Design and synthesis of 4-O-methylhonokiol analogs as inhibitors of cyclooxygenase-2 (COX-2) and PGF₁ production

Recent advances of honokiol：pharmacological activities, manmade derivatives and structure-activity relationship

Honokiol (HNK) is a typical natural biphenyl polyphenol compound. It has been proven to have a wide range of biological activities, including pharmacological effects such as anti-cancer, anti-inflammatory, neuroprotective, and antimicrobial. However, due to the poor stability, water solubility, and bioavailability of HNK, HNK has not been used in clinical treatment. This article reviews the latest research on the pharmacological activity of HNK and summarizes the HNK derivatives designed and improved by several researchers. Reviewing these contents could promote the research process of HNK and guide the design of better HNK derivatives for clinical application in the future.

Design and Development of COX-II Inhibitors: Current Scenario and Future Perspective

Innate inflammation beyond a threshold is a significant problem involved in cardiovascular diseases, cancer, and many other chronic conditions. Cyclooxygenase (COX) enzymes are key inflammatory markers as they catalyze prostaglandins production and are crucial for inflammation processes. While COX-I is constitutively expressed and is generally involved in "housekeeping" roles, the expression of the COX-II isoform is induced by the stimulation of different inflammatory cytokines and also promotes the further generation of pro-inflammatory cytokines and chemokines, which affect the prognosis of various diseases. Hence, COX-II is considered an important therapeutic target for drug development against inflammation-related illnesses. Several selective COX-II inhibitors with safe gastric safety profiles features that do not cause gastrointestinal complications associated with classic anti-inflammatory drugs have been developed. Nevertheless, there is mounting evidence of cardiovascular side effects from COX-II inhibitors that resulted in the withdrawal of market-approved anti-COX-II drugs. This necessitates the development of COX-II inhibitors that not only exhibit inhibit potency but also are free of side effects. Probing the scaffold diversity of known inhibitors is vital to achieving this goal. A systematic review and discussion on the scaffold diversity of COX inhibitors are still limited. To address this gap, herein we present an overview of chemical structures and inhibitory activity of different scaffolds of known COX-II inhibitors. The insights from this article could be helpful in seeding the development of next-generation COX-II inhibitors.

Design, Synthesis, and Structure-Activity Relationship Studies of Magnolol Derivatives as Antifungal Agents

Plant pathogenic fungi seriously affect agricultural production and are difficult to control. The discovery of new leads based on natural products is an important way to innovate fungicides. In this study, 30 natural-product-based magnolol derivatives were synthesized and characterized on the basis of NMR and mass spectroscopy. Bioactivity tests on phytopathogenic fungi (Rhizoctonia solani, Fusarium graminearum, Botrytis cinerea, and Sclerotinia sclerotiorum) in vitro of these compounds were performed systematically. The results showed that 11 compounds were active against four kinds of phytopathogenic fungi with EC₅₀ values in the range of 1.40-20.00 μg/mL, especially compound L5 that exhibited excellent antifungal properties against B. cinerea with an EC₅₀ value of 2.86 μg/mL, approximately 2.8-fold more potent than magnolol (EC₅₀ = 8.13 μg/mL). Moreover, compound L6 showed the highest antifungal activity against F. graminearum and Rhophitulus solani with EC₅₀ values of 4.39 and 1.40 μg/mL, respectively, and compound L7 showed good antifungal activity against S. sclerotiorum. Then, an in vivo experiment of compound L5 against B. cinerea was further investigated in vivo using infected tomatoes (curative effect, 50/200 and 36%/100 μg/mL). The physiological and biochemical studies illustrated that the primary action mechanism of compound L5 on B. cinerea might change the mycelium morphology, increase cell membrane permeability, and destroy the function of mitochondria. Furthermore, structure-activity relationship (SAR) studies revealed that hydroxyl groups play a key role in antifungal activity. To sum up, this study provides a reference for understanding the application of magnolol-based antifungal agents in crop protection.

