Development and optimization of halogenated vinyl sulfones as Nrf2 activators for the treatment of Parkinson’s disease
Graphical abstract
Introduction
Parkinson’s disease (PD) is one of the most common neurodegenerative diseases characterized by rigidity, resting tremor, bradykinesia, and postural instability in motor system [[1], [2], [3]]. Until now, the clear cause of PD has not been widely known, but the main pathophysiology of PD is mainly caused by the loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta (SNpc) [4,5]. Currently, the mainstay strategy is by administering drugs that mimic the action of dopamine [6,7]. A representative therapeutic agent is dopamine precursor L-3,4-dihydroxy phenylalanine (l-DOPA) that directly increases dopamine levels. Over the years, l-DOPA has been considered the most effective drug for alleviating the symptoms of PD patients. Unfortunately, long-term treatment with l-DOPA induces side effects of l-DOPA-induced dyskinesia (LID) [8,9]. Another strategy is developing therapeutic agents to prevent the death of DAergic neurons. Oxidative stress is caused by an imbalance between the generation of reactive oxygen species (ROS) and antioxidant defense system, inducing damages and death of the DAergic cells involved in dopamine production [10,11]. Furthermore, oxidative stress is a critical factor in the pathophysiology of inflammation because endothelial dysfunction of immune cells involves free radical production. Therefore, oxidative stress and inflammation are commonly known to contribute to the major signs of various diseases, including PD, Alzheimer’s disease (AD), and other neurodegenerative disorders [12,13].
Generally, the induction of nuclear factor E2-related factor 2 (Nrf2), which resists environmental stress, seems to be a promising strategy for maintaining cell homeostasis. Nrf2 is a member of the cap’n’collar family of transcription factors and plays a crucial role in antioxidant stress. Without oxidative stress, Nrf2 remains in the cytoplasm by binding to the Kelch-like ECH-associated protein 1 (Keap1) and is directed to proteasomal degradation by ubiquitination [14,15]. Upon exposure to oxidative stress and electrophiles, the cysteine residues of Keap1 are modified to alter the interaction between Nrf2 and Keap1, and release Nrf2 into the nucleus. This phenomenon causes Nrf2 to accumulate in the nucleus, allowing Nrf2 to interact with a small Maf protein and bind to a promoter, including an antioxidant response element (ARE) sequence [16,17]. Therefore, Nrf2 is known to be a major regulator of oxidative stress that induces the expression of antioxidant enzyme genes such as heme oxygenase-1 (HO-1), glutamate-cysteine ligase (GCL), and superoxide dismutase 1 (SOD1) [[18], [19], [20], [21]]. Furthermore, the Keap1-Nrf2-ARE signaling pathway mainly regulates the expression of proinflammatory genes such as interleukin-6 (Il-6) and tumor necrosis factor-α (TNF-α), and then inhibits inflammation progression [22,23].
Accordingly, Nrf2 pharmacological activation is a promising therapeutic approach to neurodegenerative diseases associated with oxidative stress and inflammation [24,25]. Sulforaphane (SFN), a representative well-known Nrf2 activator, is a type of isothiocyanate derived from broccoli and other vegetables. It is being extensively studied because it has antioxidant, anti-inflammatory, and antitumor effects through various mechanisms [[26], [27], [28], [29]]. One of the major targets of sulforaphane is the Nrf2 that regulates the expression of numerous cytoprotective enzymes with antioxidant effects [30,31]. The most successful case of Nrf2 targeting is the dimethyl fumarate (Tefidera™), which is already used as an oral drug for multiple sclerosis (MS). Dimethyl fumarate modifies Keap1 to covalent bonds to induce Nrf2 translocation and upregulation of the Nrf2-dependent antioxidant genes [32,33]. Bardoxolone methyl (CDDO-Me), derived by modifying the natural product oleanolic acid, has α,β-unsaturated scaffold that currently acts as an electrophilic Nrf2 activator with the most superior Nrf2 activation efficacy. CDDO-Me has been reported to protect cells from oxidative stress by inhibiting ROS production and diminishing proinflammatory signaling at low nanomolar concentrations [[34], [35], [36]].
