Introduction

Since Robert Koch discovered the causative agent of tuberculosis (TB) in 1882, research into new antituberculosis drugs has been a continuous effort. The first breakthrough in modern antitubercular therapy came with the discovery of streptomycin 58 years later [1]. However, prolonged use of this antibiotic led to significant side effects and the rapid development of drug resistance. In the early 1950s, the introduction of isoniazid marked the beginning of the modern era of TB treatment, as it was inexpensive, well-tolerated, and safe. Subsequently, the development of ethambutol, rifampin, and pyrazinamide in the 1970s enabled polytherapy, which remains highly effective nowadays [2]. However, the rise of drug-resistant strains underscores the ongoing need for new therapies and innovative solutions to ensure continuous progress in the fight against TB.

Although TB is a curable disease, we are still in a constant battle to eradicate it. In recent decades, the TB burden has decreased by 1.5–2% annually. However, factors such as slow and insufficient diagnosis, low cure rates, drug resistance, and the devastating impact of the COVID-19 pandemic have significantly reversed the progress made to date [3]. Given the growing threat of drug-resistant strains, it has become increasingly urgent in the management of TB treatment to develop new and more effective antituberculosis drugs [3]. According to World Health Organization (WHO) guidelines, fluoroquinolones (FQ) are the most valuable second-line agents for treating TB [4]. These drugs are effective against intracellular pathogens due to their ability to pass through porins and exert antimicrobial activity by inhibiting homologous type II and IV topoisomerases, as well as DNA gyrase. Over the years, the structural modification of FQ has led to the development of four generations of these compounds, each improving their antibacterial activity [5].

The ongoing struggle against TB therapies, particularly drug-resistant forms, requires continuous innovation in drug development [2]. FQ have long been a key role in the treatment of multidrug-resistant TB (MDR-TB; resistance to rifampicin and isoniazid). Advancements in the synthesis of FQ hold promise for overcoming the growing challenges posed by resistance. The emergence of new FQs, alongside a broader focus on combination therapies and alternative treatments, could pave the way for more effective TB therapies in the near future.

This review emphasizes the critical importance of ongoing research in the development of next-generation FQ, with a particular focus on the hybridization of chemical motifs and by combining distinct chemical structures in a targeted manner, this approach could potentially overcome current therapeutic challenges, improving treatment outcomes for patients with multidrug-resistant tuberculosis. Ensuring the continuous evolution of FQ is essential for staying ahead of the evolving threat of tuberculosis and its resistance patterns.

Tuberculosis Burden and Resistance

According to the WHO, TB remains the leading cause of infectious death worldwide. In 2023, there were 10.8 million new cases, and 1.25 million deaths attributed to TB globally [3]. The COVID-19 pandemic disrupted public health services, reversing several years of progress made under the TB END strategy. Additionally, various factors contributed to the increasing number of reported TB cases worldwide, including economic disparities and barriers to access adequate healthcare [6]. Despite these challenges, it is estimated that by 2030, 20 out of 26 developing countries will have succeeded in eradicating TB [4].

Drug-resistant TB continues to pose a significant public health threat. The most effective first-line drugs for treating TB are rifampicin and isoniazid. Resistance to both drugs define MDR-TB. MDR-TB and rifampicin-resistant TB (RR-TB) require second-line drugs for treatment. The main causes for drug-resistant TB include inadequate healthcare systems, treatment abandonment, limited access to healthcare, and direct transmission. For example, treatment coverage in 2022 was only 58%, which increased to 70% in 2023. Therefore, it is essential to assess the factors contributing to the development of drug resistance.

In 2023, 175,923 people were newly diagnosed and treated for MDR/RR-TB. Globally, around 400,000 cases of drug-resistant TB require treatment annually with second-line drug regimens, such as bedaquiline and fluoroquinolones. These regimens are more expensive and are associated with a higher risk of adverse events compared to first-line treatments for drug-susceptible TB [3].

In a worldwide scenario, isoniazid resistance (without rifampicin resistance) is the most common type of resistance among infected patients. As a result, fluroquinolones have been raised as a potentially efficient alternative to replace isoniazid. In 2018, WHO recommended the use of levofloxacin (along with rifampicin, ethambutol, and pyrazinamide) in order to treat MDR TB. In resistant TB, or any other contraindication for FQ, the use of rifampicin, ethambutol, and pyrazinamide is recommended, but only for six months. Nevertheless, further clinical trials must be carried out to fully understand the correct type of FQ and regimen needed to overcome bacterial resistance [7].

New candidates and challenges in developing antibiotics for tuberculosis treatment

Approximately 85% of patients will have a successful treatment outcome [8]. The standard treatment for active TB consists of a six-month regimen of first-line antibiotics, including isoniazid (H), rifampicin (R), ethambutol (E), and pyrazinamide (Z), with the recommended 2HRZE/4HR regimen for new pulmonary TB patients. Besides, in some countries, the bacille Calmette-Guérin (BCG) vaccine is given to babies or small children to prevent TB. The vaccine prevents extrapulmonary tuberculosis, but it is not effective against the pulmonary form [9].

