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Review

Therapeutic Potential of Tricyclic Pyridazinone-Based Molecules: An Overview

by
Battistina Asproni
*,
Gérard A. Pinna
,
Paola Corona
,
Silvia Coinu
,
Sandra Piras
,
Antonio Carta
and
Gabriele Murineddu
*
Department of Medicine, Surgery and Pharmacy, University of Sassari, Via Muroni 23/A, 07100 Sassari, Italy
*
Authors to whom correspondence should be addressed.
Int. J. Mol. Sci. 2025, 26(8), 3806; https://doi.org/10.3390/ijms26083806
Submission received: 6 March 2025 / Revised: 9 April 2025 / Accepted: 15 April 2025 / Published: 17 April 2025
(This article belongs to the Special Issue Heterocyclic Compounds: Synthesis, Design, and Biological Activity)
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Versions Notes

Abstract

:
Pyridazin-3(2H)one-based molecules have always attracted the attention of medicinal chemists due to their different pharmacological properties. The incorporation of such nuclei in therapeutically active molecules either as monocyclic units or as fused bi- or tricyclic scaffolds results in a wide range of pharmacological effects such as anti-inflammatory, analgesic, anticancer, antimicrobial, antiviral, cardiovascular-protective, antiulcer, and many other useful pharmacological activities. In accordance with our consolidated experience gained over the years in the chemistry and biology of tricyclic pyridazin-3(2H)ones, this review summarizes SAR studies of such pyridazinone-based polycyclic compounds endowed with various biological and therapeutic properties.

1. Introduction

Heterocyclic compounds, particularly those containing one or more nitrogen atoms, have long attracted interest due to their crucial role in drug discovery. These molecules can function as pharmacologically active entities alone, with various substituents, or they can be fused with other (hetero)cyclic systems to form linear or, more often, angular polycyclic structures. This duality has made them widely studied and applicable in a wide range of pharmacological effects [1,2]. Pyridazine (1,2-diazine) is a heterocyclic nucleus derived from the replacement of two carbon atoms in the benzene ring with the same number of nitrogen atoms. Among its oxygen-substituted analogs, pyridazin-3(2H)one A is a more stable oxo-form of 3-hydroxyl pyridazine B (Figure 1).
The pyridazine-3(2H)one nucleus is known as a privileged scaffold and also as the wonder nucleus because of its numerous applications in different areas, especially as a pharmacophoric element in therapeutically active drugs. Structurally, the presence of nitrogen atoms and the keto functionality can provide protonation reactions, as well as hydrogen bond formation, which can account for the variety of pharmacological properties of small molecules containing a pyridazinone ring [3,4].
A survey of the literature revealed several hundred articles encompassing appropriate substituted pyridazine-3(2H)one-based molecules or structures in which this nucleus appears in fused bicyclic scaffolds. Furthermore, the analysis of these publications highlighted more sophisticated structures incorporating this nucleus in tricyclic or even tetracyclic molecular architectures endowed with a broad range of biological activities. Pyridazine-3(2H)one-based molecules, both saturated and unsaturated, have been reported to have diverse pharmacological activities such as anti-inflammatory, analgesic, anticancer, antimicrobial, antiviral, cardiovascular-protective, antiulcer, and many other useful pharmacological activities [3,4,5,6].
Several synthetic strategies were described in the literature for the synthesis of pyridazine-3(2H)-one-based molecules [7]. A simple method for the synthesis of monocyclic 6-substituted-phenyl-4,5-dihydropyridazin-3(2H)-ones (Scheme 1, a, D) involved the addition of hydrazine hydrate to the appropriate γ-ketoacid (C) [8]. The addition of hydrazine or even substituted hydrazine to appropriate bicyclic γ-ketoacid E (b) provided several tricyclic benzocycloalkylpyridazine-3(2H)ones F in high yields, which were oxidized efficiently with bromine in acetic acid to the corresponding pyridazinones G [9,10].
Our interest in tricyclic pyridazine-3(2H)one derivatives dates back several decades, when a series of benzocycloalkylpyridazinone-based tricyclic scaffolds of general formula I–III (Figure 2) were designed as rigid analogs of 5-methyl-6-para-cyanophenyl-4,5-dihydro-3(2H)-pyridazinone 1, endowed with antihypertensive activity together with inotropic, vasodilator, and antiaggregating properties. It was observed that such pharmacological effects are at least partly mediated by the inhibition of phosphodiesterase III (PDEIII) [11].
It has been suggested that minimization of the flexibility of the lead compound 1 through the introduction of structural constraints could have some effect on the biological activity and is likely to allow the ligand to bind to its biological target with a high degree of affinity and selectivity. Modifications carried out on such scaffolds I–III, i.e., dehydrogenation, isomerization, and bioisosterism and their fine tuning (i.e., by varying R and R1 substituents), allowed the identification of hundreds of molecules, most of them endowed with different pharmacological properties (vide infra). Herein, we briefly present the bioactivities and SAR studies of pyridazinone-based tricyclic or even tetracyclic compounds, most of which are related to I–III scaffolds, that have been investigated by us and other groups. In addition, this review encompasses the biological literature of some other most relevant tricyclic-based molecules that are quite different from I–III, featuring the pyridazine-3(2H)one core in their structure.

2. Anti-Inflammatory/Analgesic/Antipyretic Activities

Inflammation is part of the biological response of the body’s tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Chronic or uncontrolled inflammation plays an important role in the initiation and development of diseases such as rheumatoid arthritis, heart disease, cancer, multiple sclerosis, diabetes, gout, psoriasis, aging, and neurodegenerative diseases [12,13]. Non-steroidal anti-inflammatory drugs, opioids, and corticosteroids are widely used in clinical practice to treat inflammatory and painful conditions, although they are associated with several side effects [14]. A survey of the recent literature revealed pyridazine/pyridazine-3(2H)one as very appealing scaffolds for anti-inflammatory and analgesic drug discovery [3,4,15].
The discovery of tricyclic pyridazine-3(2H)one derivatives as anti-inflammatory agents started in our laboratories as a part of a structure–activity relationship (SAR) study carried out by Cignarella et al., allowing the serendipitous identification of 7-cyano-4,4a-dihydro-5H-indeno[1,2-c]-3(2H)-pyridazinone 2 (Figure 2 and Figure 3) endowed with anti-inflammatory properties. This derivative lacked any detectable antihypertensive activity compared to the lead compound 1, while being active as an anti-inflammatory agent, along with 3, in a carrageenin-induced rat paw edema model, with activity comparable to that of acetylsalicylic acid (ASA, Table 1) [16]. Dehydrogenation and full aromatization of 4,4a-dihydro-5H-indeno[1,2-c]-3(2H)-pyridazinone 3 gave rise to 5H-indeno[1,2-c]-3(2H)-pyridazinone 4 and 5H-indeno[1,2-c]pyridazine 5, respectively, both endowed with anti-inflammatory activity. Oxidation of the methylene group at the C-5 position of compound 5 to ketone 6, as well as isomerization of compounds 4 and 5 to give 9H-indeno[2,1-c]-3(2H)-pyridazinone 7 and 9H-indeno[2,1-c]pyridazine 8, respectively, kept the anti-inflammatory activity comparable to ASA. These compounds, and their related analogs, exhibited analgesic and antipyretic properties to various extents [17,18].

