Clinical pharmacokinetics of tacrine

Tacrine First-Phase Biotransformation and Associated Hepatotoxicity: A Possible Way to Avoid Quinone Methide Formation

Tacrine was withdrawn from clinical use as a drug against Alzheimer's disease in 2013, mainly due to drug-induced liver injury. The culprit of tacrine-associated hepatotoxicity is believed to be the 7-OH-tacrine metabolite, a possible precursor of quinone methide (Qmeth), which binds to intracellular -SH proteins. In our study, several different animal and human models (liver microsomes, primary hepatocytes, and liver slices) were used to investigate the biotransformation and hepatotoxicity of tacrine and its 7-substituted analogues (7-methoxy-, 7-phenoxy-, and 7-OH-tacrine). Our goal was to find the most appropriate in vitro model for studying tacrine hepatotoxicity and, through rational structure modifications, to develop derivatives of tacrine that are less prone to Qmeth formation. Our results show that none of animal models tested accurately mimic human tacrine biotransformation; however, the murine model seems to be more suitable than the rat model. Tacrine metabolism was overall most accurately mimicked in three-dimensional (3D) spheroid cultures of primary human hepatocytes (PHHs). In this system, tacrine and 7-methoxytacrine were hydroxylated to 7-OH-tacrine, whereas 7-phenoxytacrine formed, as expected, only trace amounts. Surprisingly, however, our study showed that 7-OH-tacrine was the least hepatotoxic (7-OH-tacrine < tacrine < 7-methoxytacrine < 7-phenoxytacrine) even after doses had been adjusted to achieve the same intracellular concentrations. The formation of Qmeth-cysteine and Qmeth-glutathione adducts after human liver microsome incubation was confirmed by all of the studied tacrine derivatives, but these findings were not confirmed after incubation with 3D PHH spheroids. Therefore, the presented data call into question the suggested previously hypothesized mechanism of toxicity, and the results open new avenues for chemical modifications to improve the safety of novel tacrine derivatives.

Structure-Guided Design of N-Methylpropargylamino-Quinazoline Derivatives as Multipotent Agents for the Treatment of Alzheimer's Disease

Alzheimer's disease (AD) is a complex disease with an unknown etiology. Available treatments, limited to cholinesterase inhibitors and N-methyl-d-aspartate receptor (NMDAR) antagonists, provide symptomatic relief only. As single-target therapies have not proven effective, rational specific-targeted combination into a single molecule represents a more promising approach for treating AD, and is expected to yield greater benefits in alleviating symptoms and slowing disease progression. In the present study, we designed, synthesized, and biologically evaluated 24 novel N-methylpropargylamino-quinazoline derivatives. Initially, compounds were thoroughly inspected by in silico techniques determining their oral and CNS availabilities. We tested, in vitro, the compounds' effects on cholinesterases and monoamine oxidase A/B (MAO-A/B), as well as their impacts on NMDAR antagonism, dehydrogenase activity, and glutathione levels. In addition, we inspected selected compounds for their cytotoxicity on undifferentiated and differentiated neuroblastoma SH-SY5Y cells. We collectively highlighted II-6h as the best candidate endowed with a selective MAO-B inhibition profile, NMDAR antagonism, an acceptable cytotoxicity profile, and the potential to permeate through BBB. The structure-guided drug design strategy applied in this study imposed a novel concept for rational drug discovery and enhances our understanding on the development of novel therapeutic agents for treating AD.

Functionalization strategies of polymeric nanoparticles for drug delivery in Alzheimer's disease: Current trends and future perspectives

Alzheimer's disease (AD), the most common form of dementia, is a progressive and multifactorial neurodegenerative disorder whose primary causes are mostly unknown. Due to the increase in life expectancy of world population, including developing countries, AD, whose incidence rises dramatically with age, is at the forefront among neurodegenerative diseases. Moreover, a definitive cure is not yet within reach, imposing substantial medical and public health burdens at every latitude. Therefore, the effort to devise novel and effective therapeutic strategies is still of paramount importance. Genetic, functional, structural and biochemical studies all indicate that new and efficacious drug delivery strategies interfere at different levels with various cellular and molecular targets. Over the last few decades, therapeutic development of nanomedicine at preclinical stage has shown to progress at a fast pace, thus paving the way for its potential impact on human health in improving prevention, diagnosis, and treatment of age-related neurodegenerative disorders, including AD. Clinical translation of nano-based therapeutics, despite current limitations, may present important advantages and innovation to be exploited in the neuroscience field as well. In this state-of-the-art review article, we present the most promising applications of polymeric nanoparticle-mediated drug delivery for bypassing the blood-brain barrier of AD preclinical models and boost pharmacological safety and efficacy. In particular, novel strategic chemical functionalization of polymeric nanocarriers that could be successfully employed for treating AD are thoroughly described. Emphasis is also placed on nanotheranostics as both potential therapeutic and diagnostic tool for targeted treatments. Our review highlights the emerging role of nanomedicine in the management of AD, providing the readers with an overview of the nanostrategies currently available to develop future therapeutic applications against this chronic neurodegenerative disease.