Novel drugs, such as bedaquiline and delamanid, have been introduced, specifically targeting MDR-TB and significantly shortening treatment duration compared to conventional regimens. These pharmaceuticals inhibit critical processes in bacterial cells, offering a new line of defense where traditional antibiotics may fail. Bedaquiline and delamanid, two key new treatments for MDR-TB, have two different and specific mechanisms of action that enhance their efficacy against resistant strains of Mtb. For example, bedaquiline formely TMC207, selectively inhibits mycobacterial ATP synthase, a critical protein complex for the growth of Mtb, and plays a crucial role in oxidative phosphorylation [10], bedaquiline effectively disrupts the energy supply needed for the survival and replication of the bacteria. This action is particularly useful against strains resistant to first-line treatments. On the other hand, delamanid, is a prodrug that stops the synthesis of mycolic acids, this process depletes the cell wall components and destroys the mycobacteria [11]. Therefore, delamanid compromises the integrity of the bacterial cell, leading its death. This mechanism is especially important when treating MDR-TB, where traditional antibiotics may no longer be effective. Both drugs are used in combination with other antitubercular agents to enhance treatment outcomes and reduce the likelihood of resistance development. This multifaceted approach is crucial in addressing the growing challenge of drug-resistant TB, improving the overall effectiveness of treatment regimens.

Recently, several promising drugs have emerged for the treatment of TB, focusing on enhancing therapeutic efficacy, reducing treatment duration, and addressing the challenges posed by drug-resistant strains. Furthermore, the repurposing of existing drugs [12] and the use of combined therapies have demonstrated significant potential at improving treatment outcomes. Additionally, ongoing clinical trials continue to investigate the potential of new vaccine candidates that may offer more effective protection against pulmonary TB, aiming to complement current vaccination strategies like the BCG vaccine. The development of additional drugs and treatments aims to address issues related to drug resistance, prolonged treatment durations, and adverse reactions associated with currently available agents. Key candidates include: pretomanid (PA-824), a novel compound that also inhibits mycolic acid synthesis (like delamanid) has proved to be effective in the treatment of drug-resistant TB when used in combination with bedaquiline and linezolid [13]. Noxafil (posaconazole), was originally an antifungal agent. Nowadays, posaconazole is being studied for its potential effectiveness against TB, particularly in drug-resistant cases [14]. Rifapentine, currently used for latent TB infection, is being tested in combination with other agents (isoniazid) to shorten treatment durations for active TB [15]. Moxifloxacin is an 8-methoxy-fluoroquinolone already established as an antibiotic. Current research aims to determine its role in shortening TB treatment regimens, particularly for drug-susceptible strains [16]. Telacebec (Q203) is an orally active small molecule drug candidate that blocks Mtb growth by inhibiting the bc1 cytochrome complex, leading to the depletion of adenosine triphosphate (ATP) synthesis of Mtb [17]. Sutezolid inhibits the 23S rRNA protein of the 50S ribosomal subunit and inhibits protein synthesis showing antiTB activity [18]. Benzothiazinone is reported to act on the synthesis of the arabinogalactan cell wall polymer, blocking elongation growth and cell division [19]. Delpazolid (LCB01-0371) inhibits protein synthesis by binding to domain V of the 23S rRNA and thereby blocking formation of the initiation complex [20]. Ganfeborole (GSK3036656) is a first-in-class benzoxaborole investigational drug that targets the bacterial cell membrane. It has shown promise in preclinical studies and is currently being evaluated in clinical trials. It suppresses protein synthesis in MTB by selectively inhibiting the enzyme Leucyl t-ribose nucleic acid (RNA) synthetase [21]. SPR720 is a novel aminobenzimidazole which targets the ATPase subunits bacterial DNA gyrase (GyrB) which is distinct from that of the fluoroquinolones [22]. Sudapyridine (WX-081) is a modified derivative of bedaquiline that retains the pharmacologically active site of bedaquiline, while introducing structural modifications aimed at enhancing its efficacy and overcoming resistance [23]. Sanfetrinem Cilexetil (GV 118819) is a repurposed tricyclic beta-lactam antibiotic; sanfetrinem is the oral prodrug of sanfetrinem, developed by GSK in the 1990s [24]. Pretomanid has demonstrated a dual mechanism of action, showing a mixed effect by inhibiting both mycolic acid biosynthesis and inducing respiratory poisoning through the release of nitric oxide [25]. In addition to the ongoing development of new drug candidates, several novel vaccine candidates are also being explored to provide better protection against TB. M72/AS01E, a combination of the M72 antigen and the AS01E adjuvant, has shown promising results in clinical trials by boosting immune responses in individuals with latent TB infection [26]. Similarly, VPM1002, a genetically modified version of the Bacillus Calmette-Guérin (BCG) vaccine, has been engineered to improve its efficacy in preventing TB [27]. These vaccine candidates represent important advancements in the fight against TB, especially in light of the challenges posed by drug-resistant strains and the limitations of current vaccination strategies. These emerging therapies and vaccines represent significant advancements in the fight against TB, addressing the urgent need for more effective treatments, especially in the face of increasing drug resistance (Fig. 1).