3. Cardiovascular Activity

It is well established that cardiovascular disease, in particular coronary heart disease and stroke, as well as peripheral arterial disease and aortic disease, is the leading cause of mortality in modern societies [19,20]. Hypertension is a chronic medical condition in which the arterial blood pressure is persistently elevated and contributes to increased risk of heart attack, stroke, heart failure, and other complications, along with raised blood glucose, raised blood lipids, and obesity. Several pyridazinone-based compounds exhibit antihypertensive activity [21,22]. The molecular mechanisms mediating this effect may ultimately involve a decrease in intracellular calcium concentrations.
As part of our ongoing efforts to generate SARs around 7-cyano-4,4a-dihydro-5H-indeno[1,2-c]-3(2H)-pyridazinone 2 (Figure 3), it was found that when the cyano group was replaced by amino or acetylamino groups, the resulting compounds 9 and 10 (Figure 4) showed a potent blood-pressure-lowering effect (reduction of 20–50 mmHg) on spontaneously hypertensive rats (SHRs), unlike the reference derivative 2 which displayed only anti-inflammatory activity [23]. A SAR study on the antihypertensive activity of 4,4a-dihydro-5H-indeno[1,2-c]-3(2H)-pyridazinone-based molecules identified derivatives 18–20 (Figure 4), which had antihypertensive activity comparable to 9 and 10, even if with a short duration of action [24]. Furthermore, compounds 9 and 10, like most analogs 11–20, were also active as antithrombotic agents, evaluated in a mouse model as percent protection against death or paralysis of the hind limbs induced by a thrombotic mixture (collagen and adrenalin), with an activity comparable to or higher than that of ASA. In addition, compounds 9 and 10 exhibited anti-inflammatory activity and platelet aggregation-inhibiting activity, evaluated using guinea pig platelet-rich plasma and inducing aggregation by either ADP, collagen, or thrombin, with derivative 10 also showing potent antiulcer properties in a rat indomethacin-induced ulcer model (vide infra) [23].
Within this framework, several substituted benzo[h]cinnolinones and benzo[6,7]cyclohepta[1,2-c]-3(2H)-pyridazinones of general formula II and III (Figure 2) were designed as higher homologues of I in order to check whether the planarity of the structure I is in line with the maximum activity observed in derivatives previously described. These homologues were tested for antihypertensive, inotropic, antithrombotic, anti-inflammatory, and antiulcer activity. While the seven-membered ring compounds 24 and 25 (Figure 5) exhibited only antithrombotic properties comparable to ASA, most of the benzo[h]cinnolinones 21–23 displayed significant antihypertensive, inotropic, and antithrombotic properties. Furthermore, the D-isomer of 22 was more active than the racemic form in lowering blood pressure, with less tachycardic effects. Moreover, unlike the 5H-indeno[1,2-c]-3(2H)-pyridazinone series (Figure 4), these derivatives were devoid of anti-inflammatory or antiulcer activity [25].
Several 4,4a,5,6-tetrahydro-4a-substituted-benzo[h]cinnolinones were synthesized and evaluated for antihypertensive and antithrombotic activity compared to the 4a-unsubstituted congeners. Most of the tested compounds displayed significant antihypertensive activity. In particular, compound 26 (Figure 6) was more active than the 4a-unsubstituted analog, whereas compounds 27–29 showed antithrombotic activity comparable to or greater than that of ASA [26].
Warrington et al. reported the cardiotonic properties (inotropic and vasodilator activity) of a series of 4,4a-dihydro-5H-indeno[1,2-c]-3(2H)-pyridazinone-based molecules, including derivative 10 (Figure 4), and of benzo[h]cinnolinones, including compound 22 (Figure 5). The authors correlated the observed activity mainly with the ability of such compounds to inhibit PDEIII [11].
Some phenyl-4,5-dihydro-3(2H)-pyridazinones related to compound 1 (Figure 2), tricyclic indenopyridazinones 3, 9, and 10, as well as benzocinnolinones 21 and 22 and benzocycloheptapyridazinones 24 and 25 were submitted for conformational analysis to better define the relationship between their cardiovascular properties and their preferred conformation. Analogs of compound 1, indenopyridazinone and benzocinnolinone derivatives, which give rise to highly active compounds, were found to exist mainly in a conformation with a near-planar arrangement of the phenyl and pyridazinone core. Compounds belonging to the benzocycloheptapyridazinone series, whose derivatives were generally inactive, showed conformations that deviated from planarity [27].
Taking into account the potent positive inotropic activity of a series of 4,5-dihydro-6-[4-(1H-imidazol-1-yl)phenyl]-3(2H)-pyridazinones, i.e., compound 30 (Figure 7), Sircal et al. synthesized and evaluated the positive inotropic activity of a series of 5H-indeno[1,2-c]-3(2H)-pyridazinones (including 9, 10, Figure 4) and the benzo[h]cinnolinone 22 (Figure 5) [28]. Most of these tricyclic pyridazinones (i.e., 31 and 32) demonstrated strong positive inotropic activity in the same range as that of their freely rotating acyclic analogs. Interestingly, in their screening, compound 10 retained an inotropic activity comparable to or slightly higher than that of the imidazole 31. By SAR studies, the authors concluded that a generally planar ring structure is desirable for maximum positive inotropic effects. Several of these tricyclic pyridazinones were evaluated for PDEIII-inhibitory activity. Compounds 31 and 32 possessed similar potency (31: IC50 1.8 μM, 32: IC50 1.6 μM) to 30 (IC50 1 μM), which also explained the similar inotropic activity of these compounds.
Our group, by an isomerization approach, designed and synthesized two dihydrobenzo[f]cinnolin-2(3H)ones 33 and 34 (Figure 8). In comparison with isomeric derivatives 21 and 22 (Figure 5), these compounds were tested for their antihypertensive and antiaggregating activities. In vivo tests showed that such compounds revealed antihypertensive properties weaker than the model compounds. On the contrary, in inhibiting collagen-induced platelet aggregation, both compounds were more potent than 21 and 22 [29].
Bioisosterism is a useful strategy for the rational design of new drugs [30,31]. Within this framework, our group envisioned that bioisosteric modification of 4,4a,5,6-tetrahydrobenzo[h]cinnolinone core II (Figure 2) might offer new templates that could provide better pharmacological properties. Following this approach, a series of 4,4a-dihydro-5H-[1]benzopyrano[4,3-c]pyridazin-3(2H)ones were designed, synthesized, and tested for their cardiovascular-related properties. The most interesting compounds of this series are depicted in Figure 9. All derivatives were ineffective in lowering the blood pressure in spontaneously hypertensive rats, with the exception of 37, showing a short-lasting action. Derivatives 36 and 37 were found to be very active as antithrombotics in mice, more so than ASA [32]. Interestingly, many derivatives of this class showed antiulcer activity (vide infra).
Following the above-mentioned bioisosteric approach, and taking the significant antihypertensive and antithrombotic properties of 4,4a,5,6-tetrahydrobenzo[h]cinnolinone derivative 22 (Figure 5) as a reference, the thieno[2,3-h]cinnolin-3(2H)-ones, 40 and 41 (Figure 10), were designed, synthesized, and pharmacologically evaluated. Compound 41 and the unsubstituted term 40, with respect to the bioisoster 22, retained antithrombotic properties which were higher than (41) or comparable to those of ASA but without significant antihypertensive activity. Conformational analysis suggested that the absence of activity may result from a different orientation of the acetylamino group in compounds 22 and 41. Further, thieno[2,3-h]- and thieno[3,2-h]cinnolin-3(2H)-ones, 42, 43 and 44, 45, respectively (Figure 10), were designed, synthesized, and evaluated for their pharmacological profile. In vivo tests revealed that only 45 exhibited antihypertensive properties comparable to compound 22, while all derivatives had a lower hypotensive activity. All compounds, except 42, were more potent than 22 in the inhibition of collagen-induced platelet aggregation [33]. The 4a-methyl-substituted-thieno[2,3-h]cinnolin-3(2H)-ones 46–48 were also synthesized. Their pharmacological profile was evaluated in comparison to 22. In vivo tests showed that 46 displayed lower levels of antihypertensive activity with respect to 22. Tricyclic thienocinnolinones 46–48 demonstrated potent platelet-antiaggregating activity [34].
Selective inhibition of thromboxane (TXA2) synthetase may be an effective approach for antithrombotic drugs that act on platelet aggregation. TXA2, rapidly hydrolyzed to TXB2 under physiological conditions, is a potent vasoconstrictor and platelet-aggregating agent. Monge et al. synthesized a series of tricyclic pyridazinone/pyridazine indole-based compounds (49–52, Figure 11) and tested them for TXA2 synthetase inhibitor activity. While compounds 51 and 52 had no significant activity, derivatives 49 and 50 were inhibitors of platelet aggregation induced by ADP in vitro, and both compounds exhibited potent TXA2 synthetase inhibition. Compound 50 also showed antihypertensive activity similar to that of hydralazine, but its hydrochloride salt had an acute toxicity in mice about 2.2 times lower than that of hydralazine [35].
The cardiovascular-related properties of most tricyclic pyridazinones described above are summarized in Table 2.

