Synthesis and evaluation of clioquinol-rolipram/roflumilast hybrids as multitarget-directed ligands for the treatment of Alzheimer's disease

Emerging phosphodiesterase inhibitors for treatment of neurodegenerative diseases

Neurodegenerative diseases (NDs) cause progressive loss of neuron structure and ultimately lead to neuronal cell death. Since the available drugs show only limited symptomatic relief, NDs are currently considered as incurable. This review will illustrate the principal roles of the signaling systems of cyclic adenosine and guanosine 3',5'-monophosphates (cAMP and cGMP) in the neuronal functions, and summarize expression/activity changes of the associated enzymes in the ND patients, including cyclases, protein kinases, and phosphodiesterases (PDEs). As the sole enzymes hydrolyzing cAMP and cGMP, PDEs are logical targets for modification of neurodegeneration. We will focus on PDE inhibitors and their potentials as disease-modifying therapeutics for the treatment of Alzheimer's disease, Parkinson's disease, and Huntington's disease. For the overlapped but distinct contributions of cAMP and cGMP to NDs, we hypothesize that dual PDE inhibitors, which simultaneously regulate both cAMP and cGMP signaling pathways, may have complementary and synergistic effects on modifying neurodegeneration and thus represent a new direction on the discovery of ND drugs.

Opportunities and perspectives of small molecular phosphodiesterase inhibitors in neurodegenerative diseases

Phosphodiesterase (PDE) is a superfamily of enzymes that are responsible for the hydrolysis of two second messengers: cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). PDE inhibition promotes the gene transcription by activating cAMP-response element binding protein (CREB), initiating gene transcription of brain-derived neurotrophic factor (BDNF). The procedure exerts neuroprotective profile, and motor and cognitive improving efficacy. From this point of view, PDE inhibition will provide a promising therapeutic strategy for treating neurodegenerative disorders. Herein, we summarized the PDE inhibitors that have entered the clinical trials or been discovered in recent five years. Well-designed clinical or preclinical investigations have confirmed the effectiveness of PDE inhibitors, such as decreasing Aβ oligomerization and tau phosphorylation, alleviating neuro-inflammation and oxidative stress, modulating neuronal plasticity and improving long-term cognitive impairment.

Discovery of novel melatonin-mydroxyquinoline hybrids as multitarget strategies for Alzheimer's disease therapy

Alzheimer's disease (AD) is a neurodegenerative disease that seriously affects human health, and current treatment strategies are far from meeting clinical needs. Inspired by multi-target drug design strategies, a series of novel natural products-based melatonin-hydroxyquinoline hybrids were designed and synthesized, targeting anti-oxidation and metal-chelating at the same time. Most of the compounds showed significant oxygen radical absorbance capacity and Aβ1-42 aggregation inhibition. Moreover, the compounds possess good blood-brain barrier permeability. 6b and 6c have a good ability to alleviate oxidative stress induced by hydrogen peroxide. 6b and 6c possess metal-chelating properties with the chelation ratio being 2:1. Furthermore, 6b can significantly mitigate metal-induced Aβ aggregation. This work may provide a new multi-target treatment strategy for Alzheimer's disease.

Accelerating and Automating the Free Energy Perturbation Absolute Binding Free Energy Calculation with the RED-E Function

The accurate prediction of the binding affinities between small molecules and biological macromolecules plays a fundamental role in structure-based drug design, which is still challenging. The free energy perturbation-based absolute binding free energy (FEP-ABFE) approach has shown potential in its reliability. To correctly calculate the energy related to the ligand being restrained by the receptor, additional restraints between the ligand and the receptor are needed. However, determining the restraint parameters for individual ligands empirically is too trivial to be automated, and usually gives rise to numerical instabilities, which set back the applications of FEP-ABFE. To address these issues, we derived the analytical expression for the probability distribution of energy differences, P(ΔU), during the process of restraint addition, which is called the RED-E (restraint energy distribution at equilibrium position) function. Simulations indicated that the RED-E function can accurately describe P(ΔU) when restraints are added at the equilibrium position. Based on the RED-E function, an automatic restraint selection method was proposed to select the best restraint. With this method, there is a high phase-space overlap between the free and restrained states, such that using a 2-λ perturbation can accurately calculate the free energy of the restraint addition, which is a nearly 6 times acceleration compared with current widely used 12-λ perturbation method. The RED-E function gives insight into the non-Gaussian behavior of the sampled P(ΔU) in certain FEP processes in an analytical way. The highly automated and accelerated restraint selection also makes it possible for the large-scale application of FEP-ABFE in real drug discovery practices.

