Since the discovery of penicillin in 1928, antibiotics have dramatically reduced mortality from bacterial infections and extended human life expectancy [1]. In this context, antibiotics rapidly entered the golden period of development with the variety and quantity of antibiotics constantly increasing [2]. However, the overuse and abuse of antibiotics has led to the emergence of resistance mechanisms of bacteria against the available drugs. Horizontal transfer by bacteria to obtain exogenous drug-resistance genes also accelerates the production of drug-resistant strains [3]. As the problem of bacterial drug resistance became more and more serious, the World Health Organization (WHO) finally identified it as a global public health crisis that could cause about 50,000 people die every day in the worldwide [4]. Of these, the trouble of multidrug-resistant (MDR) bacteria is especially prominent. In 2017, the WHO published a list of antibiotic-resistant bacteria that should be given priority in research for new antibiotics. In particular, methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE), and Streptococcus pneumoniae (PRSP) are the most prominent and prevalent, and they make clinical treatment very difficult [5]. Therefore, it is particularly urgent to develop new antibacterial drugs to deal with the growing problem of MDR bacteria.

Oxazolidinones are the next-generation synthetic antibacterial agents with completely new chemical structures after sulfonamides and quinolones, that could effectively inhibit MDR Gram-positive bacteria, MRSA, VRE, PRSP and anaerobic bacteria [6]. Oxazolidinones exert antibacterial activity by inhibiting protein synthesis on the ribosomal 50S subunit of the bacteria and thus preventing the formation of a functional 70S initiation complex [7,8]. Due to their unique mechanism of action, oxazolidinones exhibit no cross-resistance with other antibacterial drugs.

Linezolid (Fig. 1), the first synthetic oxazolidinone, was approved for clinical use in 2000. Linezolid exhibited excellent bacteriostatic activity against methicillin-sensitive Staphylococcus aureus (MSSA), MRSA, VRE with minimal inhibitory concentrations (MIC) values of 1–4 μg/mL [9,10]. However, linezolid is limited by its toxicity issues, which center on myelosuppression and monoamine oxidase (MAO) inhibition [11]. In addition, the emerging resistance toward linezolid is a significant concern. Tedizolid phosphate (Fig. 1), the second oxazolidinone, was approved in 2014 for the treatment of sensitive strain caused acute bacterial skin and skin structure infection (ABSSSI) [12]. Tedizolid phosphate has the similar skeleton and the similar structure-activity relationships (SARs) with linezolid, in which phosphate group could improve water solubility, bioavailability, and enhanced antibacterial activity against drug-resistant strains containing cfr gene [13,14]. Like linezolid, the toxicity (mainly including myelosuppression and MAO inhibition) of tedizolid phosphate has not been significantly reduced. Contezolid (Fig. 1), developed by MicuRx, was approved in 2021 for the treatment of complex skin and soft tissue infection (CSSTI) [15]. Compared to linezolid, contezolid exhibits lower MAO-A/B inhibitory activity and myelosuppression inhibitory effect which significantly reduced the adverse reactions [16,17]. According to the previous reports, oxazolidinone ring was described as the privileged scaffold in antibacterial drug discovery, and further structural modification sites mainly focus on the modification of the morpholine ring of linezolid and the C-5 position of the parent scaffold to improve their efficacy, pharmacokinetic (PK), and safety profiles [[18], [19], [20], [21]].

A series of biaryloxazolidinone analogues bearing a hydrazone moiety have been described by our research group [22,23]. Among them, compound 14a-7 (OB-158) showed significantly improved antibacterial activity against Gram-positive bacteria and clinical isolates of selected antibiotic-susceptible and antibiotic-resistant isolates with MIC values of 0.125–0.25 μg/mL compared to linezolid with MIC values of 2–16 μg/mL (Fig. 2 and Table S1) [23]. Further studies indicated OB-158 was stable in human, monkey, dog and mouse liver microsomes, and exhibited lower inhibition rate of human MAO-A (72 % at 30 μM) [23]. In the in vivo PK profiling experiments in Sprague-Dawley (SD) rats, following intravenous administration at 1 mg/kg, OB-158 displayed moderate plasma clearance (12.9 mL/min/kg) and AUC0-t (1290 h ng/mL). However, further studies indicated that OB-158 showed low exposure Cmax (140 ng/mL), AUC0-t (469 h ng/mL), and bioavailability (F = 3.6 %) following oral administration at 10 mg/kg in SD rats, which blocked its further progression.

Herein, we embarked on a lead optimization program to search for new biaryloxazolidinone analogues with maintained/enhanced antibacterial activities and concomitantly improved PK profiles. We also believed that we might find an opportunity to reduce MAO inhibition while maintaining in vivo activity. For this purpose, the derivatives were designed and prepared by modifying the tail of hydrazone bond and C-5 position of oxazolidinone ring to improve physicochemical properties and provide late leads suitable for in vivo efficacy studies. These 26 synthesized derivatives were firstly evaluated for their antibacterial activity. For the most promising potent derivatives, we performed in-depth preclinical studies for ADME profiles, in vitro safety, and have demonstrated that they displayed favorable properties. Ultimately, the antibacterial activity of linezolid and compound 8b against linezolid-resistant Staphylococcus aureus (LRSA) was evaluated in vivo in a mouse model of LRSA systemic infection.