4. Antiulcer Activity

Peptic ulcer disease includes gastric or duodenal ulceration, which is a break in the epithelium of the gastric or duodenal mucosa. The most common causes of peptic ulcer disease are the use of non-steroidal anti-inflammatory drugs and Helicobacter pylori infection. Management of peptic ulcer disease makes use of antiacids, gastroprotective complexes and chelators, H2-receptor antagonists, synthetic prostaglandins, and H+/K+-ATPase proton pump inhibitors [14]. Despite the wide pharmacological activities of 3-(2H)-pyridazinone-based molecules, only a few compounds with gastric antisecretory and/or antiulcer properties have been described. In this context, a series of monocyclic 3(2H)-pyridazinone derivatives with a thiourea or cyanoguanidine moiety (53 and 54, Figure 12) showed marked gastric antisecretory activity in rats without histamine H2-receptor antagonistic action [36].
During the synthesis and pharmacological evaluation of 4,4a-dihydro-5H-[1]benzopyrano[4,3-c]pyridazin-3(2H)ones 35–39 (Figure 9) and bioisosteres of antihypertensive and antithrombotic benzo[h]cinnolinones (21 and 22, Figure 5), our group identified compound 38 as a potent antiulcer agent, able to protect rats from the formation of ulcers induced by ASA or phenylbutazone (PBZ) (ED50 = 12.2 mg/Kg and 25.4 mg/Kg po in ASA and PBZ models, respectively). In vitro experiments carried out on compound 38 indicated that it has no anticholinergic activity at a concentration of 1 × 10−5 M and that it has no histamine H2-receptor antagonistic activity at the same concentration [32]. Further SAR studies carried out on 5H-[1]benzopyrano[4,3-c]pyridazin-3(2H)one and benzo[h]cinnolinone scaffolds identified further tricyclic 3-(2H)-pyridazinone derivatives whose structures are depicted in Figure 12. All compounds 55–59 showed statistically significant antiulcer properties in the ethanol model, with 55 being the most active compound. Conversely, only 55 and 59 were found to be weakly active in the ASA model. In a pylorus-ligated rat model, 55 displayed significant antisecretory activity, being able to reduce the acidity of gastric secretions [37].
Taking into account the previously described antiulcer properties of tricyclic indenopyridazinone 10 (Figure 4), benzopyrano[4,3-c]pyridazin-3(2H)ones, and benzo[h]cinnolinones 55–59, Barlocco et al., by a scaffold hopping approach, synthesized a series of indenopyridazinones (Figure 13) for antisecretory and antiulcer activity, taking ranitidine as a reference drug [38]. The most interesting compound of this class was the 7,8-dimethoxy-substituted derivative 60, which, at an oral dose of 30 mg/Kg, still retained significant antisecretory activity (pylorus-ligated rat model). The dihydro-derivatives of 60 and 61, compounds 62 and 63, respectively, were more active than (63) or comparable (62) to the parent compounds. These derivatives were able to prevent hemorrhagic lesions induced by 90% ethanol in rats in a dose-dependent manner. In the indometacine model, they still showed significant activity, albeit not quite as much as in the previous test. An in-depth pharmacological study carried out on compound 60 highlighted that the molecule, contrary to ranitidine, exerted antiulcer activity mainly by improving the integrity of the gastric mucosa while at the same time inhibiting gastric acid hypersecretion induced exclusively by cholinergic pulses. The authors concluded that the mechanism of action for this derivative may involve a neuronal rather than an effectorial mechanism [39].
The antiulcer properties of tricyclic pyridazinones described above are summarized in Table 3.