Fig. 1
figure 1

Emerging drugs for the treatment of Mycobacterium tuberculosis. Highlighted are drugs currently in the pipeline, targeting both cell wall components and intracellular targets, as well as repurposed drugs that are being evaluated for their efficacy against Mycobacterium tuberculosis. Additionally, ongoing research into new vaccine candidates is shown. Blunt arrows indicating inhibition of key cellular targets

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Structure and anti-bacterial activity of fluoroquinolones

The first class of quinolones was described by George Lesher in 1962. This molecule, called nalidixic acid, had limited use due to its short duration of action and the rapid development of resistance, which decreased its effectiveness [28]. To improve the action of these molecules, the main approach was the addition of a fluorine atom substituted onto carbon 6 (C-6) of the quinolone scaffold, producing what is now known as a fluoroquinolone, which has better antibacterial activity and increased gyrase potency. Similarly, research into structural modifications, enhanced potency, and better bioavailability led to the incorporation of a piperazine ring onto C-7 and a cyclopropyl group at the C-1 position [29]. These changes define the second generation of quinolones, which exhibit improved pharmacokinetic and pharmacodynamic properties. Consequently, several molecules that are in clinical use today, such as ciprofloxacin, show systemic activity against most Gram-negative bacteria, including Enterobacteriaceae, Haemophilus, Neisseria, and other Pasteurellaceae, Vibrionaceae, as well as various species of Pseudomonas and Acinetobacter [30].

The third generation of quinolones was appeared by adding alkylated piperazine and pyrrolidinyl groups at the C-7 position. Some other structural changed were made, such as the addition of (-NH2) in the case of sparfloxacin and a methyl group (-CH3) in the case of grepafloxacin. Substitutions at the C-5 position is unusual among clinically used fluoroquinolones, and up to now, there are no approved drugs having a hydroxyl group (-OH) at this position. This generation of quinolones also includes substituents at the C-8 position, such as a chlorine atom, which enhances their effectiveness against Gram-positive bacteria [31]. Correspondingly, fourth-generation quinolones, such as moxifloxacin and gemifloxacin, retain many features from the third generation and the additional benefit of having activity against anaerobic organisms, due to the addition of a nitrogen at the C-8 position. When it comes to a 2,4-difluorophenyl group at the nitrogen position conflates different structural elements. In this scenario, only trovafloxacin (withdrawn from the market due to safety concerns) had a 2,4-difluorophenyl group, specifically at the N-1 position. This group is not a universal feature across all fourth generation quinolones (Fig. 2) [32].

Fig. 2
figure 2

Structure and Anti-Bacterial Activity of quinolones. The antibacterial activity of quinolones is improved by modifications of different substituents in different positions

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However, there are only a few FQs capable of replacing other TB therapeutic agents. Therefore, there is an increasing need to synthesize new FQ derivatives with improved properties. When developing new compounds, it is crucial to consider various factors, such as the type of structural modifications, reagents, solvents, and the activation energy required for the reaction. These factors directly impact final yields and, consequently, the potential for the compounds to serve as lead compounds or even pharmaceuticals. One of the most resourceful strategies for obtaining new bioactive molecules is to use already known bioactive molecules as starting reagents. Coupling and condensation reactions, along with the addition of new moieties to the original molecules, often result in the creation of novel bioactive compounds.

Fluoroquinolones in TB treatment

Fluoroquinolones are among the second-line antituberculosis drugs, essential in the treatment of MDR-TB [33]. Their broad spectrum of activity and good tissue and cellular distribution, as well as their few associated adverse effects, have made fluoroquinolones a useful treatment for MDR strains or patients who have adverse reactions to first-line drugs [33]. In a Table 1, we described the effectives MIC of commonly used FQs in TB. The FQ currently available within the WHO treatment regimen for drug-resistant tuberculosis are levofloxacin and moxifloxacin, useful alternatives for treating severe forms of extrapulmonary TB and TB meningitis due to their ability to penetrate the central nervous system by crossing the blood-brain barrier. Both levofloxacin and moxifloxacin are effective in treating fluoroquinolone-sensitive tuberculosis. Other fluoroquinolones such as ciprofloxacin and ofloxacin are less effective in treating MDR-TB and their use is no longer recommended [34]. The use of FQs has been associated with lower treatment failure and fewer deaths from tuberculosis; however, it is estimated that up to one-third of MDR-TB cases may have additional resistance to FQs and other second-line drugs, making treatment even more difficult [35]. These cases are classified as Pre-extensively drug-resistant TB (pre-XDR-TB) refers to MDR-TB that is also resistant to a fluoroquinolone, and extensively drug-resistant TB (XDR-TB) isolates are additionally resistant to other key drugs such as bedaquiline and/or linezolid.

