Frontiers in CNS Drug Discovery

Prion diseases are rapidly progressive neurodegenerative illnesses caused by an abnormal conformer of the native prion protein. Human prion diseases include Creutzfeldt-Jakob disease, variant Creutzfeldt-Jakob disease, fatal familial insomnia, Gerstmann-Sträussler-Scheinker disease, and variable protease sensitive prionopathy for which there are no available treatments. Several treatments have been investigated, mostly focusing on inhibiting the conversion of the native prion protein to its pathological form. Although in vitro and animal model studies have been encouraging, success has yet to be translated to clinical trials. In addition to prion protein conversion inhibitors, other avenues of research have focused on blocking the expression of the normal prion protein, hence removing the substrate required for further disease propagation. This technique is complicated by the elusive physiological role(s) of the native prion protein in humans and the unknown complications that could arise with this type of treatment. Although prion diseases are relatively rare in humans, other neurodegenerative protein misfolding disorders such as Alzheimer’s and Parkinson’s disease are now known to exhibit prion-like behavior and will also benefit from treatments originally designed to combat prion disease. The goal of this review is to examine possible treatment targets for protein misfolding disorders utilizing knowledge obtained from the field of prion disease.

Drug addiction is a chronic relapsing disorder characterized by maladaptive patterns of cognition and behavior related to drug use, which are thought to arise from long term changes in the neural circuitries underlying reward, motivation, affect, learning and memory, and executive function. Recently, a large body of evidence has been accumulated showing that epigenetic mechanisms such as DNA methylation and histone modification are involved in drug-induced maladaptive neural plasticity. Epigenetics not only provides a novel avenue for examining the molecular mechanisms underlying interactions between inheritable vulnerabilities and environmental factors that contribute to addiction and relapse, but also provides novel potential pharmacological targets for the treatment of addiction. In this chapter, we begin by introducing relevant epigenetic mechanisms that modulate gene transcription. We then review and summarize the existing literature on epigenetic changes that occur after acute and chronic exposure to or self-administration of alcohol, psychostimulants, opiates, and nicotine, and studies examining the effects of manipulation of epigenetic processes in reward-related brain regions on addiction-like behaviors. We also discuss the possible implications of epigenetic factors as predictors of addiction vulnerability prior to drug exposure. Finally, we will review findings from preclinical studies on the effects of pharmacological modifiers of epigenetic processes on addiction-related behaviors, and discuss the advantages and disadvantages of developing novel epigenetic-based CNS therapeutics for the treatment of addiction.

The mammalian brain retains the capacity to generate new neurons throughout adulthood through a process referred to as adult neurogenesis. This capacity is restricted to well-defined brain regions, namely the sub-ventricular zone (SVZ) adjacent to the lateral ventricles, and the sub-granular zone (SGZ) of the hippocampal dentate gyrus (DG). Adult neurogenesis and each one of its phases are tightly regulated and can be influenced by multiple behavioral, physiological, and pathological factors. Indeed, mounting evidence from animal models has indicated that neurodegenerative conditions such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD) may be associated with altered neurogenic function. Importantly, alterations in adult hippocampal neurogenesis may be responsible, at least in part, for some of the cognitive deficits observed in animal models of these neurodegenerative conditions as well as individuals afflicted with these disorders. On the other hand, since adult neural progenitors have been proposed as an endogenous source of healthy neurons, it has been suggested that harnessing the endogenous neurogenic capacity in the diseased brain might be of therapeutic value for these neurodegenerative conditions. In this chapter we review the results obtained in rodent models of AD, PD, and HD with regards to therapies aimed at restoring adult neurogenesis and discuss whether such therapies might have therapeutic relevance for the treatment of these devastating neurodegenerative disorders.

HIV encephalopathy covers a range of HIV-related brain dysfunctions. HIV- 1 enters the Central Nervous System (CNS) soon after it enters the body. In the CNS, it is largely impervious to highly active anti-retroviral therapeutic drugs (HAART). As survival with chronic HIV-1 infection improves, the number of people harboring the virus in their CNS increases. The prevalence of HIV-associated neurocognitive disorder (HAND) therefore continues to rise. It is also becoming clear that the brain is an important reservoir for the virus, and neurodegenerative and neuroinflammatory changes may continue despite the use of HAART.