Application of empirical scalars to enable early prediction of human hepatic clearance using IVIVE in drug discovery: an evaluation of 173 drugs

The utilization of in vitro data to predict drug pharmacokinetics (PK) in vivo has been a consistent practice in early drug discovery for decades. However, its success is hampered by mispredictions attributed to uncharacterized biological phenomena/experimental artifacts. Predicted drug clearance (CL) from experimental data (i.e. hepatocyte intrinsic clearance: CLint, fraction unbound in plasma: fu,p) is often systematically underpredicted using the well-stirred model (WSM). The objective of this study was to evaluate using empirical scalars in the WSM to correct for CL mispredictions. Drugs (N=28) were used to generate numerical scalars on CLint (α), and fu,p (β) to minimize the error (AAFE) for CL predictions. These scalars were validated using an additional dataset (N=28 drugs) and applied to a non-redundant AstraZeneca (AZ) dataset available in the literature (N=117 drugs) for a total of 173 compounds. CL predictions using the WSM were improved for most compounds using an α value of 3.66 (~64%<2-fold) compared to no scaling (~46%<2-fold). Similarly, using a β value of 0.55 or combination of α and β scalars (values of 1.74 and 0.66, respectively) resulted in a similar improvement in predictions (~64%<2-fold and ~65%<2-fold, respectively). For highly bound compounds (fu,p{less than or equal to}0.01), AAFE was substantially reduced across all scaling methods. Using the β scalar alone or a combination of α and β appeared optimal; and produce larger magnitude corrections for highly-bound compounds. Some drugs are still disproportionally mispredicted, however the improvements in prediction error and simplicity of applying these scalars suggests its utility for early-stage CL predictions. Significance Statement In early drug discovery, prediction of human clearance using in vitro experimental data plays an essential role in triaging compounds prior to in vivo studies. These predictions have been systematically underestimated. Here we introduce empirical scalars calibrated on the extent of plasma protein binding that appear to improve clearance prediction across multiple datasets. This approach can be used in early phases of drug discovery prior to the availability of pre-clinical data for early quantitative predictions of human clearance.

New In Vitro-In Silico Approach for the Prediction of In Vivo Performance of Drug Combinations

Pharmacokinetic (PK) studies improve the design of dosing regimens in preclinical and clinical settings. In complex diseases like cancer, single-agent approaches are often insufficient for an effective treatment, and drug combination therapies can be implemented. In this work, in silico PK models were developed based on in vitro assays results, with the goal of predicting the in vivo performance of drug combinations in the context of cancer therapy. Combinations of reference drugs for cancer treatment, gemcitabine and 5-fluorouracil (5-FU), and repurposed drugs itraconazole, verapamil or tacrine, were evaluated in vitro. Then, two-compartment PK models were developed based on the previous in vitro studies and on the PK profile reported in the literature for human patients. Considering the quantification parameter area under the dose-response-time curve (AUC_effect) for the combinations effect, itraconazole was the most effective in combination with either reference anticancer drugs. In addition, cell growth inhibition was itraconazole-dose dependent and an increase in effect was predicted if itraconazole administration was continued (24-h dosing interval). This work demonstrates that in silico methods and AUC_effect are powerful tools to study relationships between tissue drug concentration and the percentage of cell growth inhibition over time.

Dimeric Tacrine(10)-hupyridone as a Multitarget-Directed Ligand To Treat Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder with multiple pathological features. Therefore, a multitarget-directed ligands (MTDLs) strategy has been developed to treat AD. We have previously designed and synthesized dimeric tacrine(10)-hupyridone (A10E), a novel tacrine derivative with acetylcholinesterase (AChE) inhibition and brain-derived neurotrophic factor (BDNF) activation activity, by linking tacrine and a fragment of huperzine A. However, it was largely unknown whether A10E could act on other AD targets and produce cognitive-enhancing ability in AD animal models. In this study, A10E could prevent cognitive impairments in APP/PS1 transgenic mice and β-amyloid (Aβ) oligomers-treated mice, with higher potency than tacrine and huperzine A. Moreover, A10E could effectively inhibit Aβ production and deposition, alleviate neuroinflammation, enhance BDNF expression, and elevate cholinergic neurotransmission in vivo. At nanomolar concentrations, A10E could inhibit Aβ oligomers-induced neurotoxicity via the activation of tyrosine kinase receptor B (TrkB)/Akt pathway in SH-SY5Y cells. Furthermore, Aβ oligomerization and fibrillization could be directly disrupted by A10E. Importantly, A10E at high concentrations did not produce obvious hepatotoxicity. Our results indicated that A10E could produce anti-AD neuroprotective effects via the inhibition of Aβ aggregation, the activation of the BDNF/TrkB pathway, the alleviation of neuroinflammation, and the decrease of AChE activity. As MTDLs could produce additional benefits, such as overcoming the deficits of drug combination and enhancing the compliance of AD patients, our results also suggested that A10E might be developed as a promising MTDL lead for the treatment of AD.

Pursuing the Complexity of Alzheimer's Disease: Discovery of Fluoren-9-Amines as Selective Butyrylcholinesterase Inhibitors and N-Methyl-d-Aspartate Receptor Antagonists

Alzheimer’s disease (AD) is a complex disorder with unknown etiology. Currently, only symptomatic therapy of AD is available, comprising cholinesterase inhibitors and N-methyl-d-aspartate (NMDA) receptor antagonists. Drugs targeting only one pathological condition have generated only limited efficacy. Thus, combining two or more therapeutic interventions into one molecule is believed to provide higher benefit for the treatment of AD. In the presented study, we designed, synthesized, and biologically evaluated 15 novel fluoren-9-amine derivatives. The in silico prediction suggested both the oral availability and permeation through the blood–brain barrier (BBB). An initial assessment of the biological profile included determination of the cholinesterase inhibition and NMDA receptor antagonism at the GluN1/GluN2A and GluN1/GluN2B subunits, along with a low cytotoxicity profile in the CHO-K1 cell line. Interestingly, compounds revealed a selective butyrylcholinesterase (BChE) inhibition pattern with antagonistic activity on the NMDARs. Their interaction with butyrylcholinesterase was elucidated by studying enzyme kinetics for compound 3c in tandem with the in silico docking simulation. The docking study showed the interaction of the tricyclic core of new derivatives with Trp82 within the anionic site of the enzyme in a similar way as the template drug tacrine. From the kinetic analysis, it is apparent that 3c is a competitive inhibitor of BChE.