Table 1 Fluroquinolones on TB treatment and reported MIC
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FQ are rapidly absorbed after oral administration and have a bioavailability greater than 50% in most cases and in some others, it is close to 100%, reaching its maximum concentration between 1.5 and 2 h with plasma concentrations between 2.3 to 3 µg ml−1 after administration of ciprofloxacin at doses of 500 to 750 mg, while for levofloxacin at the same doses they can reach plasma concentrations between 5.3 and 7.1 µg ml−1. In the case of moxifloxacin, plasma concentrations between 1.6 and 3.34 µg ml−1 are shown at doses of 200 to 400 mg [36].

Fluoroquinolones are generally well tolerated, although some adverse effects may occur, such as nausea, gastrointestinal discomfort, and some central nervous system side effects such as headache, dizziness, and insomnia [37]. Other connective tissue-related symptoms include tendonitis and hamstring tendon rupture, which are more common in older adults, although these effects occur less frequently [38]. FQs can also cause arrhythmias by prolonging the QT interval; however, this effect may vary among FQs [34].

The main mechanism of fluoroquinolone resistance is through modifications of their targets, primarily in the subunits of DNA gyrase or topoisomerase [37]. In Mtb, the most important resistance mechanism to fluoroquinolones is due to mutations in genes encoding DNA gyrase in the gyrA and gyrB genes, which encode the A and B subunits, respectively. These mutations occur in the quinolone resistance-determining region (QRDR), which is specific to each subunit. For instances, the gyrA gene, mutations occur in a conserved region spanning codons 74 to 113, while mutations in the gyrB gene are less frequent and typically occur between codons 461 and 499 [39]. QRDR mutations involve amino acid substitutions in the target proteins, leading to structural changes that reduce the binding affinity of fluoroquinolones to the enzyme, thereby resulting in drug resistance [40]. gyrA mutations are more common and are associated with higher levels of resistance compared to gyrB mutations. Furthermore, double mutations in both gyrA and gyrB can lead to even higher levels of resistance [41].

Due to the impact of MDR and pre-XDR-TB, the design of novel compounds based on structural modifications is crucial. These modifications should enable the drug to remain effective despite alterations in the target enzyme, such as mutated DNA gyrase, or target alternative bacterial pathways, both of which are key strategies for preserving therapeutic effectiveness.

Synthesis of new quinolones based on hybridizing chemical motifs

Quinolones and quinolines-based compounds

The current understanding of the structure-activity relationships of quinolones has led to the investigation and development of numerous compounds via structural modifications aimed at enhancing their potency, pharmacokinetics, and tissue penetration (Table 2).

Table 2 New quinolones and quinolines based on hybridizing chemical motifs
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In recent years, some studies have demonstrated that new F-triazolequinolones and alkoxy-triazolequinolones inhibit replication of Mtb. Thus, Carta A et al. observed that five out of 21 synthetic compounds exhibited MIC values ranging from 6.6 to 57.9 µM. The authors describe that compound 21a and 30a (Table 2, structure 1) showed the most potent MIC of 6.6 µM against reference strain [42].

On the other hand, the generation of quinolone-based compounds bearing an isoniazid (INH) moiety promotes dual antimicrobial activity by interfering with essential biochemical pathways. For instance, INH inhibits the production of mycolic acids. Additionally, INH induces oxidative stress through the generation of Reactive Oxygen Species (ROS). The authors demonstrated that Structure-Activity Relationship (SAR) analysis indicated that the efficacy of the molecules improved with modifications at positions −1 and −3 on the quinolone ring (Table 2, Structure 2). Furthermore, the compounds exhibited significant anti-Mtb activity with MIC values ranging from 0.2 to 8 μM [43]. These modifications prevent the fast acetylation of INH, resulting in less toxicity and higher lipophilicity. This approach requires further research to support the safety and potency of these anti-TB agents.

Co-infection between malaria and TB has been reported primarily in endemic regions. However, there is limited knowledge regarding the pathophysiology of this coinfection, and treatment options remain scarce. To mitigate the challenges posed by potential drug-drug interactions in the management of such co-infections, it is crucial to explore new molecules with dual activity against the causative agents of both diseases. In this context, a library of quinolone-thiosemicarbazones was synthesized. Compound 10 (Table 2, structure 3), emerging as the hit; with an MIC of 2 µM against H37Rv strain of Mtb and an MIC of 1 µM against the 3D7 strain of P. falciparum [44].