Neurons themselves are rarely infected by HIV-1, and neuronal damage is felt to be mainly indirect. HIV-1 infects resident microglia, periventricular macrophages, leading to increased production of cytokines and to release of HIV-1 proteins, the most likely neurotoxins, among which are the envelope glycoprotein gp120 and HIV-1 trans-acting protein Tat. Animal model systems in which recombinant gp120, or Tat, proteins are directly injected into the striatum have been developed and recapitulate some of the features seen in HAND.

As HIV-1 infection of the brain lasts the lifetime of affected individuals, and as eradication of CNS HIV-1 is currently not possible, control of the damage caused by the virus may represent a useful approach to treatment. One way to limit the final damage could be by limiting oxidative stress-related neurotoxicity. We used SV40 vectors for long-term gene delivery of transgenes to the brain. Intracerebral injection of SV(SOD1) or SV(GPx1) carrying the antioxidant enzymes, Cu/Zn superoxide dismutase (SOD1) or glutathione peroxidase (GPx1) respectively, into the rat caudate putamen (CP), significantly protects neurons from apoptosis caused by subsequent inoculation of recombinant gp120 and Tat. Vector administration into the lateral ventricle or cisterna magna, particularly if preceded by intraperitoneal mannitol, protects from intra-CP gp120-induced neurotoxicity comparably to intra-CP vector administration. The safety of SV(SOD1) and SV(GPx1) delivered intra-CP has been demonstrated in rats and in Rhesus macaques monkeys, and resulting transgene expression is very durable. These models should provide a better understanding of the pathogenesis of HIV-1 in the brain as well as offer new therapeutic avenues.

Transient receptor potential (TRP) ion channels have been extensively investigated over the past few years and they are being ardently pursued as targets for drug discovery. Several factors make TRP ion channels appealing as drug targets. First, they are the largest group of noxious stimulus detectors in pain receptors (nociceptors). Second, although pain is currently the most advanced TRP channel-related field, an increasing number of gene deletion researches in animals and genetic association studies in humans have demonstrated that the pathophysiological roles of TRP channels extend well beyond the sensory nervous system (vision, olfaction, taste, mechano- and thermosensation, and osmoregulation). Many studies implicate them in other body systems, including pulmonary, cardiovascular, renal, and bladder systems.

Many TRP channels are expressed by the central nervous system; some are expressed at the spinal cord level (for example TRPA1, TRPM8 and TRPV1 channels), whereas others are expressed at high levels in the cerebrum (e.g., TRPC3 in cerebellar Purkinje cells, and TRPC5 in the hippocampus and amygdala). TRPM2 and TRPM7 are expressed in brain neurons and microglia and are implicated in various pathologies related to oxidative stress, including the focal ischemia model of stroke. Therefore, TRPM7 antagonists may have a role in the treatment of stroke. The TRPM2 gene is also a candidate risk factor gene for bipolar disorder.

Recent findings in the field of pain have established a subset of TRP channels that are activated by temperature (the so-called thermoTRP ion channels) and are capable of initiating sensory nerve impulses following the detection of thermal, as well as mechanical and chemical irritant stimuli. At least, a family of six thermoTRP channels (TRPA1, TRPM8, TRPV1, TRPV2, TRPV3, and TRPV4) exhibits sensitivity to increases or decreases in temperature as well as to chemical substances that elicit similar hot or cold sensations. Such irritants include menthol from mint, cinnamaldehyde, gingerol, capsaicin from chili peppers, mustard oil, camphor, eugenol from cloves, and others.

This review focuses on recent developments in the TRP ion channel-related area and highlights evidence supporting TRP channels as promising targets for new analgesic drugs at the periphery and central levels and opportunities for therapeutic intervention.

