No effect of adjunctive, repeated-dose intranasal insulin treatment on psychopathology and cognition in patients with schizophrenia

Peptide therapeutics: current status and future opportunity with focus on nose-to-brain delivery☆

Peptide drugs are a highly diverse group of therapeutic agents. Over the last decade, more than 40 peptides have been approved for clinical use. They target different structures, ranging from G protein-coupled receptors (GPCRs) to pathogens and are used to treat a variety of indications, including metabolic disorders, genetic diseases, acute illnesses and more. Structurally, peptide therapeutics are a heterogeneous class. This diversity allows them to bridge the gap between small molecules and biologics. However, limited metabolic stability and bioavailability must be addressed. Strategies to improve the half-life include backbone and sequence modification, cyclization and the addition of stabilizing moieties. Great strides have been made in recent years towards achieving sufficient drug uptake for oral application have been achieved within recent years. However, these methods require specialized peptide design or involve permeabilization of the gastrointestinal tract. Consequently, other routes of administration are being explored. One promising approach is the nasal application of peptides. This method can be used for systemic uptake, but also allows for direct nose-to-brain delivery of compounds. While successful nose-to-brain delivery is already used in the clinic, the underlining mechanisms are poorly understood. Strategies for rational optimization are needed to make this method more applicable to a wider range of compounds. Overall, approved peptide therapeutics cover a wide range of applications and have demonstrated a growing and novel potential in recent drug discovery.

Intranasal insulin for improving cognitive function in multiple sclerosis

Cognitive impairment is common in people with multiple sclerosis (PwMS). There is an urgent need to identify/develop novel therapies that can help cognitive function in MS. Insulin is critical for helping with regulation of multiple CNS functions, including learning and memory. Insulin administrated intranasally has shown to improve memory and learning in healthy people and in those with some neurodegenerative disorders. Hence, there was rationale for investigating intranasal insulin in PwMS who experience cognitive impairment. We completed a phase Ib/II, randomized, double-blind, placebo-controlled trial; participants were randomized in a 1:1:1 fashion, stratified by relapsing versus progressive MS, to intranasal insulin 10 ​international units (IU) twice a day, 20 IU twice a day, or placebo for 24 weeks. One-hundred and five PwMS were enrolled, 69 of whom had at least one follow up visit during the active treatment phase of the trial (baseline to week 24). The cohort's mean age was 52.4 ​± ​9.7years, 62 ​% were female, and ∼60 ​% had relapsing-remitting MS. The most common side effects amongst treatment groups included headache, rhinorrhea, and dizziness. There were 13 SAEs which were not deemed study drug related; there were no deaths. The main clinical outcome measure, SDMT, did not demonstrate any difference between intranasal insulin and placebo. Similar findings were noted for all secondary outcome measures. Intranasal insulin appeared safe and well-tolerated in PwMS. However, it was not superior to placebo in any of the clinical outcome measures assessed, which could have been impacted by the duration of the trial, small sample size for a three-arm trial design, data missingness (particularly during COVID-19), outcome measure insensitivity to change, baseline cognitive reserve, or other factors. Nonetheless, intranasally-administered therapeutics may be of interest to develop further as a way to get across the blood brain barrier.

Exploring the effects of an insulin challenge on neuroimaging outcomes: A scoping review

Emerging evidence demonstrates that insulin has a modulating effect on metabolic and cognitive function in the brain, highlighting the potential role of aberrant brain insulin signaling in the pathogenesis of various neuropsychiatric illnesses. Neuroimaging paradigms using intranasal insulin (INI) as a pharmacological challenge have allowed us to study the effects of insulin in the human brain. In this scoping review, we conducted a systematic database search to identify relevant research studies that employed an INI-based neuroimaging assay of brain insulin signaling. Thirty-six studies met inclusion criteria for this review. INI was found to significantly modulate activity and cerebral blood flow in brain regions related to homeostatic/hedonic control of food intake, as well as cognition. This review highlights the putative role of insulin signaling in the brain and the potential therapeutic value of INI in patients with mental health, addiction, and co-morbid metabolic disorders.