Nitrobenzenoids have shown activity against both drug-susceptible and MDR-TB by acting as suicide inhibitors of the mycobacterial decaprenylphosphoryl-β-d-ribose 2′-epimerase (DprE1), which is required for the synthesis of arabinogalactan. Consequently, the synthesis of novel 6-nitroquinolones was achieved through simple synthetic transformations to reduce the probability of side effects and to enhance the half-life and aqueous solubility of nitrobenzenoids. The results of nitro quinolone-based compounds showed that Compound 8e, was identified selective activity MIC < 0.24 µM against Mtb, non-toxic to mammalian cells, and high lipophilicity (Table 2, structure 4) [45]. Moreover, the other NO2 containing compounds, called 8 f and 8 g exhibit MIC > 4 µM. SAR analysis suggests that the presence of a nitro group at position 6 of the quinolone scaffold enhances antimycobacterial activity, while modifications at position 2 improve solubility. These findings indicate that the 6-nitro moiety is crucial for anti-TB potency, and future development should focus on targeted delivery systems and combination therapies to further optimize efficacy.

The phenanthrolines are compounds that have been studied in Staphylococcus aureus infection. Thus, the generation of new hybrid compounds based on replaced the carbon at position 6 by various halogen, amino, or nitro groups, besides an extended scaffold in the 4-oxo-2,3-dihydro-1,10-phenanthroline-3-carboxylic acid. It showed that these compounds (Table 2, structure 5) also proved effective in tuberculosis-infected macrophages, as well as XDR-TB and specific gyrA/gyrB mutants [46].

In a study conducted by Alsayed et al., the synthesis and evaluation of several arylcarboxamide derivatives were reported. These quinolone-based hybrid molecules derivatives were designed based on a homology model and the crystal structure of the mycobacterial membrane protein large 3 (MmpL3), which is responsible for the translocation of mycolic acids across the plasma membrane. Among the compounds tested, the quinolone-2-carboxamide derivative 8i (Table 2, Structure 6) demonstrated moderate overall efficacy against Mtb, with a MIC of 9.97 μM, whereas other analogs exhibited MIC values ranging from ≥32 µg ml−1 to > 80 µM. However, the potency of compound 8i remains moderate when compared to standard anti-TB drugs such as isoniazid (MIC = 0.29 µM) and ethambutol (MIC = 4.89 µM). Additionally, molecular docking studies revealed that these compounds interact effectively with the active site of MmpL3 [47].

Despite the fact that Q8b is not a fluoroquinolone but rather a chloroquine derivative (Table 2, structure 7) it demonstrated superior performance compared to the lead compound in terms of growth inhibition against Mtb, displaying a MIC of 0.06 µg ml−1, representing a 100-fold increase in activity compared to the lead compound. The N-benzylation of the oxindole motif improved the activity of the 7-chloroquinoline-bearing molecules, with Q8b showing better anti-tubercular activity. While Q8b shares a chloroquine core, its modified structure with the istain tether and N-benzylated oxindole, it is highly probable that Q8b has a different mechanism of action compared to chloroquine. Since this study is focused on quinolones, further details regarding mechanism are not reported within this study.

Additionally, compounds Q8b exhibited superior activities against both MDR and XDR Mtb strains, with MIC values of 0.24 µg ml−1 for MDR strains, and 1.95 µg ml−1 for XDR strains. These results highlight the promising potential of isatin-tethered derivatives, particularly compound Q8b, as potent anti-tubercular agents. The significant improvement in activity and the superior efficacy against MDR and XDR strains highlight the value of N-benzylation of the oxindole motif in enhancing anti-tubercular properties. These findings suggest that Q8b and similar derivatives could serve as promising candidates for the development of novel treatments for drug-resistant TB [48].

Based on the need of Mtb to synthesize arabinogalactan for the formation of its cell wall, the use of synthetic analogs has been proposed to inhibit the function of key enzymes, particularly DprE1. In this context, it is known that molecules with benzenoid rings act as prototype inhibitors of this enzyme. Ockal et al. introduced a nitro group at position C-6, a hydrogen atom at position C-8, benzylic fractions at position N1, and aliphatic amides at C-3 of the quinolone ring. In their study, the substituents at C-6 and C-8 include a trifluoromethyl fraction and a nitro fraction, respectively, to mimic the benzenoid ring of benzothiazinones. Among the most promising results, compound 25 (Table 2, structure 8) showed greater potency against Mtb strains, as well as against strains resistant to FQ. [49].

Quinolones, as a class of synthetic antibiotics, would have evolved from quinoline derivatives, incorporating modifications that would have enhanced their antimicrobial potency, broadened their spectrum of activity, and improved their pharmacokinetic properties. The structural similarities between quinolines and quinolones, along with the biological activity resulting from subtle modifications to the quinoline scaffold, would underscore the potential for further development [50]. In this regard, we present several reports of quinoline-based compounds.

