In the current landscape of new solid-state drug development, a significant challenge lies in enhancing the water solubility, stability, and biological activity of active pharmaceutical ingredients (APIs) in a safe and reliable manner. This must be achieved while ensuring that the molecular structure of the API remains intact and its efficacy is maintained (Allen and Cullis, 2013, Rautio et al., 2008).Typically, methods employed to modify the physicochemical properties of APIs involve altering the drug's active structure to improve key aspects such as solubility, stability, and bioavailability (Dahan & Miller, 2012; Zhang et al., 2021a). However, changes to the active structure of a drug may, on the one hand, affect the original pharmacological activity of the drug; on the other hand, this process of chemical modification may introduce new impurities or produce unexpected metabolites, which may be potentially toxic and increase the risk of the drug in clinical applications (Pal et al., 2024, Wang et al., 2023). Pharmaceutical co-crystals/salts, as an emerging solid-state form of drugs, has received much attention from researchers in recent years. Drug co-crystals/salts are crystals formed by combining APIs with suitable co-crystal formers (CCFs) by virtue of non-covalent bonding, and the non-covalent bonding interactions formed include hydrogen bonding, charge-assisted hydrogen bonding (CAHBs), and π···π stacking (Bazzo et al., 2020, Springuel et al., 2012, Ji et al., 2021). Pharmaceutical co-crystals/salts shows significant advantages in improving the physicochemical properties of drugs, and its most prominent application value lies in its ability to maintain the original molecular structure of drugs while changing their physicochemical properties. Drug-drug co-crystals/salts are crystals formed by combining two or more active pharmaceutical ingredients through non-covalent bonding (Haneef et al., 2021, Kaur et al., 2017, Baptista et al., 2021). The emergence of drug-drug co-crystals/salts provides completely new ideas and possibilities for solving the types of problems mentioned above. Compared with the traditional fixed dose combination (FDC) drugs, drug-drug co-crystals/salts can improve the physicochemical properties of the parent drug and at the same time play a synergistic therapeutic role as well as reduce the toxicity and increase the effect, thus effectively solving the problems associated with the conventional FDC drugs and the combination of drugs in the process of treatment (Parkes et al., 2024, Yu et al., 2022). Therefore, the synthesis of drug-drug co-crystals/salts by non-covalent bonding of two drugs undoubtedly provides a new way and method for the formation of new solid-state drugs with both good physicochemical properties and good biological activities.

Septic arthritis (SA) is a type of arthritis mainly caused by bacterial infections. Bacteria can invade joints through various routes, such as hematogenous spread, direct spread from adjacent tissue infections, and open wounds (Cimaz, 2021, Mathews et al., 2008, Wu et al., 2024). Among the bacteria that cause septic arthritis, the most common strain is the Gram-positive bacterium Staphylococcus aureus; followed by Gram-negative bacteria, such as Escherichia coli and Salmonella (Ming et al., 2021; Kwon et al., 2023, Gottlieb et al., 2019). Once bacteria enter the joints, the bacteria and their products stimulate synovial cells to secrete a large amount of inflammatory mediators, which will lead to obvious redness, swelling, heat, and pain in the joint area and limit joint movement (Otto, 2021). The main treatment methods for septic arthritis lie in eradicating the infection, relieving pain, and preserving joint function. During the treatment process, it is necessary not only to inhibit the infected bacteria but also to prevent the generated inflammation from deteriorating further. For this complex disease, broad-spectrum antibiotics are usually used in combination with non-steroidal anti-inflammatory drugs (NSAIDs) for combined drug administration treatment (Ahmed et al., 2012; Wang and Wang, 2021; Shao et al., 2015). Tolfenamic acid (TA, Scheme 1), as a traditional NSAID, is not only effective in relieving pain caused by inflammation, but also inhibits the further development of inflammation to a certain extent. TA is commonly used to treat septic arthritis with quinolones (Ahmed et al., 2018). Enrofloxacin (ENR, Scheme1) is a representative drug of the fluoroquinolone antibiotic class with broad-spectrum antimicrobial activity. However, TA and ENR, as BCS class II drugs, have very poor solubility thereby substantially reducing clinical efficacy (Wu et al., 2016). Therefore, the treatment of septic arthritis still faces great challenges. Then whether a method can be used to improve the physicochemical properties of TA and ENR without the formation of covalent compounds and at the same time enhance their activities and provide new ideas for the treatment of septic arthritis has become an urgent problem.

We have been working on the synthesis of drug co-crystals/salts of quinolones through the formation of non-covalent bonds to enhance their physicochemical properties. By summarizing the previous quinolone drug co-crystals/salts formed by our group (Table 1), we found that most of the quinolone drugs and co-formers interact with each other by non-covalent bonds (mainly charge-assisted hydrogen bonds (CAHBs)) to build stable crystal structures. For example, the solubility and permeation of the pharmaceutical salts of sparfloxacin (SPX, C19H22F2N4O), namely SPX-PCA (C19H23F2N4O3·C7H5O4) and SPX-PCA-MeOH (C19H23F2N4O3·C7H5O4·CH4O·H2O), were also improved (Zhang et al., 2021b). The physicochemical properties of the pharmaceutical salt of enrofloxacin (ENR,C19H22FN3O3), ENR-PNB (C19H23FN3O3·C7H4NO4·C7H5NO4), were also enhanced compared to ENR (Liu et al., 2022). In addition, the solubility and permeation of two pharmaceutical salts of pipemidic acid (PPA, C14H17N5O3), PPA-CCA (C14H18N5O3·C7H5O4·2(H2O)) and PPA-MHA (C14H18N5O3·C7H5O3·C7H6O3·2(H2O)), were both significantly improved (Zhang et al., 2021). From the above experience we found several commonalities in the co-formers that readily form co-crystals/salts with quinolones: (1) Co-formers are mostly acidic. (2) The co-formers possess at least one polar group. This polar group helps in the formation of intermolecular hydrogen bonds. Additionally, these co-formers can have ionized protons acting as acceptors. These ionized protons can easily form charge-assisted hydrogen bonds (CAHBs) with nitrogen atoms. (3) The co-formers that contain a benzene ring can form intermolecular interactions of π···π stacking with quinolones to build a stable crystal structure. And TA meets the above characteristics, so we hypothesized that ENR and TA can form drug-drug co-crystals/salts through non-covalent bond forms such as CAHBs. In summary, we will explore the following: (1) synthesis of TA-ENR drug-drug salt single crystals; (2) structural characterization of TA-ENR drug-drug salt single crystals to explore the role of non-covalent bonding in the process of crystal stacking; and (3) correlation between the physicochemical properties and biological activities of TA-ENR drug-drug salts and their structures.