Improved Pharmacokinetic and Pharmacodynamic Profile of Deuterium-Reinforced Tricyclic Antidepressants Doxepin, Dosulepin, and Clomipramine in Animal Models.

BACKGROUND AND OBJECTIVES: Doxepin, dosulepin, and clomipramine are tricyclic antidepressants (TCAs) that act as serotonin and noradrenaline reuptake inhibitors. The metabolites formed by N-dealkylation of these tricyclic antidepressants contribute to overall poor pharmacokinetics and efficacy. Deuteration of the methyl groups at metabolically active sites has been reported to be a useful strategy for developing more selective and potent antidepressants. This isotopic deuteration can lead to better bioavailability and overall effectiveness. The objective is to study the effect of site-selective deuteration of TCAs on their pharmacokinetic and pharmacodynamic profile by comparison with their nondeuterated counterparts. METHODS: In the current study, the pharmacokinetic profile and antidepressant behavior of deuterated TCAs were evaluated using the forced swim test (FST) and tail suspension test (TST), using male Wistar rats and male Swiss albino mice, respectively; additionally, a synaptosomal reuptake study was carried out. RESULTS: Compared with the nondeuterated parent drugs, deuterated forms showed improved efficacy in the behavior paradigm, indicating improved pharmacological activity. The pharmacokinetic parameters indicated increased maximum concentration in the plasma (Cmax), elimination half-life (t1/2), and area under the concentration-time curve (AUC)  in deuterated compounds. This can have a positive clinical impact on antidepressant treatment. Synaptosomal reuptake studies indicated marked inhibition of the reuptake mechanism of serotonin (5-HT) and norepinephrine. CONCLUSIONS: Deuterated TCAs can prove to be potentially better molecules in the treatment of neuropsychiatric disorders as compared with nondeuterated compounds. In addition, we have demonstrated a concept that metabolically active, site-selective deuteration can be beneficial for improving the pharmacokinetic and pharmacodynamic profiles of TCAs. A further toxicological study of these compounds is needed to validate their future clinical use.

Insomnia Management: A Review and Update.

KEY TAKEAWAYS: Insomnia is a distinct disorder that is common, yet underrecognized and undertreated in primary care. Treating insomnia has been shown to improve outcomes, including reduced risk of developing cardiovascular and mental health disorders. Insomnia is influenced by the brain's regulation of sleep and wake, which are mutually exclusive events. Insomnia should be treated as a distinct condition, even when occurring with a comorbid diagnosis such as depression or anxiety. Clinicians should implement a multimodal approach to insomnia management, including nonpharmacologic interventions and pharmacologic therapy (when indicated). Pharmacologic agents that are approved by the US Food and Drug Administration for insomnia include benzodiazepine receptor agonists (zolpidem, eszopiclone, and zaleplon), low-dose doxepin (tricyclic antidepressant), ramelteon (melatonin receptor agonist), and dual orexin receptor agonists (DORAs, daridorexant, lemborexant, and suvorexant). Unlike other pharmacologic agents, DORAs inhibit wakefulness rather than induce sedation. Additionally, these medications have no evidence of rebound insomnia or withdrawal, and little to no abuse potential. Daridorexant is the newest DORA, has an ideal half-life of 8 hours, and has demonstrated continued efficacy over a 12-month period. Selection of pharmacologic agent should be based on the patient's comorbid conditions, treatment goals and preferences, and other clinical characteristics.

In-vitro antiviral activity of doxepin hydrochloride against group B coxsackievirus.