Intranasal Insulin Increases Brain Glutathione and Enhances Antioxidant Capacity in Healthy Participants but Not in Those With Early Psychotic Disorders

BACKGROUND

We examined the acute effects of intranasal insulin on cognitive function and brain glutathione (GSH), a central factor in resistance to oxidative stress, in both participants with early psychosis and healthy control (HC) participants.

METHODS

Twenty-one patients with early-stage psychotic disorders and 18 HC participants underwent magnetic resonance spectroscopy (MRS) scans and cognitive assessments before and after administration of intranasal insulin 40 IU. We conducted proton MRS (1H-MRS) in the prefrontal cortex at 4T to measure GSH and glutamate metabolites. We assessed cognition using the Brief Assessment of Cognition in Schizophrenia symbol coding, digit sequencing, and verbal fluency tasks, in addition to the Stroop task.

RESULTS

The mean (SD) age of participants was 25.7 (4.6) years; 51.3% were female. There were no significant group differences at baseline in age, sex, body mass index, homeostatic model assessment of insulin resistance (HOMA-IR), or cognition. Patients had higher baseline GSH (p < .001) and glutamate (p = .007). After insulin administration, GSH increased in HC participants (mean change, 0.15; 95% CI 0.03 to 0.26; p = .015), but not in patients. Symbol coding improved in both patients (0.74; 95% CI 0.37 to 1.11; p < .001) and HC participants (0.83; 95% CI 0.58 to 1.09; p < .001), and verbal fluency improved in HC participants (0.43; 95% CI 0.14 to 0.72; p = .006). Lower baseline HOMA-IR was associated with greater change in GSH (coefficient -0.22; 95% CI -0.40 to -0.04; p = .017).

CONCLUSIONS

Intranasal insulin increased brain GSH in HC participants, but not in patients with early psychotic disorders. These novel findings demonstrate that intranasal insulin enhances antioxidant capacity and resilience to oxidative stress in HC individuals in contrast to an absent antioxidant response in those with early psychotic disorders.

Insulin-resistance as a modifiable pathway to cognitive dysfunction in schizophrenia: A systematic review

BACKGROUND

Cognitive deficits are difficult to treat and negatively influence quality of life and functional outcomes of persons with schizophrenia. In the last twenty years, extensive literature demonstrated that persons with diabetes and insulin resistance (IR) also display cognitive deficits. Being type 2 diabetes (T2DM) and IR highly frequent in persons with schizophrenia, it is plausible to hypothesize that these conditions might play a role in determining dyscognition. If that is the case, acting on glucose dysmetabolism may eventually improve cognitive functioning. This review aims at: 1. evaluating the association between IR or T2DM and cognitive dysfunction in schizophrenia; 2. reviewing the evidence that pharmacological treatment of IR or T2DM may improve dyscognition in schizophrenia.

METHODS

Two systematic searches were conducted in PubMed, PsycInfo, and Scopus. We followed the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines.

RESULTS

From the first search we included 17 studies, 8 on the effects of T2DM and 9 on the effects of IR-other prediabetes measures on cognition in persons with schizophrenia. From the second search we included 12 studies investigating the effect on cognition of glucose (4 studies), insulin (2 studies), metformin (2 studies), PPAR-γ agonists (2 studies), GLP-1 agonist (1 study), bromocriptine (1 study).

CONCLUSIONS

T2DM was associated with worse cognitive function in persons with schizophrenia, while IR was less strongly associated with cognitive dysfunction. Evidence regarding the efficacy of glucose-lowering medications on cognition in schizophrenia is inconclusive, yet methodological issues likely contribute to explain conflicting results.

