Utility of Animal Models to Understand Human Alzheimer's Disease, Using the Mastermind Research Approach to Avoid Unnecessary Further Sacrifices of Animals.

To diagnose and treat early-stage (preclinical) Alzheimer's disease (AD) patients, we need body-fluid-based biomarkers that reflect the processes that occur in this stage, but current knowledge on associated processes is lacking. As human studies on (possible) onset and early-stage AD would be extremely expensive and time-consuming, we investigate the potential value of animal AD models to help to fill this knowledge gap. We provide a comprehensive overview of processes associated with AD pathogenesis and biomarkers, current knowledge on AD-related biomarkers derived from on human and animal brains and body fluids, comparisons of biomarkers obtained in human AD and frequently used animal AD models, and emerging body-fluid-based biomarkers. In human studies, amyloid beta (Aß), hyperphosphorylated tau (P-tau), total tau (T-tau), neurogranin, SNAP-25, glial fibrillary acidic protein (GFAP), YKL-40, and especially neurofilament light (NfL) are frequently measured. In animal studies, the emphasis has been mostly on Aß. Although a direct comparison between human (familial and sporadic) AD and (mostly genetic) animal AD models cannot be made, still, in brain, cerebrospinal fluid (CSF), and blood, a majority of similar trends are observed for human AD stage and animal AD model life stage. This indicates the potential value of animal AD models in understanding of the onset and early stage of AD. Moreover, animal studies can be smartly designed to provide mechanistic information on the interrelationships between the different AD processes in a longitudinal fashion and may also include the combinations of different conditions that may reflect comorbidities in human AD, according to the Mastermind Research approach.

Physiologically Based Modeling Approach to Predict Dopamine D2 Receptor Occupancy of Antipsychotics in Brain: Translation From Rat to Human.

Receptor occupancy (RO) is a translational biomarker for assessing drug efficacy and safety. We aimed to apply a physiologically based pharmacokinetic (PBPK) modeling approach to predict the brain dopamine D2 RO time profiles of antipsychotics. Clozapine and risperidone were modeled together with their active metabolites, norclozapine and paliperidone, First, in PK-Sim a rat PBPK model was developed and optimized using literature plasma PK data. Then, blood-brain barrier parameters including the expression and efflux transport kinetics of P-glycoprotein were optimized using literature microdialysis data on brain extracellular fluid (brainECF), which were further adapted when translating the rat PBPK model into the human PBPK model. Based on the simulated drug and metabolite concentrations in brainECF, drug-D2 receptor binding kinetics (association and dissociation rates) were incorporated in MoBi to predict RO. From an extensive literature search, 32 plasma PK data sets (16 from rat and 16 from human studies) and 23 striatum RO data sets (13 from rat and 10 from human studies) were prepared and compared with the model predictions. The rat PBPK-RO model adequately predicted the plasma concentrations of the parent drugs and metabolites and the RO levels. The human PBPK-RO model also captured the plasma PK and RO levels despite the large interindividual and interstudy variability, although it tended to underestimate the plasma concentrations and RO measured at late time points after risperidone dosing. The developed human PBPK-RO model was successfully applied to predict the plasma PK and RO changes observed after risperidone dose reduction in a clinical trial in schizophrenic patients.

Intranasal delivery of a novel acetylcholinesterase inhibitor HLS-3 for treatment of Alzheimer's disease.