Group B coxsackievirus is an enterovirus that can cause a variety of diseases, including myocarditis, aseptic meningitis, and hand, foot, and mouth disease. Currently, there is no effective antiviral drug against this virus. In this study, we used a cytopathic effect-based viral inhibition assay to screen an FDA-approved drug library and found that doxepin hydrochloride had potential antiviral activity. Doxepin hydrochloride exhibited strong antiviral activity against coxsackievirus B types 1-3 with a 50% inhibitory concentration of 10.12 ± 0.85 µM. Moreover, doxepin hydrochloride did not exert antiviral activity against other enteroviruses, including enterovirus A71 (subtypes BrCr/C4) and coxsackievirus A (subtypes 6/10/16). Furthermore, doxepin hydrochloride inhibited virus replication in the early stage of the infection cycle rather than affecting the entry or assembly process. In addition, a few mechanism-related pharmacophores were discovered through gene association network analysis. These findings identify a possible lead compound for treating coxsackievirus B infection and simultaneously offer valuable clues for drug repositioning.

Repositioning of histamine H1 receptor antagonist: Doxepin inhibits viropexis of SARS-CoV-2 Spike pseudovirus by blocking ACE2.

The spread of the corona virus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been intensifying in the past year, posing a huge threat to global health. There is an urgent need for effective drugs and vaccines to fight the COVID-19, but their advent may not be quite fast. Drug repurposing is a feasible strategy in the current situation, which could greatly shorten drug development time and help to response quickly to the novel virus outbreak. It has been reported that histamine H1 receptor antagonists have broad-spectrum antiviral effects. Therefore, in this study, we aim to screen potential drugs among histamine H1 receptor antagonists that may inhibit SARS-CoV-2 infection. Based on the model of angiotensin-converting enzyme 2 (ACE2) overexpressing HEK293T cell membrane chromatography (CMC), five FDA-approved histamine H1 receptor antagonists were found to have bioaffinity to ACE2. Then we determined the interaction between these drugs and ACE2 by frontal analysis and surface plasmon resonance (SPR), which consistently demonstrated that these hits bind to ACE2 at micromolar levels of affinity. Through the pseudovirus assay, we finally identified that doxepin could inhibit SARS-CoV-2 spike pseudovirus from entering the ACE2-expressing cell, reducing the infection rate to 25.82%. These preliminary results indicate that the histamine H1 receptor antagonist, doxepin, is a viable drug candidate for clinical trials. Therefore, we hope the work timely provides rational help for developing anti-SARS-CoV-2 drugs to control the rapid spread of SARS-CoV-2.

Protective effects of doxepin cream on radiation dermatitis in breast cancer: A single arm double-blind randomized clinical trial.

AIMS: Breast cancer is the most frequently occurring cancer in women. Lumpectomy followed by radiotherapy is suggested to be as effective as a total mastectomy. Radiation-induced dermatitis often occurs as a result of breast radiotherapy. Recent studies suggest that doxepin has promising anti-inflammatory properties. This study was undertaken to evaluate the effects of doxepin therapy on radiation dermatitis. METHODS: A double-blind randomized clinical trial was launched from 2016 to 2017, with a total of 48 patients who had undergone breast-conserving surgery and received postoperative radiation therapy. Radiotherapy was applied 5 days per week for 5 weeks. Adverse dermatological effects were evaluated by a physician at the beginning of the fifth week of radiotherapy and the patients were then randomly assigned (1:1 ratio) to receive either doxepin (5%) or placebo cream for 7 days. RESULTS: There were no significant differences in the dermatitis grade between doxepin and placebo groups at baseline (P > .5). The occurrence of acute dermatitis (grade 2 or higher) was significantly lower with the use of doxepin than with placebo (P ≤ .0001, Zα = 1.96 at 95% confidence interval). CONCLUSION: Doxepin cream prevents dermatitis grade 2 or higher during post-operative breast irradiation. Doxepin cream is easy to use, affordable and prevents pain and irritation.

Anti-inflammatory effects of doxepin hydrochloride against LPS-induced C6-glioma cell inflammatory reaction by PI3K-mediated Akt signaling.

