5,8-Quinolinedione Scaffold as a Promising Moiety of Bioactive Agents

Natural 5,8-quinolinedione antibiotics exhibit a broad spectrum of activities including anticancer, antibacterial, antifungal, and antimalarial activities. The structure–activity research showed that the 5,8-quinolinedione scaffold is responsible for its biological effect. The subject of this review report is a presentation of the pharmacological activity of synthetic 5,8-quinolinedione compounds containing different groups at C-6 and/or C-7 positions. The relationship between the activity and the mechanism of action is included if these data have been included in the original literature. The review mostly covers the period between 2000 and 2019. Previously published literature data were used to present historical points.


Introduction
Natural products are an important source of medicinal substances used in the therapy of various ailments including infectious, cardiovascular, neurological, and oncological diseases. Such substances are produced by plants, microorganisms, and invertebrates. About half of the currently used antibiotics come from the strain of Streptomyces bacteria, a microorganism which is widespread in nature [1].
In 1959, the bioactive 7-aminoquinone called Streptonigrin 1 (Figure 1) was isolated from Streptomyces flocculus [2]. This alkaloid was also found in Actinomyces albus var bruneomycini, Streptomyces rufochromogenes, and Streptomyces echinatus [3,4].  Based on chemical degradative methods and spectral analysis, Rao et al. developed the chemical structure of Streptonigrin 1 [5]. Finally, the structure of compound 1 was confirmed by x-ray crystallography in 1975 [6]. The total synthesis of the alkaloid 1 has been carried out by two research groups (Weinreb et al. and Kende et al.) from 1980 to 1982 [7,8].
The multidirectional biological activity of natural compounds 1-5 results from their structural diversity and the possibility of interaction with various molecular targets. The mechanism of biological action of the compounds containing the 5,8-quinolinedione moiety largely depends on their ability to form radicals in vivo. Numerous studies have confirmed that nicotinamide adenosine diphosphate (NADP) or NADPH-dependent quinone oxidoreductase (NQO1) are important molecular targets for new derivatives based on the structure of 5,8-quinolinedione [18,20,23].
Research into the structure-activity relationship for natural antibiotics 1-5 has shown that the 5,8-quinolinedione scaffold is essential for ensuring biological activity [9,18,24,25]. Introduction of various groups at the C-6 and C-7 positions significantly affects the biological properties of the compounds. In many cases, modification of the 5,8-quinolinedione moiety at the C-2 position reduces activity when compared to compounds not substituted at this position [26][27][28].
The multidirectional biological activity of natural compounds 1-5 results from their structural diversity and the possibility of interaction with various molecular targets. The mechanism of biological action of the compounds containing the 5,8-quinolinedione moiety largely depends on their ability to form radicals in vivo. Numerous studies have confirmed that nicotinamide adenosine diphosphate (NADP) or NADPH-dependent quinone oxidoreductase (NQO1) are important molecular targets for new derivatives based on the structure of 5,8-quinolinedione [18,20,23].
Research into the structure-activity relationship for natural antibiotics 1-5 has shown that the 5,8-quinolinedione scaffold is essential for ensuring biological activity [9,18,24,25]. Introduction of various groups at the C-6 and C-7 positions significantly affects the biological properties of the compounds. In many cases, modification of the 5,8-quinolinedione moiety at the C-2 position reduces activity when compared to compounds not substituted at this position [26][27][28].
The World Health Organization (WHO) reports that cancer is the first or second main cause of death in 71 of 127 countries. The International Agency for Research on Cancer (IARC) estimated that only in 2018, 18.1 million new cases were diagnosed and 9.6 million people died of cancer. According to the GLOBOCAN 2018 research, the most common types of cancers are lung and breast cancer, which accounted for over 23% of new oncological cases [54].
Several compounds containing the 5,8-quinolinedione moiety that exhibit anticancer activity have been described in the literature. Ling et al. obtained a series of 6-and 7-arylamino-5,8quinolinediones that were tested against drug sensitive (HeLeS3) and multidrug resistant (KB-vin) cell lines [37].
Compounds 10-11 ( Figure 2) were characterized by high anticancer activity against both cell lines and their IC50 values were within the range of 0.59-1.52 µM. The 7-substituted derivative 10 exhibited 1.4-time and 1.6-time higher activity than compound 11 against HeLeS3 and KB-vin, respectively. Further research has shown that both compounds cause an NQO1-dependent antiproliferative effect and induce a dose-dependent lethal mitochondrial dysfunction in the tested cell lines. Derivative 10 exhibited anticancer activity, but did not affect a normal cell line. Moreover, it induced apoptosis by upregulating Bcl-2 protein and reducing Bax protein and cleaved caspase-3 [37]. Reaction of 5,8-quinolinediones 7-9 with an amine under basic conditions led to a mixture of 6-or 7-mono-substituted compounds, but the ratio of the products depended on the type of solvent. Use of an aprotic solvent such as tetrahydrofuran (THF) or dimethylformamide (DMF) led to the 7-amino compound as a major product. The 6-substituted compounds were formed in a protic solvent like water or ethanol (Scheme 2) [40,[45][46][47][48]. Moreover, 6-amino-5,8-quinolinedione could be obtained selectively in reaction with the addition of a catalyst like CeCl 3 or NiCl 2 in the presence of ethanol (Scheme 2) [24,26,49,50]. Modification of 6-amino-5,8-quinolinedione derivatives resulted in 6-amino-7-thiol-5,8-quinolinedione and multicyclic compounds [24,26,38].