5. Anticancer Activity

According to the WHO, cancer is the second leading cause of death in the world, and was estimated to be responsible for 9.6 million deaths, or 1 in 6 deaths, in 2018. Lung, prostate, colorectal, stomach, and liver cancers are the most common cancers in men, while breast, colorectal, lung, cervical, and thyroid cancers are the most common in women. Cancer is a broad group of diseases characterized by the high proliferation and spread of aberrant cells from their site of origin. Treatment options include surgery, cancer medicines, and/or radiotherapy, which may be given alone or in combination [40]. There are three main periods or “waves” in the history of anticancer drugs, which emerged in succession. Each period is characterized by distinct antitumor activity and toxicity [41]. The first period (classical chemotherapeutics) comprised drugs that primarily interfere with the integrity and/or replication of DNA, such as DNA cross-linking agents [42], mitosis inhibitors or topoisomerase poisons [43,44], and DNA intercalators [45]. In the second period, drugs targeted signaling intermediates that contribute to cancer growth, particularly tyrosine kinase and related inhibitors [46]. The third period encompassed drugs that target a wide range of cellular machineries that are not directly involved in DNA replication or cell division, but are essential for the growth and survival of tumors (proteasome, PARP, protein chaperones, DNA methyltransferase) such as proteasome inhibitors [47], PARP inhibitors [48], chromatin modifiers [49,50], or protein chaperone inhibitors [51]. In this context, several research groups have developed tricyclic pyridazinone-based compounds as anti-proliferative agents, most of which with promising anticancer potential.
DNA intercalators are a class of drugs featuring a planar poly-aromatic system that can be inserted between the base pairs of DNA through hydrophobic interactions, van der Waals attractions, hydrogen bonding, and/or charge transfer processes. Such interactions hamper DNA replication. A further structural feature of DNA intercalators is the presence of at least one flexible cationic side chain linked to the heteroaromatic ring, which interacts with the negatively charged phosphate DNA backbone, playing a critical role in the affinity and selectivity of such compounds. On this basis, Cignarella et al. undertook a research program designed to verify whether the tricyclic indenopyridazinone of general formula IV and its dihydro analog V (Figure 14) could be appropriate scaffolds for the development of DNA-interacting anticancer agents. SAR studies were carried out on 14 compounds by testing their cytotoxic activity on LoVo and LoVo/DX human colon carcinoma cell lines and on L1210 and L1210/CDDP murine leukemia cell lines, and indicated that the double bond in position 4,4a is essential for activity. Compounds 64 and 65 exhibited comparable activity to cisplatin and melphalan against resistant cell lines and against both sensitive and resistant cell lines, respectively. DNA-binding studies on most of the active compounds have suggested that DNA is an important biological target for the cytotoxic activity of the reported indenopyridazinones. The authors concluded that further investigation is needed to understand the mechanisms (i.e., topoisomerase poisoning and free radical formation) through which 64 and 65 exert their cytotoxic effects [52].
Our interest in the chemistry and pharmacology of polycyclic pyridazinones continued with the design and synthesis of a series of tetracyclic pyrrole[2,3-d]pyridazine-4-one derivatives (66–69, Figure 15) with an almost planar structure, potentially intercalating with DNA. These were evaluated in vitro by the National Cancer Institute against 60 human tumor cell lines derived from nine cancer cell types [53]. Moreover, to compare the variation in biological effects due to the loss of planarity of the ring system, 1-methyl-2-phenylpyrrole[2,3-d]pyridazine-4-one 70 and 1,3-dimethyl-2-phenylpyrrole[2,3-d]pyridazine-4-one 71, derived from 66 by the removal or cleavage of the carbon bridge between the phenyl and pyrrole moieties, were synthesized and included in the screening. The biological results showed that, with potency increasing in the order of 71 < 67 ≤ 68 < 69 < 66, the antitumor activities of these compounds were related to the planarity of their ring systems. Among these, the most potent compound 66 showed significant cell line cytotoxicity, particularly against the renal cancer subpanel (growth inhibition: GI50 5.07 μM) and significant potency against leukemia (MOLT-4, SR), non-small-cell lung (NCI-H460), colon (HCT-116), and CNS (SF-295) cancer cells (GI50 3.04–4.32 μM).
In a mini-review, Roman encompasses the biological activity of Mannich bases generated from various substrates through the introduction of an aminomethyl function by means of the Mannich reaction. Among various biological properties, Roman extensively reviewed the anticancer and cytotoxic properties of Mannich bases belonging to different chemotypes [54]. A few years earlier, our research group reported a series of twelve tricyclic hexahydrothienocycloheptapyridazinone Mannich bases of general formula VI (Figure 16) endowed with anticancer activity. A sulforhodamine B assay (SRB assay) was used for in vitro screening to evaluate growth inhibition properties, at a single 10 μM dose, against sixty different human tumor cell lines. Most of the compounds exhibited no activity on the cell lines tested, but two derivatives, namely 72 and 73, mildly inhibited the growth of non-small-cell lung cancer (EKVX and HOP-92) and glioblastoma (SNB-75) cell lines, respectively [55].
Signal transducer and activator of transcription 3 (STAT3) is one of seven members of the STAT family that regulates a variety of biological processes, including proliferation, metastasis, angiogenesis, immune response, and chemoresistance. STAT3 is widely regarded as an attractive target for the development of inhibitors for the treatment of STAT3-related diseases, such as cancers, autoimmune, and inflammatory diseases [56]. Villa et al. reported a series of twenty tricyclic pyridazinones of general formula VII (Figure 17) as potential STAT3 inhibitors. All compounds were evaluated in a dual-luciferase assay in human colorectal carcinoma cells, HCT-116, which are characterized by uncontrolled expression of STAT3. After 24 h, the percentages of inhibition were determined at a concentration of 2 μM of the compounds in comparison with cryptotanshinone, a natural compound endowed with apparent similarity to the investigated tricyclic pyridazinones, and endowed with STAT3 inhibition. SAR studies identified the benzocinnolinone derivative 74 as a promising STAT3 inhibitor with 46% inhibitory activity, versus cryptotanshinone endowed with 25% inhibition. Furthermore, enantiomeric separation of compound 74 furnished (S)-(−)-74, which was twice as potent as (R)-(+)-74 [57].
Pyruvate kinase muscle 2 (PKM2), a subtype of pyruvate kinase (PK), regulates the final step of the glycolytic pathway: the transformation of phosphoenolpyruvate and ADP to pyruvate and ATP. It plays a key role in regulating cell metabolism. Many reports highlight the importance of this kinase in cancer progression and different inhibitors, and agonists of PKM2 have been described for future cancer treatment [58]. Activators of PKM2 based on a substituted thieno[3,2-b]pyrrole[3,2-d]pyridazinone scaffold were described by Thomas et al., from the National Institutes of Health Chemical Genomics Center (general formula VIII, Figure 18). These compounds were discovered through a high-throughput quantitative screen of nearly 300.000 small molecules using the firefly luciferase assay system [59]. Compound 75, with a maximum response at 57 μM of 122% relative to FBP (fructose-1,6-bis-phosphate), showed good and selective potency to activate PKM2 (AC50 = 63 nM). Its optimization led to improved compounds in terms of activity and solubility, such as the methylsulfoxide-substituted derivative 76, which shows activation properties (AC50 = 73 nM, 99% maximal response) similar to those of the parent pyridazinone 75 but with improved aqueous solubility (37.4 μg mL−1).
Reduced or deficient red-blood-cell-specific forms of PK (PKR) appear to play a crucial role in hemolytic anemias such as sickle cell disease (SCD) or in bone marrow disorders as myelodysplastic syndromes (MDSs). Recently, assuming derivatives 76 and 77 (Figure 18) as reference compounds, Dang et al. described the structure-based drug design of compound 78 (AG-946) as an activator of PK isoforms, including PKR. This strategy allowed the optimization of AG-946 for improved selectivity, especially eliminating the undesirable off-target activity of the PDEIII isoform observed in the original scaffold. The authors summarize the following favorable properties of AG-946: (a) potent activation of human wild-type PK (AC50 = 5 nM) and a panel of mutated PK proteins (K410E: AC50 = 4 nM) and R510Q (AC50 = 7nM); (b) significantly longer half-time of activation (>150-fold) compared with the lead compound 76; and (c) stabilization of PKR R510Q, an unstable mutant PKR enzyme, and maintenance of its catalytic activity under increasingly denaturing conditions. Currently, AG-946 is being evaluated as a potent, oral, small-molecule allosteric activator of wild-type and mutant PKR in clinical trials for low-risk MDS and SCD [60].
Bruel et al. reported the SAR study of a series of pyridazino[4,5-b]indol-4-ones of general formula IX (Figure 19), which were evaluated in selected kinase assays, notably the dual-specificity kinase 1A DYRK1A, the serine/threonine kinase CDK5, and the phosphoinositide 3-kinases PI3Ks [61], described as therapeutic targets in neurodegenerative diseases or cancer [62,63,64]. The authors proposed that, through competitive inhibition at the ATP-binding site, the pyridazino[4,5-b]indol-4-one system may be a suitable structure for providing effective interaction with the enzyme. The N-5-benzylated derivatives showed promising inhibitory activities against PI3Kα as well as significant anti-proliferative effects in a variety of cell types. Most IC50 values were in the μM range. Derivative 79 emerged for its PI3Kα submicromolar affinity (IC50 = 0.091 μM). Interestingly, further SAR studies carried out on the pyridazino[4,5-b]indol-4-one scaffold, in comparison with the ring-opened analogs of general formula X, provided compound 80, which exhibited submicromolar IC50 values against DYRK1A (IC50 0.22 μM) with no activities against the other kinases CDK5/p25, GSK3α/β, and the p110-α isoform of PI3K. Furthermore, derivative 80 displayed anti-proliferative activities in the Huh-7, Caco2, and MDA-MB-231 cell lines [65].
A few years later, Panathur et al. reported a series of thirty compounds of general formula XI (Figure 20) featuring the pyridazino[4,5-b]indol-4-one scaffold, whose indole nitrogen bore the 1,2,3-triazolylmethyl unit, a motif of several small molecules exhibiting several biological actions, including anticancer activity [66]. Such hybrid derivatives were evaluated in vitro against four cancer cell lines (breast cancer MDA-MB-231 and MCF7, human primary glioblastoma U-87, and human neuroblastoma IMR-32), and most of the compounds showed potent inhibition of cancer cell growth at very low μM concentrations. Compounds 81 and 82, with IC50 values of 0.07 and 0.04 μM, respectively, emerged for their potent activity against the human neuroblastoma cell line IMR-32. Molecular docking studies of all compounds within the ATP-binding site of PI3 kinase (PI3K) suggested that these derivatives could be potential lead inhibitors of the PI3K pathway, which is overexpressed in human tumors.
Recently, Sarhan et al. described further pyridazino[4,5-b]indol-4-ones as new PI3K inhibitors for breast cancer therapy [67]. The hydrazides 83 and 84 (Figure 21) had promising, potent, and significant cytotoxic activity against the MCF-7 breast cancer cell line, with IC50 values of 4.25 and 5.35 μM, respectively, compared to the standard drug 5-FU (IC50 6.98 μM).
In the last decade, poly(ADP-ribose) polymerases (PARPs) have emerged as a new target in cancer treatment [68]. PARPs are known to catalyze the process of PARylation, transferring ADP-ribose residues from nicotinamide adenine dinucleotide (NAD) to target substrates. PARP-1, a subtype of a family of at least seventeen members, is involved in the maintenance of genomic stability, DNA repair, and regulating DNA transcriptional processes. Several PARP-targeted agents, including niraparib, talazoparib, pamiparib, and fluzoparib, are currently approved for the treatment of malignant tumors, such as pancreatic cancer, breast cancer, and prostate cancer [69]. From a structural point of view, several PARP-1 inhibitors incorporate a bicyclic pyridazinone core or even a tricyclic core, such as talazoparib 85 (Figure 22), which was developed by Pfizer and approved to treat adults with deleterious or suspected germline BRCA-mutated, HER2-negative, locally advanced, or metastatic breast cancer [14,70]. Guilford/MGI/Eisai Pharmaceuticals discovered a series of tetracyclic PARP-1 inhibitors of general formula XII (Figure 22), featuring the pyridazinone core assembled with a xanthene unit. Currently, compound 86, namely E-7016 (PARP-1 IC50 = 0.04 μM), is under evaluation in combination with temozolomide in patients with advanced solid tumors or with wild-type BRAF stage IV or unresectable stage III melanoma [71].
Considering the pharmacological relevance of pyridazinones along with quinazolines as PARP-1 inhibitors, Li et al. designed and synthesized a series of eight hybrid pyridazino[3,4,5-de]quinazolin-3(2H)-ones. All derivatives were tested as PARP-1 inhibitors, and analogs 87 and 88 (Figure 23), endowed with IC50 = 0.148 μM and 0.152 μM, respectively, were reported as the most active compounds of the series with neuroprotective effects in a PC12 cell model injured by H2O2 [72].
Dihydrofolate reductase (DHFR) is a ubiquitous enzyme that catalyzes the reduction of folic acid to tetrahydro-folic acid (THF) using NADPH as a cofactor. DHFR pairs with thymidylate synthase (TS), which catalyzes the reductive methylation of deoxyuridine monophosphate (dUMP) in deoxythymidine monophosphate (dTMP) using N5-N10-methylenetetrahydrofolate (5,10-methylene THF) as a cofactor. Inhibition of DHFR, which is responsible for the generation of raw materials for DNA replication, is the basis for the treatment of several infectious diseases and is the target of antibacterial drugs like trimethoprim as well as the anticancer drug methotrexate [73].
Using lead compound 89 (Figure 24), which carries carboxyl ester function along with a thioureido function and has an IC50 inhibition affinity of 0.05 μM, as a reference, Ewida et al. designed and synthesized a series of 1,3-thiazole-containing heterocycles acting at the DHFR enzyme active site. Modification carried out on compound 89, notably the replacement of the thiazole scaffold with imidazo[2,1-b]thiazole and the annexation of carboxyl ester function into a pyridazinone unit, provided the tricyclic imidazo[2′,1′:2,3]thiazolo[4,5-d]pyridazinone 90 with an IC50 value of 0.06 μM [74]. Interestingly, SAR studies have shown that ring annexation of the active 1,3-thiazole ring of 89 in the bicyclic thiazolo[4,5-d]pyridazinone (i.e., 91, IC50 = 0.35 μM) or imidazo[2,1-b]thiazoles (i.e., 92, IC50 = 49.67 μM) reduced DHFR inhibition activity, whereas the formation of the tricyclic imidazo[2′,1′:2,3]thiazolo[4,5-d]pyridazinone increased the potency, with 90 being the most representative of these novel DHFR inhibitors. Compound 90 proved to be lethal for OVCAR-3 ovarian cancer and MDA-MB-435 melanoma cells at IC50 = 0.32 and 0.46 μM, respectively.
The anticancer properties of the representative tricyclic pyridazinones described above are summarized in Table 4.