Structure activity analyses of different Nrf2 activators have shown that an α,β-unsaturated carbonyl group, a key structure for the Michael addition, is vital for exhibiting the potent antioxidant and anti-inflammatory effects [[24], [25], [26],28]. Chalcone can also activate Nrf2 by modifying the cysteine residues of Keap1 by Michael addition [[37], [38], [39]]. In our previous study, compound 1 was developed by introducing vinyl sulfone based on the chalcone structure and exhibited both antioxidant and anti-inflammatory effects [40,41]. Recently, compound 2 was developed with potent Nrf2 activation compared to compound 1 (1: EC50 = 530 nM vs 2: EC50 = 326 nM) and dose-dependently induced the expression of Nrf2-dependent antioxidant enzymes in DAergic cells. Compound 2 also protected DAergic neuronal cells from oxidative damages in vitro and in vivo and attenuated PD-related movement deficits in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model of PD [42].
It has recently been demonstrated that the introduction of halogen atoms at a specific position on active molecules can substantially enhance its biological activity [43,44]. Furthermore, pyridine ring is the second most commonly introduced aromatic nitrogen heterocycle among all U.S. FDA-approved drugs and has moderate activities against a number of biological targets [[45], [46], [47]]. In this study, we attempted to maximize Nrf2 activating effect of vinyl sulfones by introducing halogens and nitrogen heterocycle. The optimized vinyl sulfones were evaluated for Nrf2 activation in cell-based assays and showed dramatically superior Nrf2 activation than the previous compounds (1 and 2). The most potent Nrf2 activator was evaluated for its ability to induce expression of the Nrf2-dependent antioxidant enzyme gene and suppress inflammatory responses in BV-2 microglial cells. We also examined whether the optimized vinyl sulfone effectively protect DAergic neurons and attenuate PD-associated behavioral deficits in the MPTP-induced mouse model.
Section snippets
Chemistry
Scheme 1 shows the synthetic route to access the target compounds (6–21). Synthesis was conducted by coupling with commercially available benzenethiols and (diethoxyphosphoryl) methyl-4-methylbenzenesulfonate (3) in the presence of cesium carbonate (Cs2CO3) to obtain substituted sulfides (4). The substituted sulfides were oxidized with 2.2 equivalents of m-chloroperoxybenzoic acid (mCPBA) at room temperature to give the desired sulfones (5). Finally, the target compounds 6–21 were obtained from
Conclusions
Optimization of potency on Nrf2 was achieved through SAR data collected during our hit-to-lead medicinal chemistry on the key structure of vinyl sulfone that contributed to the efficacy of the previously developed compounds (1 and 2). These optimizations included the following: (1) the halogen substitution in the ortho position of the benzene ring, (2) substitution of the vinyl group in beta-position with a pyridine ring, and (3) insertion of a halogen in R2 on pyridine ring.
As a result of SAR
General methods
Commercially available chemicals, solvents, and reagents were purchased from chemical suppliers and used without further purification. The desired compounds were purified by column chromatography using silica gel (Merck, Cat No. 1.07734 and 1.09385). 1H and 13C NMR spectra were obtained using Bruker spectrometers (400 MHz or 300 MHz, respectively) with deuterated solvents (DMSO‑d6 and CDCl3). All NMR spectra were reported in ppm downfield from tetramethylsilane (TMS). Analytical HPLC for final
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by a National Research Council of Science & Technology grant by the South Korean government (MSIP, No. CRC-15-04-KIST), the National Research Foundation of Korea (NRF-2018M3A9C8016849).
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2022, European Journal of Medicinal ChemistryCitation Excerpt :Compound 65a also protected dopaminergic neurons, attenuating microglial activation against MPTP in the Striatum and substantia nigra pars compacta. Recently, the same research group have synthesized a series of 61 halogenated derivatives of vinyl sulfones (Fig. 39) through introducing halogens and a pyridine heterocycle with the purpose of maximizing the Nrf2 activation efficacy for the treatment of Parkinson's disease [81]. Of these N-heterocyclic vinyl sulfones, compound 70a (Fig. 39) displayed dramatically superior ability to activate Nrf2 with EC50 value of 26 nM which was superior than the previously developed compounds 61a (EC50 = 530 nM) and 64a (EC50 = 326 nM).
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These authors contributed equally to this work.