Cholinesterase Inhibitors for Alzheimer's Disease: Multitargeting Strategy Based on Anti-Alzheimer's Drugs Repositioning

In the brain, acetylcholine (ACh) is regarded as one of the major neurotransmitters. During the advancement of Alzheimer's disease (AD) cholinergic deficits occur and this can lead to extensive cognitive dysfunction and decline. Acetylcholinesterase (AChE) remains a highly feasible target for the symptomatic improvement of AD. Acetylcholinesterase (AChE) remains a highly viable target for the symptomatic improvement in AD because cholinergic deficit is a consistent and early finding in AD. The treatment approach of inhibiting peripheral AChE for myasthenia gravis had effectively proven that AChE inhibition was a reachable therapeutic target. Subsequently tacrine, donepezil, rivastigmine, and galantamine were developed and approved for the symptomatic treatment of AD. Since then, multiple cholinesterase inhibitors (ChEIs) have been continued to be developed. These include newer ChEIs, naturally derived ChEIs, hybrids, and synthetic analogues. In this paper, we summarize the different types of ChEIs which are under development and their respective mechanisms of actions.

Neurotherapeutic applications of nanomedicine for treating Alzheimer's disease

Alzheimer's disease (AD) is a progressive, irreversible, fatal brain disease which disturbs cognitive functions. It affects 35 million people worldwide and the number of people suffering may increase to 100 million by 2050 if no effective treatments are available. The present treatment improves cognitive functions and provide temporary symptomatic relief, but do not stop or delay the disease progression. Moreover, they are mainly available as conventional oral dosage forms and these conventional oral medications lack brain specificity and also produce side effects which leads to poor patient compliance. Brain drug targeting by nanomedicines is a promising approach to improve brain targeting specificity, brain bioavailability and patient compliance. The present review discuses about the currently available pharmacotherapy for AD and the neurotherapeutic applications as well as the advancements of nanomedicine for treating AD. It also highlights the recent advancements of various nanomedicines containing phytopharmaceuticals for treating AD. It is believed that nanomedicines containing approved drugs can be transformed into the clinics hence improve the life style of AD patients.

Design and Development of Novel Transdermal Nanoemulgel for Alzheimer’s Disease: Pharmacokinetic, Pharmacodynamic and Biochemical Investigations

Background:

Alzheimer’s disease is a chronic progressive neurodegenerative disorder associated with depletion of acetylcholine. Oral treatment with tacrine hydrochloride; a reversible inhibitor of acetylcholinesterase, finds limited use in Alzheimer’s disease due to frequent dosing, hepatotoxicity and extensive pre-systemic metabolism.

Objectives:

The objective of the study was to evaluate pharmacokinetic, pharmacodynamic, safety and stability profile of transdermal w/o nanoemulsion gel of tacrine hydrochloride and determine its relative bioavailability from transdermal nanogel in contrast to marketed capsule and conventional hydrogel.

Methods:

The optimized nanoemulsion gel NEGT4 (droplet size 156.4 ±0.48 nm, with poly dispersity index 0.36 ±0.4, permeation flux 6.172±2.94 µg/cm2/h across rat skin) was prepared by spontaneous emulsification followed by sonication. NEGT4 contained 7 mg of drug in 10% w/w distilled water, 30% w/w surfactant (Labrafil M) and cosurfactant (Transcutol P) mixture in ratio 1:4 and 60 % Capryol 90 as oily phase thickened with 98.9 mg ethyl cellulose (20 cps). In vivo studies were carried out on male Wistar rats following standard guidelines. Scopolamine was used to induce amnesia in rats which is a characteristic of Alzheimer’s disease. Various formulations were compared by performing pharmacokinetic, histopathological, behavioural and biochemical studies on rats. Stability studies on nanoemulsion gels were carried out in accordance with The International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) guidelines.

Results:

Pharmacokinetic studies exhibited significantly greater extent of absorption from NEGT4 in comparison to capsule and hydrogel with a 2.18 and 5.26-fold increase respectively. Significant improvement in neurobehavioral parameters was observed with NEGT4 in scopolamine-induced amnesic rats. Biochemical assessment showed superior anti-amnesic activity of NEGT4 through augmentation of antioxidant enzymes, decreased lipid peroxidation and acetylcholinesterase activity. Low value of serum aminotransferase in rats treated with NEGT4 indicated the absence of hepatotoxicity. NEGT4 was found to be non-irritant and possessed a shelf life of 4.11 years.

Conclusion:

Developed nanoemulsion gel of tacrine hydrochloride was found to be safe, stable, and efficacious and has immense potential to be used in the therapy of Alzheimer’s disease.

Prediction of Drug-Induced Hepatotoxicity Using Long-Term Stable Primary Hepatic 3D Spheroid Cultures in Chemically Defined Conditions

High failure rates of drug candidates in the clinics, restricted-use warnings as well as withdrawals of drugs in postmarketing stages are of substantial concern for the pharmaceutical industry and drug-induced liver injury (DILI) constitutes one of the most frequent reasons for such safety failures. Importantly, as DILI cannot be accurately predicted using animal models, animal safety tests are commonly complemented with assessments in human in vitro systems. 3D spheroid cultures of primary human hepatocytes in chemically defined conditions, hereafter termed CD-spheroids, have recently emerged as a microphysiological model system in which hepatocytes retain their molecular phenotypes and hepatic functions for multiple weeks in culture. However, their predictive power for the detection of hepatotoxic liabilities has not been systematically assessed. Therefore, we here evaluated the hepatotoxicity of 123 drugs with or without direct implication in clinical DILI events. Importantly, using ATP quantifications as the single endpoint, the model accurately distinguished between hepatotoxic and nontoxic structural analogues and exceeded both sensitivity and specificity of all previously published in vitro assays at substantially lower exposure levels, successfully detecting 69% of all hepatotoxic compounds without producing any false positive results (100% specificity). Furthermore, the platform supports the culture of spheroids of primary hepatocytes from preclinical animal models, thereby allowing the identification of animal-specific toxicity events. We anticipate that CD-spheroids represent a powerful and versatile tool in drug discovery and preclinical drug development that can reliably flag hepatotoxic drug candidates and provide guidance for the selection of the most suitable animal models.