AIM: The present study aims to investigate the pharmacokinetics and pharmacodynamics of HLS-3, a tacrine dimer with high anti-acetylcholinesterase activity for the treatment of Alzheimer's disease. MAIN METHODS: In vitro Calu-3 and Caco-2 cell monolayer transport and liver microsomal incubation studies of HLS-3 were carried out to evaluate its nasal epithelium and intestinal membrane permeability, transporters involved in absorption and hepatic metabolism. In vivo pharmacokinetics of HLS-3 followed by central and peripheral cholinergic mediated responses and ex vivo AChE activities in rats via oral and intranasal administrations were further investigated and compared. KEY FINDINGS: Our in vitro studies suggested that HLS-3 is the substrate of both P-gp and MRPs with no significant hepatic oxidation and glucuronidation metabolism. Oral administration only delivered trace amount of HLS-3 in systemic circulation with a high faecal recovery of 70.7%, whereas intranasal administration demonstrated an absolute bioavailability of 28.9% with urinary and faecal recoveries of 1.5% and 34.0%, respectively. In comparison to oral administration of HLS-3, intranasally delivered HLS-3 exhibited significant higher central cholinergic mediated responses without obvious peripheral side effect. SIGNIFICANCE: Intranasal delivery of HLS-3 with better pharmacokinetics and pharmacodynamics performances provides a promising approach for treatment of Alzheimer's disease.

Fingerprints of CNS drug effects: a plasma neuroendocrine reflection of D2 receptor activation using multi-biomarker pharmacokinetic/pharmacodynamic modelling.

BACKGROUND AND PURPOSE: Because biological systems behave as networks, multi-biomarker approaches increasingly replace single biomarker approaches in drug development. To improve the mechanistic insights into CNS drug effects, a plasma neuroendocrine fingerprint was identified using multi-biomarker pharmacokinetic/pharmacodynamic (PK/PD) modelling. Short- and long-term D2 receptor activation was evaluated using quinpirole as a paradigm compound. EXPERIMENTAL APPROACH: Rats received 0, 0.17 or 0.86 mg·kg-1 of the D2 agonist quinpirole i.v. Quinpirole concentrations in plasma and brain extracellular fluid (brainECF ), as well as plasma concentrations of 13 hormones and neuropeptides, were measured. Experiments were performed at day 1 and repeated after 7-day s.c. drug administration. PK/PD modelling was applied to identify the in vivo concentration-effect relations and neuroendocrine dynamics. KEY RESULTS: The quinpirole pharmacokinetics were adequately described by a two-compartment model with an unbound brainECF -to-plasma concentration ratio of 5. The release of adenocorticotropic hormone (ACTH), growth hormone, prolactin and thyroid-stimulating hormone (TSH) from the pituitary was influenced. Except for ACTH, D2 receptor expression levels on the pituitary hormone-releasing cells predicted the concentration-effect relationship differences. Baseline levels (ACTH, prolactin, TSH), hormone release (ACTH) and potency (TSH) changed with treatment duration. CONCLUSIONS AND IMPLICATIONS: The integrated multi-biomarker PK/PD approach revealed a fingerprint reflecting D2 receptor activation. This forms the conceptual basis for in vivo evaluation of on- and off-target CNS drug effects. The effect of treatment duration is highly relevant given the long-term use of D2 agonists in clinical practice. Further development towards quantitative systems pharmacology models will eventually facilitate mechanistic drug development.

Development of a population pharmacokinetic model to predict brain distribution and dopamine D2 receptor occupancy of raclopride in non-anesthetized rat.

