Towards selective CNS PET imaging of the 5-HT7 receptor system: Past, present and future

Discovery and Model-Informed Drug Development of a Controlled-Release Formulation of Nonracemic Amisulpride that Reduces Plasma Exposure but Achieves Pharmacodynamic Bioequivalence in the Brain

Nonracemic amisulpride (SEP-4199) is an investigational 85:15 ratio of aramisulpride to esamisulpride and currently in clinical trials for the treatment of bipolar depression. During testing of SEP-4199, a pharmacokinetic/pharmacodynamic (PK/PD) disconnect was discovered that prompted the development of a controlled-release (CR) formulation with improved therapeutic index for QT prolongation. Observations that supported the development of a CR formulation included (i) plasma concentrations of amisulpride enantiomers were cleared within 24-hours, but brain dopamine D2 receptor (D2R) occupancies, although achieving stable levels during this time, required 5 days to return to baseline; (ii) nonracemic amisulpride administered to non-human primates produced significantly greater D2R occupancies during a gradual 6-hour administration compared with a single bolus; (iii) concentration-occupancy curves were left-shifted in humans when nonracemic amisulpride was gradually administered over 3 and 6 hours compared with immediate delivery; (iv) CR solid oral dose formulations of nonracemic amisulpride were able to slow drug dissolution in vitro and reduce peak plasma exposures in vivo in human subjects. By mathematically solving for a drug distribution step into an effect compartment, and for binding to target receptors, the discovery of a novel PK/PD model (termed here as Distribution Model) accounted for hysteresis between plasma and brain, a lack of receptor saturation, and an absence of accumulation of drug occupancy with daily doses. The PK/PD disconnect solved by the Distribution Model provided model-informed drug development to continue in Phase III using the non-bioequivalent CR formulation with diminished QT prolongation as dose-equivalent to the immediate release (IR) formulation utilized in Phase II.

The effectiveness of vortioxetine on neurobiochemical metabolites and cognitive of major depressive disorders patients: A 8-week follow-up study

OBJECTIVE

Vortioxetine has been shown to improve cognitive performance in people with depression. This study will look at the changes in neurobiochemical metabolites that occur when vortioxetine improves cognitive performance in MDD patients, with the goal of determining the neuroimaging mechanism through which vortioxetine improves cognitive function.

METHODS

30 depressed patients and 30 demographically matched healthy controls (HC) underwent MCCB cognitive assessment and 1H-MRS. After 8 weeks of vortioxetine medication, MCCB and 1H-MRS tests were retested in the MDD group. Before and after therapy, changes in cognitive performance, NAA/Cr, and Cho/Cr were examined in the MDD group.

RESULTS

Compared with the HC group, the MDD group had significant reduced in verbal learning, social cognition, and total cognition (all p < 0.05). And the MDD group had lower NAA/Cr in Right thalamus and Left PFC; the Cho/Cr in Right thalamus was lower than HC; the Cho/Cr in Left ACC had significantly increase (all p < 0.05). The MDD group showed significant improvements in the areas of verbal learning, attention/alertness, and total cognitive function before and after Vortioxetine treatment (all p < 0.05). The NAA/Cr ratio of the right PFC before and after treatment (t = 2.338, p = 0.026) showed significant changes.

CONCLUSIONS

Vortioxetine can enhance not just the depression symptoms of MDD patients in the initial period, but also their verbal learning, social cognition, and general cognitive capacities after 8 weeks of treatment. Furthermore, vortioxetine has been shown to enhance cognitive function in MDD patients by altering NAA/Cr and Cho/Cr levels in the frontal-thalamic-ACC.

Synthesis and evaluation of 99mTc-labeled 1-(2-Pyridyl)piperazine derivatives as radioligands for 5HT7 receptors

Glioblastoma multiform (GBM) is one of the most aggressive tumors of the central nervous system in humans. GBM overexpresses serotonin-7 receptors (5-HT₇Rs); hence, this study aims to develop 5-HT₇R targeted radiotracers. Aryl piperazine derivatives can act as ligands for 5-HT₇R. Therefore, compounds 6 and 7 as 1-(3-nitropyridin-2-yl)piperazine derivatives were synthesized and radiolabeled with ^99mTcN²⁺ core. Radiolabeled 6 and 7 (^99mTcN-[6] and ^99mTcN-[7]) were prepared with high radiochemical purity (RCP > 96%). They displayed high affinity toward U-87 MG cell line 5-HT₇R. The calculated K_i for ^99mTcN-[7] was lower than that of ^99mTcN-[6] (14.85 ± 0.32 vs 22.57 ± 0.73 nM) which indicates the higher affinity of ^99mTcN-[7] toward 5-HT₇R. A molecular docking study also confirmed the binding of these radiotracers to 5-HT₇R. The biodistribution study in normal mice revealed that ^99mTcN-[7] has the highest brain accumulation at 30 min post-injection (0.54 ± 0.12 %ID/g) while the uptake of ^99mTcN-[6] is much lower (0.14 ± 0.02 %ID/g). The biodistribution study in the xenograft model confirms that the radiotracers recognize the tumor site. ^99mTcN-[6], and ^99mTcN-[7] showed the highest tumor uptake at 1-hour post-injection (5.44 ± 0.58 vs 4.94 ± 1.65 %ID/g) and tumor-to-muscle ratios were (4.61 vs. 5.61). The injection of pimozide blocks the receptors and significantly reduces the tumor-to-muscle ratios at 1-hour post-injection to 0.81 and 0.31, respectively. In correlation with in vitro study, ^99mTcN-[6] and ^99mTcN-[7] visualize the tumor site in U-87 MG glioma xenografted nude mice and display the tumor-to-muscle ratios of 7.05 and 6.03.