Anticancer Activity
The World Health Organization (WHO) reports that cancer is the first or second main cause of death in 71 of 127 countries. The International Agency for Research on Cancer (IARC) estimated that only in 2018, 18.1 million new cases were diagnosed and 9.6 million people died of cancer. According to the GLOBOCAN 2018 research, the most common types of cancers are lung and breast cancer, which accounted for over 23% of new oncological cases [54].
Several compounds containing the 5,8-quinolinedione moiety that exhibit anticancer activity have been described in the literature. Ling et al. obtained a series of 6-and 7-arylamino-5,8-quinolinediones that were tested against drug sensitive (HeLeS3) and multidrug resistant (KB-vin) cell lines [37].
Compounds 10-11 ( Figure 2) were characterized by high anticancer activity against both cell lines and their IC 50 values were within the range of 0.59-1.52 µM. The 7-substituted derivative 10 exhibited 1.4-time and 1.6-time higher activity than compound 11 against HeLeS3 and KB-vin, respectively. Further research has shown that both compounds cause an NQO1-dependent antiproliferative effect and induce a dose-dependent lethal mitochondrial dysfunction in the tested cell lines. Derivative 10 exhibited anticancer activity, but did not affect a normal cell line. Moreover, it induced apoptosis by upregulating Bcl-2 protein and reducing Bax protein and cleaved caspase-3 [37].  A small library of arylamine derivatives of 5,8-quinolinedione were prepared by Ryu et al. [50,55,56]. Derivatives 12-23 ( Figure 3) were tested against lung cancer (A549), ovary adenocarcinoma (SK-OV-3), and malignant melanoma (SK-MEL-2) cell lines. In the series of 12-15, compound 13 exhibited high activity against all tested cell lines with the IC 50 range of 0.36-0.85 µg/mL. Moreover, this derivative had a higher activity than cisplatin. Compound 12 demonstrated a selective activity against the A549 cell line, which was 2-and 3-times higher than Streptonigrin 1 and cisplatin, respectively. Additionally, for compounds 12-15, the effect on NQO1 activity was evaluated using the cytosolic fractions of the A549 cell line. This research showed that the type of substituent affects the NQO1 activity as follows: propyl (15) > chloride (12) > ethyl (14) > trifluorometoxy (13) [50]. In order to check the influence of the substituent at the C-7 position, 6-arylamino-7-halogen-5,8-quinolinediones 16-20 were obtained ( Figure 3) [55]. Compounds 12, 16, and 19 differed only in the C-7 position substituent. Comparing their anticancer activity against the A549, SK-OV-3, and SK-MEL-2 cell lines, it showed that the introduction of a halogen atom at the C-7 position causes a decrease in activity [50,55]. However, the derivatives containing a chlorine atom (16)(17) at this position exhibited higher activity than those containing a bromine atom (19)(20). The structure-activity relationship showed that the R group affects the anticancer activity, but there was no correlation between the properties of the R group and the activity against the tested cell lines [55]. A small library of arylamine derivatives of 5,8-quinolinedione were prepared by Ryu et al. [50,55,56].
The 6-arylamine derivatives 24-26 were used as substrates for the synthesis of tetracyclic compounds 27-29 ( Figure 4). Both series of compounds were tested against ovary adenocarcinoma (SK-OV-3), malignant melanoma (SK-MEL-2), lung (A549), brain cancer (XF 498), and colon (HCT 15) cancer cell lines [24].  A comparison of the activity of derivatives 24-25 and 27-28 showed that tetracyclic compounds 27-28 exhibited a higher anticancer activity in relation to all tested cell lines. The IC50 value for 29 was in the range of 0.12-0.21 µM, and was comparable with the activity of doxorubicin against the SK-OV-3 and XF 498 cell lines [24]. Suh et al. described tri-and tetracyclic (30-32 and 33-34, respectively) ( Figure 4) imidazo-5,8quinolinedione analogues, which exhibited potential anticancer activity against the A549, SK-OV-3, SK-MEL-2, XF 498, and HCT 15 cell lines. A comparison of the IC50 values of 30-32 derivatives against the tested cell lines suggested that side chain elongation of the substituent led to a decrease in activity. Compounds 33 and 34 showed a similar anticancer effect against the tested cell lines. In the case of the SK-OV-3 line, compound 33 exhibited an activity 3.5-times higher than 34 [21].