Inhibition of phosphodiesterase: A novel therapeutic target for the treatment of mild cognitive impairment and Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia and is ranked as the 6th leading cause of death in the US. The prevalence of AD and dementia is steadily increasing and expected cases in USA is 14.8 million by 2050. Neuroinflammation and gradual neurodegeneration occurs in Alzheimer's disease. However, existing medications has limitation to completely abolish, delay, or prevent disease progression. Phosphodiesterases (PDEs) are large family of enzymes to hydrolyze the 3'-phosphodiester links in cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) in signal-transduction pathways for generation of 5'-cyclic nucleotides. It plays vital role to orchestrate several pharmacological activities for proper cell functioning and regulating the levels of cAMP and cGMP. Several evidence has suggested that abnormal cAMP signaling is linked to cognitive problems in neurodegenerative disorders like AD. Therefore, the PDE family has become a widely accepted and multipotential therapeutic target for neurodegenerative diseases. Notably, modulation of cAMP/cGMP by phytonutrients has a huge potential for the management of AD. Natural compounds have been known to inhibit phosphodiesterase by targeting key enzymes of cGMP synthesis pathway, however, the mechanism of action and their therapeutic efficacy has not been explored extensively. Currently, few PDE inhibitors such as Vinpocetine and Nicergoline have been used for treatment of central nervous system (CNS) disorders. Considering the role of flavonoids to inhibit PDE, this review discussed the therapeutic potential of natural compounds with PDE inhibitory activity for the treatment of AD and related dementia.

Advancements in the development of multi-target directed ligands for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is a multifactorial irreversible neurological disorder which results in cognitive impairment, loss of cholinergic neurons in synapses of the basal forebrain and neuronal death. Exact pathology of the disease is not yet known however, many hypotheses have been proposed for its treatment. The available treatments including monotherapies and combination therapies are not able to combat the disease effectively because of its complex pathological mechanism. A multipotent drug for AD has the potential to bind or inhibit multiple targets responsible for the progression of the disease like aggregated Aβ, hyperphosphorylated tau proteins, cholinergic and adrenergic receptors, MAO enzymes, overactivated N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor etc. The traditional approach of one disease-one target-one drug has been rationalized to one drug-multi targets for the chronic diseases like AD and cancer. Thus, over the last decade research focus has been shifted towards the development of multi target directed ligands (MTDLs) which can simultaneously inhibit multiple targets and stop or slow the progression of the disease. The MTDLs can be more effective against AD and eliminate any possibility of drug-drug interactions. Many important active pharmacophore units have been fused, merged or incorporated into different scaffolds to synthesize new potent drugs. In the current article, we have described various hypothesis for AD and effectiveness of the MTDLs treatment strategy is discussed in detail. Different chemical scaffolds and their synthetic strategies have been described and important functionalities are identified in the chemical scaffold that have the potential to bind to the multiple targets. The important leads identified in this study with MTDL characteristics have the potential to be developed as drug candidates for the effective treatment of AD.

Novel pharmacological strategies to treat cognitive dysfunction in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a major incurable global health burden and currently the 3rd largest cause of death in the world, with approximately 3.23 million deaths per year. Globally, the financial burden of COPD is approximately €82 billion per year and causes substantial morbidity and mortality. Importantly, much of the disease burden and health care utilisation in COPD is associated with the management of its comorbidities and viral and bacterial-induced acute exacerbations (AECOPD). Recent clinical studies have shown that cognitive dysfunction is present in up to 60% of people with COPD, with impairments in executive function, memory, and attention, impacting on important outcomes such as quality of life, hospitalisation and survival. The high prevalence of cognitive dysfunction in COPD may also help explain the insufficient adherence to therapeutic plans and strategies, thus worsening disease progression in people with COPD. However, the mechanisms underlying the impaired neuropathology and cognition in COPD remain largely unknown. In this review, we propose that the observed pulmonary oxidative burden and inflammatory response of people with COPD 'spills over' into the systemic circulation, resulting in damage to the brain and leading to cognitive dysfunction. As such, drugs targeting the lungs and comorbidities concurrently represent an exciting and unique therapeutic opportunity to treat COPD and cognitive impairments, which may lead to the production of novel targets to prevent and reverse the debilitating and life-threatening effects of cognitive dysfunction in COPD.