Recent studies have shown that tricyclic antidepressants (TCAs) may have anti-inflammatory and anticonvulsant effects in addition to its antidepressant effects. So far, the nonantidepressant effects of TCAs and their molecular pharmacological mechanisms remain completely unclear. Chronic inflammation in the brain parenchyma may be related to the pathogenesis and progression of various neurodegenerative diseases. As a common antidepressant and anti-insomnia drug, doxepin also may be a potential anti-inflammatory and anticonvulsant drug, so the study on the anti-inflammatory protective effect of doxepin and its molecular mechanism has become a very important issue in pharmacology and clinical medicine. Further elucidating the anti-inflammatory and neuroprotective effects of doxepin and its molecular mechanism may provide the important theoretical and clinical basis for the prevention and treatment of neurodegenerative disease. This study was designed to understand the glio-protective mechanism of doxepin against the inflammatory damage induced by lipopolysaccharide (LPS) exposure in C6-glioma cells. We found the treatment of C6-glioma cells with LPS results in deleterious effects, including the augmentation of inflammatory cytokine levels (tumor necrosis factor-α, interleukin-1ß), and suppresses the Akt phosphorylation. Furthermore, our outcomes demonstrated that doxepin was able to suppress these effects induced by LPS, through activation of the phosphatidylinositol-3-kinase-mediated protein kinase B (Akt) pathway. To sum up, these results highlight the potential role of doxepin against neuroinflammatory-related disease in the brain.

Acid sphingomyelinase mediates the noise-induced liver disorder in mice.

Noise-induced structural and functional disorder of the liver has been realized, but the underlying mechanism remains to be characterized, which has limited the introduction of precautious measures. Over-activation of acid sphingomyelinase (ASM)/ceramide (Cer) pathway takes centre stage in hepatocyte injury entailed by various stimulus. We aimed to investigate whether it mediated the noise elicited liver disorder on infrastructure, lipid metabolism, apoptosis, and oxidative stress. Mice were exposed to broad band noise (20-20k Hz, 90-110 dB) for 1, 3, 5 or 7 days by 3 hr/d. Doxepin hydrochloride (DOX), an ASM inhibitor was given by 5 mg/kg/d gavage. We showed that 5 or 7 days intense, broad band noise exposure caused significant infrastructure derangement and lipid droplets storage in hepatocytes. The content of cholesterol, free fatty acids or triglyceride was increased significantly in liver tissue upon noise stimulation. Moreover, the noise promoted apoptosis and superoxide generation in hepatocytes significantly, enhancing activity of aspartate aminotransferase (AST) or alanine amino transferase (ALT) in serum. Acid sphingomyelinase activity and Cer generation in liver tissue were elevated by noise exposure, which was normalized with DOX administrated. Accordingly, DOX alleviated steatosis, apoptosis, oxidative stress and enzymatic change in hepatocytes or serum of noise exposed mice substantially. In summary, our results suggest the ASM/Cer pathway contributes to the broad band noise elicited liver damage in mice.

Tricyclic antidepressants inhibit Candida albicans growth and biofilm formation.

Candida albicans is a commensal yeast of the human body, able to form biofilms on solid surfaces such as implanted medical devices, and contributes to nosocomial infections. Biofilms have the capacity to resist higher levels of antifungals compared with planktonic cells, and can develop tolerance to commonly used treatments. The necessity to overcome acquired drug resistance and identify new active molecules with low toxicity is a significant problem. It has been reported that some antidepressants have antibacterial properties, but little is known regarding the effect of these drugs on fungi. This study demonstrated the capacity of three tricyclic antidepressants (doxepin, imipramine and nortriptyline) to inhibit the growth and biofilm formation of Candida spp. The antimicrobial potential of the drugs was assessed by studying gene expression, hyphae formation, biofilm growth and maturation. Their negative impact on the growth of C. albicans and other Candida spp. is shown in vitro and with the hepatic S9 system, which is preliminary to any in-vivo test. This study found that the antidepressants considered can inhibit not only hyphae and biofilm formation, but also kill cells in a mature biofilm. Moreover, cell lysis by nortriptyline was observed, along with its synergistic activity with amphotericin B. These findings suggest that tricyclic antidepressants, particularly nortriptyline, should be studied further in drug repositioning programmes to assess their antimycotic capacity in full.

Doxepin Mitigates Noise-induced Neuronal Damage in Primary Auditory Cortex of Mice via Suppression of Acid Sphingomyelinase/Ceramide Pathway.