The indoleamine 2,3-dioxygenase 1 (IDO1) enzyme catalyzes degradation of tryptophan to Nformyl-kynurenine. This reaction initiates the kynurenine pathway leading to the synthesis of nicotinamide adenine dinucleotide (NAD), which is used to transfer electrons in many enzymatic reactions. Many studies have confirmed that IDO1 has an influence on the development of many types of human tumors [57][58][59]. One of the group that exhibited promising IDO1 inhibitory activity was imidazo-5,8-quinolindione compounds 35-36 and thiazolo-5,8-quinolinediones 37-38 ( Figure 5) [38]. The inhibitory rates of compounds 35-36 and 37-38 at 1 µM were 65.5-76.0%. However, derivatives containing 4-thiazolyl substituent as the R group (36 and 38) showed a higher activity against IDO1 than epacadostat (INCB24360) used as the positive control. The IC50 values determined for 36 and 38 were equal to 61 nM and 18 nM, respectively. The cytotoxic activity was evaluated against human breast (MCF-7) and cervical (HeLa) cancer cell lines. The in vivo test showed that 38 reduced kynurenine levels in plasma by 30%. The CC50 values were significantly higher than the IC50 values, meaning that the tested compounds were not cytotoxic in their effective concentration against IDO1 [38]. derivatives against the tested cell lines suggested that side chain elongation of the substituent led to a decrease in activity. Compounds 33 and 34 showed a similar anticancer effect against the tested cell lines. In the case of the SK-OV-3 line, compound 33 exhibited an activity 3.5-times higher than 34 [21].
The indoleamine 2,3-dioxygenase 1 (IDO1) enzyme catalyzes degradation of tryptophan to N-formyl-kynurenine. This reaction initiates the kynurenine pathway leading to the synthesis of nicotinamide adenine dinucleotide (NAD), which is used to transfer electrons in many enzymatic reactions. Many studies have confirmed that IDO1 has an influence on the development of many types of human tumors [57][58][59]. One of the group that exhibited promising IDO1 inhibitory activity was imidazo-5,8-quinolindione compounds 35-36 and thiazolo-5,8-quinolinediones 37-38 ( Figure 5) [38].  A comparison of the activity of derivatives 24-25 and 27-28 showed that tetracyclic compounds 27-28 exhibited a higher anticancer activity in relation to all tested cell lines. The IC50 value for 29 was in the range of 0.12-0.21 µM, and was comparable with the activity of doxorubicin against the SK-OV-3 and XF 498 cell lines [24]. Suh et al. described tri-and tetracyclic (30-32 and 33-34, respectively) ( Figure 4) imidazo-5,8quinolinedione analogues, which exhibited potential anticancer activity against the A549, SK-OV-3, SK-MEL-2, XF 498, and HCT 15 cell lines. A comparison of the IC50 values of 30-32 derivatives against the tested cell lines suggested that side chain elongation of the substituent led to a decrease in activity. Compounds 33 and 34 showed a similar anticancer effect against the tested cell lines. In the case of the SK-OV-3 line, compound 33 exhibited an activity 3.5-times higher than 34 [21].
The indoleamine 2,3-dioxygenase 1 (IDO1) enzyme catalyzes degradation of tryptophan to Nformyl-kynurenine. This reaction initiates the kynurenine pathway leading to the synthesis of nicotinamide adenine dinucleotide (NAD), which is used to transfer electrons in many enzymatic reactions. Many studies have confirmed that IDO1 has an influence on the development of many types of human tumors [57][58][59]. One of the group that exhibited promising IDO1 inhibitory activity was imidazo-5,8-quinolindione compounds 35-36 and thiazolo-5,8-quinolinediones 37-38 ( Figure 5) [38]. The inhibitory rates of compounds 35-36 and 37-38 at 1 µM were 65.5-76.0%. However, derivatives containing 4-thiazolyl substituent as the R group (36 and 38) showed a higher activity against IDO1 than epacadostat (INCB24360) used as the positive control. The IC50 values determined for 36 and 38 were equal to 61 nM and 18 nM, respectively. The cytotoxic activity was evaluated against human breast (MCF-7) and cervical (HeLa) cancer cell lines. The in vivo test showed that 38 reduced kynurenine levels in plasma by 30%. The CC50 values were significantly higher than the IC50 values, meaning that the tested compounds were not cytotoxic in their effective concentration against IDO1 [38]. The inhibitory rates of compounds 35-36 and 37-38 at 1 µM were 65.5-76.0%. However, derivatives containing 4-thiazolyl substituent as the R group (36 and 38) showed a higher activity against IDO1 than epacadostat (INCB24360) used as the positive control. The IC 50 values determined for 36 and 38 were equal to 61 nM and 18 nM, respectively. The cytotoxic activity was evaluated against human breast (MCF-7) and cervical (HeLa) cancer cell lines. The in vivo test showed that 38 reduced kynurenine levels in plasma by 30%. The CC 50 values were significantly higher than the IC 50 values, meaning that the tested compounds were not cytotoxic in their effective concentration against IDO1 [38].