Highly Sensitive LC-MS-MS Method for the Determination of Tacrine in Rat Plasma: Application to Pharmacokinetic Studies in Rats

A rapid and highly sensitive assay method has been developed and validated for the estimation of tacrine in rat plasma using liquid chromatography coupled to tandem mass spectrometry with electrospray ionization in the positive-ion mode. The assay procedure involves a simple liquid-liquid extraction of tacrine and phenacetin (internal standard, IS) from rat plasma using ethyl acetate. Chromatographic separation was achieved with 0.2% formic acid : acetonitrile (30 : 70, v/v) at a flow rate of 0.50 mL/min on an Atlantis dC18 column with a total run time of 3.0 min. The MS-MS ion transitions monitored were 199.10 → 171.20 for tacrine and 180.10 → 110.10 for IS. Method validation was performed as per United States Food and Drug Administration (US FDA) guidelines and the results met the acceptance criteria. The lower limit of quantification achieved was 0.008 ng/mL and linearity was observed from 0.008 to 53.4 ng/mL. The intra- and inter-day precision was in the range of 2.76-12.5 and 5.15-12.8%, respectively. This novel method has been applied to a pharmacokinetic study in rats.

Tacrine sinusoidal uptake and biliary excretion in sandwich-cultured primary rat hepatocytes

PURPOSE. The knowledge of hepatic disposition kinetics of tacrine, a first cholinesterase inhibitor was approved by FDA for the treatment of Alzheimer's disease (AD), would help to understand its hepatotoxicity, its therapeutic effect, and improve the management of patients with AD. The current study aims to characterize tacrine hepatic transport kinetics and study the role of organic cation transporters (OCTs), P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP2) in tacrine sinusoidal uptake and biliary excretion. METHODS. Modulation of tacrine hepatic uptake and efflux, biliary excretion index (BEI%), were performed in sandwich-cultured primary rat hepatocytes (SCHs) using transporters inhibitors. Conformation of the integrity of SCHs model was established by capturing images with light-contrast and fluorescence microscopy. RESULTS. Tacrine uptake in SCHs was carrier-mediated process and saturable with apparent Km of 31.5±9.6 µM and Vmax of 908±72 pmol/min/mg protein. Tetraethyl ammonium (TEA), cimetidine and verapamil significantly reduced tacrine uptake with more pronounced effect observed with verapamil which caused 3-fold reduction in tacrine uptake, indicating role for OCTs. Tacrine has a biliary excretion in SCHs with maximum BEI% value of 22.9±1.9% at 10 min of incubation. Addition of MK571 and valspodar decreased the BEI% of tacrine by 40 and 60% suggesting roles for canalicular MRP2 and P-gp, respectively. CONCLUSIONS. Our results show that in addition to metabolism, tacrine hepatic disposition is carrier-mediated process mediated by sinusoidal OCTs, and canalicular MRP2 and P-gp.

Enduring effects of tacrine on cocaine-reinforced behavior: Analysis by conditioned-place preference, temporal separation from drug reward, and reinstatement

Previous work by our laboratory has shown that tacrine can produce long-lasting reductions in cocaine-reinforced behavior, when administered to rats as daily intravenous infusions over four days. Tacrine causes dose-related liver toxicity in different species, and its manufacture for human use was recently discontinued. This study was conducted to further characterize its actions on cocaine reward. Cocaine-experienced animals that had no contact with drug over one week resumed self-administration at levels similar to their initial baseline. When tacrine was administered over four days which were preceded and followed by washout periods to allow elimination of cocaine and tacrine respectively, subsequent cocaine self-administration was attenuated by more than one-half. Tacrine administered at 10 mg/kg-day as a chronic infusion by osmotic pump did not modify cocaine-induced increases in locomotor activity or conditioned-place preference. In rats that exhibited persistent attenuation of cocaine-self-administration after receiving tacrine, cocaine-induced reinstatement was also attenuated. No changes in plasma measures of renal or hepatic function were observed in rats receiving tacrine. In conclusion, pretreatment with tacrine can decrease cocaine-motivated behavior measured by self-administration or reinstatement, but not conditioned-place preference. Reductions in cocaine self-administration following pretreatment with tacrine do not require direct interaction with cocaine and are not secondary to either liver or kidney toxicity.

In-vitro stability and metabolism of a tacrine–silibinin codrug

Objectives

A tacrine-silibinin codrug showed promising results in pharmacological and toxicity testing, superior to an equimolar mixture of tacrine and silibinin. The aim of this study was to get more information about its stability, possible degradation products, metabolites, and especially its active principle in vitro and in vivo.

Methods

The stability of the codrug was analysed under in-vitro assay conditions. Additionally, its metabolism was investigated using pooled human liver microsomes. Metabolites were identified via liquid chromatography-high resolution electrospray ionization mass spectrometry. Furthermore, the influence of one of the main cleavage products, tacrine hemi succinamide, on viability and mitochondria of hepatic stellate cells was analysed.

Key findings

The codrug remained stable in culture medium (Dulbecco's modified Eagle's medium) over an incubation period of 24 h, whereas exposition to microsomal enzymes led to rapid cleavage of the ester bond to form silibinin and a tacrine hemi succinamide. In addition, glucuronidated metabolites of both silibinin and the codrug were detected. For the tacrine hemi succinamide, no effects were observed with regard to cell viability and mitochondrial impairment.