Pharmacological management of neurocognitive impairment in schizophrenia: A narrative review

BACKGROUND

Cognitive impairment are among the core features of schizophrenia, experienced by up to 75% of patients. Available treatment options for schizophrenia including dopamine antagonists and traditional antipsychotic medications have not been shown to confer significant benefits on cognitive deficits. Contrary to the focus on management of positive symptoms in schizophrenia, cognitive abilities are main predictor of independent living skills, functional abilities, employment, engagement in relapse prevention, and patients' subjective sense of well-being and quality of life. This review aims to provide a summary of recent literature on pharmacological options for the treatment of cognitive deficits in schizophrenia.

METHODS

We conducted a literature search of studies from 2011 to 2021 across four electronic databases including PubMed, PsycInfo, MEDLINE, and Embase. Human studies using a pharmacological treatment for cognitive impairment in schizophrenia were included.

RESULTS

Fifty-eight eligible publications, representing 11 pharmacological classes, were included in this review. Major limitations involved small sample size, methodological limitations as well as heterogeneity of participants and outcome measures.

CONCLUSIONS

Overall evidence remains inconclusive for any pharmacological classes studied for the treatment of cognitive deficits in schizophrenia. Methodological limitations in a majority of the studies rendered their findings preliminary. We further discuss possible explanations for these findings that could guide future research.

Outcomes and clinical implications of intranasal insulin on cognition in humans: A systematic review and meta-analysis

BACKGROUND

Aberrant brain insulin signaling has been posited to lie at the crossroads of several metabolic and cognitive disorders. Intranasal insulin (INI) is a non-invasive approach that allows investigation and modulation of insulin signaling in the brain while limiting peripheral side effects.

OBJECTIVES

The objective of this systematic review and meta-analysis is to evaluate the effects of INI on cognition in diverse patient populations and healthy individuals.

METHODS

MEDLINE, EMBASE, PsycINFO, and Cochrane CENTRAL were systematically searched from 2000 to July 2021. Eligible studies were randomized controlled trials that studied the effects of INI on cognition. Two independent reviewers determined study eligibility and extracted relevant descriptive and outcome data.

RESULTS

Twenty-nine studies (pooled N = 1,726) in healthy individuals as well as those with Alzheimer's disease (AD)/mild cognitive impairment (MCI), mental health disorders, metabolic disorders, among others, were included in the quantitative meta-analysis. Patients with AD/MCI treated with INI were more likely to show an improvement in global cognition (SMD = 0.22, 95% CI: 0.05-0.38 p = <0.00001, N = 12 studies). Among studies with healthy individuals and other patient populations, no significant effects of INI were found for global cognition.

CONCLUSIONS

This review demonstrates that INI may be associated with pro-cognitive benefits for global cognition, specifically for individuals with AD/MCI. Further studies are required to better understand the neurobiological mechanisms and differences in etiology to dissect the intrinsic and extrinsic factors contributing to the treatment response of INI.

Intranasal Peptide Therapeutics: A Promising Avenue for Overcoming the Challenges of Traditional CNS Drug Development

The central nervous system (CNS) has, among all organ systems in the human body, the highest failure rate of traditional small-molecule drug development, ranging from 80-100% depending on the area of disease research. This has led to widespread abandonment by the pharmaceutical industry of research and development for CNS disorders, despite increased diagnoses of neurodegenerative disorders and the continued lack of adequate treatment options for brain injuries, stroke, neurodevelopmental disorders, and neuropsychiatric illness. However, new approaches, concurrent with the development of sophisticated bioinformatic and genomic tools, are being used to explore peptide-based therapeutics to manipulate endogenous pathways and targets, including "undruggable" intracellular protein-protein interactions (PPIs). The development of peptide-based therapeutics was previously rejected due to systemic off-target effects and poor bioavailability arising from traditional oral and systemic delivery methods. However, targeted nose-to-brain, or intranasal (IN), approaches have begun to emerge that allow CNS-specific delivery of therapeutics via the trigeminal and olfactory nerve pathways, laying the foundation for improved alternatives to systemic drug delivery. Here we review a dozen promising IN peptide therapeutics in preclinical and clinical development for neurodegenerative (Alzheimer's, Parkinson's), neuropsychiatric (depression, PTSD, schizophrenia), and neurodevelopmental disorders (autism), with insulin, NAP (davunetide), IGF-1, PACAP, NPY, oxytocin, and GLP-1 agonists prominent among them.