Thiosemicarbazones, due to the versatility of sulfur and nitrogen atoms as donor sites, can form a variety of coordination modes with bacterial proteins. In the development of new dihydroquinolone-based chemical entities as potential antitubercular agents, a series of novel 7,8-dihydroquinoline-5(6H)-ylidenehydrazinecarbothioamide derivatives were synthesized. Among them, compounds 4e (Table 2, structure 9a) and 4j (Table 1, structure 9b) MIC of 0.39 µg ml−1 emerged as the most promising antitubercular candidates, having been tested in TB zebrafish models [51].

A series of novels 7-chloro-N-phenylquinolin-4-amine derivatives containing thiosemicarbazone were tested as potential drug candidates in in vitro assays. The authors demonstrated that the most effective molecules were 7c and 7 g (Table 2, structure 10), with an MIC of 1.56 µg ml-1. Furthermore, tolerability in a cytotoxicity assay using human fibroblasts was observed, suggesting that these compounds could be tested in future experiments to ensure their safety and efficacy [52].

Organic compounds containing a C = N bond, have demonstrated various biological and pharmacological properties. One approach to enhancing anti-TB activity involves the creation of quinoline-based hybrid compounds, combining hydrazones with other heterocyclic structures, such as quinoline. In this context, Shruthi et al. synthesized eleven quinoline hydrazone derivatives, including QH-02, QH-04, and QH-05 (Table 2, structure 11), which showed a MIC value of 4 μg ml-1 against H37Rv. However, the authors emphasized the need for further studies, as the compounds have not yet been tested in vitro assays [53].

Fluoroquinolone-based hybrid molecules

A promising strategy to improve FQ activity is the design of hybrid molecules. For instance, adding pyrrolobenzodiazepines (PBDs), which are sequence-specific DNA minor groove binding agents, causes distortion of the double-helical structure and results in damaged chromosomal DNA, disrupting essential processes such as replication and transcription by impeding polymerase activity. However, PBDs have been shown to exhibit high cytotoxicity in mammalian cells. To address this, the antitubercular agent ciprofloxacin was added to the C8 substituent of PBDs, preventing the cytotoxicity associated with PBDs. This dual-action hybrid compound displayed minimum inhibitory concentration (MIC) values of 0.4 μg ml−1 against drug-sensitive strains and 2.1 μg ml−1 against MDR-TB strains. Furthermore, in the in-silico analysis, the compound showed a binding free energy and ChemScore higher than those obtained for ciprofloxacin (Table 3, Structure 1) [54].

Table 3 New fluroquinolones based on hybridizing chemical motifs
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Similarly, Rongxing et al. designed and synthesized twelve new hybrids of ciprofloxacin with 1,2,3-triazole-isatin as a linker and the addition of amides (compounds 7a-l), which has a great influence on antimicrobial activity. A novel 1H-1,2,3-triazole tethered isatin–moxifloxacin compound was designed. Isatin (1H-indole-2,3-dione) is an endogenous compound identified in anti-TB activity through ROS production and inhibition of enoyl-ACP reductase (InhA), that disrupts mycolic acid biosynthesis, a critical component of the Mtb cell wall. The overall results of the hybrids showed considerable in vitro activity against TB H37Rv and MDR-TB with MIC of 0.12 to 32 μg mL−1. However, compound 7a was shown to require concentrations like those of the unmodified ciprofloxacin (Table 3, structure 2) [55]. Therefore, this compound serves as a valuable foundation for future scientific investigations.

The evaluation of propionyl analogue or large cholesteryl hybrid in position N-15 of ciprofloxacin (Table 3, structure 3), showed a MIC of 1.6 μM to MIC 2.0 μM, respectively [56]. The results suggest that both the propionyl analogue and the cholesteryl hybrid exhibit potent antimicrobial activity, though with slight differences in inhibitory potency. Likewise Azoles, as privileged structures, are highly valued due to their broad range of biological activities. These include significant antitumor and antibacterial properties. A series of 34 clinafloxacin-azole conjugates were designed and synthesized (Table 2, structure 4). The results showed that most of the compounds exhibited anti-TB activity, with 14 of them displaying strong anti-TB efficacy (MIC < 2 μM) [57]. These results indicate the potential of azoles derivatives as antitubercular therapy.

By improving the lipophilicity of fluoroquinolones at the C-7 position, it could improve the anti-TB activity. Higher lipophilicity is highly desirable to effectively target Mtb within lung granulomas. 1,2,3-Triazole can serve as a useful tool to manipulate lipophilicity, polarity, and hydrogen bonding capacity of molecules, and consequently improve the pharmacological properties of drug candidates. The results showed that the most active hybrid 5c (Table 3, structure 5) showed a MIC of 0.025 μgmL−1 and 0.06 μg mL−1 against Mtb H37Rv and MDR–TB respectively [58]. Structural modifications through rational drug design, particularly hybrid molecules combining isatin with stabilized TB drugs warrant enhanced efficacy and overcome resistance.