BACKGROUND: Raclopride is a selective antagonist of the dopamine D2 receptor. It is one of the most frequently used in vivo D2 tracers (at low doses) for assessing drug-induced receptor occupancy (RO) in animals and humans. It is also commonly used as a pharmacological blocker (at high doses) to occupy the available D2 receptors and antagonize the action of dopamine or drugs on D2 in preclinical studies. The aims of this study were to comprehensively evaluate its pharmacokinetic (PK) profiles in different brain compartments and to establish a PK-RO model that could predict the brain distribution and RO of raclopride in the freely moving rat using a LC-MS based approach. METHODS: Rats (n=24) received a 10-min IV infusion of non-radiolabeled raclopride (1.61µmol/kg, i.e. 0.56mg/kg). Plasma and the brain tissues of striatum (with high density of D2 receptors) and cerebellum (with negligible amount of D2 receptors) were collected. Additional microdialysis experiments were performed in some rats (n=7) to measure the free drug concentration in the extracellular fluid of the striatum and cerebellum. Raclopride concentrations in all samples were analyzed by LC-MS. A population PK-RO model was constructed in NONMEM to describe the concentration-time profiles in the unbound plasma, brain extracellular fluid and brain tissue compartments and to estimate the RO based on raclopride-D2 receptor binding kinetics. RESULTS: In plasma raclopride showed a rapid distribution phase followed by a slower elimination phase. The striatum tissue concentrations were consistently higher than that of cerebellum tissue throughout the whole experimental period (10-h) due to higher non-specific tissue binding and D2 receptor binding in the striatum. Model-based simulations accurately predicted the literature data on rat plasma PK, brain tissue PK and D2 RO at different time points after intravenous or subcutaneous administration of raclopride at tracer dose (RO <10%), sub-pharmacological dose (RO 10%-30%) and pharmacological dose (RO >30%). CONCLUSION: For the first time a predictive model that could describe the quantitative in vivo relationship between dose, PK and D2 RO of raclopride in non-anesthetized rat was established. The PK-RO model could facilitate the selection of optimal dose and dosing time when raclopride is used as tracer or as pharmacological blocker in various rat studies. The LC-MS based approach, which doses and quantifies a non-radiolabeled tracer, could be useful in evaluating the systemic disposition and brain kinetics of tracers.

Prediction of human CNS pharmacokinetics using a physiologically-based pharmacokinetic modeling approach.

Knowledge of drug concentration-time profiles at the central nervous system (CNS) target-site is critically important for rational development of CNS targeted drugs. Our aim was to translate a recently published comprehensive CNS physiologically-based pharmacokinetic (PBPK) model from rat to human, and to predict drug concentration-time profiles in multiple CNS compartments on available human data of four drugs (acetaminophen, oxycodone, morphine and phenytoin). Values of the system-specific parameters in the rat CNS PBPK model were replaced by corresponding human values. The contribution of active transporters for the four selected drugs was scaled based on differences in expression of the pertinent transporters in both species. Model predictions were evaluated with available pharmacokinetic (PK) data in human brain extracellular fluid and/or cerebrospinal fluid, obtained under physiologically healthy CNS conditions (acetaminophen, oxycodone, and morphine) and under pathophysiological CNS conditions where CNS physiology could be affected (acetaminophen, morphine and phenytoin). The human CNS PBPK model could successfully predict their concentration-time profiles in multiple human CNS compartments in physiological CNS conditions within a 1.6-fold error. Furthermore, the model allowed investigation of the potential underlying mechanisms that can explain differences in CNS PK associated with pathophysiological changes. This analysis supports the relevance of the developed model to allow more effective selection of CNS drug candidates since it enables the prediction of CNS target-site concentrations in humans, which are essential for drug development and patient treatment.

Novel CNS drug discovery and development approach: model-based integration to predict neuro-pharmacokinetics and pharmacodynamics.

INTRODUCTION: CNS drug development has been hampered by inadequate consideration of CNS pharmacokinetic (PK), pharmacodynamics (PD) and disease complexity (reductionist approach). Improvement is required via integrative model-based approaches. Areas covered: The authors summarize factors that have played a role in the high attrition rate of CNS compounds. Recent advances in CNS research and drug discovery are presented, especially with regard to assessment of relevant neuro-PK parameters. Suggestions for further improvements are also discussed. Expert opinion: Understanding time- and condition dependent interrelationships between neuro-PK and neuro-PD processes is key to predictions in different conditions. As a first screen, it is suggested to use in silico/in vitro derived molecular properties of candidate compounds and predict concentration-time profiles of compounds in multiple compartments of the human CNS, using time-course based physiology-based (PB) PK models. Then, for selected compounds, one can include in vitro drug-target binding kinetics to predict target occupancy (TO)-time profiles in humans. This will improve neuro-PD prediction. Furthermore, a pharmaco-omics approach is suggested, providing multilevel and paralleled data on systems processes from individuals in a systems-wide manner. Thus, clinical trials will be better informed, using fewer animals, while also, needing fewer individuals and samples per individual for proof of concept in humans.