Enzymes controlling the cell cycle process include phosphatase Cdc25, which exists as three homologues denoted as Cdc24A, Cdc25B, and Cdc25c. Overexpression of Cdc25B and Cdc25c has been observed in many human tumors and is considered to be oncogenic [60]. Lazo et al. examined 5,8-quinolinedione compounds as potential inhibitors of recombinant human Cdc25B 2 [47].
In the series of tested compounds, the highest inhibitory activity was shown by compounds containing the 2-morpholin-4-ylethylamine substituent, 39 and 40 ( Figure 6). A comparison of the activity of these two compounds showed that the position of amine substituent affects the activity. The IC 50 value for 39 was 3-times lower than that calculated for 40, and they were equal to 0.21 µM and 0.82 µM, respectively [47]. The in vitro tests against breast cancer cells showed that derivative 39 selectively and irreversibly inhibits Cdc25 phosphatases, which results in a decrease in the Cdc2 kinase level and causes the arrest of the cells in the G1 and G2/M phases of the cell cycle [47,61]. Enzymes controlling the cell cycle process include phosphatase Cdc25, which exists as three homologues denoted as Cdc24A, Cdc25B, and Cdc25c. Overexpression of Cdc25B and Cdc25c has been observed in many human tumors and is considered to be oncogenic [60]. Lazo et al. examined 5,8-quinolinedione compounds as potential inhibitors of recombinant human Cdc25B2 [47].
In the series of tested compounds, the highest inhibitory activity was shown by compounds containing the 2-morpholin-4-ylethylamine substituent, 39 and 40 ( Figure 6). A comparison of the activity of these two compounds showed that the position of amine substituent affects the activity. The IC50 value for 39 was 3-times lower than that calculated for 40, and they were equal to 0.21 µM and 0.82 µM, respectively [47]. The in vitro tests against breast cancer cells showed that derivative 39 selectively and irreversibly inhibits Cdc25 phosphatases, which results in a decrease in the Cdc2 kinase level and causes the arrest of the cells in the G1 and G2/M phases of the cell cycle [47,61]. Compounds 41-42 ( Figure 6) were designed as inhibitors of Cdc2 and ERK proteins [62]. Extracellular signal-regulated kinase (ERK) plays an important role in the signal pathway, which controls cancer cell proliferation and transformation [63]. The preliminary test showed that only compound 42 induces cytotoxicity in the lung cancer cell line (A549) significantly, and the IC50 value was estimated at 5 µM. Hsu et al. found that 42 inhibited the phosphorylated sites of Cdc2 and increased the G2/M arrest in lung cancer cells. The blockade of ERK by 42 may result in the inhibition of proliferation and apoptosis of A549 cells [62].
The alkyne moiety, which is one of the most important groups in medical chemistry, affects the biological and chemical properties of compounds that contain this fragment [64][65][66]. For this reason, acetylenic amine derivatives of 5,8-quinolinedione 43-46 ( Figure 7) were prepared. The antiproliferative activity was tested against melanoma (C-32), glioblastoma (SNB- 19), and breast cancer (T47D) cell lines [48]. Derivatives 43-44 and 45-46 exhibited a higher efficiency against the tested cell lines than cisplatin. In both groups of compounds, it was observed that an introduction of the methyl group led to a decrease in the activity. A comparison of the activity of 6-and 7-amino substituted derivatives showed that 43-44 had a lower anticancer efficiency than 45-46 [48]. Compounds 41-42 ( Figure 6) were designed as inhibitors of Cdc2 and ERK proteins [62]. Extracellular signal-regulated kinase (ERK) plays an important role in the signal pathway, which controls cancer cell proliferation and transformation [63]. The preliminary test showed that only compound 42 induces cytotoxicity in the lung cancer cell line (A549) significantly, and the IC 50 value was estimated at 5 µM. Hsu et al. found that 42 inhibited the phosphorylated sites of Cdc2 and increased the G 2 /M arrest in lung cancer cells. The blockade of ERK by 42 may result in the inhibition of proliferation and apoptosis of A549 cells [62].
The alkyne moiety, which is one of the most important groups in medical chemistry, affects the biological and chemical properties of compounds that contain this fragment [64][65][66]. For this reason, acetylenic amine derivatives of 5,8-quinolinedione 43-46 ( Figure 7) were prepared. The antiproliferative activity was tested against melanoma (C-32), glioblastoma (SNB- 19), and breast cancer (T47D) cell lines [48]. Enzymes controlling the cell cycle process include phosphatase Cdc25, which exists as three homologues denoted as Cdc24A, Cdc25B, and Cdc25c. Overexpression of Cdc25B and Cdc25c has been observed in many human tumors and is considered to be oncogenic [60]. Lazo et al. examined 5,8-quinolinedione compounds as potential inhibitors of recombinant human Cdc25B2 [47].