Therapeutic Potential and Limitation of Serotonin Type 7 Receptor Modulation

Although a number of mood-stabilising atypical antipsychotics and antidepressants modulate serotonin type 7 receptor (5-HT7), the detailed contributions of 5-HT7 function to clinical efficacy and pathophysiology have not been fully understood. The mood-stabilising antipsychotic agent, lurasidone, and the serotonin partial agonist reuptake inhibitor, vortioxetine, exhibit higher binding affinity to 5-HT7 than other conventional antipsychotics and antidepressants. To date, the initially expected rapid onset of antidepressant effects-in comparison with conventional antidepressants or mood-stabilising antipsychotics-due to 5-HT7 inhibition has not been observed with lurasidone and vortioxetine; however, several clinical studies suggest that 5-HT7 inhibition likely contributes to quality of life of patients with schizophrenia and mood disorders via the improvement of cognition. Furthermore, recent preclinical studies reported that 5-HT7 inhibition might mitigate antipsychotic-induced weight gain and metabolic complication by blocking other monoamine receptors. Further preclinical studies for the development of 5-HT7 modulation against neurodevelopmental disorders and neurodegenerative diseases have been ongoing. To date, various findings from various preclinical studies indicate the possibility that 5-HT7 modifications can provide two independent strategies. The first is that 5-HT7 inhibition ameliorates the dysfunction of inter-neuronal transmission in mature networks. The other is that activation of 5-HT7 can improve transmission dysfunction due to microstructure abnormality in the neurotransmission network-which could be unaffected by conventional therapeutic agents-via modulating intracellular signalling during the neurodevelopmental stage or via loss of neural networks with aging. This review attempts to describe the current and novel clinical applications of 5-HT7 modulation based on preclinical findings.

Radiotracers for the Central Serotoninergic System

This review lists the most important radiotracers described so far for imaging the central serotoninergic system. Single-photon emission computed tomography and positron emission tomography radiotracers are reviewed and critically discussed for each receptor.

Benzoxazolone-arylpiperazinyl scaffold-based PET ligand for 5-HT7 : Synthesis and biological evaluation

Efforts are underway to improve the diagnosis and treatment for neurological disorders like depression, anxiety, epilepsy, and schizophrenia. The G-protein-coupled receptors (GPCRs) 5-HT₇   receptor, the most recently identified member of 5-HT receptor family dysregulation has an association with various central nervous system (CNS) disorders and its ligands have an edge as potential therapeutics. Here, we report the synthesis, characterization, and biological evaluation of diversely substituted methoxy derivatives of 2-benzoxazolone arylpiperazine for targeting 5-HT₇  receptors. Out of all derivatives, only C-2 substituted derivative, 3-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)benzoxazol-2(3H)-one/ABO demonstrate a high affinity for human 5-HT₇ receptors. [¹¹ C]ABO was obtained by O-methylation of desmethyl-precursor using [¹¹ C]CH₃ OTf in the presence of NaOH giving a high radiochemical yield of 25 ± 12% (decay-corrected, n = 7) with stability up to 1.5 h postradiolabeling. In vitro autoradiography displays binding of [¹¹ C]ABO in accordance with 5-HT₇ distribution with a decrease of approximately 80% and 40% activity in the hippocampus and cerebellum brain region when administered with 10 µM cold ligand. Prefatory positron emission tomography scan results in Sprague-Dawley (SD) rat brain revealed fast and high radioactivity build-up in 5-HT₇ receptor-rich regions, namely, the hippocampus (2.75 ± 0.16 SUV) and the cerebral cortex (2.27 ± 0.02 SUV) establishing selective targeting of [¹¹ C]ABO. In summary, these pieces of data designate [¹¹ C]ABO as a promising 5-HT₇  receptor ligand that can have possible roles in clinics after its further optimization on different animal models.