Treatment with antipsychotics, APs has been a cornerstone of schizophrenia and other severe mental disorders for more than fifty years. Antipsychotics have been used in monotherapy or in a combination of two, or more drugs. Empirical polypharmacy, common after First Generation Antipsychotics, FGAs introduction, in 1960s, was replaced by monotherapy and the principle of “Minimally effective APs drug strategies” later on, at the end of the 20th Century. Despite current guidelines recommendations, promoting monotherapy with the newly introduced Second Generation Antipsychotics, SGAs, the practice of combining two antipsychotic drugs is routinely utilized. The aim of this review is to address the boundaries of the polypharmacy with APs, from epidemiological features to its clinical significance. At first APs prescription patterns in Italy and other countries were outlined. Then, a comparison between the theoretical rationale of monotheraphy and the pharmacokinetic and pharmachodynamic properties of the combined selective associations of two APs was performed, to assess if this practice might help in the care of treatment-resistant forms of schizophrenia. Finally, a PubMed literature search from 1957 to February 2013 led to the extraction of 118 papers regarding APs combined treatment, with data from larger RCTs to single case reports described in detail to provide reasons for rationale use of polypharmacy in routine practice. Although RCTs led to inconclusive results, case series revisions outlined the feasibility of combining APs with a complementary pharmacodynamic profile. The need for adequately targeted, clinically effective, lowcost APs combinations improving functional autonomy of schizophrenic persons is warranted.

Neurodegenerative disorders such as Parkinson’s disease (PD), Alzheimer’s disease (AD), Huntington’s disease (HD) and amyotrophic lateral sclerosis (ALS) are incurable pathologies with huge social and economic impacts closely related to the increasing of life expectancy in modern times. Although the clinical and neuropathological aspects of these debilitating disorders are distinct, they have a unified feature that is the characteristic pattern of neurodegeneration in anatomically or functionally related regions. All of them remain uncured and presently available treatments are only symptomatic and do not alter the course or progression of the underlying diseases. In this context, the search for new effective chemical entities, capable to act in diverse biochemical targets, with new mechanisms of action and low toxicity stands as a challenge to research groups and Pharmaceutical Industry. This scenery has been promising to the reemerging of modern natural products chemistry to provide active, sophisticated and complex new lead molecules to drug discovery and development. In this publication, we discuss some of the main contributions of the natural products chemistry including more than thirty plant species and the main pharmacological advances for the discovery of active constituents in plants, herbs and extracts prescribed by Traditional Medicine practices to treat senile neurodegenerative disorders, especially for AD and PD, in the period after the 2000s. Many of the recent reported data are resultant of studies carried out during the 90s decade, and the most important contributions are also cited, as well as the main advances in the pharmacological basis to understand the mechanisms of action and to explain the effects of some of these active compounds in the improvement on memory and cognition, in neurovascular function, and as neuroprotective agents.

Parkinson´s disease (PD) is a major public health problem worldwide that affects millions of people, increasingly prevalent as the population ages. This disease, the most common human neurodegenerative motor disorder, is characterized by a progressive decrease in striatal dopamine content of dopaminergic neurons in the substantia nigra pars compacta. Converging pathogenic factors such as oxidative stress, inflammation, mitochondrial impairment and altered calcium homeostasis, among others, have been described as biochemical mechanisms of neurodegeneration in PD. Presently, quite a few natural flavonoids with potential antioxidants and signaling properties have been investigated and are still in progress to identify hopefully preventive neuroprotective compounds to forestall clinical progression of PD.

Flavonoids are the most abundant plant polyphenolic substances (over 4000 different ones) and they are found in main dietary sources (fruits, vegetables and plant-derived beverages). Chemically, this group of natural products shares a 2-phenylbenzopyran as basic structure (C6-C3-C6), and it is further subdivided into different classes (i.e. flavones, flavanones, flavonols anthocyanins, flavan-3-ols). Related to their structural characteristics, flavonoids can transfer a hydrogen atom to scavenge reactive oxygen species (ROS), chelate metal ions (i.e. iron, copper) and stabilize unpaired electrons by resonance. Structural differences found among individual types of flavonoids as well as glycosylation patterns determine the biological activities of these promising chemoprotective compounds. As neuroprotective agents, flavonoids have been reported to act as direct ROS scavengers, modulate the endogenous enzymatic and nonenzymatic antioxidant defense system and activate and regulate different pro-survival pathways.