In the series of tested compounds, the highest inhibitory activity was shown by compounds containing the 2-morpholin-4-ylethylamine substituent, 39 and 40 ( Figure 6). A comparison of the activity of these two compounds showed that the position of amine substituent affects the activity. The IC50 value for 39 was 3-times lower than that calculated for 40, and they were equal to 0.21 µM and 0.82 µM, respectively [47]. The in vitro tests against breast cancer cells showed that derivative 39 selectively and irreversibly inhibits Cdc25 phosphatases, which results in a decrease in the Cdc2 kinase level and causes the arrest of the cells in the G1 and G2/M phases of the cell cycle [47,61]. Compounds 41-42 ( Figure 6) were designed as inhibitors of Cdc2 and ERK proteins [62]. Extracellular signal-regulated kinase (ERK) plays an important role in the signal pathway, which controls cancer cell proliferation and transformation [63]. The preliminary test showed that only compound 42 induces cytotoxicity in the lung cancer cell line (A549) significantly, and the IC50 value was estimated at 5 µM. Hsu et al. found that 42 inhibited the phosphorylated sites of Cdc2 and increased the G2/M arrest in lung cancer cells. The blockade of ERK by 42 may result in the inhibition of proliferation and apoptosis of A549 cells [62].
The alkyne moiety, which is one of the most important groups in medical chemistry, affects the biological and chemical properties of compounds that contain this fragment [64][65][66]. For this reason, acetylenic amine derivatives of 5,8-quinolinedione 43-46 ( Figure 7) were prepared. The antiproliferative activity was tested against melanoma (C-32), glioblastoma (SNB- 19), and breast cancer (T47D) cell lines [48]. Derivatives 43-44 and 45-46 exhibited a higher efficiency against the tested cell lines than cisplatin. In both groups of compounds, it was observed that an introduction of the methyl group led to a decrease in the activity. A comparison of the activity of 6-and 7-amino substituted derivatives showed that 43-44 had a lower anticancer efficiency than 45-46 [48]. Derivatives 43-44 and 45-46 exhibited a higher efficiency against the tested cell lines than cisplatin. In both groups of compounds, it was observed that an introduction of the methyl group led to a decrease in the activity. A comparison of the activity of 6-and 7-amino substituted derivatives showed that 43-44 had a lower anticancer efficiency than 45-46 [48].
In the literature data, compounds containing an alkoxy, alkynyloxy, or enediyne substituent at the C-7 or/and C-6 position were described. Derivatives of this type seem to be potential anticancer agents [28,51,52]. A comparison of activities in both series of compounds showed that the derivatives containing the aziridinyl moiety at the C-6 position (58-60) were characterized by a higher cytotoxicity against the SK-OV-3, SK-MEL-2, and XF498 cell lines than the C-7 analogues 55-57 [72].
In the literature data, compounds containing an alkoxy, alkynyloxy, or enediyne substituent at the C-7 or/and C-6 position were described. Derivatives of this type seem to be potential anticancer agents [28,51,52].
Betulin was one of the first chemically pure compounds isolated from plant material. Betulin and its semi-synthetic derivatives are characterized by high biological activity including anticancer, antimicrobial, and antiviral activities [76][77][78]. The 5,8-quinolinedione moiety was connected with betulin derivatives to obtain hybrids 75-77 ( Figure 12). The compounds were tested for their anticancer activity against glioblastoma (SNB- 19), melanoma (C-32 and Colo-829), breast (MCF-7, T47D and MDA-MB-231), and lung (A549) cancer cell lines [79].   (75). However, compounds 75-77 exhibited the highest activity against the A549 cell line; the IC50 values were in the range 0. 45-8.58 µM. An apoptosis assay showed that compound 76 caused a significant increase in TP53 gene expression in A549 and MCF-7 cells. The TP53 gene encodes the p53 protein, which can affect expression of proapoptotic (Bax) and antiapoptotic (Bcl-2) protein [80][81][82]. Hybrid 76 induced a significant expression of Bax and did not affect the expression of Bcl-2 and the ratio of Bax and Bcl-2. The obtained results suggested that the cytotoxic effect of derivative 76 is associated with the mitochondrial apoptosis pathway in the A549 and MCF-7 cell lines. Molecular docking study showed that hybrid 76 was bonded to the NQO1 active site by hydrophobic interaction between the enzyme and the 5,8-quinolinedione moiety [79].  protein [80][81][82]. Hybrid 76 induced a significant expression of Bax and did not affect the expression of Bcl-2 and the ratio of Bax and Bcl-2. The obtained results suggested that the cytotoxic effect of derivative 76 is associated with the mitochondrial apoptosis pathway in the A549 and MCF-7 cell lines. Molecular docking study showed that hybrid 76 was bonded to the NQO1 active site by hydrophobic interaction between the enzyme and the 5,8-quinolinedione moiety [79].