Repurposing of Anti-Diabetic Agents as a New Opportunity to Alleviate Cognitive Impairment in Neurodegenerative and Neuropsychiatric Disorders

Cognitive impairment is a shared abnormality between type 2 diabetes mellitus (T2DM) and many neurodegenerative and neuropsychiatric disorders, such as Alzheimer’s disease (AD) and schizophrenia. Emerging evidence suggests that brain insulin resistance plays a significant role in cognitive deficits, which provides the possibility of anti-diabetic agents repositioning to alleviate cognitive deficits. Both preclinical and clinical studies have evaluated the potential cognitive enhancement effects of anti-diabetic agents targeting the insulin pathway. Repurposing of anti-diabetic agents is considered to be promising for cognitive deficits prevention or control in these neurodegenerative and neuropsychiatric disorders. This article reviewed the possible relationship between brain insulin resistance and cognitive deficits. In addition, promising therapeutic interventions, especially current advances in anti-diabetic agents targeting the insulin pathway to alleviate cognitive impairment in AD and schizophrenia were also summarized.

Non-Invasive Strategies for Nose-to-Brain Drug Delivery

Intranasal drug administration is a promising method for delivering drugs directly to the brain. Animal studies have described pathways and potential brain targets, but nose-to-brain delivery and treatment efficacy in humans remains debated. We describe the proposed pathways and barriers for nose-to-brain drug delivery in humans, drug properties that influence central nervous system delivery, clinically tested methods to enhance absorption, and the devices used in clinical trials. This review compiles the available evidence for nose-to-brain drug delivery in humans and summarizes the factors involved in nose-to-brain drug delivery.

Bioenergetic adaptations to HIV infection. Could modulation of energy substrate utilization improve brain health in people living with HIV-1?

The human brain consumes more energy than any other organ in the body and it relies on an uninterrupted supply of energy in the form of adenosine triphosphate (ATP) to maintain normal cognitive function. This constant supply of energy is made available through an interdependent system of metabolic pathways in neurons, glia and endothelial cells that each have specialized roles in the delivery and metabolism of multiple energetic substrates. Perturbations in brain energy metabolism is associated with a number of different neurodegenerative conditions including impairments in cognition associated with infection by the Human Immunodeficiency Type 1 Virus (HIV-1). Adaptive changes in brain energy metabolism are apparent early following infection, do not fully normalize with the initiation of antiretroviral therapy (ART), and often worsen with length of infection and duration of anti-retroviral therapeutic use. There is now a considerable amount of cumulative evidence that suggests mild forms of cognitive impairments in people living with HIV-1 (PLWH) may be reversible and are associated with specific modifications in brain energy metabolism. In this review we discuss brain energy metabolism with an emphasis on adaptations that occur in response to HIV-1 infection. The potential for interventions that target brain energy metabolism to preserve or restore cognition in PLWH are also discussed.

Brain insulin action: Implications for the treatment of schizophrenia

Insulin action in the central nervous system is a major regulator of energy balance and cognitive processes. The development of central insulin resistance is associated with alterations in dopaminergic reward systems and homeostatic signals affecting food intake, glucose metabolism, body weight and cognitive performance. Emerging evidence has highlighted a role for antipsychotics (APs) to modulate central insulin-mediated pathways. Although APs remain the cornerstone treatment for schizophrenia they are associated with severe metabolic complications and fail to address premorbid cognitive deficits, which characterize the disorder of schizophrenia. In this review, we first explore how the hypothesized association between schizophrenia and CNS insulin dysregulation aligns with the use of APs. We then investigate the proposed relationship between CNS insulin action and AP-mediated effects on metabolic homeostasis, and different domains of psychopathology, including cognition. We briefly discuss a potential role of CNS insulin signaling to explain the hypothesized, but somewhat controversial association between therapeutic efficacy and metabolic side effects of APs. Finally, we propose how this knowledge might inform novel treatment strategies to target difficult to treat domains of schizophrenia. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