A series of twenty-one novel alkyl/acyl/sulfonyl substituted fluoroquinolone derivatives were synthesized and evaluated for their anti-tuberculosis activities. These compounds were designed by modifying the secondary amine group of moxifloxacin with various alkyl, acyl, or sulfonyl groups. In vitro testing against Mtb H37Rv showed that compounds 1, and 2 exhibited outstanding anti-mycobacterial activity, with MIC values of 0.39 μg mL−1. Compounds 1 and 2 (Table 3, structure 6) demonstrated remarkable anti-mycobacterial activity against Mtb H37Rv. These compounds show promise as potential agents for the treatment of TB [59].

The incorporation of isatin and 1,2,3-triazole groups into the structure of compounds, such as moxifloxacin hybrids, provides a strategy to enhance their antimicrobial activity, particularly against Mtb including resistant strains, while reducing side effects. Some researchers designed moxifloxacin [60] and gatifloxacin [61] hybrids incorporating these molecules. The structure-activity and structure-cytotoxicity relationships revealed that substituents on the C-3 and C-5/C-7 positions of the isatin framework significantly influenced the anti-mycobacterial activity. Among the tested hybrids, compounds 8 h (Table 3, structure 7) and 8 l exhibited excellent anti-mycobacterial activity MIC 0.12–0.5 μg mL−1, which was comparable to moxifloxacin, demonstrating their potential as novel anti-tuberculosis candidates [60]. At the same time, one hybrid was found to be the most active against the tested strains, demonstrating efficacy comparable to the parent gatifloxacin, even against MDR strains (Table 3, structure 8) [61]. These findings highlight the potential of these novel hybrids as effective candidates for combating drug-resistant TB.

The development of isatin–fluoroquinolone–triazole hybrids leverages a multi-pharmacophore approach to enhance antimycobacterial activity and circumvent resistance mechanisms in MDR-TB. Collectively, these results show that the nature of the fluoroquinolone core and the substitution pattern on the isatin moiety greatly influence antimycobacterial activity and selectivity. This type of result, obtained through various assays such as QSAR, allows us to predict the biological activity of a chemical compound based on its molecular structure, specific characteristics, and the types of bonds it forms. Therefore, using different fluoroquinolone cores such as ciprofloxacin, moxifloxacin, and gatifloxacin broadens the possibility of discovering new antibiotics capable of being effective against MDR-TB strains. The performance of moxifloxacin- and gatifloxacin-based hybrids, compared to those derived from ciprofloxacin, supports their further development as promising candidates against drug-resistant.

The new fluoroquinolone, WQ-3810 (Table 3, Structure 9), demonstrated inhibitory activity against variants carrying resistance-associated mutations, such as G88C, at a concentration of 5 μg mL−1. In contrast, levofloxacin and moxifloxacin exhibited MIC of 16 μg mL−1 and 8 μg mL−1, respectively. These mutations alter the conformation of the loop-connecting domain of GyrA and affect the depth of the fluoroquinolone binding pocket, particularly at residues involved in the water–magnesium bridge, a critical interaction site for drug binding thereby reducing fluoroquinolone efficacy in clinical isolates. The authors proposed that C7-azetidinyl-substituted fluoroquinolones may bind to the DNA gyrase fluoroquinolone-binding pocket without relying heavily on this interaction. Moreover, reports have shown that WQ-3810, in combination with ethambutol, exhibits strong synergistic activity against recombinant strains. Notably, this synergistic effect with ethambutol suggests a promising role for WQ-3810 in combination therapy for fluoroquinolone-resistant tuberculosis [62]. This indicates that the newly developed FQs might increase the occurrence of the G88C mutation in Mtb clinical isolates.

The use of isatins as describes previously, phthalimides, and oxadiazoles, which have shown promise in inhibiting enoyl-ACP reductase, to create chimeric fluoroquinolones, has demonstrated enhanced effectiveness in antimicrobial activity. Among these, chimeric-isatins showed the best results. For instance, Niveditha et al. reported that the modification of carboxylic group at 3 position with isatin substituted ciprofloxacin derivatives exhibited potent activity with MIC values as low as 0.0625 µg mL−1, outperforming ciprofloxacin (MIC = 2 to > 8 µg mL−1) against drug-sensitive, MDR, and XDR Mtb. These hybrids also demonstrated dual binding affinity for DNA gyrase and InhA, disrupting mycolic acid biosynthesis, which suggests a potential dual-target mechanism of action. Furthermore, the authors reported that the incorporation of these compounds, particularly compound 6, showed a stronger binding affinity for DNA gyrase (Table 3, Structure 10) [63].

Collectively, the development of chimeric fluoroquinolones incorporating isatin and other bioactive fragments presents a promising strategy for enhancing antimicrobial efficacy. These hybrid molecules exhibit dual-target activity and superior potency compared to traditional agents, indicating their potential as effective therapeutic candidates against drug-resistant strains of Mtb.