6. Antidiabetic Activity

Diabetes is a group of metabolic disorders affecting more than 400 million people worldwide, becoming a global public health concern [75]. The hyperglycemic condition, in both type 1 and 2 diabetes, is associated with long-term complications and dysfunction of vital organs such as the kidney, heart, nerves, and eyes [76]. There is growing evidence that the toxic effects of hyperglycemia are associated with the activation of the polyol metabolic pathway, where glucose is converted to sorbitol by the first enzyme of the polyol pathway, aldose reductase (ALR2), with the oxidation of NADPH. Sorbitol is subsequently converted to fructose by the enzyme sorbitol dehydrogenase (SDH), with concomitant reduction of NAD+ [77]. ALR2 has been considered for many years as a valid target for the development of drugs capable of preventing or delaying the appearance of long-term diabetic complications without the risk of hypoglycemia. This is because ALR2 inhibitors have no effects on plasma glucose [78,79]. Different ALR2 inhibitors have been studied as potential drug candidates, such as the pyridazinone derivatives ponalrestat (93) and zopolrestat (94) for the treatment of diabetic complications, with IC50 values against ALR2 of 20 and 2.1 nM, respectively (Figure 25) [80]. However, there are several problems associated with ALR2 inhibition therapy, including poor selectivity and loss of efficacy with prolonged treatment. To date, epalrestat (95) is the only compound successfully marketed in Japan, India, and China to treat diabetic neuropathy [78].
Within this frame, Barlocco et al. described the SAR studies of a series of tricyclic pyridazinones bearing acidic side chains at the N2 or C4 position, of general formulas XIII and XIV (Figure 25), which were evaluated as ALR2 inhibitors; sorbitol dehydrogenase and other enzymes are not implicated in the polyol pathway like aldehyde reductase and glutathione reductase. From this study, the acetic and butyric acid benzocinnolinones 96–101 emerged as the most active and selective ALR2 inhibitors, with IC50 values ranging from 4.25 (101) to 12.6 μM (96), even if less effective than ponalrestat or zopolrestat which act in the nM range [81,82].
Using a structure-based drug design strategy and the ALR2 inhibitor zopolrestat (94) as a reference compound, along with the benzocinnolinone derivative 96, Rastelli et al. discovered the hybrid compound 102 (Figure 26), which is two orders of magnitude more potent than the model 96 (IC50 = 0.15 μM) [83]. Within this frame, some years later, our group disclosed a series of thienocinnolinone derivatives with activity as ALR2 inhibitors. Compounds 103–105 (Figure 26), featuring a thienocinnolinone core linked through a pentamethylene spacer to a carboxylic function, exhibited IC50 values of 7.6, 18, and 31.4 μM, respectively [84].
Further, tricyclic pyridazinones structurally related to ponalrestat and zopolrestat were disclosed by Pfizer [85]. Compounds 106 and 107 (Figure 27) exhibited ALR2 inhibition activities of 420 and 46 nM, respectively.
ALR2 enzyme inhibition data for tricyclic pyridazinones described above are summarized in Table 5.

7. Anxiety Disorders

In the Diagnostic and Statistical Manual of Mental Disorders of the American Psychiatric Association (DSM-5), anxiety disorders include generalized anxiety disorder, separation anxiety disorder, social phobia, specific phobia, panic disorder, agoraphobia, and selective mutism. Benzodiazepines are efficacious drugs for treating anxiety disorders, and several of them have received approval for clinical use. The pharmacological effects of benzodiazepines result from their affinity for a specific binding site on the GABAA receptor, the benzodiazepine site (BZR). They exhibit a continuum of intrinsic activity, ranging from full agonists (positive allosteric modulators of GABA-evoked Cl− currents with anxiolytic, hypnotic, and anticonvulsant activities), antagonists with no intrinsic activity, and inverse agonists (negative allosteric modulators of GABA-evoked Cl− currents with proconvulsant, convulsant, and anxiogenic activities) [86].
Most of the drugs in clinical use, such as diazepam (108, Figure 28), are full agonists at the BZR and have anticonvulsant, sedative, and muscle-relaxant effects along with anxiolytic properties [14]. Partial agonists of the BZR such as imidazenil offer some theoretical and practical advantages over full agonists, including fewer side effects such as sedation, ataxia, and abuse potential, even though no GABAA partial agonist has received approval for treating anxiety disorders. In the past, several research groups, both from academia and industry, dedicated many efforts to discovering drugs targeting the BZR, and a few structural classes of nonbenzodiazepine compounds showed desired anxiolytic or other pharmacological activities in vivo, such as zoplicone and zolpidem, which were marketed to treat insomnia [14].
As part of this research, Yoshitomi Pharmaceutical Industrie has been involved for several years in the design and synthesis of BZR ligands of the general formulas XV and XVI, incorporating a tricyclic pyridazinone system (Figure 28). SAR studies of synthesized compounds identified the benzothiepino[5,4-c]pyridazine-3(2H)-one-7-oxide 109 (Y-23684) as a partial BZR agonist, exhibiting Ki = 42 nM versus diazepam, Ki = 5.8 nM (radioligand-binding assay with [3H]diazepam in the cerebral cortex of rats). An in-depth pharmacological study conducted on this compound in comparison with diazepam suggested that 109 would be a potent and selective anxiolytic agent in humans with fewer side effects than conventional BZ-anxiolytics [87].
Taking into consideration the pharmacological relevance of these compounds, further tricyclic pyridazinones have been designed and synthesized, taking 109 as a reference compound. Isosteric replacement of the condensed benzene ring of 109 with a thiophene ring, along with the introduction of various substituents into the 9-position of the condensed-ring system, and variation of the oxidation level of the sulfoxide function (from sulfoxide to sulfide or sulfone) at the 7-position of the same system, gave several thieno[2′,3′:2,3]thiepino[4,5-c]pyridazine-3(2H)-ones of general formula XVII (Figure 29) as potent ligands at the BZR, most of them endowed with Ki values ranging from 0.61 to 6.8 nM. Modifications carried out on the lead compound 109 had a remarkable impact both on affinity values to the BZR and in vivo tests. Electrophysiological studies conducted in vitro (GABA-induced chloride current in frog sensory neurons) along with in vivo tests such as an anticonvulsant test (anti-BCL test), anticonflict test (water-lick test), and rotarod test, indicated that the scaffold thieno[2′,3′:2,3]thiepino[4,5-c]pyridazine-3(2H)-one provided compounds with a broad spectrum of activities ranging from full agonists to partial agonists. Compounds 110 (Ki = 1.1 nM) and 111 (Ki = 0.61 nM), featuring a sulfone function, exhibited significantly greater BZR affinity than diazepam and were classified as a full agonist and partial agonist, respectively [87].
The SAR studies carried out on such tricyclic pyridazinones were extended to thieno[2′,3′:6,7]cyclohepta[1,2-c]pyridazine-3-(2H)ones and thieno[2,3-h]cinnoline-3(2H)ones of general formulas XVIII and XIX, respectively (Figure 30) [88]. Among the tested compounds, several had affinity for the BZR in the nM range. Particularly, the partial agonist 112 exhibited an anxioselective feature.
In this context, some years later, Primofiore et al. designed, synthesized, and evaluated the binding affinity of a series of pyrido[3′,2′:5,6]thiopyrano[4,3-c]pyridazine-3(2H,5H)-one [89] to the BZR. The target compounds exhibited an affinity in the micromolar/submicromolar range, with derivative 113 (Figure 30) exhibiting the most potent affinity (Ki = 0.695 μM).
The BZR-binding affinities of the tricyclic pyridazinones described above are summarized in Table 6.