The past and future of novel, non-dopamine-2 receptor therapeutics for schizophrenia: A critical and comprehensive review

Since the discovery of chlorpromazine in the 1950's, antipsychotic drugs have been the cornerstone of treatment of schizophrenia, and all attenuate dopamine transmission at the dopamine-2 receptor. Drug development for schizophrenia since that time has led to improvements in side effects and tolerability, and limited improvements in efficacy, with the exception of clozapine. However, the reasons for clozapine's greater efficacy remain unclear, despite the great efforts and resources invested therewith. We performed a comprehensive review of the literature to determine the fate of previously tested, non-dopamine-2 receptor experimental treatments. Overall we included 250 studies in the review from the period 1970 to 2017 including treatments with glutamatergic, serotonergic, cholinergic, neuropeptidergic, hormone-based, dopaminergic, metabolic, vitamin/naturopathic, histaminergic, infection/inflammation-based, and miscellaneous mechanisms. Despite there being several promising targets, such as allosteric modulation of the NMDA and α7 nicotinic receptors, we cannot confidently state that any of the mechanistically novel experimental treatments covered in this review are definitely effective for the treatment of schizophrenia and ready for clinical use. We discuss potential reasons for the relative lack of progress in developing non-dopamine-2 receptor treatments for schizophrenia and provide recommendations for future efforts pursuing novel drug development for schizophrenia.

Safety of intranasal human insulin: A review

AIMS

To conduct a review in order to assess the safety of intranasal human insulin in clinical studies as well as the temporal stability of nasal insulin sprays.

MATERIAL AND METHODS

An electronic search was performed using MEDLINE. We selected original research on intranasal human insulin without further additives in humans. The studies included could be of any design as long as they used human intranasal insulin as their study product. All outcomes and adverse side effects were extracted.

RESULTS

A total of 38 studies in 1092 individuals receiving acute human intranasal insulin treatment and 18 studies in 832 individuals receiving human intranasal insulin treatment lasting between 21 days and 9.7 years were identified. No cases of symptomatic hypoglycaemia or severe adverse events (AEs) were reported. Transient local side effects in the nasal area were frequently experienced after intranasal insulin and placebo spray, while other AEs were less commonly reported. There were no reports of participants being excluded as a result of AEs. No instances of temporal stability of nasal insulin were reported in the literature. Tests on insulin that had been repacked into spray flasks showed that it had a chemical stability of up to 57 days.

CONCLUSIONS

Our retrospective review of published studies on intranasal insulin did not reveal any safety concerns; however, there were insufficient data to ensure the long-term safety of this method of chronic insulin administration. Improved insulin preparations that cause less nasal irritation would be desirable for future treatment.

Learning and memory performance following acute intranasal insulin administration in abstinent smokers

The highest incidence of relapse to smoking occurs within the first 2 weeks of a cessation attempt. In addition to enhanced nicotine craving, this phase of smoking cessation is also marked by learning and memory dysfunction. Many smokers are not able to overcome these symptoms, and they relapse to smoking shortly after trying to quit. In two clinical studies, we evaluated intranasal insulin for efficacy in improving learning and memory function during nicotine withdrawal. Our first study was a crossover evaluation (N = 19) following 20 hr of smoking abstinence. Study 2 was a parallel design study (N = 50) following 16 hr of abstinence. Intranasal insulin (60 IU) dose was administered in both studies and cognitive function was measured using California Verbal Learning Test-II. Intranasal insulin did not improve learning over the 5 verbal learning trials. In addition, intranasal insulin did not improve either short- or long-delay recall in either study. In summary, the one-time administration of intranasal insulin does not improve verbal learning and memory in smokers. Whether longer administration schedules may be of benefit should be evaluated in future studies.

