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Study on the physicochemical properties and pharmacokinetic profiles of bis(7)-tacrine, a promising anti-Alzheimer's dimer /

English Abstract

Alzheimer’s Disease (AD) is a chronic neurodegenerative disease that attacks the brain and results in impaired memory, thinking and behavior in the elderly. There are many different theories on the causes of AD and one of the well-established theories suggests that the neurotransmitter acetylcholine levels are too low in the brains of AD patients. Although several acetylcholinesterase (AChE) inhibitors have been developed, they are only useful for treating patients with mild to moderate AD. Tacrine, the first approved AChE inhibitor for AD, has been found to have potential hepatotoxicity and is therefore seldom used nowadays. As a result, the development of more effective anti-AD agents is still an important area of AD-associated research. With the identification of the three-dimensional structure of AChE and the assistance of computer docking programs, three series of novel dimers have been designed based on the structures of tacrine and and huperzine A. Of the newer dimers, bis(7)-tacrine (B7T), bis(12)-hupyridone (B12H) and tacrine-(10)-hupyridone (T10H) are three typical compounds from those candidates. Among them, B7T has been demonstrated to have selective and potent AChE inhibitory effects both in vitro and in vivo, but also shown to have multiple actions such as antagonizing N-methyl- d-aspartate (NMDA) receptors and gamma-aminobutyric acid (GABAA) receptors and inhibiting nitric oxide synthase , which may play synergistical roles in the treatment of AD. Although these dimers are expected to be the promising drugs for the treatment of AD in clinic, lack of information on the physicochemical properties as well as pharmacokinetic profile and related pharmacokinetic parameters delays its further development. Therefore, the objective of this project is focused on the measurement of the pharmacokinetic profiles and parameters as well as the pharmacokinetics related physicochemical properties for B7T because of its advantages identified in our previous pharmacological studies. In Chapter 1, the background of our project is introduced. The basic principle for the molecular design and development of the novel dimeric agents are also displayed. Although we have gotten three series of different homo- and hetero-dimer from tacrine and huperzine A, but only B7T is selected in this study because of its advantages and clear pharmacological mechanisms. Moreover, the significances of this project are also presented in this chapter. In Chapter 2, we measured some main physicochemical properties of B7T including solubility, stability, ionization constants and lipophilicity. The results indicate that B7T is poorly soluble in water (12.71 mg•mL-1), and sparingly soluble in saline (0.36 mg/ml) and phosphate buffers (0.33–0.47 mg•mL-1). The solubilities are considerably affected by pH value and the ionic strength (main the [Cl- ]) of the solution. Two pKa values are determined as 8.7±0.1 (pKa1) and 10.7±0.4 (pKa2) for B7T in water. The high logP value for B7T is found to be 8.2, indicating that the B7T are highly lipophilic. All these physiochemical characteristics suggest that B7T maybe have the favorite properties for membrane permeability in vivo. In Chapter 3, based on the results obtained from our previous studies, the in vivo dose-response relationship and response-time curve for B7T are evaluated to rats. As the results, the ED50 is calculated as 1.44 mg/kg (i.v.) according to the inhibition to the AChE in brain. However, higher dose (>5 mg•kg-1, i.v.) may cause to the side-effects even to death (>7 mg•kg-1, i.v.). Finally, the dose of 1 mg/kg is selected for the i.v. administration in our later studies. Moreover, the response-time curve suggests that B7T can quickly exhibit its AChE inhibition in rat brain in 15~30 min, and duration of this effect can be kept more than 4 h after administration. However, the fast blood elimination can be observed by measuring the BChE inhibitory effect in rat serum. To directly measure the B7T concentrations in blood and brain, the further pharmacokinetic studies are designed and carried out to establish the relationship between the B7T concentrations to time. In Chapter 4, the analytical methods (HPLC-DAD and HPLC-MS) are established and validated for the bio-sample quantification. Furthermore, the pharmacokinetic profiles of B7T including Absorption, Distribution, Metabolism and Excretion are systemic studied using different administration routes and the relative parameters are also compared. As results, B7T was found poorly absorbed on the Caco-2 cell monolayer and rat intestinal perfusion models with the passive transport although it has the high lipophilicity. However, much extensive tissue distribution was measured by determining the B7T contents in different organs (heart, liver, spleen, lung, kidney, brain and blood) at different time after dosing. Renal excretion is considered as the main elimination route as presented as the highest AUCorgan/AUCblood value (5789%). The in vitro incubation of B7T with intestine or liver microsomes suggests that B7T is very stable in intestine but first-pass maybe occur in liver, and the hydroxylation is the main metabolic route of B7T in vivo. Further study the pharmacokinetic profiles of B7T after various administration routes (i.v., i.p., i.m. and p.o.), B7T shows a very low absolute bioavailability after oral administration (12.6% with dose of 20 mg•kg-1 and 9.4% of 50 mg•kg-1). Although we try to increase the oral bioavailability by using of the nonionic surfactant, Pluronic F-68, it only extends the absorption rate of B7T from the gastrointestinal tract but change the absorption extent. Further study on the enhancing of the bioavailability with the assistance of absorption enhancer or pharmaceutical techniques need to be carefully considered. In Chapter 5, the overviews of the present study are discussed. The contributors of this study are not only help us to well understand the physicochemical properties and in vivo characteristics of the promising anti-Alzheimer’s dimer, B7T, but also give us many useful directions on how to develop and optimize more effective candidates with favorable physicochemical and pharmacokinetic properties. Keywords: Alzheimer’s disease, dimer, bis(7)-tacrine, acetylcholinesterase inhibitor, multiple functions, physicochemical properties, pharmacodynamics, pharmacokinetics, HPLC-MS, HPLC-DAD.

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Institute of Chinese Medical Sciences





Alzheimer's disease -- Treatment



Han, Yi Fan


Liang, Chun

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