WFQ-228 (Table 3, Structure 11) is a novel FQ derivative that features unique substitutions at the N1 position of the quinolone ring. These modifications have been specifically designed to enhance its antibacterial activity, particularly against Mtb, including strains harboring gyrA mutations. The presence of these mutations in the gyrA gene, which encodes a subunit of the bacterial DNA gyrase, often leads to resistance to conventional fluoroquinolones, making treatment more challenging. In a study involving 75 Mtb isolates, WFQ-228 demonstrated superior efficacy compared to levofloxacin. Notably, it showed enhanced activity against strains with mutations at critical sites within the gyrA gene. WFQ-228 represents a processing next generations gyrase inhibitor for TB [64]. These approaches involving changes at the N1 position should be further explored to enhance the activity of fluoroquinolone derivatives.

Given the important antimicrobial role attributed to nitric oxide (NO), Aziz et al. decided to investigate the use of hybrid molecules containing NO-donating oxime moieties to enhance the antimicrobial potential of selected fluoroquinolones. The antitubercular evaluation of the synthesized compounds indicated that ketone derivatives 2b (Table 3, structure 12a) and 2e (Table 3, structure 12b), as well as oximes 3b (Table 3, structure 12c) and 3 d (Table 3, structure 12 d), exhibited slightly higher activity than their respective parent fluoroquinolones. These results suggest that the incorporation of NO-donating groups could provide a promising strategy for improving the efficacy of FQ against Mtb [65].

Fatty acids are a part of bacterial membranes; therefore, the synthesis of fatty acids is essential for the survival of the bacteria. Nevertheless, several reports have shown the antibacterial effect of unsaturated fatty acids, due to the presence of double bonds in their chain transferring the aqueous phase into the membrane and increasing their antibacterial efficacy. Despite, the exact mechanism of action of long-chain fatty acids is not fully understood. Thus, Novel conjugates of moxifloxacin with Sorbic acid, an unsaturated aliphatic straight-chain monocarboxylic fatty acid, 2,4-hexadienoic acid, were synthesized using amide chemistry. The sorbic acid conjugate 2 m (Table 3, structure 13) showed high activity against standard bacterial isolates and was particularly effective against Mtb Spec 210. Among the derivatives, sorbic acid demonstrated the highest bactericidal potential against clinical strains. The mechanism of action was studied using topoisomerase IV decatenation and DNA gyrase supercoiling assays, with molecular docking studies supporting its antibacterial activity [66]. However, the authors do not demonstrate the advantages conferred by the addition of fatty acids, although the influence of fatty acids on membrane penetration and cellular efflux may raise a hypothesis about the potential role of these processes in the antibacterial activity of the tested conjugates. Its strong bactericidal activity against Mtb emphasizes its promising therapeutic potential, which should be studied further.

In research carried out by Pais et al. they developed esters as prodrugs of the levofloxacin and ciprofloxacin, with long-chain fatty alcohols (Table 3, structure 14). The carboxylic acid group of fluoroquinolones is essential to the mode of action but is also responsible for many of its side effects and metal-chelating properties. The results show a reduction in the antitubercular activity of the synthesized derivatives, probably due to deficient activation of the ester prodrug. Despite this, it was found that the derivatives exhibit bioactivity against other fluoroquinolone-resistant bacteria, indicating a different mode of action and suggesting that it may be worthwhile to research further modifications to the carboxylic acid group. This might lead to new compounds that are efficient against resistant strains [67]. This research field remains open for innovation and will keep progressing as even changes are developed.

Conclusions

Ever since the discovery of quinolones in the early 70 s, there has been an increasing number of bioactive FQ derivatives with promising activity against Mtb. In this matter, there are still some challenges that need to be overcome to make these FQ (and other molecules) an established therapeutic agent. By modifying particular moieties in already known FQ, researchers are able to access different biological targets otherwise unaccusable for lead quinolone compounds. In many cases, new derivatives show an even higher activity than reference pharmaceuticals, however, this activity has only been measured in vitro. For this reason, there may be other variables related to the ADME process that need to be considered if these compounds are to be tested clinically. For this reason, a meticulous design is needed in order to guarantee a proper solubility, hence, a proper distribution in the organism. It is known that the best way to make a new and effective drug is to use an already known one as starting material. This review has mentioned several of these types of molecules, but most of them lack clinical information to support the so-called efficiency in a human organism. Not only the development of drugs is being investigated for the eradication of tuberculosis; researchers have also focused on other alternatives such a vaccination. It does represent a powerful tool for TB prophylaxis. Even when most molecules may never reach a clinical testing phase, the search for more bioactive molecules is also necessary to stand a chance in the fight against tuberculosis, hat even nowadays is still a deadly and worldwide disease.