Intracranial pancreatic islet transplantation increases islet hormone expression in the rat brain and attenuates behavioral dysfunctions induced by MK-801 (dizocilpine)

The treatment of rodents with non-competitive antagonist of the N-Methyl-D-aspartate (NMDA) receptor, MK-801 (dizocilpine), induces symptoms of psychosis, deficits in spatial memory and impairment of synaptic plasticity. Recent studies have suggested that insulin administration might attenuate the cognitive dysfunctions through the modulatory effect on the expression of NMDA receptors and on the brain insulin signaling. Intrahepatic pancreatic islet transplantation is known as an efficient tool for correcting impaired insulin signaling. We examined the capacity of syngeneic islets grafted into the cranial subarachnoid cavity to attenuate behavioral dysfunctions in rats exposed to MK-801. Animals were examined in the open field (OF) and the Morris Water Maze (MWM) tests following acute or subchronic administration of MK-801. We found well-vascularized grafted islets expressing insulin, glucagon and somatostatin onto the olfactory bulb and prefrontal cortex. Significantly higher levels of insulin were detected in the hippocampus and prefrontal cortex of transplanted animals compared to the non-transplanted rats. All animals expressed normal peripheral glucose homeostasis for two months after transplantation. OF tests revealed that rats exposed to MK-801 treatment, showed hyper-responsiveness in motility parameters and augmented center field exploration compared to intact controls and these effects were attenuated by the grafted islets. Moreover, in the MWM, the rats treated with MK-801 showed impairment of spatial memory that were partially corrected by the grafted islets. In conclusion, intracranial islet transplantation leads to the expression of islet hormones in the brain and attenuates behavioral and cognitive dysfunctions in rats exposed to MK-801 administration without altering the peripheral glucose homeostasis.

The Potential Role of Insulin on the Shank-Postsynaptic Platform in Neurodegenerative Diseases Involving Cognition

Loss of synaptic function is critical in the pathogenesis of Alzheimer's disease (AD) and other central nervous system (CNS) degenerations. A promising candidate in the regulation of synaptic function is Shank, a protein that serves as a scaffold for excitatory synaptic receptors and proteins. Loss of Shank alters structure and function of the postsynaptic density (PSD). Shank proteins are associated with N-methyl-d-aspartate and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor loss at the PSD in AD; mutations in Shank also lead to autism spectrum disorders (ASDs) and schizophrenia, both of which affect cognition, suggesting that Shank may play a common pathologic role in AD, ASD, and schizophrenia. Shank protein directly associates with insulin receptor substrate protein p53 in PSD. Insulin and insulin sensitizers have been used in clinical trials for these diseases; this suggests that insulin signals may alter protein homeostasis at the shank-postsynaptic platform in PSDs; insulin could improve the function of synapses in these diseases.

No effect of adjunctive, repeated dose intranasal insulin treatment on body metabolism in patients with schizophrenia

OBJECTIVE

This study examined the effect of adjunctive intranasal insulin therapy on body metabolism in patients with schizophrenia.

METHOD

Each subject had a DSM-IV diagnosis of schizophrenia or schizoaffective disorder and had been on stable dose of antipsychotic agent for at least one month. In an 8-week randomized, double-blind, placebo-controlled study, subjects received either intranasal insulin (40 IU 4 times per day) or placebo. The whole body dual-energy X-ray absorptiometry (DXA) was used to assess body composition. Lipid particles were assessed using nuclear magnetic resonance (NMR) spectroscopy. All assessments were conducted at baseline, and repeated at week 8.

RESULTS

A total number of 39 subjects completed the study (18 in the insulin group, 21 in the placebo group). There were no significant differences between the two groups in week 8 changes for body weight, body mass index, waist circumference, as well as various measures of lipid particles (p's>0.100). The DXA assessment showed no significant differences between the two groups in week 8 changes for fat mass, lean mass or total mass (p's>0.100).

CONCLUSION

In the present study, adjunctive therapy of intranasal insulin did not seem to improve body metabolism in patients with schizophrenia. The implications for future studies were discussed.