Conclusions

This study helps understand and interpret previous results concerning the effects and the absence of toxicity of the tacrine–silibinin codrug and supplies important information for further identification of the active principles of the codrug in vivo.

Oral bioavailability of the novel cannabinoid CB1 antagonist AM6527: effects on food-reinforced behavior and comparisons with AM4113

Drugs that interfere with cannabinoid CB1 transmission suppress food-motivated behaviors, and may be clinically useful as appetite suppressants. Several CB1 receptor inverse agonists, such as rimonabant and AM251, as well as the CB1 receptor neutral antagonist, AM4113, have been assessed for their effects on food-motivated behavior. One important criterion for establishing if a drug may be useful clinically is the determination of its oral bioavailability. The present studies compared the effects of AM4113 and a novel CB1 antagonist, AM6527, on the suppression of food-reinforced behavior following intraperitoneal (IP) and oral administration. AM4113 and AM6527 both suppressed lever pressing after IP injections. The ED50 for the effect on FR5 responding was 0.78 mg/kg for IP AM4113, and 0.5763 mg/kg for IP AM6527. AM6527 also was effective after oral administration (ED50=1.49 mg/kg), however, AM 4113 was ineffective up to oral doses of 32.0 mg/kg. AM 4113 may be very useful as a research tool, but its lack of oral activity suggests that this drug might not be effective if orally administered in humans. In contrast, AM 6527 is an orally active CB1 antagonist, which may be useful for clinical research on the appetite suppressant effects of CB1 antagonists.

Rofecoxib is a potent inhibitor of cytochrome P450 1A2: studies with tizanidine and caffeine in healthy subjects

AIMS

Case reports suggest an interaction between rofecoxib and the CYP1A2 substrate tizanidine. Our objectives were to explore the extent and mechanism of this possible interaction and to determine the CYP1A2 inhibitory potency of rofecoxib.

METHODS

In a randomized, double-blind, two-phase cross-over study, nine healthy subjects took 25 mg rofecoxib or placebo daily for 4 days and, on day 4, each ingested 4 mg tizanidine. Plasma concentrations and the urinary excretion of tizanidine, its metabolites (M) and rofecoxib, and pharmacodynamic variables were measured up to 24 h. On day 3, a caffeine test was performed to estimate CYP1A2 activity.

RESULTS

Rofecoxib increased the area under the plasma concentration-time curve (AUC(0-infinity)) of tizanidine by 13.6-fold [95% confidence interval (CI) 8.0, 15.6; P < 0.001), peak plasma concentration (C(max)) by 6.1-fold (4.8, 7.3; P < 0.001) and elimination half-life (t(1/2)) from 1.6 to 3.0 h (P < 0.001). Consequently, rofecoxib markedly increased the blood pressure-lowering and sedative effects of tizanidine (P < 0.05). Rofecoxib increased several fold the tizanidine/M-3 and tizanidine/M-4 ratios in plasma and urine and the tizanidine/M-5, tizanidine/M-9 and tizanidine/M-10 ratios in urine (P < 0.05). In addition, it increased the plasma caffeine/paraxanthine ratio by 2.4-fold (95% CI 1.4, 3.4; P = 0.008) and this ratio correlated with the tizanidine/metabolite ratios. Finally, the AUC(0-25) of rofecoxib correlated with the placebo phase caffeine/paraxanthine ratio (r = 0.80, P = 0.01).

CONCLUSIONS

Rofecoxib is a potent inhibitor of CYP1A2 and it greatly increases the plasma concentrations and adverse effects of tizanidine. The findings suggest that rofecoxib itself is also metabolized by CYP1A2, raising concerns about interactions between rofecoxib and other CYP1A2 substrate and inhibitor drugs.

Mechanism for increased bioavailability of tacrine in fasted rats

The mechanism associated with the increased bioavailability of tacrine as a result of a 24-h period of fasting was examined in rats. The AUC value for tacrine after the fasting was 52% higher compared with normal feeding when 4 mg kg(-1) of tacrine was orally administered, but the value for velnacrine, a hydroxylated metabolite of tacrine, was reduced by 10%. The relative metabolic ratio of tacrine in urinary excretion (Au(tacrine) divided by Au(velnacrine)) was lower in fasted rats compared with normally fed rats. This clearly shows that metabolism of tacrine is reduced with 24-h fasting after oral administration. Altered intestinal permeation in the fasting state was hypothesized, and the transport of tacrine across the rat intestine was studied. When a fasted intestine was mounted in an Ussing chamber, the mucosal-to-serosal permeability of tacrine was increased to double that for a fed rat intestine. To examine the effect of absorption rate on the hepatic metabolism of tacrine, a direct pyloric vein infusion study was carried out. Compared with an infusion of tacrine for 5 min, a slow infusion of tacrine over a period of 30 or 60 min increased the hepatic metabolism of tacrine and decreased its systemic clearance in rats. Collectively, these results suggest that rapid transport across the intestine aids tacrine in avoiding hepatic first-pass metabolism and enhances its bioavailability in fasted rats. From these findings, we conclude that both oral administration before a meal and a reduction in the dose might be recommended in tacrine therapy considering the serious hepatotoxicity of tacrine in clinical use.

