) or placebo in 3 dose cohorts of 12 d-serine and 3 placebo-treated participants each. In AudRem, participants indicated which paired tone was higher in pitch. The primary outcome was plasticity improvement, operationalized as change in pitch threshold between AudRem tones [(test tone Hz - reference tone Hz)/reference tone Hz] between the initial plateau pitch threshold (mean of trials 20-30 of treatment visit 1) to pitch threshold at the end of visit(s). Target engagement was assessed by electroencephalography outcomes, including mismatch negativity (pitch primary).

RESULTS

There was a significant overall treatment effect for plasticity improvement (p = .014). Plasticity improvement was largest within the 80 and 100 mg/kg groups (p < .001, d > 0.67), while 120 mg/kg and placebo-treated participants showed nonsignificant within-group changes. Plasticity improvement was seen after a single treatment and was sustained on subsequent treatments. Target engagement was demonstrated by significantly larger mismatch negativity (p = .049, d = 1.0) for the 100 mg/kg dose versus placebo.

CONCLUSIONS

Our results demonstrate sufficient proof of principle for continued development of both the d-serine+AudRem combination and our target engagement methodology. The ultimate utility is dependent on the results of an ongoing larger, longer study of the combination for clinically relevant outcomes.

Candidate biomarkers in psychiatric disorders: state of the field

The field of psychiatry is hampered by a lack of robust, reliable and valid biomarkers that can aid in objectively diagnosing patients and providing individualized treatment recommendations. Here we review and critically evaluate the evidence for the most promising biomarkers in the psychiatric neuroscience literature for autism spectrum disorder, schizophrenia, anxiety disorders and post-traumatic stress disorder, major depression and bipolar disorder, and substance use disorders. Candidate biomarkers reviewed include various neuroimaging, genetic, molecular and peripheral assays, for the purposes of determining susceptibility or presence of illness, and predicting treatment response or safety. This review highlights a critical gap in the biomarker validation process. An enormous societal investment over the past 50 years has identified numerous candidate biomarkers. However, to date, the overwhelming majority of these measures have not been proven sufficiently reliable, valid and useful to be adopted clinically. It is time to consider whether strategic investments might break this impasse, focusing on a limited number of promising candidates to advance through a process of definitive testing for a specific indication. Some promising candidates for definitive testing include the N170 signal, an event-related brain potential measured using electroencephalography, for subgroup identification within autism spectrum disorder; striatal resting-state functional magnetic resonance imaging (fMRI) measures, such as the striatal connectivity index (SCI) and the functional striatal abnormalities (FSA) index, for prediction of treatment response in schizophrenia; error-related negativity (ERN), an electrophysiological index, for prediction of first onset of generalized anxiety disorder, and resting-state and structural brain connectomic measures for prediction of treatment response in social anxiety disorder. Alternate forms of classification may be useful for conceptualizing and testing potential biomarkers. Collaborative efforts allowing the inclusion of biosystems beyond genetics and neuroimaging are needed, and online remote acquisition of selected measures in a naturalistic setting using mobile health tools may significantly advance the field. Setting specific benchmarks for well-defined target application, along with development of appropriate funding and partnership mechanisms, would also be crucial. Finally, it should never be forgotten that, for a biomarker to be actionable, it will need to be clinically predictive at the individual level and viable in clinical settings.

Augmentation of learning in schizophrenia by D-serine is related to auditory and frontally-generated biomarkers: A randomized, double-blind, placebo-controlled study

Auditory cognition is impaired in schizophrenia, and typically engages a complex, distributed, hierarchical network, including both auditory and frontal input. We recently demonstrated proof of principle for the target engagement of an N-methyl-D-aspartate-type glutamate receptor (NMDAR) agonist + auditory targeted remediation (d-serine+AudRem) combination, showing significant improvement in auditory-learning induced plasticity and mismatch negativity. In this secondary analysis, we report on frontal EEG outcomes, assessing for both generalized effects and the mechanism of auditory plasticity. 21 schizophrenia or schizoaffective disorder participants were randomized to three 1x weekly AudRem + double-blind d-serine (100 mg/kg) visits. In AudRem, participants indicated which paired tone was higher in pitch. The focus of this secondary analysis was a frontally (premotor) mediated EEG outcome- event-related desynchronization in the b band (b-ERD), which was shown to be sensitive to AudRem in previous studies. d-Serine+AudRem led to significant improvement in b-ERD power across the retention and motor preparation intervals (F 1,18 =6.0, p=0.025) vs. AudRem alone. b-ERD was significantly related to baseline cognition, but not auditory-learning induced plasticity. The principal finding of this prespecified secondary analysis are that in addition to improving auditory based biomarkers, the d-serine+AudRem combination led to significant improvement in biomarkers thought to represent frontally mediated dysfunction, suggesting potential generalization of effects. Changes in auditory-learning induced plasticity were independent of these frontally mediated biomarkers. Ongoing work will assess whether d-serine+AudRem is sufficient to remediate cognition or whether targeting frontal NMDAR deficits with higher-level remediation may also be required. Trial Registration: NCT03711500.

Working memory and sensory memory in subclinical high schizotypy: An avenue for understanding schizophrenia?