Synthesis of Either C2- or C4'-Alkylated Derivatives of Honokiol and Their Biological Evaluation for Anti-inflammatory Activity

Honokiol, a biphenolic neolignan isolated from Magnolia officinalis, was reported to have a promising anti-inflammatory activity for the treatment of various diseases. There are many efforts on the synthesis and structure-activity relationship of honokiol derivatives. However, regioselective O-alkylation of honokiol remains a challenge and serves as a tool to provide not only some derivatives but also chemical probes for target identification and mode of action. In this study, we examined the reaction condition for regioselective O-alkylation, in which C2 and C4'-alkylated analogs of honokiol were synthesized and evaluated for inhibitory activity on nitric oxide production and cyclooxygenase-2 expression. Furthermore, we successfully synthesized a potential photoaffinity probe consisting of biotin and benzophenone based on a C4'-alkylated derivative.

Synthesis and anti-neuroinflammatory activity of N-heterocyclic analogs based on natural biphenyl-neolignan honokiol

Novel isoxazole and pyrazole analogs based on natural biphenyl-neolignan honokiol were synthesized and evaluated for their inhibitory activities against nitric oxide production in lipopolysaccharide-activated BV-2 microglial cells. The isoxazole skeleton was constructed via nitrile oxide cycloaddition from oxime 3 and pyrazole was generated by condensation of 4-chromone and alkylhydrazine. Among the analogs, 13b and 14a showed stronger inhibitory activities with IC₅₀ values of 8.9 and 1.2 µM, respectively, than honokiol.

Derivatization of honokiol by integrated acetylation and methylation for improved cutaneous delivery and anti-inflammatory potency

A set of honokiol derivatives was synthesized to evaluate skin permeation and bioactivity. The reaction for derivatization included acetylation and methylation. The anti-inflammatory activity against neutrophils and macrophages was examined. The experimental setup for the assessment of cutaneous absorption was the in vitro Franz diffusion assembly. Honokiol and its derivatives significantly downregulated superoxide anion and elastase production in neutrophils, with honokiol showing the greatest inhibition. All derivatives could be completely hydrolyzed to the parent compounds after passing into the skin. The skin deposition of honokiol at an infinite dose (3mM) was 0.33nmol/mg 4'-O-acetylhonokiol (AH), and 2,4'-diacetylhonokiol (DAH) exhibited comparable or less absorption than honokiol. The integrated acetylation and methylation (2-O-acetyl-4'-O-methylhonokiol, AMH) led to a 10.5-fold improvement of absorption compared to honokiol. AMH was advantageous for the targeted cutaneous treatment due to the high skin deposition and minimal penetration across the skin (8.40nmol/cm² compared to 93.49nmol/cm² for honokiol). The predicted therapeutic index for superoxide and interleukin (IL)-6 inhibition was much higher for topically applied AMH than for the other penetrants tested. The total polarity surface and hydrogen bond acceptor number calculated by molecular modeling were the parameters used to anticipate the cutaneous absorption. Our data suggest that AMH is a potent and safe candidate for cutaneous inflammation therapy.

Chemoenzymatic Synthesis and α-Glucosidase Inhibitory Activity of Dimeric Neolignans Inspired by Magnolol

A chemoenzymatic synthesis of a small library of dimeric neolignans inspired by magnolol (1) is reported. The 2-iodoxybenzoic acid (IBX)-mediated regioselective ortho-hydroxylation of magnolol is described, affording the bisphenols 6 and 7. Further magnolol analogues (12, 13, 15-17, 19-23) were obtained from eugenol (3), tyrosol (4), and homovanillic alcohol (5), through horseradish peroxidase (HRP)-mediated oxidative coupling and regioselective ortho-hydroxylation or ortho-demethylation in the presence of IBX, followed by reductive treatment with Na₂S₂O₄. A chemoselective protection/deprotection of the alcoholic group of 4 and 5 was carried out by lipase-mediated acetylation/deacetylation. The dimeric neolignans, together with 1 and honokiol (2), were evaluated as inhibitors of yeast α-glucosidase, in view of their possible utilization and optimization as antidiabetic drugs. The synthetic analogues of magnolol showed a strong inhibitory activity with IC₅₀ values in the range 0.15-4.1 μM, much lower than those of honokiol and the reference compounds quercetin and acarbose. In particular, a very potent inhibitory activity, with an IC₅₀ of 0.15 μM, was observed for 1,1'-dityrosol-8,8'-diacetate (15), and comparable inhibitory activities were also shown by bisphenols 6 (0.49 μM), 13 (0.50 μM), and 22 (0.86 μM). A kinetic study showed that 15 acts as a competitive inhibitor, with a K_i value of 0.86 μM.