Neuronal damage in primary auditory cortex (A1) underlies complex manifestations of noise exposure, prevention of which is critical for health maintenance. Acid sphingomyelinase (ASM) catalyzes generation of ceramide (Cer) which if over-activated mediates neuronal disorders in various diseases. Tricyclic antidepressants (TCAs), by restraining ASM/Cer, benefits multiple neuronal anomalies, so we aimed to elucidate the effect of TCA on noise induced hearing loss and auditory cortex derangement, unraveling mechanism involved. The mice were exposed to noise with frequencies of 20-20 KHz and intensity of 95 dB. Doxepin hydrochloride (DOX), a kind of TCAs, was given intragastrically by 5 mg kg-1  days-1 . Morphology of neurons was examined using hematoxylin-eosin (HE) and Nissl staining. Apoptosis was assayed through transferase-mediated dUTP nick end labeling (TUNEL). The content of ASM, Cer or acid ceramidase (AC) was detected by western blot and immunohistochemistry analysis. We demonstrated intense, broad band noise caused upward shift of auditory brainstem response (ABR) threshold to sound over frequencies 4-32 KHz, with prominent morphologic changes and enhanced apoptosis in neurons of primary auditory cortex (A1) (P < 0.05). DOX partly restored noise-caused hearing loss alleviating morphologic changes or apoptosis remarkably (P < 0.05). Both ASM and Cer abundance were elevated significantly by noise which was reversed upon DOX treatment (P < 0.05), but neither noise nor DOX altered AC content. DOX had no influence on hearing, neuronal morphology or ASM/Cer in control mice. Our result suggests DOX palliates noise induced hearing loss and neuronal damage in auditory cortex by correcting over-activation of ASM/Cer without hampering intrinsic behavior of it. Anat Rec, 300:2220-2232, 2017. © 2017 Wiley Periodicals, Inc.

A novel neurological function of rice bran: a standardized rice bran supplement promotes non-rapid eye movement sleep in mice through histamine H1 receptors.

SCOPE: Although rice bran has been shown to be associated with a wide spectrum of health benefits, to date, there are no reports on its effects on sleep. We investigated the effect of rice bran on sleep and the mechanism underlying this effect. METHODS AND RESULTS: Electroencephalography was used to evaluate the effects of standardized rice bran supplement (RBS) and doxepin hydrochloride (DH), a histamine H1 receptor (H1 R) antagonist used as a positive control, on sleep in mice. The mechanism of RBS action was investigated using knockout (KO) mice and ex vivo electrophysiological recordings. Oral administration of RBS and DH significantly decreased sleep latency and increased the amount of non-rapid eye movement sleep (NREMS) in mice. Similar to DH, RBS fully inhibited H1 R agonist-induced increase in action potential frequency in tuberomammillary nucleus neurons. In H1 R KO mice, neither RBS nor DH administration led to the increase in NREMS and decrease in sleep latency observed in WT mice. These results indicate that the sleep-promoting effect of RBS is completely dependent on H1 R antagonism. CONCLUSIONS: RBS decreases sleep latency and promotes NREMS through the inhibition of H1 R, suggesting that it could be a promising therapeutic agent for insomnia.

Arousability and Fall Risk During Forced Awakenings From Nocturnal Sleep Among Healthy Males Following Administration of Zolpidem 10 mg and Doxepin 6 mg: A Randomized, Placebo-Controlled, Four-Way Crossover Trial.