Predicting Drug Concentration-Time Profiles in Multiple CNS Compartments Using a Comprehensive Physiologically-Based Pharmacokinetic Model.

Drug development targeting the central nervous system (CNS) is challenging due to poor predictability of drug concentrations in various CNS compartments. We developed a generic physiologically based pharmacokinetic (PBPK) model for prediction of drug concentrations in physiologically relevant CNS compartments. System-specific and drug-specific model parameters were derived from literature and in silico predictions. The model was validated using detailed concentration-time profiles from 10 drugs in rat plasma, brain extracellular fluid, 2 cerebrospinal fluid sites, and total brain tissue. These drugs, all small molecules, were selected to cover a wide range of physicochemical properties. The concentration-time profiles for these drugs were adequately predicted across the CNS compartments (symmetric mean absolute percentage error for the model prediction was <91%). In conclusion, the developed PBPK model can be used to predict temporal concentration profiles of drugs in multiple relevant CNS compartments, which we consider valuable information for efficient CNS drug development.

Brain Uptake of Bioactive Flavones in Scutellariae Radix and Its Relationship to Anxiolytic Effect in Mice.

Scutellariae Radix (SR) and its bioactive flavones elicit a variety of effects in the brain. However, the brain uptake of individual SR flavones and its relationship to the elicited effects after SR administration remain unknown. Moreover, previous studies seldom measured pharmacokinetic and pharmacodynamic outcomes simultaneously. In the current study, the brain uptake of six major SR flavones and the anxiolytic behavior following oral administration of a SR extract at two clinically relevant doses (600 and 1200 mg/kg twice daily) were simultaneously investigated in mice (n = 18 per group). Brain and plasma concentrations of the flavones were measured by LC-MS/MS, while the anxiolytic effect was evaluated using the elevated plus maze. To further investigate the mechanism behind the differential brain uptake of the six SR flavones, these flavones were separately administered to mice at an equivalent molar oral dose (n = 6). The brain tissue bindings of the SR flavones were also measured with the in vitro brain slice method. Our results indicated that all six SR flavones including three aglycons (baicalein, wogonin, and oroxylin A) and three glucuronides (baicalin, wogonoside, and oroxyloside) could pass through the blood-brain barrier, with brain concentrations ranging from 7.9 to 224.0 pmol/g. It provided novel evidence that oroxylin A had the highest brain uptake among the six SR flavones regardless of its limited content in SR extract, in which 3.6-3.9% of the administered oroxylin A dose was present in the brain 6 h postdosing and with a brain-to-plasma ratio of 0.42-0.46. Although SR extract contains flavones that are positive modulators of the benzodiazepine binding site of GABAA receptors (baicalein, wogonin, and baicalin), our behavioral study for the first time indicated that SR extract (a mixture of six flavones) did not elicit significant anxiolytic effect at the studied doses. Oroxylin A also demonstrated the highest brain uptake when the six flavones were separately administered to mice, and the highest affinity to brain tissues in the in vitro tissue binding assay. The high brain uptake of oroxylin A, a GABAA antagonist which had been reported to antagonize diazepam-induced anxiolytic effect, might have suppressed the anxiolytic effects of the other flavones and account for the lack of overall anxiolytic effect of SR extract. The current study illustrates the importance of monitoring pharmacokinetics in a behavioral study, particularly for herbal medicines which consist of multiple components that might have different or even opposite pharmacological effects on the same target.

Kinetics for Drug Discovery: an industry-driven effort to target drug residence time.

A considerable number of approved drugs show non-equilibrium binding characteristics, emphasizing the potential role of drug residence times for in vivo efficacy. Therefore, a detailed understanding of the kinetics of association and dissociation of a target-ligand complex might provide crucial insight into the molecular mechanism-of-action of a compound. This deeper understanding will help to improve decision making in drug discovery, thus leading to a better selection of interesting compounds to be profiled further. In this review, we highlight the contributions of the Kinetics for Drug Discovery (K4DD) Consortium, which targets major open questions related to binding kinetics in an industry-driven public-private partnership.