Alpha-Synuclein PET Tracer Development-An Overview about Current Efforts

Neurodegenerative diseases such as Parkinson’s disease (PD) are manifested by inclusion bodies of alpha-synuclein (α-syn) also called α-synucleinopathies. Detection of these inclusions is thus far only possible by histological examination of postmortem brain tissue. The possibility of non-invasively detecting α-syn will therefore provide valuable insights into the disease progression of α-synucleinopathies. In particular, α-syn imaging can quantify changes in monomeric, oligomeric, and fibrillic α-syn over time and improve early diagnosis of various α-synucleinopathies or monitor treatment progress. Positron emission tomography (PET) is a non-invasive in vivo imaging technique that can quantify target expression and drug occupancies when a suitable tracer exists. As such, novel α-syn PET tracers are highly sought after. The development of an α-syn PET tracer faces several challenges. For example, the low abundance of α-syn within the brain necessitates the development of a high-affinity ligand. Moreover, α-syn depositions are, in contrast to amyloid proteins, predominantly localized intracellularly, limiting their accessibility. Furthermore, another challenge is the ligand selectivity over structurally similar amyloids such as amyloid-beta or tau, which are often co-localized with α-syn pathology. The lack of a defined crystal structure of α-syn has also hindered rational drug and tracer design efforts. Our objective for this review is to provide a comprehensive overview of current efforts in the development of selective α-syn PET tracers.

Aliphatic 18 F-Radiofluorination: Recent Advances in the Labeling of Base-Sensitive Substrates*

Aliphatic fluorine-18 radiolabeling is the most commonly used method to synthesize tracers for PET-imaging. With an increasing demand for ¹⁸ F-radiotracers for clinical applications, new labeling strategies aiming to increase radiochemical yields of established tracers or, more importantly, to enable ¹⁸ F-labeling of new scaffolds have been developed. In recent years, increased attention has been focused on the direct aliphatic ¹⁸ F-fluorination of base-sensitive substrates in this respect. This minireview gives a concise overview of the recent advances within this field and aims to highlight the advantages and limitations of these methods.

Effects of Subchronic Administrations of Vortioxetine, Lurasidone, and Escitalopram on Thalamocortical Glutamatergic Transmission Associated with Serotonin 5-HT7 Receptor

The functional suppression of serotonin (5-HT) type 7 receptor (5-HT7R) is forming a basis for scientific discussion in psychopharmacology due to its rapid-acting antidepressant-like action. A novel mood-stabilizing atypical antipsychotic agent, lurasidone, exhibits a unique receptor-binding profile, including a high affinity for 5-HT7R antagonism. A member of a novel class of antidepressants, vortioxetine, which is a serotonin partial agonist reuptake inhibitor (SPARI), also exhibits a higher affinity for serotonin transporter, serotonin receptors type 1A (5-HT1AR) and type 3 (5-HT3R), and 5-HT7R. However, the effects of chronic administration of lurasidone, vortioxetine, and the selective serotonin reuptake inhibitor (SSRI), escitalopram, on 5-HT7R function remained to be clarified. Thus, to explore the mechanisms underlying the clinical effects of vortioxetine, escitalopram, and lurasidone, the present study determined the effects of these agents on thalamocortical glutamatergic transmission, which contributes to emotional/mood perception, using multiprobe microdialysis and 5-HT7R expression using capillary immunoblotting. Acute local administration of a 5-HT7R agonist and antagonist into the mediodorsal thalamic nucleus (MDTN) enhanced and reduced thalamocortical glutamatergic transmission, induced by N-methyl-D-aspartate (NMDA)/glutamate receptor inhibition in the reticular thalamic nucleus (RTN). Acute local administration of a relevant therapeutic concentration of vortioxetine and lurasidone into the MDTN suppressed the thalamocortical glutamatergic transmission via 5-HT7R inhibition, whereas that of escitalopram activated 5-HT7R. Subchronic administration of effective doses of vortioxetine and lurasidone (for 7 days) reduced the thalamocortical glutamatergic transmission, but escitalopram did not affect it, whereas subchronic administration of these three agents attenuated the stimulatory effects of the 5-HT7R agonist on thalamocortical glutamatergic transmission. Subchronic administration of effective doses of vortioxetine, lurasidone, and escitalopram downregulated the 5-HT7R expression of the plasma membrane in the MDTN; the 5-HT7R downregulation induced by vortioxetine and lurasidone was observed at 3 days, but that induced by escitalopram required a longer duration of 7 days. These results indicate that chronic administration of vortioxetine, escitalopram, and lurasidone generate downregulation of 5-HT7R in the thalamus; however, the direct inhibition of 5-HT7R associated with vortioxetine and lurasidone generates more rapid downregulation than the indirect elevation of the extracellular serotonin level via serotonin transporter inhibition by escitalopram.