Study Objectives: To examine and compare the arousability threshold and fall risk upon awakening of doxepin (6 mg) versus zolpidem (10 mg). Methods: A total of 52 healthy adult males were included in a double-blind, placebo-controlled, four-way crossover study. The experimental procedure included four nights with polysomnography in the lab (zolpidem, doxepin, and their respective placebo conditions). Arousability was measured using an auditory awakening threshold delivered at the peak-plasma concentration for the active hypnotics and at matched times for the respective placebo conditions. Fall risk during the night was measured following awakening using the Berg Balance Scale and the Tandem Walk Task. Results: Both arousability and fall risk were lower in the doxepin condition compared to the zolpidem condition. Furthermore, arousability and fall risk for doxepin did not differ significantly from the placebo conditions. A significantly greater proportion of participants in the zolpidem condition (63.5%) did not wake until receiving the loudest tone (110 dB) as compared to the doxepin (17.6%) and placebo conditions (17.3%, 5.8%). Conclusions: Results suggest that zolpidem has greater risks for balance and awakening threshold compared with low-dose doxepin. Future prospective studies should extend results to clinical samples with population-level risk of injury and arousability.

Sleep-Promoting Effects and Possible Mechanisms of Action Associated with a Standardized Rice Bran Supplement.

Natural sleep aids are becoming more popular due to the widespread occurrence of sleep disorders. The objective of this study was to assess the sleep-promoting effects of rice bran-a product that is considered as a functional ingredient. To evaluate the sleep-promoting effects of a standardized rice bran supplement (RBS), we employed a pentobarbital-induced sleep test and conducted analyses of sleep architecture. In addition, the effect of RBS on a caffeine-induced sleep disturbance was investigated. Oral administration of RBS (500 and 1000 mg/kg) produced a significant decrease in sleep latency and increase in sleep duration in pentobarbital-induced sleep in mice. Moreover, both RBS (1000 mg/kg) and doxepin hydrochloride (histamine H1 receptor antagonist, 30 mg/kg) counteracted a caffeine-induced sleep disturbance in mice. In terms of sleep phases, RBS (500 mg/kg) promoted non-rapid eye movement sleep for the first 3 h following its administration. Lastly, we unveiled a possible mechanism for RBS action as the hypnotic effect of RBS was blocked by a histamine H1 receptor agonist. The present study revealed sleep-promoting effects of RBS using various animal assays. Such effects seem to be mediated through the histaminergic system. Our findings suggest that RBS may be a promising natural aid for relieving sleep problems.

Doxepin inhibits GPVI-dependent platelet Ca2+ signaling and collagen-dependent thrombus formation.

Platelet adhesion, activation, and aggregation are essential for primary hemostasis, but are also critically involved in the development of acute arterial thrombotic occlusion. Stimulation of the collagen receptor glycoprotein VI (GPVI) leads to phospholipase Cγ2-dependent inositol triphosphate (IP3) production with subsequent platelet activation, due to increased intracellular Ca2+ concentration ([Ca2+]i). Although tricyclic antidepressants have been shown to potentially impair platelet activation, nothing is hitherto known about potential effects of the tricyclic antidepressant doxepin on platelet Ca2+ signaling and thrombus formation. As shown in the present study, doxepin significantly diminished the stimulatory effect of GPVI agonist collagen-related peptide (CRP) on intracellular Ca2+ release as well as subsequent extracellular Ca2+ influx. Doxepin was partially effective by impairment of CRP-dependent IP3 production. Moreover, doxepin abrogated CRP-induced platelet degranulation and integrin αIIbß3 activation and aggregation. Finally, doxepin markedly blunted in vitro platelet adhesion to collagen and thrombus formation under high arterial shear rates (1,700-s). In conclusion, doxepin is a powerful inhibitor of GPVI-dependent platelet Ca2+ signaling, platelet activation, and thrombus formation.

A Review of Suvorexant, Doxepin, Ramelteon, and Tasimelteon for the Treatment of Insomnia in Geriatric Patients.

Geriatric patients often experience insomnia because of physiological and neurological changes that occur during the aging process. Use of benzodiazepines, nonbenzodiazepine hypnotics, and diphenhydramine for the treatment of insomnia pose an increased risk of cognitive impairment, falls, and fractures in this patient population. Therapeutic alternatives approved by the Food and Drug Administration include suvorexant, doxepin, ramelteon, and tasimelteon, which have shown efficacy and safety in various studies. This paper explores and outlines the available safety and efficacy data associated with these medications and reviews their potential place in therapy in treating insomnia in the geriatric population.