This chapter, based on highlighted research articles, focuses on the multiple neuroprotection mechanisms of natural flavonoids in PD, covering the most recent preclinical in vitro and in vivo PD animal models’ studies and clinical trials and providing an overview and challenges that may be helpful for future research.

Inflammation plays an important role in the onset and progress of neurodegenerative diseases. Inflammation may trigger or exacerbate neuronal apoptosis and death through glucocorticoid secretion, oxidative stress, and changes in neurotransmission. Therefore, the use of anti-inflammatory drugs might diminish the cumulative effects of inflammation in the brain. Indeed, some epidemiological studies showed that sustained use of anti-inflammatory drugs or natural products may prevent or slow down the progression of neurodegenerative diseases.

Among several new products, omega (n)-3 fatty acids have anti-inflammatory and neuroprotective effects with few side effects. Essential polyunsaturated fatty acids (PUFA), including n-3 and 6 fatty acids can change brain and immune functions. The effects may be via modulating 1) membrane structure and fluidity; 2) the interaction between genes and proteins; 3) channel and receptor functions; 4) neurotransmitter release and long-term potentiation process, 5) the function of glial cells in the brain and 6) cellular and humoral inflammatory responses and more. Many studies have demonstrated that n-3 and n-6 fatty acids cooperate and compete with each other to maintain the homeostasis. Over intake of n-6 fatty acids may induce inflammation and neurodegeneration, while n-3 fatty acids have been tried clinically and experimentally to treat patients with neurodegenerative diseases or to explore therapeutic mechanisms in animal models.

More interestingly, the combination of n-3 and n-6 fatty acids at different ratios showed the enhancement of anti-inflammation, neuroprotection or gene and protein modulation.

This chapter will (1) review the new findings from studies in relationship between inflammation and neurodegenerative disease, mainly Alzheimer’s disease (AD); (2) introduce the important role of PUFA in the brain and the immune system; (3) discuss clinical trials of n-3 fatty acids used for treatments of neurodegeneration and (4) explore/summarize possible mechanisms by which PUFA can be used for treatment of neurodegeneration. In addition, the limitation of current studies and further research directions will be raised.

Monoterpenoids and their derivatives play an important role in the creation of new bioactive substances including drugs. Many of these compounds possess substantial CNS activities such as antinociceptive, neuroprotective, anticonvulsant. In the past decades, interest in investigating the possibility of using monoterpenoids for the development of new drugs has increased significantly due to improvement of both chemical methods of modification of natural compounds, and a substantial deficit of breakthrough research to create drugs to treat diseases associated with central nervous system. The review covers the literature on monoterpenoids and their derivatives exhibit various types of CNS activities published up to early 2012.

Zebrafish (Danio rerio) are a vertebrate animal model with advantages for screening and development of therapeutic agents. The ease of growth and handling and the ability of zebrafish to be treated with compounds in a multi-well format are advantages for high-throughput screening (HTS) work to identify new drugs. Zebrafish have also been used in target confirmation after a lead compound has been identified. In vivo structure activity relationship (SAR) studies in zebrafish have also been performed. Indeed, zebrafish can be used at several points in the drug discovery process. Zebrafish are ideal for testing drug toxicity on a large scale, thus saving much time, money and effort to further develop a compound with toxicity in vertebrates. Many behavioral assays developed in other animals, and which are used to assay drugs targeted to several neurological diseases, are available in zebrafish These include assays for locomotion, avoidance behaviors, learning, and conditioned place preference. The use of zebrafish allows one to combine the ability to perform behavioral assays with HTS and thus perform high-throughput in vivo drug screening. Many biochemical pathways and genes present in humans are conserved in zebrafish, including those involving the nicotinic cholinergic and dopaminergic systems. Zebrafish is an exciting new system amenable to identification of new drugs to treat disorders due to nicotinic cholinergic and dopaminergic disregulation including nicotine addiction, schizophrenia, Alzheimer's disease and Parkinson's disease.