Significant delivery of tacrine into the brain using magnetic chitosan microparticles for treating Alzheimer's disease

Abstract

Alzheimer's disease (AD) is a progressive degenerative disorder of the brain characterized by a slow, progressive decline in cognitive function and behavior. As the disease advances, persons have a tough time with daily tasks like using the phone, cooking, handling money or driving the car. AD affects 15 million people worldwide and it has been estimated that AD affects 4.5 million Americans. Tacrine is a reversible cholinesterase inhibitor used for treating mild to moderate AD. In the present study, an attempt was made to target the anti-Alzheimer's drug tacrine in the brain by using magnetic chitosan microparticles. The magnetic chitosan microparticles were prepared by emulsion cross-linking. The formulated microparticles were characterized for process yield, drug loading capacity, particle size, in vitro release, release kinetics and magnetite content. The particle size was analyzed by scanning electron microscope. The magnetite content of the microparticles was determined by atomic absorption spectroscopy. For animal testing, the microparticles were injected intravenously after keeping a suitable magnet at the target region. The concentrations of tacrine at the target and non-target organs were analyzed by HPLC. The magnetic chitosan microparticles significantly increased the concentration of tacrine in the brain in comparison with the free drug.

Introduction

Drug targeting is the delivery of drugs to receptors or organs or any other specific part of the body to which one wishes to deliver the drug exclusively. The concept of designing a specified delivery system to achieve selective drug targeting has been originated from the perception of Paul Ehrlich, who proposed drug delivery to be as a ‘magic bullet’ (Ehrlich, 1902), where a drug-carrier complex/conjugate, delivers drug(s) exclusively to the preselected target cells in a specific manner. The objective of drug targeting is to achieve desired pharmacological response at a selected site without undesirable interactions at other sites. The magnetic targeted drug delivery system is one of the most attractive strategies of delivering drugs to the area of interest (Asmatulu et al., 2005). This is because the targeted systems improve the therapeutic index of drug molecules by minimizing the toxic side effects on healthy cells and tissues (Goodwin et al., 1999, Lübbe et al., 1999, Rudge et al., 2000, Lübbe et al., 2001, Sershen and West, 2002).

Magnetic carrier technology was first used in the early 1940s as a new methodology in waste water treatment (Arias et al., 2001). Interest in the possibilities of this technique in obtaining suitable magnetic particles for the delivery of pharmaceutical drugs to specific locations in the body developed much later (Pieters et al., 1994). The polymeric magnetic microparticles contain a magnetic nucleus and a biodegradable polymer shell. The magnetic core could make it possible to direct the particles to the specified location by means of external magnetic fields. This, in turn, would reduce the side effects of the drug because its level in the general circulation is lowered (Craig, 1994). Finally, the polymeric shell would have the role of transporting the drug and releasing it during its biodegradation (Arias et al., 2001).

Targeting of drug with magnetic microspheres were first described by Widder et al. (1979), who used magnetically responsive biodegradable drug carrier with the capacity to localize both carrier and therapeutic agent, by magnetic means to a specified in vivo target site. For more information, about the preparation and application of magnetic particles in biomedicine, the readers are requested to see the reviews written by Pankhurst et al. (2003), Tartaj et al. (2003), Arruebo et al. (2007) and Durán et al. (2008). The magnetic microspheres consist of magnetite (Fe₃O₄) particles, which are responsible for magnetic property, and a therapeutic agent entrapped in the biodegradable polymer matrix. Magnetite are biocompatible iron (II, III) oxide particles with no toxicity, hence this material is mainly utilized in the preparation of magnetic microparticles, to target toxic drugs particularly for anti-cancer therapy (Kato, 1983, Gupta and Hung, 1989). Suitable biodegradable polymers, such as chitosan, polylactides, poly(ɛ-caprolactone), poly(alkylcyanoacrylate), polyglycolides, poly(lactide-co-glycolides), polyanhydrides or polyorthoesters are usually chosen to make drug-carrying particles. These natural or synthetic polymers are incorporated with magnetic particles and drugs using various techniques (Asmatulu et al., 2005). Retention of magnetic carrier at target site will delay reticuloendothelial clearance, facilitate extravasation thus prolong action of drug (Vyas and Khar, 2002).

Tacrine was the first acetylcholinestearse inhibitor licensed for the treatment of Alzheimer's disease (Reichman, 2004). Chemically, tacrine is 9-amino-1,2,3,4-tetrahydroacridine. The chemical formula is C₁₃H₁₄N₂ and molecular weight is 198.26. The melting point is ranged between 183 and 187 °C. Tacrine hydrochloride is yellow colored and needle shaped. It has a bitter taste and soluble in water. The pH of 1.5% solution is 4.5–6.0 (Anonymous, 2001). It has a pKa value of 9.85 (Freeman and Dawson, 1991). The solubility of tacrine in water is 0.25 ± 0.02 mg/ml (Sathyan et al., 1995). The primary action of tacrine is the reversible inhibition of cholinesterase, butyrylcholinesterase more than acetylcholineestearse (Kumar and Becker, 1989, Freeman and Dawson, 1991, Reichman, 2004). This inhibition is believed to increase the acetylcholine level in the brain.

AD is a progressive and fatal neurodegenerative disorder manifested by cognitive and memory deterioration, progressive impairment of activities of daily living, and a variety of neuropsychiatric symptoms and behavioral disturbances (Cummings, 2004). Alzheimer's disease is characterized by marked atrophy of the cerebral cortex and loss of cortical neurons. The neuropathologic hallmarks of the disorder that are generally noted on postmortem brain examination: amyloid rich senile plaques (Selkoe, 2000), neurofibrillary tangles (Spillantini and Goedert, 1998), and neuronal degeneration. Impairment of short-term memory is the first clinical feature. As the condition progresses, additional cognitive abilities are impaired (Standaert and Young, 2001). The classic clinical features of Alzheimer's disease are an amnesic type of memory impairment (Green et al., 1996), deterioration of language (Price et al., 1993), and visuospatial deficits (Kirk and Kertesz, 1991). Alzheimer's disease has recently received lot of attention, especially in areas related to novel treatments (Cirrito and Holtzman, 2003).

Alzheimer's disease affects 10% of people over the age of 65% and 50% of people over the age of 85 (Evans, 1990). Women are about 1.5 times more likely to develop Alzheimer's disease than men are (Gao et al., 1998). Alzheimer's disease affects 15 million people worldwide (Anonymous, 2003). It has been estimated that Alzheimer's disease affects 4.5 million Americans (Hebert et al., 2003). The cost of caring for patients with Alzheimer's disease is extraordinary; annual expenditures total 83.9 billion (in 1996 U.S. dollars) (Wimo and Winblad, 2001). With population trends creating a larger percentage of elderly, Alzheimer's disease is projected to nearly triple (Hebert et al., 2003), if not quadruple (Brookmeyer et al., 1998), in prevalence, afflicting many millions in the United States and around the world in the next 50 years, unless new discoveries facilitate prevention of the disease (Hebert et al., 2003). In the present study, an attempt was made to significantly deliver the anti-Alzheimer's drug tacrine in the brain using magnetic chitosan microparticles.