A Generic Multi-Compartmental CNS Distribution Model Structure for 9 Drugs Allows Prediction of Human Brain Target Site Concentrations.

PURPOSE: Predicting target site drug concentration in the brain is of key importance for the successful development of drugs acting on the central nervous system. We propose a generic mathematical model to describe the pharmacokinetics in brain compartments, and apply this model to predict human brain disposition. METHODS: A mathematical model consisting of several physiological brain compartments in the rat was developed using rich concentration-time profiles from nine structurally diverse drugs in plasma, brain extracellular fluid, and two cerebrospinal fluid compartments. The effect of active drug transporters was also accounted for. Subsequently, the model was translated to predict human concentration-time profiles for acetaminophen and morphine, by scaling or replacing system- and drug-specific parameters in the model. RESULTS: A common model structure was identified that adequately described the rat pharmacokinetic profiles for each of the nine drugs across brain compartments, with good precision of structural model parameters (relative standard error <37.5%). The model predicted the human concentration-time profiles in different brain compartments well (symmetric mean absolute percentage error <90%). CONCLUSIONS: A multi-compartmental brain pharmacokinetic model was developed and its structure could adequately describe data across nine different drugs. The model could be successfully translated to predict human brain concentrations.

Mechanistic models enable the rational use of in vitro drug-target binding kinetics for better drug effects in patients.

INTRODUCTION: Drug-target binding kinetics are major determinants of the time course of drug action for several drugs, as clearly described for the irreversible binders omeprazole and aspirin. This supports the increasing interest to incorporate newly developed high-throughput assays for drug-target binding kinetics in drug discovery. A meaningful application of in vitro drug-target binding kinetics in drug discovery requires insight into the relation between in vivo drug effect and in vitro measured drug-target binding kinetics. AREAS COVERED: In this review, the authors discuss both the relation between in vitro and in vivo measured binding kinetics and the relation between in vivo binding kinetics, target occupancy and effect profiles. EXPERT OPINION: More scientific evidence is required for the rational selection and development of drug-candidates on the basis of in vitro estimates of drug-target binding kinetics. To elucidate the value of in vitro binding kinetics measurements, it is necessary to obtain information on system-specific properties which influence the kinetics of target occupancy and drug effect. Mathematical integration of this information enables the identification of drug-specific properties which lead to optimal target occupancy and drug effect in patients.

Identification and disposition of novel mono-hydroxyl mefenamic acid and their potentially toxic 1-O-acyl-glucuronides in vivo.

Mefenamic acid (MEF) is a widely prescribed non-steroidal anti-inflammatory drug that has been found associated with rare but severe cases of hepatotoxicity, nephrotoxicity and gastrointestinal toxicity. The formation of protein-reactive acylating metabolites such as 1-O-acyl-MEF glucuronide (MEFG) and 3'-hydroxymethyl-MEF 1-O-acyl-glucuronide is one proposed cause. In addition to the well-reported 3'-hydroxymethyl-MEF, two mono-hydroxyl-MEF (OH-MEFs) were recently identified in vitro. However, in vivo evidence is lacking and whether these OH-MEFs would be further glucuronidated to the potentially reactive 1-O-acyl-glucuronides (OH-MEFGs) is unknown. Utilizing UPLC-Q-TOF/MS and LC-MS/MS, the current study identified, for the first time, four OH-MEFs and their corresponding OH-MEFGs from plasma after a single oral administration of MEF (40 mg/kg) to rats, including an OH-MEF that has not been reported previously. The systemic exposure of these identified metabolites was high, with metabolic to parent AUC0 → 24 h ratios reaching 23-52% (OH-MEFs) and 8-29% (OH-MEFGs). These metabolites also had a long systemic exposure time in both single and 5 day multiple oral MEF-treated rats, with elimination half-lives between 9 h and > 24 h. In addition to these novel metabolites, the previously reported MEFG was also identified and its systemic exposure was found to be doubled after multiple MEF administrations. These pharmacokinetic results suggest that systemic toxicities caused by the potentially reactive MEFG and OH-MEFGs could be considerable, especially after repeated MEF treatment. Nevertheless, MEFG and OH-MEFGs had negligible uptake in the brain, indicating a minimal risk of brain toxicities. Furthermore, an in situ intestinal perfusion study revealed that during MEF absorption, it was extensively metabolized to MEFG while < 5% was metabolized to OH-MEFs and OH-MEFGs.