Methylation and Esterification of Magnolol for Ameliorating Cutaneous Targeting and Therapeutic Index by Topical Application

PURPOSE

As a continuing effort to elucidate the impact of structure modification upon cutaneous absorption behavior, we attempted to assess the skin permeation of magnolol by methylation and acetylation.

METHODS

Diacetylmagnolol and 2-O-acetyl-2'-O-methylmagnolol (AMM) were designed and synthesized in this study. The anti-inflammatory activity against stimulated neutrophils and keratinocytes was evaluated to check the bioactivity of the analogues. In vitro skin absorption was investigated using nude mouse and pig skin models at both equimolar and saturated doses.

RESULTS

Magnolol generally showed the strongest anti-inflammatory potential, followed by diacetylmagnolol and AMM. The antibacterial activity was observed for magnolol and diacetylmagnolol but not AMM. Diacetylmagnolol and AMM could be partly hydrolyzed to magnolol and 2-O-methylmagnolol after entering the skin. The hydrolysis rate of diacetylmagnolol was faster than that of AMM. The lipophilicity played a crucial role in cutaneous absorption, with AMM exhibiting the highest skin deposition. AMM accumulation within nude mouse skin was about 2.5-fold greater than that of magnolol and diacetylmagnolol. On the other hand, the transdermal penetration across the skin was lessened by methylation and esterification. This led to a superior skin targeting of AMM. Although the pharmacological activity of AMM was low, the high skin uptake and bioconversion into 2-O-methylmagnolol in the skin contributed to a greater therapeutic index (TI, skin deposition x inflammatory inhibition percentage) compared to the others. The accumulation of AMM in the hair follicles was 77.12 nmol/cm(2), which was significantly greater than that with magnolol (44.84 nmol/cm(2)) and diacetylmagnolol (26.96 nmol/cm(2)). The synthetic analogues were tolerable to the nude mouse skin.

CONCLUSIONS

Based on the experimental results, we may suggest topically applied AMM as a potent and safe candidate for the treatment of cutaneous inflammation.

The new 4-O-methylhonokiol analog GS12021 inhibits inflammation and macrophage chemotaxis: role of AMP-activated protein kinase α activation

Preventing pathologic tissue inflammation is key to treating obesity-induced insulin resistance and type 2 diabetes. Previously, we synthesized a series of methylhonokiol analogs and reported that compounds with a carbamate structure had inhibitory function against cyclooxygenase-2 in a cell-free enzyme assay. However, whether these compounds could inhibit the expression of inflammatory genes in macrophages has not been investigated. Here, we found that a new 4-O-methylhonokiol analog, 3′,5-diallyl-4′-methoxy-[1,1′-biphenyl]-2-yl morpholine-4-carboxylate (GS12021) inhibited LPS- or TNFα-stimulated inflammation in macrophages and adipocytes, respectively. LPS-induced phosphorylation of nuclear factor-kappa B (NF-κB)/p65 was significantly decreased, whereas NF-κB luciferase activities were slightly inhibited, by GS12021 treatment in RAW 264.7 cells. Either mitogen-activated protein kinase phosphorylation or AP-1 luciferase activity was not altered by GS12021. GS12021 increased the phosphorylation of AMP-activated protein kinase (AMPK) α and the expression of sirtuin (SIRT) 1. Inhibition of mRNA expression of inflammatory genes by GS12021 was abolished in AMPKα1-knockdown cells, but not in SIRT1 knockout cells, demonstrating that GS12021 exerts anti-inflammatory effects through AMPKα activation. The transwell migration assay results showed that GS12021 treatment of macrophages prevented the cell migration promoted by incubation with conditioned medium obtained from adipocytes. GS12021 suppression of p65 phosphorylation and macrophage chemotaxis were preserved in AMPKα1-knockdown cells, indicating AMPK is not required for these functions of GS12021. Identification of this novel methylhonokiol analog could enable studies of the structure-activity relationship of this class of compounds and further evaluation of its in vivo potential for the treatment of insulin-resistant states and other chronic inflammatory diseases.

