Neurotrophic factors regulate the survival and differentiation of neurons during development (Davies, 1994) and are known to be involved in a diversity of different functions including the maintenance of neuronal structures, the activity of ion channels, the release of neurotransmitters, and axon path-finding during an organism's life span (Schnell et al., 1994; Song and Poo, 1999; Schinder and Poo, 2000). There are several neurotrophic factors such as, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5)
BDNF, acting on neurons of the central nervous system (CNS), is expressed predominantly in the hippocampus, the cortex, and the synapses of the basal forebrain, which are areas vital to learning, memory, and higher thinking. The BNDF expressed in these areas serves as an important regulator of synaptic transmission and synaptic plasticity (which forms a neurobiochemical basis for learning and memory formation and a recognition process) (Lu B, 2003). Particularly, BDNF promotes long-term potentiation (LTP), which is one of the cellular mechanisms which underlies learning and memory, while reducing long-term depression (LTD) (Ikegaya, Y. et al., 2002; Huber, K. et al., 1998). The roles of BDNF arise in part because BDNF and its receptor TrkB (tropomyosin-related kinase B) are localized to glutamate synapses.
As explained above, BDNF plays a regulatory role in synaptic transmission and synaptic plasticity, and these roles are significantly important with respect to various diseases, as well as inter alia, cerebral degenerative diseases, especially, Alzheimer's disease (AD), Parkinson's disease (PD), stress-induced depression, stroke, Huntington's disease, cerebral ischemia, neurodegenerative diseases and diabetic neuropathy. BDNF is highly correlated with these diseases.
Cholinergic neurons of the forebrain degenerate in Alzheimer's disease (“AD”), leading to acetylcholine reduction and subsequent cognitive deterioration (Murer M G et al., 2001). With respect to AD, BDNF has been shown to promote the survival and differentiation of basal forebrain cholinergic neurons. Interestingly, in these neurons, BDNF is also known to stimulate the release of acetylcholine (Knipper M et al., 1994). These preclinical observations suggest that deficits of BDNF synthesis might participate in the deterioration of the cellular homeostasis that leads to AD. In addition, it has been proposed that AD is due to the failure of neuroplasticity, which causes the loss of synaptic contacts and may lead to neuropathological and clinical manifestations (Mesulam M M., 1999). Postmortem clinical evidence of AD patients has shown that the expression of BDNF and its receptor trk B is significantly decreased in the hippocampus and the cortex, which are cerebral areas responsible for learning and memory (Phillips H S et al., 1991; Holsinger R M et al., 2000; Allen S J et al., 1999). BDNF is known to regulate LTP (long-term potentiation), which is in these areas a cellular mechanism of learning and memory through synaptic plasticity (Figurov, A. et al., 1996). Therefore, the decrease in BDNF expression is thought to induce the functional reduction of recognition and memory-related processes, causing Alzheimer's disease (C Zuccato and Elena Cattaneo, 2009).
Parkinson's disease is a debilitating movement disorder resulting from a massive loss of substantia nigral dopaminergic neurons and a depletion of striatal dopamine. Cognitive impairment is another feature of patients who have Parkinson's disease. Among the theories suggested to explain the etiology of Parkinson's disease, neurotrophic factors are expected to play an important role in protecting dopaminergic neurons (Siegel G J and Chauhan N B., 2000). Inter alia, BDNF is well known to interact with dopaminergic neurons. Dopaminergic neurons are in all of the ventral midbrain, the substantia nigra and the ventral tegmental area (Seroogy K B et al., 1994). Reduced expression of BDNF within the substantia nigra is accompanied by a significant deterioration in the dopaminergic neurons (Porritt M J et al., 2005). Also, BDNF is required for the establishment of the proper number of dopaminergic neurons in the substantia nigra pars compacta (Baguet Z C et al., 2005).
Postmortem studies of PD patients has revealed that the expression level of BDNF was remarkably reduced in the striatal dopaminergic neurons of such patients, indicating that there is a correlation between the reduced number of dopaminergic neurons and a shortfall in BDNF biosynthesis in Parkinson's disease (Mogi M et al., 1999; Howells D W et al., 2000).
In society these days, there is a rapid increase in the population of people who are suffering from stress-related mood disorders such as major depression. Stress is now known to cause nervous prostration and to reduce the volume of various brain regions including the hippocampus (Duman, R. S, and Monteggia, L. M. 2006) as well as the mRNA expression level of hippocampal BDNF (Duman, R. S, and Monteggia, L. M. 2006 Smith, M. A. et al., 1995). In practice, postmortem evidence has shown that the expression level of BDNF in the brain of depressed patients was significantly lower than in the brain of healthy persons. In addition, imaging studies have revealed a shrinkage of the hippocampus in the brain patients with major depression (Sheline, Y. I et al., 2003). It has been suggested that a variety of antidepressants can be used to treat stress-related mood disorders. These antidepressants are commonly intended to increase BDNF mRNA levels in the hippocampus or prefrontal cortex or in both regions (Duman, R. S, and Monteggia, L. M. 2006). On the basis of these results, the so-called ‘neurotrophin hypothesis of depression’ has been proposed. This hypothesis states that antidepressant treatments achieve their therapeutic effects by increasing the expression of BDNF in the hippocampus or the prefrontal cortex (Duman, R. S, and Monteggia, L. M. 2006).
As described hereinbefore, BDNF is involved in the maintenance of neural structures, the activity of ion channels and the release of neurotransmitters as well as playing an important role in the growth and differentiation of neurons, learning and memory, and anti-depression activity. Together with these various functions, BDNF is highly correlated with the onset of various diseases including Alzheimer's disease, Parkinson's disease, chronic stress-related mood disorder such as major depression, stroke (Schabitz et al., Stroke, 38:2165-2172, 2007), Huntington's disease (Zuccato C et al., Science 293, 20, July 2001), cerebral ischemia (Han B H et al., the Journal of neuroscience, 2000, 20(15):5775-5781, Aug. 1, 2000), neurodegenerative disease (Tsuzaka K et al., Muscle Nerve. 24(4):474-80, April 2001) and diabetic neuropathy (Nitta A et al., Neurotoxicology and Teratology, 24:695-701, 2002).
Accordingly, a variety of methods have been suggested for applying BDNF to the treatment of these diseases. PCT Publication No. WO2003/056925 describes the treatment of neurodegenerative diseases by delivering BDNF to the entorhinal cortex with a micropump. U.S. Pat. No. 5,512,661 discloses a chimeric protein which has neurotrophic activity and which consists essentially of BDNF and partially of NGF. However, BDNF itself, which is a macromolecule with a molecular weight of 14 kDa, cannot pass through the blood-brain barrier (BBB). The likelihood of enzymatic degradation degrades the reliability of BDNF because it cannot be delivered safely to the targets. Further, a limitation is imparted to the oral dosage of the neutrophic factor. As solutions to these problems, non-peptide mimetics of BDNF were suggested in PCT WO 2000/075176 and U.S. Patent Publication No. 2007-060526. Thanks to their low molecular weights, these mimetics can advantageously pass through the blood brain barrier and can overcome the problem of short life span; however, they are cytotoxic and cause side effects