Tackling Alzheimer's Disease with Existing Drugs: A Promising Strategy for Bypassing Obstacles

The unmet need for the development of effective drugs to treat Alzheimer 's disease has been steadily growing, representing a major challenge in drug discovery. In this context, drug repurposing, namely the identification of novel therapeutic indications for approved or investigational compounds, can be seen as an attractive attempt to obtain new medications reducing both the time and the economic burden usually required for research and development programs. In the last years, several classes of drugs have evidenced promising beneficial effects in neurodegenerative diseases, and for some of them, preliminary clinical trials have been started. This review aims to illustrate some of the most recent examples of drugs reprofiled for Alzheimer's disease, considering not only the finding of new uses for existing drugs but also the new hypotheses on disease pathogenesis that could promote previously unconsidered therapeutic regimens. Moreover, some examples of structural modifications performed on existing drugs in order to obtain multifunctional compounds will also be described.

Insights of 8-hydroxyquinolines: A novel target in medicinal chemistry

8-Hydroxyquinoline (8-HQ) is a significant heterocyclic scaffold in organic and analytical chemistry because of the properties of chromophore and is used to detect various metal ions and anions. But from the last 2 decades, this moiety has been drawn great attention of medicinal chemists due to its significant biological activities. Synthetic modification of 8-hydroxyquinoline is under exploration on large scale to develop more potent target-based broad spectrum drug molecules for the treatment of several life-threatening diseases such as anti-cancer, HIV, neurodegenerative disorders, etc. Metal chelation properties of 8-hydroxyquinoline and its derivatives also make these potent drug candidates for the treatment of various diseases. This review comprises 8-hydroxyquinoline derivatives reported in the literature in last five years (2016-2020) and we anticipate that it will assist medicinal chemists in the synthesis of novel and pharmacologically potent agents for various therapeutic targets, mainly anti-proliferative, anti-microbial, anti-fungal and anti-viral as well as for the treatment of neurodegenerative disorders.

Carnoquinolines Target Copper Dyshomeostasis, Aberrant Protein-Protein Interactions, and Oxidative Stress

Metal dysregulation, oxidative stress, protein modification, and aggregation are factors strictly interrelated and associated with neurodegenerative pathologies. As such, all of these aspects represent valid targets to counteract neurodegeneration and, therefore, the development of metal-binding compounds with other properties to combat multifactorial disorders is definitely on the rise. Herein, the synthesis and in-depth analysis of the first hybrids of carnosine and 8-hydroxyquinoline, carnoquinolines (CarHQs), which combine the properties of the dipeptide with those of 8-hydroxyquinoline, are reported. CarHQs and their copper complexes were characterized through several techniques, such as ESI-MS and NMR, UV/Vis, and circular dichroism spectroscopy. CarHQs can modulate self- and copper-induced amyloid-β aggregation. These hybrids combine the antioxidant activity of their parent compounds. Therefore, they can simultaneously scavenge free radicals and reactive carbonyl species, thanks to the phenolic group and imidazole ring. These results indicate that CarHQs are promising multifunctional candidates for neurodegenerative disorders and they are worthy of further studies.

Biological evaluation of 7-O-amide hesperetin derivatives as multitarget-directed ligands for the treatment of Alzheimer's disease

A series of 7-O-amide hesperetin derivatives were subjected to multi-target biological evaluation of anti-Alzheimer's disease. Most of the compounds showed good in vitro inhibitory activity against cholinesterase, of which compound 7c (7-O-(4-(morpholinoethyl)-acetamide) hesperetin) was the most effective anti-eqBuChE derivative (IC₅₀ = 0.28 ± 0.05 μM) and exerted neuroprotective effects. Further biological evaluation found that compounds 4d, 4e and 7c showed strong antioxidant, anti-Aβ self-aggregation and anti-neuroinflammatory activities. Compound 7c could inhibit the expression of iNOS and COX-2 proteins and prevent LPS-induced inflammatory response in BV2 cells. In addition, compound 7c could chelate biometal ions such as Cu²⁺ and Zn²⁺. In the vivo study, the MWM test confirmed that compound 7c could improve the cognitive impairment caused by scopolamine. In summary, the above studies have shown that the optimized compound 7c has great development potential as MTDL for the treatment of AD.