Structural Analysis of the Histamine H1 Receptor.

The crystal structure of the human histamine H1 receptor (H1R) has been determined in complex with its inverse agonist doxepin, a first-generation antihistamine. The crystal structure showed that doxepin sits deeply inside the ligand-binding pocket and predominantly interacts with residues highly conserved among other aminergic receptors. This binding mode is considered to result in the low selectivity of the first-generation antihistamines for H1R. The crystal structure also revealed the mechanism of receptor inactivation by the inverse agonist doxepin. On the other hand, the crystal structure elucidated the anion-binding site near the extracellular portion of the receptor. This site consists of residues not conserved among other aminergic receptors, which are specific for H1R. Docking simulation and biochemical experimentation demonstrated that a carboxyl group on the second-generation antihistamines interacts with the anion-binding site. These results imply that the anion-binding site is a key site for the development of highly selective antihistamine drugs.

Antipruritic Effect of Cold-induced and Transient Receptor Potential-agonist-induced Counter-irritation on Histaminergic Itch in Humans.

A frequent empirical observation is that cold-induced counter-irritation may attenuate itch. The aim of this randomized, single-blinded, exploratory study was to evaluate the counter-irritation effects of cold-stimulation and topical application of transient receptor potential TRPA1/M8-agonists (trans-cinnamaldehyde/L-menthol, respectively), on histamine-induced itch, wheals and neurogenic inflammation in 13 healthy volunteers. Histamine 1% was applied to the volar forearms using skin prick-test lancets. Recorded outcome-parameters were itch intensity, wheal reactions, and neurogenic inflammation (measured by laser-speckle perfusion-imaging). Homotopic thermal counter-irritation was performed with 6 temperatures, ranging from 4°C to 37°C, using a 3 × 3-cm thermal stimulator. Chemical "cold-like" counter-irritation was conducted with 40% L-menthol and 10% trans-cinnamaldehyde, while 5% doxepin was used as a positive antipruritic control/comparator. Cold counter-irritation stimuli from 4°C to 22°C inhibited itch in a stimulus-intensity-dependent manner (p < 0.05) and, to a lesser extent, also wheal reactions and neurogenic inflammation. Chemical "cold-like" counter-irritation with both L-menthol and trans-cinnamaldehyde had antipruritic efficacy similar to doxepin (p < 0.05). Cold-induced counter-irritation had an inhibitory effect on histaminergic itch, suggesting that agonists of cold transduction receptors could be of potential antipruritic value.

Ovariectomy changes the response to antidepressant drugs in tail suspension test in mice.

Depressive symptoms are very frequent over a lifetime, especially for women. Menopause is a period of higher depressive vulnerability. There are suggestive data that estrogen deficiency may increase the susceptibility for depression. We studied whether a bilateral ovariectomy (OVX) modifies mice behaviors and antidepressant drug effects through tail suspension test (TST). We evaluated behavioral changes at 1 week, 2 weeks, and up to 2 months after OVX. The behavior responses to doxepin, paroxetine, and venlafaxine at 1 week, 2 weeks, and 2 months after OVX were evaluated. No obvious difference was detected on the duration of immobility among control group, sham group, and OVX group in the TST at 1 week and 2 weeks after OVX. But the duration of immobility of OVX group was distinctly longer than that of both control group and sham operation group at 2 months after OVX. At 1 and 2 weeks after OVX, only the antidepressant response to venlafaxine was observed, while response to paroxetine increased 2 months after OVX. Response to antidepressant drugs was strongly modified in OVX mice. The present results suggest that not all antidepressant drugs are appropriate for depression with estrogen deficiency.

The effect of histamine on changes in mental energy and fatigue after a single bout of exercise.