Antimicrobial Activity
Infections caused by antibiotic-resistant bacteria are one of the most serious problems of modern medicine. The EU and the European Economic Area estimates that in 2015, the median number of infections was 671,689 and more than 33,000 people died of infection [85]. Only one antibiotic, daptomycin, belonging to lipopeptide antibiotics, has been discovered and registered as a drug in the past 50 years [86]. The search for new antibiotics is therefore a particularly important research problem.
Many compounds containing the 1,4-quinone moiety are being investigated for their antibacterial and antifungal properties [87][88][89]. One of the groups that has exhibited promising antibacterial and antifungal activity was constituted by the 5,8-quinolinedione compounds containing the arylamine group [90].
The tested compounds 80-82 ( Figure 14) did not exhibit any activity against Gram-negative bacteria like E. coli and P. aeruginosa. Activity of derivatives 80-82 against Gram-positive bacteria were higher or comparable with that of ampicillin, and depended on the R substituent. The research showed that the R group affects the activity against MRS aureus, and this effect decreases in the following order: bromine (81) > chloride (82) > fluorine (80). The arylamine compounds 80-82 had a potent antifungal activity against C. albicans, C. albicans L., and A. niger. Their activities were higher than those of fluconazole and griseofulvin, respectively [90]. The kind of substituent at the nitrogen atom in derivatives 78-79 affected their anticancer activity. Compound 78, which contained an ethyl group, exhibited a higher activity against all tested cell lines. In addition, this derivative had activity that was 3-times and 10-times higher against A549 and HCT116 cells than that of doxorubicin, respectively [84]. Most chemotherapeutic agents interact or inhibit topoisomerase I and II, which are involved in the DNA replication process [84]. Compounds 78-79 at a 5 µM concentration strongly inhibited the activity of Topo II, and did not interact with Topo I [83].

Antimicrobial Activity
Infections caused by antibiotic-resistant bacteria are one of the most serious problems of modern medicine. The EU and the European Economic Area estimates that in 2015, the median number of infections was 671,689 and more than 33,000 people died of infection [85]. Only one antibiotic, daptomycin, belonging to lipopeptide antibiotics, has been discovered and registered as a drug in the past 50 years [86]. The search for new antibiotics is therefore a particularly important research problem.
Many compounds containing the 1,4-quinone moiety are being investigated for their antibacterial and antifungal properties [87][88][89]. One of the groups that has exhibited promising antibacterial and antifungal activity was constituted by the 5,8-quinolinedione compounds containing the arylamine group [90].
The tested compounds 80-82 ( Figure 14) did not exhibit any activity against Gram-negative bacteria like E. coli and P. aeruginosa. Activity of derivatives 80-82 against Gram-positive bacteria were higher or comparable with that of ampicillin, and depended on the R substituent. The research showed that the R group affects the activity against MRS aureus, and this effect decreases in the following order: bromine (81) > chloride (82) > fluorine (80). The arylamine compounds 80-82 had a potent antifungal activity against C. albicans, C. albicans L., and A. niger. Their activities were higher than those of fluconazole and griseofulvin, respectively [90].
bacteria like E. coli and P. aeruginosa. Activity of derivatives 80-82 against Gram-positive bacteria were higher or comparable with that of ampicillin, and depended on the R substituent. The research showed that the R group affects the activity against MRS aureus, and this effect decreases in the following order: bromine (81) > chloride (82) > fluorine (80). The arylamine compounds 80-82 had a potent antifungal activity against C. albicans, C. albicans L., and A. niger. Their activities were higher than those of fluconazole and griseofulvin, respectively [90].     Compounds 90-93 exhibited the highest antifungal activity against Candida tropicalis and the MIC was within the range of 0.6-6.3 µg/mL. For these species, the order of activities was as follows: 91 > 93 > 92 > 90 [92].
Natural enediyne antibiotics are characterized by a high antibacterial activity against Grampositive and Gram-negative strains,74]. The 5,8-quinolinedione moiety contains a double bond between the C-6 and C-7 carbon atom, which enables the enediyne fragment to be obtained by a reaction with terminal alkynes. Ezeokonkwa et al. described the synthesis of mono-(94-96) ( Figure  16) and disubstituent (97-99) (Figure 16) hybrids of 5,8-quinolinedione with terminal alkynes and their antibacterial activity against E. coli 12, K. pneumonia, S. aureus, and P. aeruiginosa [93,94].    Gram-negative bacteria possess a specific outer membrane, lipid A, as a major component. The LpxC enzyme is responsible for the synthesis of lipid A and is a promising target for the preparation of selective anti-Gram-negative antibiotics [95,96]. The molecular docking study showed that compounds 94-99 could bind to LpxC and the obtained ∆G was higher than that of the co-crystalized inhibitor. However, dialkyne derivatives 97-99 interacted slightly less with the active site of enzyme [95,96].
