Inflammation is a physiological process initiated when organisms respond to harmful stimuli such as pathogens, viruses, and foreign particles [1,2]. The inflammatory pathway comprises inducers, sensors, mediators and target tissues. Inducers (such as lipopolysaccharides (LPS)) trigger an inflammatory response and can be detected by the sensors [3]. On the other hand, sensors such as TLRs are activated on specific cells, including tissue-resident macrophages [4], dendritic cells (DCs), and mast cells [5]. Sensors stimulate the production of mediators, such as cytokines [6], chemokines [7], arachidonic acids [8], and proteolytic cascades (including bradykinins) [9]. Inflammatory mediators target different tissues, leading to alterations in their functional status. Inflammation plays a pivotal role in the advancement of numerous conditions, such as acute lung injury (ALI) and inflammatory bowel disease (IBD) [10,11]. Both of these illnesses involve the production of pro-inflammatory factors such as interleukin-6 (IL-6), leading to tissue damage and various symptoms [12,13].

Injury to alveolar epithelial cells and capillary endothelial cells induced by various direct and indirect factors is a major cause of ALI. The disease is often characterized by diffuse pulmonary interstitial and alveolar edema, ultimately leading to acute hypoxic respiratory insufficiency [14]. Bacterial or viral infections can also cause ALI, such as pneumonia resulting from the recently widely publicized novel coronavirus [15,16]. On the other hand, Crohn's disease and ulcerative colitis (UC) are some of the examples of IBD. UC mainly involves the colon and rectum [17]. In the disease, the range of lesions is generally limited to the intestinal mucosal layer, which is characterized by continuous mucosal inflammation, often accompanied by diarrhea, hematochezia, abdominal pain and other symptoms. While UC could easily progress to colon cancer, its exact cause is very complex and remains elusive [18]. In clinical practice, the management of ALI focuses on inflammation control through corticosteroids. The treatment of UC typically involves corticosteroids or non-steroidal anti-inflammatory drugs (5-ASA) to manage inflammation and enhance quality of life, thereby alleviating the disease [19]. Since steroid or non-steroidal anti-inflammatory drugs still have many limitations in clinical practice, developing new drugs for controlling ALI and UC is imperative.

Many marketed drugs are derived from natural products, such as Galanthamine, Calanolide A, Arteether, Warfarin, and Methoxsalen as shown in Fig. 1, which were discovered by retaining the active skeleton of natural products and then making corresponding modification to enhance their biological activity and drug-likeness. Galanthamine has been used to treat neurodegenerative diseases [20]. Arteether is widely used in malaria treatment [21]. Calanolide Ais is used as an anti-HIV drug [22]. Our research group has been engaged in the modification on the natural products for more than a decade, such as the work of cinnamamide derivatives 2i and L26 published on J. Med. Chem. [23] and Eur. J. Med. Chem. [24], respectively, as illustrated in Fig. 1. Coumarin exhibits a broad spectrum of pharmacological activities including anti-HIV [25], anti-tumor [26], anti-hypertension [27], anti-hyperlipidemia [28], anti-inflammatory [29] and analgesic effects [30]. Many coumarin-based medications, such as Methoxsalen and Warfarin, are already on the market. Moreover, both coumarin and 4-Hydroxycoumarin (4-HC) have demonstrated anti-inflammatory activity in TNBS-induced colitis in rat models [31]. Additionally, C-4-modified coumarin compounds exhibited potent anti-inflammatory and analgesic activities, as well as a positive regulation and inhibition of inflammatory factors [32]. Therefore, synthesizing new derivatives with improved pharmacological or pharmacokinetic (PK) properties by chemically modifying the 4-HC framework is crucial for drug research and development.

In this study, we used 4-HC as the lead compound to make modification by retaining the dominant fragment of coumarin skeleton. The primary objective of the modification is to introduce flexible hydrophilic or lipophilic segments at the 4-hydroxyl position. The structure of enrofloxacin, an anti-inflammatory drug, contains a hydrophilic piperazine ring (Fig. 2). Some commercially available or to-be-marketed drugs, such as Ensaculin and Morniflumate, also have hydrophilic piperazine rings and morpholine rings. Inspired by this, hydrophilic fragments with similar structures, such as ethylpiperazine and acetylpiperazine, were introduced. Furthermore, benzene ring and thiophene ring are common in the structure of drugs, such as cephalothin sodium, phenylbutazone, ensaculin, and morniflumate. Additionally, fat-soluble groups like benzene and thiophene rings were introduced to investigate their impact on biological activity. Therefore, we modified 4-HC from the two dimensions by introducing fat solubility or water solubility fragments to design the target compounds.

Herein, 25 4-HC derivatives were successfully synthesized, in which 15 compounds showed better inhibition effect than 4-HC on the LPS-induced IL-6 release from J774A.1 mouse cells in ELISA assay. Compound B8 showed 3 times more active than the lead compound 4-HC and exhibited a concentration gradient inhibition with the IC50 of 4.57 μM and 6.51 μM for IL-6 release on mouse cells J774A.1 and human cells THP-1 cells, respectively. Furthermore, B8 demonstrated minimal cytotoxicity and animal toxicity, affirming its safety profile. Overall, our investigations, both in vitro and in vivo, suggest that B8 holds promise as a prospective therapeutic agent for addressing LPS-induced ALI and DSS-induced colitis.