The search for robust, reliable biomarkers of schizophrenia remains a high priority in psychiatry. Biomarkers are valuable because they can reveal the underlying mechanisms of symptoms and monitor treatment progress and may predict future risk of developing schizophrenia. Despite the existence of various promising biomarkers that relate to symptoms across the schizophrenia spectrum, and despite published recommendations encouraging multivariate metrics, they are rarely investigated simultaneously within the same individuals. In those with schizophrenia, the magnitude of purported biomarkers is complicated by comorbid diagnoses, medications and other treatments. Here, we argue three points. First, we reiterate the importance of assessing multiple biomarkers simultaneously. Second, we argue that investigating biomarkers in those with schizophrenia-related traits (schizotypy) in the general population can accelerate progress in understanding the mechanisms of schizophrenia. We focus on biomarkers of sensory and working memory in schizophrenia and their smaller effects in individuals with nonclinical schizotypy. Third, we note irregularities across research domains leading to the current situation in which there is a preponderance of data on auditory sensory memory and visual working memory, but markedly less in visual (iconic) memory and auditory working memory, particularly when focusing on schizotypy where data are either scarce or inconsistent. Together, this review highlights opportunities for researchers without access to clinical populations to address gaps in knowledge. We conclude by highlighting the theory that early sensory memory deficits contribute negatively to working memory and vice versa. This presents a mechanistic perspective where biomarkers may interact with one another and impact schizophrenia-related symptoms.

Early auditory processing dysfunction in schizophrenia: Mechanisms and implications

Schizophrenia is a major mental disorder that affects approximately 1% of the population worldwide. Cognitive deficits are a key feature of the disorder and a primary cause of long-term disability. Over the past decades, significant literature has accumulated demonstrating impairments in early auditory perceptual processes in schizophrenia. In this review, we first describe early auditory dysfunction in schizophrenia from both a behavioral and neurophysiological perspective and examine their interrelationship with both higher order cognitive constructs and social cognitive processes. Then, we provide insights into underlying pathological processes, especially in relationship to glutamatergic and N-methyl-D-aspartate receptor (NMDAR) dysfunction models. Finally, we discuss the utility of early auditory measures as both treatment targets for precision intervention and as translational biomarkers for etiological investigation. Altogether, this review points out the crucial role of early auditory deficits in the pathophysiology of schizophrenia, in addition to major implications for early intervention and auditory-targeted approaches.

Connecting Neurobiological Features with Interregional Dysconnectivity in Social-Cognitive Impairments of Schizophrenia

Schizophrenia (SZ) is a devastating psychiatric disorder affecting about 1% of the world's population. Social-cognitive impairments in SZ prevent positive social interactions and lead to progressive social withdrawal. The neurobiological underpinnings of social-cognitive symptoms remain poorly understood, which hinders the development of novel treatments. At the whole-brain level, an abnormal activation of social brain regions and interregional dysconnectivity within social-cognitive brain networks have been identified as major contributors to these symptoms. At the cellular and subcellular levels, an interplay between oxidative stress, neuroinflammation and N-methyl-D-aspartate receptor hypofunction is thought to underly SZ pathology. However, it is not clear how these molecular processes are linked with interregional dysconnectivity in the genesis of social-cognitive symptoms. Here, we aim to bridge the gap between macroscale (connectivity analyses) and microscale (molecular and cellular mechanistic) knowledge by proposing impaired myelination and the disinhibition of local microcircuits as possible causative biological pathways leading to dysconnectivity and abnormal activity of the social brain. Furthermore, we recommend electroencephalography as a promising translational technique that can foster pre-clinical drug development and discuss attractive drug targets for the treatment of social-cognitive symptoms in SZ.

Cognitive Impairment Associated with Schizophrenia: From Pathophysiology to Treatment

Cognitive impairment is a core feature of schizophrenia and a major contributor to poor functional outcomes. Methods for assessment of cognitive dysfunction in schizophrenia are now well established. In addition, there has been increasing appreciation in recent years of the additional role of social cognitive impairment in driving functional outcomes and of the contributions of sensory-level dysfunction to higher-order impairments. At the neurochemical level, acute administration of N-methyl-d-aspartate receptor (NMDAR) antagonists reproduces the pattern of neurocognitive dysfunction associated with schizophrenia, encouraging the development of treatments targeted at both NMDAR and its interactome. At the local-circuit level, an auditory neurophysiological measure, mismatch negativity, has emerged both as a veridical index of NMDAR dysfunction and excitatory/inhibitory imbalance in schizophrenia and as a critical biomarker for early-stage translational drug development. Although no compounds have yet been approved for treatment of cognitive impairment associated with schizophrenia, several candidates are showing promise in early-phase testing.

Challenges and Strategies for the Recruitment of Patients With Schizophrenia in a Research Setting

BACKGROUND

With numerous potentially novel targets and pharmacodynamic biomarkers for schizophrenia entering late-stage testing, the next decade will bring an urgent need for well-conducted clinical trials. A critically important step for the successful execution of clinical research trials is timely and appropriate recruitment of participants. Patients with schizophrenia can be especially challenging to recruit because of the disability inherent in psychotic spectrum disorders. Research on how best to recruit for clinical trials is understudied. Clearly defining a model for recruitment procedures would be valuable for researchers and, by extension, the patient populations that may benefit from the insight gained by future clinical research.

METHODS

This article aims to offer suggestions for recruitment based on years of experience at the Columbia Schizophrenia Research Clinic (CSRC), a hub for clinical trials focusing on the etiology and treatment of various psychotic disorders.

RESULTS

The present report provides practical, step-by-step recommendations for implementing the highly effective CSRC recruitment model, including the benefits of 2 recruitment initiatives that were instituted in 2018: hiring a dedicated recruiter and targeted chart reviews at affiliated clinics. Other topics discussed include our umbrella protocol and database, advertising, and tips for collaborating with external sites.

CONCLUSIONS

Despite ongoing complications from coronavirus disease 2019, these strategies have been successful, increasing the rate of both consents and study enrollments by approximately 40% and enabling the CSRC to conduct multiple studies simultaneously.