The most deadly bacterial disease is tuberculosis, which caused the death of 1.3 million people in 2017. People infected by the HIV virus suffer from the active form of tuberculosis 20-30 times more often than people without HIV [97]. Tuberculosis in HIV-negative people is caused by Mycobacterium tuberculosis, but in HIV-positive patients, the etiological factor of the disease may also be Mycobacterium bovis, Mycobacterium africanum, Mycobacterium canetti, and Mycobacterium microti [98].
One of the first compounds containing the 5,8-quinolinedione moiety that exhibited activity against M. tuberculosis was compound 100 ( Figure 17) with a MIC equal to 3.5 µM. Mulchin et al. synthesized a series of amine and thiol derivatives and tested these compounds regarding their anticancer and tuberculostatic activities [26,40]. Gram-negative bacteria possess a specific outer membrane, lipid A, as a major component. The LpxC enzyme is responsible for the synthesis of lipid A and is a promising target for the preparation of selective anti-Gram-negative antibiotics [95,96]. The molecular docking study showed that compounds 94-99 could bind to LpxC and the obtained ΔG was higher than that of the co-crystalized inhibitor. However, dialkyne derivatives 97-99 interacted slightly less with the active site of enzyme [95,96].
The most deadly bacterial disease is tuberculosis, which caused the death of 1.3 million people in 2017. People infected by the HIV virus suffer from the active form of tuberculosis 20-30 times more often than people without HIV [97]. Tuberculosis in HIV-negative people is caused by Mycobacterium tuberculosis, but in HIV-positive patients, the etiological factor of the disease may also be Mycobacterium bovis, Mycobacterium africanum, Mycobacterium canetti, and Mycobacterium microti [98].
One of the first compounds containing the 5,8-quinolinedione moiety that exhibited activity against M. tuberculosis was compound 100 ( Figure 17) with a MIC equal to 3.5 µM. Mulchin et al. synthesized a series of amine and thiol derivatives and tested these compounds regarding their anticancer and tuberculostatic activities [26,40]. In the series of compounds 47-50 ( Figure 8) and 101-105 (Figure 17), the highest activity against M. bovis was shown by 6-amino-7-chloro-5,8-quinolinedione 101. Introduction of a thiomethyl or thiophenyl group at the C-7 position decreases the activity. Derivatives containing an amine group at the C-6 position (50, 105) were characterized by higher activity against M. bovis than 7aminosubstituted compounds (48,104). A comparison of the activities of 2-chloroethylamine (48 and 50) (Figure 8) and 2-bromoethylamine (104-105) (Figure 17) compounds showed that replacing the chloride atom with a bromine atom increased the tuberculostatic activity [26].
Tuberculostatic activities of 6-106-109 and 7-aminosubstituted 110-113 5,8-quinolinediones ( Figure 18) were tested against M. tuberculosis [40]. Analyzing the obtained results, a general trend was observed. The research proved that the type of substituent and its position had an influence on the activity. A comparison of activities of compounds 106-107 and 110-111 showed that the derivatives containing a phenyl moiety at the C-7 position (106 and 107) had higher activity than the 6-substituted 110-111. An inverse relationship was Analyzing the obtained results, a general trend was observed. The research proved that the type of substituent and its position had an influence on the activity. A comparison of activities of compounds 106-107 and 110-111 showed that the derivatives containing a phenyl moiety at the C-7 position (106 and 107) had higher activity than the 6-substituted 110-111. An inverse relationship was observed for alkylamine derivatives, and the MIC values for 112-113 were lower than those for 108-109. The highest activity against M. tuberculosis was exhibited by derivative 45 (Figure 7) with a MIC equal to 8 µM [40].
The most important problems in Sub-Saharan Africa include malaria caused by the Plasmodium parasite. Quinine isolated from cinchona trees was the first anti-malarial drug and is the scaffold for obtaining more effective substances [99,100]. Lanfranchi et al. synthetized a series of derivatives that were tested on two types of P. falciparum ( Figure 19) [101].  (Figure 7) with a MIC equal to 8 µM [40].