Molecular Hybridization as a Tool in the Design of Multi-target Directed Drug Candidates for Neurodegenerative Diseases

Neurodegenerative Diseases (NDs) are progressive multifactorial neurological pathologies related to neuronal impairment and functional loss from different brain regions. Currently, no effective treatments are available for any NDs, and this lack of efficacy has been attributed to the multitude of interconnected factors involved in their pathophysiology. In the last two decades, a new approach for the rational design of new drug candidates, also called multitarget-directed ligands (MTDLs) strategy, has emerged and has been used in the design and for the development of a variety of hybrid compounds capable to act simultaneously in diverse biological targets. Based on the polypharmacology concept, this new paradigm has been thought as a more secure and effective way for modulating concomitantly two or more biochemical pathways responsible for the onset and progress of NDs, trying to overcome low therapeutical effectiveness. As a complement to our previous review article (Curr. Med. Chem. 2007, 14 (17), 1829-1852. https://doi.org/10.2174/092986707781058805), herein we aimed to cover the period from 2008 to 2019 and highlight the most recent advances of the exploitation of Molecular Hybridization (MH) as a tool in the rational design of innovative multifunctional drug candidate prototypes for the treatment of NDs, specially focused on AD, PD, HD and ALS.

Recent Advances in the Synthesis and Biological Activity of 8-Hydroxyquinolines

Compounds containing the 8-hydroxyquinoline (8-HQ) 1 nucleus exhibit a wide range of biological activities, including antimicrobial, anticancer, and antifungal effects. The chemistry and biology of this group have attracted the attention of chemists, medicinal chemists, and professionals in health sciences. A number of prescribed drugs incorporate this group, and numerous 8-HQ- based molecules can be used to develop potent lead compounds with good efficacy and low toxicity. This review focusses on the recent advances in the synthesis of 8-HQ derivatives with different pharmacological properties, including anticancer, antiviral, and antibacterial activities. For this purpose, recent relevant references were searched in different known databases and search engines, such as MEDLINE (PubMed), Google Scholar, Science Direct, Scopus, Cochrane, Scientific Information Database (SID), SciFinder, and Institute for Scientific Information (ISI) Web of Knowledge. This review article provides a literature overview of the various synthetic strategies and biological activities of 8-HQ derivatives and covers the recent related literature. Taken together, compounds containing the 8-HQ moiety have huge therapeutic value and can act as potential building blocks for various pharmacologically active scaffolds. In addition, several described compounds in this review could act leads for the development of drugs against numerous diseases including cancer.

Direct electrochemical reductive amination between aldehydes and amines with a H/D-donor solvent

A novel electrochemical synthesis protocol has been achieved for reductive amination between aldehydes and amines in undivided cells at room temperature. Under metal-free and external-reductant-free electrolysis conditions, various important secondary amine products are obtained in moderate-to-high yields. Deuterium-labeling experiments have demonstrated that low-toxicity DMSO acts both as a solvent and a H-donor in the reaction. On this basis, various deuterium-labeled products with good-to-excellent D-incorporation have been synthesized by using DMSO-d6 as a solvent. Furthermore, a molecule with GR-antagonistic activity has been synthesized through further sulfonylation.