8. Antiviral Activity

The integrase (IN) enzyme represents an important chemotherapeutic target as its inhibition blocks infection by human immunodeficiency virus type 1 (HIV-1), which is responsible for acquired immune deficiency syndrome (AIDS). In fact, integrase strand-transfer inhibitors (INSTIs) represent one of the most significant advances in HIV care, being the latest combination antiretroviral therapy (cART) drugs developed [90]. Integrase is one of three core enzymes encoded by the HIV genome and is essential for the integration of viral genetic material into the host cell DNA and successful replication of the virus. INSTIs bind to the integrase–viral DNA complex in the catalytic core domain. As a result, they interfere with the integration of viral DNA into host DNA. It has been proposed that INSTIs bind to key metals that are involved in the action of integrase and thus block the viral enzyme [91]. Raltegravir and, more recently, elvitegravir, dolutegravir, and cabotegravir were approved compounds within cART formulations [14,90].
Wiscount et al., from Merck laboratories, described a tricyclic hydroxypyrrole scaffold XX (Figure 31) as a new mimetic of the metal-binding pharmacophore of diketoacid integrase inhibitors. In spite of the unique opportunities of such compounds to maintain activity against integrase mutants, in pharmacokinetic experiments, they were susceptible to air oxidation. The authors assumed that the tricyclic pyridazinone scaffold XXI could provide compounds with improved chemical stability and better pharmacokinetic properties, keeping the same activity against the integrase enzyme. Among the synthesized compounds, analog 114 inhibited integrase strand-transfer activity with an IC50 less than 10 nM and also inhibited HIV-1 cell culture replication with an IC95 value of 35 nM in the presence of 50% normal human serum. This compound may represent a promising lead for further optimization studies toward second-generation integrase strand-transfer inhibitors, having shown modest pharmacokinetic properties in rats (iv t1/2 = 5.3 h, F = 17%) [92]. No further data are available for this class of tricyclic pyridazinones.

9. Other Biological Activities

9.1. Matrix Metalloproteinase Inhibitors

Matrix metalloproteinases (MMPs) belong to a family of zinc2+-dependent enzymes including about 28 members involved in the degradation of different protein targets. MMPs play a major role in the pathogenesis of cancer, autoimmune diseases, cardiovascular diseases, inflammation such as periodontal diseases, and neurodegenerative disease states [93,94]. Our group reported a series of thieno[3,2-h]cinnolinones which were tested against the Met80-Gly242 domain of human neutrophil collagenase (MMP-8) expressed in Escherichia coli. Compounds 115 and 116 (Figure 32), featuring an NH-acetyl group in position 8 of the thieno[3,2-h]cinnolinone scaffold, were found to be the most potent analogs with IC50 values of 17 and 36 μM, respectively [95].

9.2. Phosphodiesterase 5 Inhibitors

Phosphodiesterase 5 (PDE5) is 1 of the 11 members of the cyclic nucleotide phosphodiesterase (PDE) family. It specifically targets cyclic guanosine monophosphate (cGMP), produced by nitric oxide-driven activation of the soluble guanylyl cyclase, an important mediator of vasodilation. PDE5 inhibition extends the duration of cGMP and promotes vasodilation. PDE5 inhibitors, including sildenafil and tadalafil, are widely used to treat erectile dysfunction, pulmonary arterial hypertension, and certain urological disorders. Preclinical studies have also revealed promising effects of PDE5 inhibitors in the treatment of myocardial infarction, cardiac hypertrophy, heart failure, cancer and anticancer-drug-associated cardiotoxicity, Alzheimer’s disease, and other aging-related conditions [96]. The research group of Dal Piaz and Giovannoni synthesized a series of tricyclic pyrazolopyrimidopyridazinones of general formula XXII (Figure 33) with potent and selective PDE5 inhibition properties as potential agents for the treatment of erectile dysfunction. Such compounds were evaluated as inhibitors for PDE5 along with PDE6, the off-target isoform of sildenafil. Several of the tested compounds showed IC50 values in the low nM range compared to sildenafil (PDE5 IC50 = 0.020 μM, PDE6 IC50 = 0.040 μM) and high selectivity versus the PDE6 isoform. Compounds 117 and 118 emerged as the most interesting compounds (117: PDE5 IC50 = 0.034 μM, PDE6 IC50 = 42.5 μM; 118: PDE5 IC50 = 0.0083 μM, PDE6 IC50 = 22.8 μM) [97,98].

9.3. Adenosine Inhibitors

Adenosine is a ubiquitous endogenous autacoid. It is involved in a number of physiological and pathological effects in the cardiovascular, renal, nervous, and immune systems, through G-protein-coupled receptors, namely A1, A2A A2B, and A3 adenosine receptors. Several types of agonists, partial agonists or antagonists, and allosteric substances have been identified for these receptors as new therapeutic drug candidates [99]. Giovannoni et al. synthesized a new series of tricyclic pyrazolopyrimidopyridazinones XXII (Figure 33) and evaluated their affinity towards human A1, A2A, and A3 adenosine receptors [100,101]. Most of the tested compounds showed high selectivity of action towards the A1 receptor subtype and high affinity with Ki values in the low nM range. The most interesting compound was 119 (hA1 Ki = 1.4 nM, selectivity 7%, and 20% of inhibition at 10 μM for hA2A and hA3, respectively). Further pharmacological studies carried out on 119 evidenced antagonistic properties at the A1 receptor and antiamnesic effects in the mouse passive avoidance test at doses of 10 and 30 mg/Kg.

9.4. Translocator Protein 18 kDa (TSPO) Ligands

An interesting biological target for the discovery of promising neuropsychotropic compounds is the translocator protein 18 kDa (TSPO), formerly known as the peripheral benzodiazepine receptor (PBR). This protein is predominantly localized in the mitochondrial membrane and is found in both the peripheral and central nervous systems. It is involved in cholesterol translocation, which is essential for the production of steroid hormones and neurosteroids [102]. TSPO is overexpressed in neuroinflammation conditions. It may be a suitable neuroinflammation biomarker and a target for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging. TSPO also has a role in apoptosis, cell proliferation and differentiation and many other physiological functions such as porphyrin transport and heme biosynthesis [103]. Numerous classes of selective synthetic ligands have been reported to further the understanding of TSPO functions in neurodegenerative and neuroinflammatory conditions [104]. Compound 120 (SSR180575), belonging to the chemical class of pyridazino[4,5-b]indole-1-acetamides (Figure 34), binds to the TSPO with a high affinity (Ki = 0.83 nM). It represents an attractive lead for the development of TSPO ligands as therapeutic drugs. This compound has also been tagged with carbon-11 (121) and has shown encouraging in vivo PET imaging properties in both rodents and non-human primates. It also has the ability to support neuronal survival and regeneration in animal models of axotomy and neuropathy by facilitating the local synthesis of neurosteroids [103]. A series of related compounds exhibiting strong affinity, with Ki values similar to those of SSR180575 (120), were discovered by functionalization at the N-3 position, allowing for substitution with F-18. Among them, derivative [18F]FPSSR180575 (122) emerged as a promising radioligand for in vivo PET imaging. Further SAR studies carried out on SSR180575 (120) provided several derivatives exhibiting potent TSPO affinity, with Ki values in the nM and sub-nM ranges. Among such derivatives, compounds 123 and 124 (123: Ki = 0.40 nM; 124: Ki = 0.42 nM) emerged as promising candidates for drug development. They also offer unique opportunities for fluorine-18 labeling and in vivo PET imaging of neuroinflammation [104].