The most important problems in Sub-Saharan Africa include malaria caused by the Plasmodium parasite. Quinine isolated from cinchona trees was the first anti-malarial drug and is the scaffold for obtaining more effective substances [99,100]. Lanfranchi et al. synthetized a series of derivatives that were tested on two types of P. falciparum ( Figure 19) [101]. Compounds 115 and 117 ( Figure 19) containing a phenyl substituent at the C-6 position did not exhibit any activity against chloroquine-sensitive 3D7 and multi-resistance Dd2 strains of P. falciparum. Derivative 114 showed a higher activity against the 3D7 strain with an IC50 equal to 727 nM. Compound 116 exhibited a comparable activity against both strains of P. falciparum [101].  Compounds 115 and 117 ( Figure 19) containing a phenyl substituent at the C-6 position did not exhibit any activity against chloroquine-sensitive 3D7 and multi-resistance Dd2 strains of P. falciparum. Derivative 114 showed a higher activity against the 3D7 strain with an IC 50 equal to 727 nM. Compound 116 exhibited a comparable activity against both strains of P. falciparum [101].
Compounds 115 and 117 ( Figure 19) containing a phenyl substituent at the C-6 position did not exhibit any activity against chloroquine-sensitive 3D7 and multi-resistance Dd2 strains of P. falciparum. Derivative 114 showed a higher activity against the 3D7 strain with an IC50 equal to 727 nM. Compound 116 exhibited a comparable activity against both strains of P. falciparum [101].

Anti-HIV Activity
Human immunodeficiency virus (HIV), which causes acquired immune deficiency syndrome (AIDS), is one of the most serious health problems in the worldwide. It is estimated that in 2018, 37.9 million people lived with the HIV virus and 5000 people are infected by this virus every day [103]. Streptonigrin 1 and its derivatives inhibit HIV reverse transcriptase (RT) and the postulated mechanism of action assumes an interaction of the quinone moiety with the active site of this enzyme [104]. Derivatives 124-125 at a concentration 10 µg/mL caused over 80% inhibition of HIV reverse transcriptase activity ( Figure 21). The activity of 7-methoxy-5,8-quinolinedione 124 was slightly In the series of tested compounds, only 118 showed antiplasmodial activity (IC 50 equal to 3 µM). Comparing the activities of 5,8-quinoinediones 118 and 122, it was found that the bromine atom at the C-7 position was necessary to maintain the antimalarial activity [102].

Anti-HIV Activity
Human immunodeficiency virus (HIV), which causes acquired immune deficiency syndrome (AIDS), is one of the most serious health problems in the worldwide. It is estimated that in 2018, 37.9 million people lived with the HIV virus and 5000 people are infected by this virus every day [103]. Streptonigrin 1 and its derivatives inhibit HIV reverse transcriptase (RT) and the postulated mechanism of action assumes an interaction of the quinone moiety with the active site of this enzyme [104]. Derivatives 124-125 at a concentration 10 µg/mL caused over 80% inhibition of HIV reverse transcriptase activity ( Figure 21). The activity of 7-methoxy-5,8-quinolinedione 124 was slightly higher. At the same concentration, derivatives 124-125 did not affect cellular DNA polymerases, which could suggest a non-toxic effect of these compounds [104].  The 5,8-quinolinedione compounds were also tested as anti-inflammatory agents [26], inhibitors of enzymes such as nitric synthase, cyclooxygenase-2 [106], acetylcholine induced vasorelaxation of rat aorta with the endothelium [107,108], tyrosine phosphatase [109], and picornavirus 3C protease [110].

Conclusions
The 5,8-quinolinedione scaffold occurs in many natural and synthetic compounds. In recent years, interest in its potential therapeutic properties against many diseases has increased. A comparison of activities of the 6-and 7-substituted compounds showed that it depended on the type of substituent and the substitution site in the 5,8-quinolinedione moiety. According to this review, in most cases, the 6-substituted derivatives had higher activities, but is not a rule.
Despite the large number of natural and synthetic active derivatives, only a few of them have qualified for clinical trials [7][8][9][10]. The most important problem in this class of compounds is due to their toxicity. For this reason, further research in all areas of the presented biological activity of 5,8quinolinedione compounds is still needed to design novel, more effective, and less toxic structures.
The discovery of novel modifications of the 5,8-quinolinedione moiety at C-6 and C-7 positions may lead to interesting medical applications in the future.  Unfortunately, the compounds were characterized by a high toxicity against the human T-cell line [105].

Conclusions
The 5,8-quinolinedione scaffold occurs in many natural and synthetic compounds. In recent years, interest in its potential therapeutic properties against many diseases has increased. A comparison of activities of the 6-and 7-substituted compounds showed that it depended on the type of substituent and the substitution site in the 5,8-quinolinedione moiety. According to this review, in most cases, the 6-substituted derivatives had higher activities, but is not a rule.
Despite the large number of natural and synthetic active derivatives, only a few of them have qualified for clinical trials [7][8][9][10]. The most important problem in this class of compounds is due to their toxicity. For this reason, further research in all areas of the presented biological activity of 5,8-quinolinedione compounds is still needed to design novel, more effective, and less toxic structures.
The discovery of novel modifications of the 5,8-quinolinedione moiety at C-6 and C-7 positions may lead to interesting medical applications in the future.