Design, Synthesis, and Biological Evaluation of Novel Multifunctional Rolipram-Tranilast Hybrids As Potential Treatment for Traumatic Brain Injury

Traumatic brain injury (TBI) is a prevalent public healthcare concern frequently instigated by mechanical shock, traffic, or violence incidents, leading to permanent nerve damage, and there is no ideal treatment for it yet. In this study, a series of Rolipram-Tranilast hybrids were designed and synthesized. The neuroprotective activities of the Rolipram-Tranilast hybrids were evaluated both in vitro and in vivo. Compound 5 has been identified as the strongest neuroprotective molecule among the series with robust anti-oxidant and anti-inflammatory potentials. Compound 5 significantly increased the heme oxygenase-1 (HO-1) levels and the phosphorylated cAMP response elements binding protein (p-CREB) while it down-regulated phosphodiesterase-4 B (PDE4B) expression in vitro. Furthermore, compound 5 remarkably attenuated TBI and had a good safety profile in mice. Taken together, our findings suggested that compound 5 could serve as a novel promising lead compound in the treatment of TBI and other central nervous system (CNS) diseases associated with PDE4B and oxidative stress.

Free radical scavengers: An overview on heterocyclic advances and medicinal prospects

In the present scenario, there has been a lot of consideration toward the field of free radical chemistry. Free radicals responsive oxygen species are produced by different endogenous frameworks, exposure to various physicochemical conditions, radiation, toxins, metabolized drug by-product, and pathological states. On the off chance that free radical overpowers the body's capacity, it generates a condition known as oxidative stress, which can alter physiological conditions of the body and results in several diseases. For appropriate physiological function, it is necessary to have a proper balance between free radicals and antioxidants. Antioxidants chemically inhibit the oxidation process; they are also known as free radical scavengers. For tackling the problem of oxidative stress application of an external source of antioxidant is helpful. A lot of antioxidants of natural, semi-synthetic and synthetic origin are in use, with time search of more effective, nontoxic, safe antioxidant is intensified. The present review, discuss different synthetic derivatives bearing various heterocyclic scaffolds as radical scavengers.

Multitarget Therapeutic Strategies for Alzheimer's Disease: Review on Emerging Target Combinations

Neurodegenerative diseases represent nowadays one of the major health problems. Despite the efforts made to unveil the mechanism leading to neurodegeneration, it is still not entirely clear what triggers this phenomenon and what allows its progression. Nevertheless, it is accepted that neurodegeneration is a consequence of several detrimental processes, such as protein aggregation, oxidative stress, and neuroinflammation, finally resulting in the loss of neuronal functions. Starting from these evidences, there has been a wide search for novel agents able to address more than a single event at the same time, the so-called multitarget-directed ligands (MTDLs). These compounds originated from the combination of different pharmacophoric elements which endowed them with the ability to interfere with different enzymatic and/or receptor systems, or to exert neuroprotective effects by modulating proteins and metal homeostasis. MTDLs have been the focus of the latest strategies to discover a new treatment for Alzheimer's disease (AD), which is considered the most common form of dementia characterized by neurodegeneration and cognitive dysfunctions. This review is aimed at collecting the latest and most interesting target combinations for the treatment of AD, with a detailed discussion on new agents with favorable in vitro properties and on optimized structures that have already been assessed in vivo in animal models of dementia.

Novel small molecule therapeutic agents for Alzheimer disease: Focusing on BACE1 and multi-target directed ligands

Alzheimer's Disease (AD) is a progressive neurodegenerative disorder that effects 50 million people worldwide. In this review, AD pathology and the development of novel therapeutic agents targeting AD were fully discussed. In particular, common approaches to prevent Aβ production and/or accumulation in the brain including α-secretase activators, specific γ-secretase modulators and small molecules BACE1 inhibitors were reviewed. Additionally, natural-origin bioactive compounds that provide AD therapeutic advances have been introduced. Considering AD is a multifactorial disease, the therapeutic potential of diverse multi target-directed ligands (MTDLs) that combine the efficacy of cholinesterase (ChE) inhibitors, MAO (monoamine oxidase) inhibitors, BACE1 inhibitors, phosphodiesterase 4D (PDE4D) inhibitors, for the treatment of AD are also reviewed. This article also highlights descriptions on the regulator of serotonin receptor (5-HT), metal chelators, anti-aggregants, antioxidants and neuroprotective agents targeting AD. Finally, current computational methods for evaluating the structure-activity relationships (SAR) and virtual screening (VS) of AD drugs are discussed and evaluated.