9.5. Multitarget-Directed Ligands

Alzheimer’s disease (AD) is a chronic, progressive, and incurable neurological disorder that affects millions of people worldwide. Notably, low levels of acetylcholine in the cerebral cortex and other brain areas appear to play a critical role in the development of cognitive and neurodegenerative disorders in AD (cholinergic hypothesis). Acetylcholinesterase inhibitors (AChEIs), such as donepezil, rivastigmine, or galantamine, or N-methyl-D-aspartate receptor antagonists, such as memantine, are currently the pillar of pharmacological therapy for AD [14].
Taking into consideration the knowledge of the multifactorial nature of AD, along with the inability of current therapies to stop the progression of the disease, several efforts have been made to identify novel and safer AChEIs or new biological targets to treat AD. Within this framework, the multitarget-directed ligand (MTDL) strategy “one molecule, multiple targets” has been applied to the design and synthesis of new drug candidates capable of interacting with multiple targets involved in the pathogenesis of AD [105].
Our research group focused on the design, synthesis, and pharmacological evaluation of a library of thienocycloalkylpyridazinones of general formulas XXIII and XXIV (Figure 35) using the MTDL approach, in the pursuit of potential AD treatment discovery [106]. These compounds featured pyridazinone-based tricyclic scaffolds connected through alkyl chains of variable length to proper amine moieties for G-protein-coupled receptors (especially serotonin receptors), and cholinesterase (AChE and BChE) molecular recognition. Notably, serotonin receptors, especially 5-HT1A, 5-HT4, 5-HT6, and 5-HT7, have attracted much interest as important players in influencing different aspects of cognitive deficits, learning and memory decline in AD. Our SAR studies carried out on the tricyclic pyridazinones XXIII and XXIV allowed us to identify several compounds showing AChE/BChE inhibition with a wide range of potencies, depending on the correct combination of the linker and the amine moiety. Compounds 125 and 126, containing the thieno[2,3-h]cinnolinone nucleus, exhibited high affinity for AChE with low affinity for BChE (125: AChE IC50 = 0.86 μM, BChE IC50 = 3.3 μM; 126: AChE IC50 = 0.64 μM, BChE IC50 = 2.1 μM). Interestingly, compound 127, which contains the thieno[3,2-h]cinnolinone core, maintained an AChE IC50 value very similar to that of its positional isomer 125, exhibiting improved selectivity versus BChE (AChE IC50 = 0.75 μM, BChE 45% of inhibition at 10 μM). In general, our SAR study revealed an objective difficulty in obtaining ligands with potent and mixed properties at cholinesterase enzyme and serotonin receptors, i.e., compounds 127 and 128, exhibiting the highest potency at AChE (128: AChE IC50 = 0.17 μM, BChE IC50 = 5.1 μM) among all tested compounds, had poor or not detectable affinity for serotonin 5-HT1A, 5-HT4, 5-HT6 and 5-HT7 receptors. Replacement of the benzyl linked to the piperazine with the phenylsulfonyl indole moiety significantly improved the affinity for 5HT6 and selectivity over other serotoninergic receptors, with a slight reduction in inhibitory activity at AChE. Compound 130, bearing a phenylsulfonyl-indol-piperazine motif with a thieno[3,2-h]cinnolinone core connected by a pentamethylene linker, was identified as the best compromise with respect to multitarget affinity (AChE IC50 = 4.44 μM, 5-HT6 Ki = 63 nM). On the other hand, the replacement of the thieno[3,2-h]cinnolinone nucleus in 130 with 2,4,4a,5-tetrahydro-3H-thieno[3′,2′:4,5]cyclopenta[1,2-c]pyridazin-3-one resulted in compound 129, the most potent and selective ligand for the 5-HT6 receptor subtype, reaching an outstanding Ki value in the low nM range (AChE IC50 = 6.9 μM, 5-HT6 Ki = 8.4 nM).

9.6. PKM2 Activators to Reduce Photoreceptor Apoptosis

Photoreceptor cell degeneration in many retinal disorders causes significant diminution of the retina’s ability to detect light, resulting in loss of vision. In order to restore vision, ocular regenerative medicine uses neuroprotective therapy, gene therapy, cell replacement therapy, and visual prostheses. Several of these treatments are still at an early stage. They require extensive additional preclinical and clinical studies, before restoration of visual function can be observed [107]. Besirli’s research group reported that the anticancer thieno[3,2-b]pyrrole[3,2-d]pyridazinone 131 (ML-265) (Figure 36), a potent activator of PKM2 (AC50 70 nM, maximum activation of 292%), exhibited photoreceptor neuroprotection, evaluated in vivo in the rat retina, reducing entry into apoptosis and improving photoreceptor viability in both in vitro and in vivo models of outer retinal stress. In spite of such promising results demonstrating the beneficial use of PKM2 activators as neuroprotective agents, ML-265 suffers from a lack of aqueous solubility and the presence of structural alerts (methyl sulfoxide and aniline functional groups) that prevent it from being further developed as an ideal intraocular clinical candidate. SAR studies carried out on compound ML-265 identified pyridazino[4,5-b]indol-4-one 132 as a potent activator of PKM2 with AC50 and maximal activation values close to the parent compound ML-265. Compound 132 prevented photoreceptor apoptosis in commonly used in vitro and in vivo preclinical models of outer retinal stress. Such results paved the way for novel pyridazino[4,5-b]indol-4-one PKM2 activators, whose best representatives are compounds 133–135 (Figure 36), retaining nM potency versus PKM2 (62–64 nM range), with 2- to 7-fold greater aqueous solubilities than ML265. The authors highlighted the importance of their work in providing such novel PKM2 activators as the basis for new therapeutic agents useful for treating retinal degenerative diseases [108].
The different biological properties of the representative tricyclic pyridazinones described above are summarized in Table 7.

10. Conclusions

Heterocyclic compounds, especially those containing one or more nitrogen atoms in their structures, have always attracted attention for their therapeutic importance. Among them, the six-membered nitrogen-containing heterocycle, pyridazin-3(2H)one, represents an interesting scaffold in several drugs, both those bearing various substituents and forming linear or, more often, angular polycyclic systems, when condensed with other (hetero)cyclic systems.
In particular, this review focused on pyridazin-3(2H)one-based tricyclic or tetracyclic molecules and provides an overview of their main potential therapeutic applications, although these types of templates have been tested for many other biological targets. In addition, to highlight the therapeutic potential of these compounds, SAR studies using both in vitro and in vivo assays have been reported.
The pharmacological relevance of tricyclic- or tetracyclic-based pyridazin-3(2H)one molecules has been well documented since the last century. However, the identification of new polycyclic pyridazin-3(2H)one-based molecules with these structures is still an important field of research due to their pharmacological ductility, responsible for their anti-inflammatory, anticancer, antimicrobial, and many other pharmacological activities. Therefore, the drug design and discovery process of novel polycyclic pyridazin-3(2H)one-based scaffolds could still prove useful in the future to identify new molecules able to modulate known and new biological targets involved in different diseases.
In conclusion, pyridazin-3(2H)one-based scaffolds have proven to be versatile and valuable in medicinal chemistry. Their sometimes-easy synthesis and broad spectrum of biological activities have made them attractive candidates for drug development across a range of therapeutic areas. While significant progress has been made in the understanding of the SARs of various pyridazinone derivatives, further research is essential to unlock the full therapeutic potential of these compounds. Future efforts should focus on the development of more potent and selective compounds with improved pharmacokinetic properties, and they should explore novel applications of pyridazinones in different unexplored therapeutic areas. The continued exploration and optimization of this privileged scaffold promise to yield innovative solutions to address unmet challenges in human health.

Funding

This research was funded by University of Sassari grant numbers: Fondo di Ateneo per la ricercar (FAR 2019 and FAR 2020).

Conflicts of Interest

The authors declare no conflict of interest.