Herb-drug interactions between Scutellariae Radix and mefenamic acid: Simultaneous investigation of pharmacokinetics, anti-inflammatory effect and gastric damage in rats.

ETHNOPHARMACOLOGICAL RELEVANCE: Scutellariae Radix (SR), the dried root of Scutellariae baicalensis Georgi, has a lot in common with non-steroidal anti-inflammatory drugs (NSAIDs). Their similarities in therapeutic action (anti-inflammation) and metabolic pathways (phase II metabolisms) may lead to co-administration by patients with the potential of pharmacokinetic and/or pharmacodynamic interactions. The current study aims to investigate the potential interactions between SR and an NSAID, mefenamic acid (MEF), on the overall pharmacokinetic dispositions, anti-inflammatory effects and adverse effects in rats. MATERIALS AND METHODS: The current study simultaneously monitored the pharmacokinetic and pharmacodynamic interactions in a single animal. Four groups of Sprague-Dawley rats (n=7 each) received oral doses of a standardized SR extract (300mg/kg, twice daily), MEF (40mg/kg, daily), combination of SR extract and MEF, and vehicle control, respectively, for 5 days. On Day 5, blood samples were collected after first dose over 24h for the determination of (1) plasma concentrations of SR bioactive components, MEF and its metabolites by LC-MS/MS, and (2) prostaglandin E2 (PGE2) production and cyclooxygenase-2 (COX-2) gene expression by ex vivo analyses using LPS-stimulated RAW264.7 macrophage cells, ELISA and real time-PCR. After the rats were sacrificed, stomachs were isolated to assess their gross mucosal damage. Statistical comparisons were conducted using ANOVA and t-test. RESULTS: Minimal pharmacokinetic interaction between SR extract and MEF was observed. Co-administration of SR extract and MEF did not significantly alter the plasma concentration-time profile or the pharmacokinetic parameters such as Cmax, AUC0→24, Tmax or clearance. Pharmacodynamic interaction via the COX-2 pathway was observed. The PGE2 level in LPS-stimulated RAW264.7 cells treated with plasma collected from control group over the 24h sampling (AUC0→24[PGE2]) was 191981±8789pg/mlhr, which was significantly reduced to 174,780±6531 and 46,225±1915pg/mlhr by plasma collected from rats administered with SR extract and MEF, respectively. Co-administration of SR extract and MEF further potentiated the PGE2 inhibition, with an AUC0→24[PGE2] of 37013±2354pg/mlhr (p<0.05, compared to SR or MEF group). By analyzing the COX-2 gene expression, SR extract significantly prolonged the COX-2 inhibitory effect of MEF over the 24h (p<0.05). Furthermore, the MEF-induced stomach ulcer after the 5-day treatment, as evidenced by the increased gross ulcer index and sum of lesion length (p<0.05, compared to control), could be alleviated by co-administration with SR extract (p<0.05). CONCLUSIONS: Co-administration of SR extract and MEF potentiated the anti-inflammatory effects, alleviated the MEF-induced stomach adverse effect while having minimal pharmacokinetic interactions. Our findings provide insight for combination therapy of SR extract and MEF against inflammatory diseases.

Development of a SPE-LC/MS/MS method for simultaneous quantification of baicalein, wogonin, oroxylin A and their glucuronides baicalin, wogonoside and oroxyloside in rats and its application to brain uptake and plasma pharmacokinetic studies.