A novel class of multitarget anti-Alzheimer benzohomoadamantane‒chlorotacrine hybrids modulating cholinesterases and glutamate NMDA receptors

The development of multitarget compounds against multifactorial diseases, such as Alzheimer's disease, is an area of very intensive research, due to the expected superior therapeutic efficacy that should arise from the simultaneous modulation of several key targets of the complex pathological network. Here we describe the synthesis and multitarget biological profiling of a new class of compounds designed by molecular hybridization of an NMDA receptor antagonist fluorobenzohomoadamantanamine with the potent acetylcholinesterase (AChE) inhibitor 6-chlorotacrine, using two different linker lengths and linkage positions, to preserve or not the memantine-like polycyclic unsubstituted primary amine. The best hybrids exhibit greater potencies than parent compounds against AChE (IC₅₀ 0.33 nM in the best case, 44-fold increased potency over 6-chlorotacrine), butyrylcholinesterase (IC₅₀ 21 nM in the best case, 24-fold increased potency over 6-chlorotacrine), and NMDA receptors (IC₅₀ 0.89 μM in the best case, 2-fold increased potency over the parent benzohomoadamantanamine and memantine), which suggests an additive effect of both pharmacophoric moieties in the interaction with the primary targets. Moreover, most of these compounds have been predicted to be brain permeable. This set of biological properties makes them promising leads for further anti-Alzheimer drug development.

Alzheimer's disease: Key developments support promising perspectives for therapy

Alzheimer's is the neurodegenerative disease affecting the largest number of patients in the world. In spite of the intense research of the last decades, progress about its knowledge and therapy was limited. In particular, various cytotoxic processes remained debated, while the few drugs approved for therapy were of only marginal relevance. Recent studies have identified key aspects of the disease, such as the mechanisms governing the development of pathology. In order to operate the Aβ peptide, known as the key factor, requires a complex assembled by its high affinity binding to PrP^c, a cell surface prion protein, and mGluR5, a metabotropic glutamate receptor. Aβ and its associates bind also phosphorylated tau transferred to the extracellular space, with final activation of intracellular cytotoxic signals. Pathology is further affected by factors (including genes, receptors and their agonists) and by glial cells governing (via vesicles, cytokines and enzymes) cell immunology, inflammation and oxidative stress. Concomitant to pathology studies, strong attempts have been made for the development of new, effective therapies. Critical for this are biomarkers, by which Alzheimer's patients are recognized even before appearance of their symptoms. The question was whether patients take advantage from drugs not yet approved. The latter, first identified in mice, were found effective also in men, however only before appearance or at early stage of the disease. In other words, the drugs not yet approved induce effective protection of patients still healthy or in a preliminary stage of the disease. In contrast, developed Alzheimer's disease is practically irreversible.

Unravelling the interaction mechanism between clioquinol and bovine serum albumin by multi-spectroscopic and molecular docking approaches

Clioquinol has recently been proposed for the treatment of Alzheimer's disease. It is able to diminish β-amyloid protein aggregation and to restore cognition of Alzheimer's mice. However, its therapeutic benefits for Alzheimer's disease in human remain controversy and need further confirmation. Herein, we have explored the interaction mechanism of clioquinol toward bovine serum albumin (BSA) by means of multi-spectroscopic and docking simulation approaches. Clioquinol interacts with BSA by a combined mechanism of static and dynamic processes. Application of the Hill's equation to fluorescence quenching experiment revealed that the binding constant of the BSA-clioquinol complex is extremely high at 10⁸ M^-1 level. Competitive displacement and docking analysis consistently suggested that there are the multiple binding modes of clioquinol toward BSA. Competitive binding study showed that clioquinol shares the binding sites with ibuprofen and digitoxin on albumin, referring to be site II and site III binding compounds. Besides, partial binding in site I was also observed. Docking simulation confirmed that clioquinol favors to bind in site I, site II, site III, fatty acid binding site 5, and the protein cleft between subdomain IB and IIIB of the BSA. Due to its small size and electric dipole property, clioquinol may easily fit in multiple pockets of the BSA. Our finding suggests the potential role of BSA as a clioquinol carrier in the vascular system. Nonetheless, clioquinol-induced BSA aggregation has been observed by the three-dimensional fluorescence technique. This phenomenon may not only impair the BSA, but may also affect other endogenous proteins, which eventually causes adverse effects to human. Therefore, the redesigned or modified molecular structure of clioquinol may reduce its toxicity and improve its bioavailability.