Current Limitations and Candidate Potential of 5-HT7 Receptor Antagonism in Psychiatric Pharmacotherapy

Several mood-stabilizing atypical antipsychotics and antidepressants weakly block serotonin (5-HT) receptor type-7 (5-HT7R); however, the contributions of 5-HT7R antagonism to clinical efficacy and pathophysiology are yet to be clarified. A novel mood-stabilizing antipsychotic agent, lurasidone exhibits predominant binding affinity to 5-HT7R when compared with other monoamine receptors. To date, we have failed to discover the superior clinical efficacy of lurasidone on schizophrenia, mood, or anxiety disorders when compared with conventional mood-stabilizing atypical antipsychotics; however, numerous preclinical findings have indicated the possible potential of 5-HT7R antagonism against several neuropsychiatric disorders, as well as the generation of novel therapeutic options that could not be expected with conventional atypical antipsychotics. Traditional experimental techniques, electrophysiology, and microdialysis have demonstrated that the effects of 5-HT receptor type-1A (5-HT1AR) and 5-HT7R on neurotransmission are in contrast, but the effect of 5-HT1AR is more predominant than that of 5-HT7R, resulting in an insufficient understanding of the 5-HT7R function in the field of psychopharmacology. Accumulating knowledge regarding the pharmacodynamic profiles of 5-HT7R suggests that 5-HT7R is one of the key players in the establishment and remodeling of neural development and cytoarchitecture during the early developmental stage to the mature brain, and dysfunction or modulation of 5-HT7R is linked to the pathogenesis/pathophysiology of neuropsychiatric and neurodevelopmental disorders. In this review, to explore candidate novel applications for the treatment of several neuropsychiatric disorders, including mood disorders, schizophrenia, and other cognitive disturbance disorders, we discuss perspectives of psychopharmacology regarding the effects of 5-HT7R antagonism on transmission and intracellular signaling systems, based on preclinical findings.

Blocking of efflux transporters in rats improves translational validation of brain radioligands

Background

Positron emission tomography (PET) is a molecular imaging technique that can be used to investigate the in vivo pharmacology of drugs. Initial preclinical evaluation of PET tracers is often conducted in rodents due to the accessibility of disease models as well as economic considerations. Compared to larger species, rodents display a higher expression and/or activity of efflux transporters such as the P-glycoprotein (P-gp). Low brain uptake could, therefore, be species-specific and uptake in rodents not be predictive for that in humans. We hypothesized that a better prediction from rodent data could be achieved when a tracer is evaluated under P-gp inhibition. Consequently, we compared the performance of eight neuroreceptor tracers in rats with and without P-gp inhibition including a specific binding blockade. This data set was then used to predict the binding of these eight tracers in pigs.

Methods

PET tracers targeting serotonin 5-HT_2A receptors ([¹⁸F]MH.MZ, [¹⁸F]Altanserin, [¹¹C]Cimbi-36, [¹¹C]Pimavanserin), serotonin 5-HT₇ receptors ([¹¹C]Cimbi-701, [¹¹C]Cimbi-717 and [¹¹C]BA-10) and dopamine D_2/3 receptors ([¹⁸F]Fallypride) were used in the study. The brain uptake and target-specific binding of these PET radiotracers were evaluated in rats with and without inhibition of P-gp. Rat data were subsequently compared to the results obtained in pigs.

Results

Without P-gp inhibition, the amount of target-specific binding in the rat brain was sufficient to justify further translation for three out of eight evaluated tracers. With P-gp inhibition, results for five out of eight tracers justified further translation. The performance in pigs could correctly be predicted for six out of eight tracers when rat data obtained under P-gp inhibition were used, compared to four out of eight tracers without P-gp inhibition.

Conclusions

P-gp strongly affects the uptake of PET tracers in rodents, but false prediction outcomes can be reduced by evaluating a tracer under P-gp inhibition.

Serotonin research: Crossing scales and boundaries

Nearly 100 years after the discovery of serotonin, its role remains elusive. Modulation of serotonin transmission is considered in numerous central nervous system (CNS) diseases including depression, anxiety, schizophrenia, obsessive-compulsive disorders, addiction, Parkinson's disease, and Alzheimer's disease. The therapeutic strategies based on serotonin systems have evolved thanks to better identification of the involvement of serotonin in various diseases, the better use of animal models, a better understanding of the molecular environment of serotonin receptors, and ultimately the better understanding of the interaction of serotonin neurotransmission with other biological systems. Some 5-HT receptors are still the object of numerous investigations including 5-HT_1A, 5-HT_2A, and 5-HT₆ receptor subtypes. It is noteworthy that the direction of research is moving towards a simultaneous action at multiple targets either through different 5-HT targets or the consideration of both 5-HT and other targets to achieve better therapeutic responses.