Validation of cyclooxygenase-2 as a direct anti-inflammatory target of 4-O-methylhonokiol in zymosan-induced animal models

4-O-methylhonokiol (MH) is known to inhibit inflammation by partially understood mechanisms. Here, the anti-inflammatory mechanisms of MH were examined using enzymatic, cellular, and animal assays. In enzymatic assays, MH inhibited COX-2 activity with an IC50 of 0.062 μM, and also COX-1 with an IC50 of 2.4 μM. In cellular assays, MH was immunotoxic above 10 μM. At non-toxic concentrations (below 3 μM), MH strongly inhibited COX-2-mediated prostaglandin production with an IC50 of 0.1 μM, whereas did not or slightly affect other functions of B cells, T cells, dendritic cells, and macrophages. In an animal model, MH inhibited the increase in footpad thickness and popliteal lymph node weight in zymosan-injected mice. When analyzed the draining pLNs of zymosan-injected mice on day 5, MH inhibited the overall inflammatory responses. However, MH inhibited cyclooxygenase (COX)-2-mediated prostaglandin production without affecting tumor necrosis factor-α production in inflamed tissues within 6 h after zymosan injection. In summary, our data suggest that COX-2 may be a direct anti-inflammatory target of MH in vitro and in vivo.

Honokiol analogs: a novel class of anticancer agents targeting cell signaling pathways and other bioactivities

Honokiol (3,5-di-(2-propenyl)-1,1-biphenyl-2,2-diol) is a natural bioactive neolignan isolated from the genus Magnolia. In recent studies, honokiol has been observed to have anti-angiogenic, anticancer, anti-inflammatory, neuroprotective and GABA-modulating properties in vitro and in preclinical models. Honokiol and its analogs target multiple signaling pathways including NF-κB, STAT3, EGFR, mTOR and caspase-mediated common pathway, which regulate cancer initiation and progression. Honokiol and its targets of action may be helpful in the development of effective analogs and targeted cancer therapy. In this review, recent data describing the molecular targets of honokiol and its analogs with anticancer and some other bioactivities are discussed.

Pharmacokinetics and metabolism of 4-O-methylhonokiol in rats

The purpose of this study was to characterize the pharmacokinetics and metabolism of 4-O-methylhonokiol in rats. The absorption and disposition of 4-O-methylhonokiol were investigated in male Sprague-Dawley rats following a single intravenous (2 mg/kg) or oral (10 mg/kg) dose. Its metabolism was studied in vitro using rat liver microsomes and cytosol. 4-O-Methylhonokiol exhibited a high systemic plasma clearance and a large volume of distribution. The oral dose gave a peak plasma concentration of 24.1±3.3 ng/mL at 2.9±1.9 h and a low estimated bioavailability. 4-O-Methylhonokiol was rapidly metabolized and converted at least in part to honokiol in a concentration-dependent manner by cytochrome P450 in rat liver microsomes, predicting a high systemic clearance consistent with the pharmacokinetic results. It was also shown to be metabolized by glucuronidation and sulfation in rat liver microsomes and cytosol, respectively. 4-O-Methylhonokiol showed a moderate permeability with no apparent vectorial transport across Caco-2 cells, suggesting that intestinal permeation process is not likely to limit its oral absorption. Taken together, these results suggest that the rapid hepatic metabolism of 4-O-methylhonokiol could be the major reason for its high systemic clearance and low oral bioavailability.