The purpose of this research was to determine if histamine, acting on brain H1 receptors, influences changes in feelings of energy and fatigue or cognitive test performance after acute exercise. Women (n=20) with low vigor and high fatigue were administered the H1 antagonist drug doxepin hydrocholoride (6 mg) in tomato juice and tomato juice alone (placebo) in a randomized, double-blinded, cross-over experiment before performing 30 min of light intensity cycling exercise and completing energy, fatigue, sleepiness, and motivation scales, and cognitive tasks. After exercise, mental fatigue increased for the doxepin condition (p=0.014) but not placebo (p=0.700), while mental energy decreased for both PLA and DOX (p<0.001) and cognitive task performance was unaffected. It is inferred that histamine binding to H1 receptors in the brain has a role in exercise-induced reductions in mental fatigue, but not increases in energy.

Doxepin and imipramine but not fluoxetine reduce the activity of the rat glutamate transporter EAAT3 expressed in Xenopus oocytes.

BACKGROUND: Many researchers have suggested that the glutamatergic system may be involved in the effects of antidepressant therapies. We investigated the effects of doxepin, imipramine, and fluoxetine on the excitatory amino acid transporter type 3 (EAAT3). METHODS: EAAT3 was expressed in Xenopus oocytes by injection of EAAT3 mRNA. Membrane currents were recorded after application of L-glutamate (30 µM) in the presence or absence of various concentrations of doxepin, imipramine, and fluoxetine. To study the effects of protein kinase C (PKC) activation on EAAT3 activity, oocytes were pre-incubated with phorbol 12-myristate-13-acetate (PMA) before application of imipramine and doxepin. RESULTS: Doxepin at 0.063-1.58 µM significantly decreased EAAT3 activity. Imipramine reduced EAAT3 activity in a concentration-dependent manner at 0.16-0.95 µM. However, fluoxetine did not affect EAAT3 activity, and PMA increased EAAT3 activity. At 0.32 µM, imipramine caused an equivalent decrease in EAAT3 activity in the presence or absence of PMA. However, 0.79 µM doxepin did not abolish the enhancement of EAAT3 activity by PMA. CONCLUSIONS: We showed that doxepin and imipramine, but not fluoxetine, inhibited EAAT3 activity at clinically relevant concentrations. This reveals a novel mechanism of action for doxepin and imipramine; that they increase glutamatergic neurotransmission. PKC may be involved in the effects of doxepin on EAAT3, but is not involved in the effects of imipramine at the concentrations studied.

Histamine H1 receptor occupancy by the new-generation antipsychotics olanzapine and quetiapine: a positron emission tomography study in healthy volunteers.

RATIONALE: Histamine H1 antagonists have hypnotic, appetite-promoting, and sedative side effects. Most second-generation antipsychotics have potent antagonistic effects on histamine H1 receptor (H1R). Positron emission tomography (PET) can measure the H1R occupancy (H1RO) in vivo, although there are no reports regarding antipsychotics. OBJECTIVES: We studied the H1RO of olanzapine and quetiapine in vivo with respect to their plasma concentrations and subjective drowsiness by performing human PET imaging studies with [(11)C]doxepin, a potent PET ligand of H1R. METHODS: Six healthy Japanese male volunteers were enrolled. Cross-randomized PET imaging was performed after a single oral administration of olanzapine (2.5 mg), quetiapine (25 mg), or placebo. PET data were analyzed by region of interest and voxel-by-voxel analysis. We concurrently measured plasma drug concentrations by liquid chromatography/tandem mass spectrometry and evaluated subjective sleepiness. RESULTS: The binding potential ratios of olanzapine and quetiapine in the cerebral cortex were significantly lower than that of the placebo. The H1RO values of olanzapine and quetiapine in the cortex were approximately 61-80 and 56-81%, respectively. The binding potential ratios of the drugs were significantly lower than that of the placebo in the dorsolateral prefrontal and lateral temporal cortices, and anterior and posterior cingulate gyri. The H1RO values in the cortex were significantly correlated with subjective sleepiness but not plasma drug concentrations. CONCLUSIONS: Olanzapine and quetiapine have high H1RO values in the human brain under their clinical minimum doses. This study provides a foundation of the properties by which new-generation antipsychotics block the central histaminergic system in humans.