This study aims to identify and quantify the six major bioactive flavones of the traditional Chinese medicine Scutellariae Radix (RS), including baicalein, baicalin, wogonin, wogonoside, oroxylin A and oroxyloside in rat after oral administration of a standardized RS extract. A novel, sensitive and selective method for simultaneous determination of these six analytes in rat brain and plasma using solid phase extraction-liquid chromatography-tandem mass spectrometry (SPE-LC/MS/MS) was developed and fully validated. The lower limits of quantification (LLOQs) for the six RS flavones in brain tissue were 0.02nmol/g. The LLOQs in plasma were 0.005nmol/ml for B, W and OA, 0.025nmol/ml for WG and OAG, and 0.1875nmol/ml for BG. The current study provides novel evidence of the presence of all the tested RS flavones and an isoform of BG (BG', probably baicalein-6-O-glucuronide) in the rat brain after oral administration of RS extract, suggesting their ability to permeate through the blood-brain barrier. The method was also successfully applied to the pharmacokinetic study of all these analytes in plasma after oral administration of RS extract (300mg/kg) to Sprague-Dawley rats. The developed assay method provides a useful tool for both preclinical and clinical investigations on the disposition of RS flavones in brain and plasma.

Development, characterization and application of in situ gel systems for intranasal delivery of tacrine.

The present study aimed to develop an in situ gel formulation for intranasal delivery of tacrine (THA), an anti-Alzheimer's drug. Thermosensitive polymer Pluronic F-127 was used to prepare THA in situ gels. Sol-gel transition temperature (Tsol-gel), rheological properties, in vitro release, and in vivo nasal mucociliary transport time were optimized. The pharmacokinetics and brain dispositions of in situ gel were compared with that from THA oral solution in rats. The in situ gel demonstrated a liquid state with Newtonian fluid behavior under 20 °C, while it exhibited as non-flowing gel with pseudoplastic fluid behavior beyond its Tsol-gel of 28.5 °C. Based on nasal mucociliary transport time, the in situ gel significantly prolonged its retention in nasal cavity compared to solution form. Moreover, the in situ gel achieved 2-3 fold higher peak plasma concentration (Cmax) and area under the curve (AUC) of THA in plasma and brain tissue, but lowered Cmax and AUC of the THA metabolites compared to that of oral solution. The enhanced nasal residence time, improved bioavailability, increased brain uptake of parent drug and decreased exposure of metabolites suggested that the in situ gel could be an effective intranasal formulation for THA.

Pharmacokinetic comparison between the long-term anesthetized, short-term anesthetized and conscious rat models in nasal drug delivery.

PURPOSE: To investigate the pharmacokinetic differences between the common nasal delivery models. METHODS: In three different rat models [long-term anesthetized (with nasal surgery), short-term anesthetized (without nasal surgery) and conscious models], tacrine and loxapine were administered via nasal, intravenous and oral routes, and the plasma pharmacokinetics were compared among different models. RESULTS: Systemic exposures of both drugs and their metabolites were consistently higher in long-term anesthetized model after all routes of administration in comparison to that of conscious model. Nasal bioavailabilities in long-term anesthetized model (tacrine 83%, loxapine 97%) were much higher than that in conscious model (tacrine 10%, loxapine 46%). Further studies on tacrine and its metabolites demonstrated no significant difference in t1/2 between short-term anesthetized and conscious models after all routes of administration; however, long-term anesthetized model showed significantly longer t1/2. Regarding the pharmacokinetic parameters (Cmax, Tmax, AUC, bioavailability) of tacrine and its metabolites, short-term anesthetized model resembled closer to conscious model than long-term anesthetized model. CONCLUSIONS: Plasma clearances of tacrine, loxapine, and their metabolites were much slower in the long-term anesthetized model of nasal delivery probably due to suppressed hepatic and renal clearances, while the short-term anesthetized model imposed less impact on tacrine pharmacokinetics and metabolism.

Alterations in the CNS effects of anti-epileptic drugs by Chinese herbal medicines.