Rofecoxib is a potent, metabolism-dependent inhibitor of CYP1A2: implications for in vitro prediction of drug interactions

Rofecoxib was recently found to greatly increase plasma concentrations of the CYP1A2 substrate drug tizanidine in humans, but there are no published in vitro studies on the CYP1A2-inhibiting effects of rofecoxib. Our objective was to investigate whether rofecoxib is a direct-acting or metabolism-dependent inhibitor of CYP1A2 in vitro. The effect of rofecoxib on the O-deethylation of phenacetin (20 microM) was studied using human liver microsomes. The effect of preincubation time on the inhibitory potential of rofecoxib was also studied, and the inhibitor concentration that supports half the maximal rate of inactivation (KI) and the maximal rate of inactivation (kinact) were determined. Rofecoxib moderately inhibited phenacetin O-deethylation (IC50 23.0 microM), and a 30-min preincubation with microsomes and NADPH considerably increased its inhibitory effect (IC50 4.2 microM). Inactivation of CYP1A2 by rofecoxib required NADPH, and was characterized by a KI of 4.8 microM and a kinact of 0.07 min(-1). Glutathione, superoxide dismutase, mannitol, or dialysis could not reverse the inactivation of CYP1A2 caused by rofecoxib. Fluvoxamine decreased the rofecoxib-caused inactivation of CYP1A2 in a concentration-dependent manner. In conclusion, rofecoxib is a potent, metabolism-dependent inhibitor of CYP1A2, a cytochrome P450 form contributing to rofecoxib metabolism. The results provide a mechanistic explanation for the interactions of rofecoxib with CYP1A2 substrates and may partially explain its nonlinear pharmacokinetics.

Pharmacotherapeutic approaches to the treatment of Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD), a progressive degenerative disorder of the brain, is the most common cause of cognitive impairment in the elderly. The pharmacotherapy of AD is evolving rapidly. Cholinergic stabilization with cholinesterase-inhibitor (ChEI) therapy implies neuroprotection and a resultant slowing of disability and disease progression. The moderate-affinity N-methyl-d-aspartate (NMDA)-receptor antagonist memantine may block neural excitotoxicity.

OBJECTIVE

The purpose of this review was to examine the evidence for the responsiveness to pharmacotherapy of established AD; specifically, the extent to which the benefits of therapy have been proved, the extent to which currently available ChEIs support cholinergic neurotransmission, and the extent to which currently available ChEIs and memantine provide neuroprotection.

METHODS

Relevant studies were identified through a comprehensive search of MEDLINE for articles published between January 1999 and February 2004 using the terms Alzheimer's pharmacotherapy, cholinesterase inhibitor therapy, Alzheimer's disease, donepezil, rivastigmine, galantamine, glutamatergic system modifiers, and memantine; a search of the reference lists of identified articles; and a manual search of pertinent journals. Articles were selected that contained higher-level evidence, based on explicit validated criteria.

RESULTS

ChEI therapy was associated with quality-of-life improvements that included enhanced performance of activities of daily living, reduced behavioral disturbances, stabilized cognitive impairment, decreased caregiver stress, and delay in the first dementia-related nursing home placement. In large clinical trials in moderate to severe AD (a stage that is associated with distress for patients and caregiver burden, and for which other treatments are not available), memantine showed an ability to delay cognitive and functional deterioration. The combination of memantine and ChEI therapy was significantly more efficacious than ChEI therapy alone (P < 0.001) and was well tolerated.

CONCLUSIONS

The idea that AD is pharmacologically unresponsive appears to be changing. With the use of ChEI and NMDA-receptor antagonist therapy, the symptoms and outcomes of this devastating neurodegenerative disease can be improved and its course altered.

Clinical pharmacokinetics and pharmacodynamics of cholinesterase inhibitors

Cholinesterase inhibitors are the 'first-line' agents in the treatment of Alzheimer's disease. This article presents the latest information on their pharmacokinetic properties and pharmacodynamic activity. Tacrine was the first cholinesterase inhibitor approved by regulatory agencies, followed by donepezil, rivastigmine and recently galantamine. With the exception of low doses of tacrine, the cholinesterase inhibitors exhibit a linear relationship between dose and area under the plasma concentration-time curve. Cholinesterase inhibitors are rapidly absorbed through the gastrointestinal tract, with time to peak concentration usually less than 2 hours; donepezil has the longest absorption time of 3 to 5 hours. Donepezil and tacrine are highly protein bound, whereas protein binding of rivastigmine and galantamine is less than 40%. Tacrine is metabolised by hepatic cytochrome P450 (CYP) 1A2, and donepezil and galantamine are metabolised by CYP3A4 and CYP2D6. Rivastigmine is metabolised by sulfate conjugation. Two cholinesterase enzymes are present in the body, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Tacrine and rivastigmine inhibit both enzymes, whereas donepezil and galantamine specifically inhibit AChE. Galantamine also modulates nicotine receptors, thereby enhancing acetylcholinergic activity at the synapse. These different pharmacological profiles provide distinctions between these agents. Cholinesterase inhibitors show a nonlinear relationship between dose and cholinesterase inhibition, where a plateau effect occurs. Cholinesterase inhibitors display a different profile as each agent achieves its plateau at different doses. In clinical trials, cholinesterase inhibitors demonstrate a dose-dependent effect on cognition and functional activities. Improvement in behavioural symptoms also occurs, but without a dose-response relationship. Gastrointestinal adverse events are dose-related. Clinical improvement occurs with between 40 and 70% inhibition of cholinesterase. A conceptual model for cholinesterase inhibitors has been proposed, linking enzyme inhibition, clinical efficacy and adverse effects. Currently, measurement of enzyme inhibition is used as the biomarker for cholinesterase inhibitors. New approaches to determining the efficacy of cholinesterase inhibitors in the brain could involve the use of various imaging techniques. The knowledge base for the pharmacokinetics and pharmacodynamics of cholinesterase inhibitors continues to expand. The increased information available to clinicians can optimise the use of these agents in the management of patients with Alzheimer's disease.

Pharmacodynamic-tolerability relationships of cholinesterase inhibitors for Alzheimer's disease

According to the cholinergic hypothesis, the impairment of cognitive function and the behavioural disturbances that affect patients with Alzheimer's disease are mainly due to cortical deficiencies in cholinergic transmission. Numerous cholinesterase inhibitors have been investigated for treatment of this disease, the rationale being to support the cholinergic system by blocking the degradation of acetylcholine released from presynaptic neurons. These drugs can be classified as reversible (tacrine, donepezil and galantamine), pseudo-reversible (physostigmine, eptastigmine and rivastigmine) or irreversible (metrifonate) enzyme inhibitors. This article reviews efficacy and tolerability results from 6-month placebo-controlled studies of 7 cholinesterase inhibitors: tacrine (80 to 160 mg/day), donepezil (5 to 10 mg/day), rivastigmine (1 to 12 mg/day), metrifonate (30 to 80 mg/day), eptastigmine (30 to 60 mg/day), physostigmine (30 to 36 mg/day) and galantamine (8 to 32 mg/day). All these agents have demonstrated a statistically significant, although modest, effect versus placebo on the cognitive and global performance of patients with Alzheimer's disease. Dramatic clinical response has been seen in only 3 to 5% of patients. There are no major differences in terms of efficacy between the different drugs. The mean difference between drug and placebo effects on standardised psychometric scales is about 2 to 4 points on the cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-Cog; a 70-point cognitive scale) and 0.2 to 0.5 points on the Clinician's Interview-Based Impression of Change with Caregiver Input (CIBIC-Plus; a 7-point global scale), or 5 to 14% of the average value of the scales. The most common adverse effects observed after administration of cholinesterase inhibitors are nausea, vomiting, diarrhoea, dizziness, asthenia and anorexia, all symptoms linked to cholinergic overstimulation. These effects are dose related and largely depend on the degree of cholinesterase inhibition. Also important is the rate of onset of cholinesterase inhibition, which depends on the kinetics of enzyme inhibition, the presence and rate of titration, and the pharmacodynamic peak-to-trough fluctuations. A model predicting the incidence of nausea based on acetylcholinesterase inhibition and the half-life of acetylcholinesterase recovery is proposed. In conclusion, cholinesterase inhibitors are the only pharmacological agents proved to be effective for the treatment of Alzheimer's disease in large, long term, double-blind, placebo-controlled trials. While the efficacy of different cholinesterase inhibitors is similar, their tolerability profiles differ. For example, the incidence of nausea (in excess of that seen with placebo) at cognitively effective dosages ranges from 1% with eptastigmine 60 mg/day to 53% with physostigmine 30 mg/day. Differences in tolerability profile may be due to the extent of peripheral acetylcholinesterase inhibition needed to reach clinical efficacy. Other contributing pharmacodynamic factors are the rate of onset of and fluctuations in acetylcholinesterase inhibition at steady state.

Clinically significant pharmacokinetic interactions between dietary caffeine and medications

Caffeine from dietary sources (mainly coffee, tea and soft drinks) is the most frequently and widely consumed CNS stimulant in the world today. Because of its enormous popularity, the consumption of caffeine is generally thought to be safe and long term caffeine intake may be disregarded as a medical problem. However, it is clear that this compound has many of the features usually associated with a drug of abuse. Furthermore, physicians should be aware of the possible contribution of dietary caffeine to the presenting signs and symptoms of patients. The toxic effects of caffeine are extensions of their pharmacological effects. The most serious caffeine-related CNS effects include seizures and delirium. Other symptoms affecting the cardiovascular system range from moderate increases in heart rate to more severe cardiac arrhythmia. Although tolerance develops to many of the pharmacological effects of caffeine, tolerance may be overwhelmed by the nonlinear accumulation of caffeine when its metabolism becomes saturated. This might occur with high levels of consumption or as the result of a pharmacokinetic interaction between caffeine and over-the-counter or prescription medications. The polycyclic aromatic hydrocarbon-inducible cytochrome P450 (CYP) 1A2 participates in the metabolism of caffeine as well as of a number of clinically important drugs. A number of drugs, including certain selective serotonin reuptake inhibitors (particularly fluvoxamine), antiarrhythmics (mexiletine), antipsychotics (clozapine), psoralens, idrocilamide and phenylpropanolamine, bronchodilators (furafylline and theophylline) and quinolones (enoxacin), have been reported to be potent inhibitors of this isoenzyme. This has important clinical implications, since drugs that are metabolised by, or bind to, the same CYP enzyme have a high potential for pharmacokinetic interactions due to inhibition of drug metabolism. Thus, pharmacokinetic interactions at the CYP1A2 enzyme level may cause toxic effects during concomitant administration of caffeine and certain drugs used for cardiovascular, CNS (an excessive dietary intake of caffeine has also been observed in psychiatric patients), gastrointestinal, infectious, respiratory and skin disorders. Unless a lack of interaction has already been demonstrated for the potentially interacting drug, dietary caffeine intake should be considered when planning, or assessing response to, drug therapy. Some of the reported interactions of caffeine, irrespective of clinical relevance, might inadvertently cause athletes to exceed the urinary caffeine concentration limit set by sports authorities at 12 mg/L. Finally, caffeine is a useful and reliable probe drug for the assessment of CYP1A2 activity, which is of considerable interest for metabolic studies in human populations.

Determination of tacrine and its metabolites in human plasma and urine by high-performance liquid chromatography and fluorescence detection

A new method for the simultaneous quantitation of tacrine and the three metabolites, 1-hydroxytacrine (velnacrine, maleate), 2-hydroxytacrine and 4-hydroxytacrine, in human plasma and urine has been developed. The method was based on simple one-step liquid-liquid extraction with ethyl acetate followed by isocratic, reversed-phase high-performance liquid chromatography and fluorescence detection (excitation: 330 nm and emission: 365 nm). The limit of detection in plasma was 0.5 nM for 2-hydroxytacrine and 4-hydroxytacrine, 2 nM for 1-hydroxytacrine and tacrine. In urine it was 60 nM for 2-hydroxytacrine and 4-hydroxytacrine, 30 nM for 1-hydroxytacrine and 80 nM for tacrine. The limit of quantification in plasma was 2.5 nM for 2-hydroxytacrine and 4-hydroxytacrine, 10 nM for 1-hydroxytacrine and 2 nM for tacrine. In urine it was 120 nM for all components. The overall mean recoveries ranged from 84 to 105% in plasma and from 64 to 100% in urine for all four compounds.