INTRODUCTION: Concomitant use of anti-epileptic drugs (AEDs) and Chinese herbal medicines (CHMs) is increasing globally. However, information summarizing how CHMs might alter the CNS effects of AEDs is lacking. AREAS COVERED: A systematic review of the English-language articles in evidence-based databases was performed. It identified CHMs that interact with AEDs and lead to alterations in the CNS effects of AEDs. This review provides a descriptive summary of the existing information on CHM-induced changes of both the therapeutic and adverse CNS effects of AEDs, including i) anti-epileptic effect, ii) sedative effect, iii) anxiolytic effect and iv) memory impairment effect. The proposed mechanisms behind the interactions are also summarized. EXPERT OPINION: Despite the popularity of both AEDs and CHMs, the availability of information on CHM-AED interactions that could result in altered CNS outcomes is considerably limited. Moreover, there are some insufficiencies in the study designs of the identified reports. More research, including both mechanistic and human studies, with improved study design is necessary to ensure the safety and efficacy of combinational use of AEDs with CHMs.

Brain disposition and catalepsy after intranasal delivery of loxapine: role of metabolism in PK/PD of intranasal CNS drugs.

PURPOSE: To elucidate the role of metabolism in the pharmacokinetics and pharmacodynamics of intranasal loxapine in conscious animals. METHODS: At pre-determined time points after intranasal or oral loxapine administration, levels of loxapine, loxapine metabolites, and neurotransmitters in rat brain were quantified after catalepsy assessments (block test and paw test). Cataleptogenicity of loxapine was also compared with its metabolites. RESULTS: Intranasally administered loxapine was efficiently absorbed into systemic circulation followed by entering brain, with tmax ≤15 min in all brain regions. Oral route delivered minimal amounts of loxapine to plasma and brain. Brain AUC0-240min values of 7-hydroxy-loxapine were similar after intranasal and oral administration. Intranasal loxapine tended to induce less catalepsy than oral loxapine, although statistical significance was not reached. The catalepsy score was positively and significantly correlated with the striatal concentration of 7-hydroxy-loxapine, but not with loxapine. 7-hydroxy-loxapine was more cataleptogenic than loxapine, while the presence of loxapine tended to reduce rather than intensify 7-hydroxy-loxapine-induced catalepsy. The increases in striatal dopamine turnover were comparable after intranasal and oral loxapine administration. CONCLUSIONS: The metabolite 7-hydroxy-loxapine, but not loxapine, was the main contributor to the catalepsy observed after intranasal and oral loxapine treatment. Intranasal route could effectively deliver loxapine to brain.

Regioselective biotransformation of CNS drugs and its clinical impact on adverse drug reactions.

INTRODUCTION: Adverse drug reactions (ADRs) continue to be one of the major causes of failure in drug development while limiting the clinical utilities of many drugs. Contribution of the metabolites formed in vivo to ADRs could be more significant than we might have expected. AREAS COVERED: This review focuses on the relationship between regioselectivity in biotransformation and the ADRs of drugs acting on the central nervous system (CNS). "Regioselectivity" is defined as an exclusively or significantly preferential metabolic reaction at one (or several) site(s) on the substrate molecule. Several CNS drugs and toxicants, of which the metabolites play pivotal roles in ADRs, are summarized in details with the highlight on the roles of metabolism in both toxification and detoxification. The article also discusses in silico predictions of regioselectivity and the formation of toxic metabolites which are becoming increasingly important. EXPERT OPINION: Researchers working on CNS drugs face particular challenges in predicting drug metabolism and potential toxicities of their metabolites. A number of factors contribute to the difficulty of accurate prediction of metabolite disposition in the human brain. Better knowledge of regioselectivity in biotransformation and elucidation of the relationships between biotransformations and ADRs would definitely help designing new compounds with lower bioactivation potentials and rejuvenating the older drugs whose clinical applications are restricted by their ADRs. Administrating drugs by alternative routes such as the intranasal, transdermal, sublingual, and buccal routes could also be a strategy to reduce unwanted metabolite formations.