Evaluation of selection criteria used in Alzheimer's disease clinical trials

BACKGROUND

In the absence of a biological marker for Alzheimer's disease (AD), diagnosis has to be achieved using clinical criteria sets such as those outlined in DSM-IV, NINCDS-ADRDA, or ICD-10. As these criteria are quite broadly defined, there may be inter-rater variability in interpretation.

METHODS

Using a previously published CT scan measuring technique which correlates well with diagnoses achieved using the NINCDS-ADRDA criteria as interpreted at our clinic, we chose to independently examine and reach a diagnosis in patients selected for participation in clinical trials of therapeutic agents for the treatment of AD. Forty-four CT scans from six investigators across Canada were examined using this model. All patients had been diagnosed as having AD by NINCDS-ADRDA criteria and were deemed acceptable to participate in a clinical trial.

RESULTS

The diagnostic concordance achieved in the original published model was 91.5%. The diagnostic concordance in the population currently being studied was 77.3%. However when examined by site, results ranged from 57.1% to 100%. Using the model, an index of atrophy and a probability of diagnosis of AD can be determined. Across sites, there were statistically significant differences in these measures (p < or = 0.035). The mean probability of diagnosis of AD across sites ranged from 0.56 to 0.94. Although the sites with lower probabilities had slightly lower mean ages and slightly less atrophy, there was no overall correlation of the atrophy measures with age.

CONCLUSIONS

Current results raise the possibility that the selection of patients for AD clinical trials using current diagnostic criteria sets may not be adequate and conclusions with respect to agent efficacy could be flawed.

Characterization of the induction of rat microsomal cytochrome P450 by tacrine

The effect of multiple-dose tacrine (THA) administration at 2 and 20 mg/kg (single oral doses for 2 weeks) on cytochrome P450 (CYP) was examined in male and female Wistar rats. Changes in CYP were determined by measuring total spectral CYP, the rates of ethoxy- and pentoxyresorufin dealkylations, and the protein expression of several CYPs by western blot analysis of liver microsomes. Animals treated with beta-naphthoflavone or phenobarbital were employed as positive controls. No physiological or metabolic changes were observed in male or female rats treated with 2 mg/kg THA for 2 weeks. Male and female animals treated with 20 mg/kg THA for 2 weeks demonstrated increased CYP1A activity (increased ethoxyresorufin deethylase activity) and increased expression of CYP1A1 with only minor increases in CYP1A2 expression. Compared with the effects of beta-naphthoflavone induction of CYP1A, the induction observed with THA at 20 mg/kg was considered minor.

Riluzole: a new agent for amyotrophic lateral sclerosis

OBJECTIVE

To provide a comprehensive review of riluzole, including its mechanism of action, pharmacokinetics, adverse drug reactions, drug interactions, efficacy, and administration. A brief review of amyotrophic lateral sclerosis (ALS) is also included.

DATA SOURCES

A computerized search of the MEDLINE database in May 1996 was used to identify publications regarding ALS, riluzole, and metabolism by CYP1A2. Manufacturer's information on riluzole was used when there was no primary literature.

DATA SYNTHESIS

Riluzole is approximately 90% absorbed following an oral dose. Its bioavailability is 60%. Peak concentrations occur within 1-1.5 hours of administration. Riluzole extensively binds to lipoproteins and albumin. This agent primarily undergoes CYP1A2 hydroxylation and glucuronidation, after which it is eliminated by the kidneys. Clearance is reduced in native Japanese healthy subjects and may be reduced in patients with hepatic impairment. Two trials with a total of 1114 patients addressed the efficacy of riluzole in ALS. Riluzole extended the time to tracheostomy or death, and the effect was greatest with dosages of 100 mg/d or greater. No effect on patients' symptoms or global assessment was detected at 18 or 21 months. Several flaws in these trials have led to questions concerning the validity of these results. The most commonly reported adverse effects of riluzole have been transient elevation of liver enzyme concentrations (2-5 times the upper limit of normal), worsening of asthenia, nausea, vomiting, diarrhea, anorexia, dizziness, vertigo, somnolence, and mouth paresthesia. Not as commonly reported, but still very serious, is neutropenia, which occurred in 3 of 4000 patients.

CONCLUSIONS

Although the benefits of riluzole are questionable and it is expensive, this agent may extend the time to tracheostomy or death in patients with ALS. At present, this is the only agent approved for the treatment of ALS and should be made available for these patients.

Identification of a 3-hydroxylated tacrine metabolite in rat and man: metabolic profiling implications and pharmacology

Discrepancies in urinary metabolic profiles in rats administered tacrine (1) suggested the presence of an unidentified metabolite of 1. Chromatographic methods were developed that allowed isolation of a metabolite fraction containing both 1-hydroxytacrine (2) and an unknown metabolite from rat urine. Mass spectral analysis indicated this metabolite to be a monohydroxylated derivative, which upon two dimensional COSY NMR analysis could be assigned as 3-hydroxytacrine (4). This structural assignment was confirmed by independent synthesis of 4. Compound 4 was also identified as a human urinary metabolite of 1. Biologically, 4 was found to have in vitro human red blood cell acetylcholinesterase inhibitory activity similar to that of 2 and 4-hydroxytacrine (5) and approximately 8-fold less than that of 1. These results underscore the need to conduct rigorous structural identification studies, especially in cases where isomeric metabolites are possible, in assessing the accuracy of chromatographic profiling techniques.