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Ijms 26 03806 g001
Figure 1. Keto-enol tautomerism of pyridazine-3(2H)one.
Figure 1. Keto-enol tautomerism of pyridazine-3(2H)one.
Ijms 26 03806 g001
Ijms 26 03806 sch001
Scheme 1. Synthesis of 6-substituted-phenyl-4,5-dihydropyridazin-3(2H)-ones D and benzocycloalkylpyridazine-3(2H)ones F and G.
Scheme 1. Synthesis of 6-substituted-phenyl-4,5-dihydropyridazin-3(2H)-ones D and benzocycloalkylpyridazine-3(2H)ones F and G.
Ijms 26 03806 sch001
Ijms 26 03806 g002
Figure 2. Design of benzocycloalkylpyridazinone-based tricyclic scaffolds I–III.
Figure 2. Design of benzocycloalkylpyridazinone-based tricyclic scaffolds I–III.
Ijms 26 03806 g002
Ijms 26 03806 g003
Figure 3. Structures of indeno-pyridazinones 4 and 7 and indeno-pyridazines 5, 6, and 8.
Figure 3. Structures of indeno-pyridazinones 4 and 7 and indeno-pyridazines 5, 6, and 8.
Ijms 26 03806 g003
Ijms 26 03806 g004
Figure 4. Structure of 5H-indeno[1,2-c]-3(2H)-pyridazinones 9–20.
Figure 4. Structure of 5H-indeno[1,2-c]-3(2H)-pyridazinones 9–20.
Ijms 26 03806 g004
Ijms 26 03806 g005
Figure 5. Structure of 4,4a,5,6-tetrahydrobenzo[h]cinnolinones 21 and 22, 5,6-dihydrobenzo[h]cinnolinone 23, and benzo[6,7]cyclohepta[1,2-c]-3(2H)-pyridazinones 24 and 25.
Figure 5. Structure of 4,4a,5,6-tetrahydrobenzo[h]cinnolinones 21 and 22, 5,6-dihydrobenzo[h]cinnolinone 23, and benzo[6,7]cyclohepta[1,2-c]-3(2H)-pyridazinones 24 and 25.
Ijms 26 03806 g005
Ijms 26 03806 g006
Figure 6. Structure of 4,4a,5,6-tetrahydro-4a-substituted-benzo[h]cinnolinones 26–29.
Figure 6. Structure of 4,4a,5,6-tetrahydro-4a-substituted-benzo[h]cinnolinones 26–29.
Ijms 26 03806 g006
Ijms 26 03806 g007
Figure 7. Design of imidazole-based indeno[1,2-c]-pyridazinones 31 and 32.
Figure 7. Design of imidazole-based indeno[1,2-c]-pyridazinones 31 and 32.
Ijms 26 03806 g007
Ijms 26 03806 g008
Figure 8. Structure of 5,6-dihydrobenzo[f]cinnolin-2(3H)ones 33 and 34.
Figure 8. Structure of 5,6-dihydrobenzo[f]cinnolin-2(3H)ones 33 and 34.
Ijms 26 03806 g008
Ijms 26 03806 g009
Figure 9. Structure of 4,4a-dihydro-5H-[1]benzopyrano[4,3-c]pyridazin-3(2H)ones 35–39.
Figure 9. Structure of 4,4a-dihydro-5H-[1]benzopyrano[4,3-c]pyridazin-3(2H)ones 35–39.
Ijms 26 03806 g009
Ijms 26 03806 g010
Figure 10. Structure of thieno[2,3-h]cinnolin-3(2H)-ones 40–43 and 46–48 and thieno[3,2-h]cinnolin-3(2H)-ones 44 and 45.
Figure 10. Structure of thieno[2,3-h]cinnolin-3(2H)-ones 40–43 and 46–48 and thieno[3,2-h]cinnolin-3(2H)-ones 44 and 45.
Ijms 26 03806 g010
Ijms 26 03806 g011
Figure 11. Structure of pyridazino[4,5-b]indoles 49, 50 and 1,2,4-triazino[4,5-a]indoles 51, 52.
Figure 11. Structure of pyridazino[4,5-b]indoles 49, 50 and 1,2,4-triazino[4,5-a]indoles 51, 52.
Ijms 26 03806 g011
Ijms 26 03806 g012
Figure 12. Structure of thiourea or 2-cyanoguanidine-based 3(2H)-pyridazinone derivatives 53 and 54, 5H-[1]benzopyrano[4,3-c]pyridazin-3(2H)ones 55, 58, and 59, and benzo[h]cinnolinones 56 and 57.
Figure 12. Structure of thiourea or 2-cyanoguanidine-based 3(2H)-pyridazinone derivatives 53 and 54, 5H-[1]benzopyrano[4,3-c]pyridazin-3(2H)ones 55, 58, and 59, and benzo[h]cinnolinones 56 and 57.
Ijms 26 03806 g012
Ijms 26 03806 g013
Figure 13. Structure of 5H-indeno[1,2-c]-3(2H)-pyridazinones 60 and 61 and 4,5-dihydro-5H-indeno[1,2-c]-3(2H)-pyridazinones 62 and 63.
Figure 13. Structure of 5H-indeno[1,2-c]-3(2H)-pyridazinones 60 and 61 and 4,5-dihydro-5H-indeno[1,2-c]-3(2H)-pyridazinones 62 and 63.
Ijms 26 03806 g013
Ijms 26 03806 g014
Figure 14. Structure of 5H-indeno[1,2-c]-3(2H)-pyridazinones 64 and 65.
Figure 14. Structure of 5H-indeno[1,2-c]-3(2H)-pyridazinones 64 and 65.
Ijms 26 03806 g014
Ijms 26 03806 g015
Figure 15. Structure of pyrrolpyridazinones 66–71.
Figure 15. Structure of pyrrolpyridazinones 66–71.
Ijms 26 03806 g015
Ijms 26 03806 g016
Figure 16. Structure of hexahydrothienocycloheptapyridazinones 72 and 73.
Figure 16. Structure of hexahydrothienocycloheptapyridazinones 72 and 73.
Ijms 26 03806 g016
Ijms 26 03806 g017
Figure 17. Structure of tetrahydrobenzo[h]cinnolinone 74 and (S)-(−)-74.
Figure 17. Structure of tetrahydrobenzo[h]cinnolinone 74 and (S)-(−)-74.
Ijms 26 03806 g017
Ijms 26 03806 g018
Figure 18. Structure of thieno[3,2-b]pyrrole[3,2-d]pyridazinones 75–77 and thiazolo[5′,4′:4,5]pyrrolo[2,3-d]pyridazin-5-one 78 (AG-946).
Figure 18. Structure of thieno[3,2-b]pyrrole[3,2-d]pyridazinones 75–77 and thiazolo[5′,4′:4,5]pyrrolo[2,3-d]pyridazin-5-one 78 (AG-946).
Ijms 26 03806 g018
Ijms 26 03806 g019
Figure 19. Structure of pyridazino[4,5-b]indol-4-ones 79 and 80.
Figure 19. Structure of pyridazino[4,5-b]indol-4-ones 79 and 80.
Ijms 26 03806 g019
Ijms 26 03806 g020
Figure 20. Structure of pyridazino[4,5-b]indol-4-one-substituted 1,2,3-triazolylmethyl unit 81 and 82.
Figure 20. Structure of pyridazino[4,5-b]indol-4-one-substituted 1,2,3-triazolylmethyl unit 81 and 82.
Ijms 26 03806 g020
Ijms 26 03806 g021
Figure 21. Structure of hydrazide-based pyridazino[4,5-b]indol-4-one derivatives 83 and 84.
Figure 21. Structure of hydrazide-based pyridazino[4,5-b]indol-4-one derivatives 83 and 84.
Ijms 26 03806 g021
Ijms 26 03806 g022
Figure 22. Structure of talazoparib (85) and E-7016 (86).
Figure 22. Structure of talazoparib (85) and E-7016 (86).
Ijms 26 03806 g022
Ijms 26 03806 g023
Figure 23. Structure of pyridazino[3,4,5-de]quinazolin-3(2H)-ones 87 and 88.
Figure 23. Structure of pyridazino[3,4,5-de]quinazolin-3(2H)-ones 87 and 88.
Ijms 26 03806 g023
Ijms 26 03806 g024
Figure 24. Structure of 1,3-thiazole-based compounds 89 and 92 and polycyclic pyridazinones 90 and 91.
Figure 24. Structure of 1,3-thiazole-based compounds 89 and 92 and polycyclic pyridazinones 90 and 91.
Ijms 26 03806 g024
Ijms 26 03806 g025
Figure 25. Structure of ponalrestat (93), zopolrestat (94), epalrestat (95), and benzocinnolinones 96–101.
Figure 25. Structure of ponalrestat (93), zopolrestat (94), epalrestat (95), and benzocinnolinones 96–101.
Ijms 26 03806 g025
Ijms 26 03806 g026
Figure 26. Structure of benzocinnolinone 102 and thienocinnolinones 103–105.
Figure 26. Structure of benzocinnolinone 102 and thienocinnolinones 103–105.
Ijms 26 03806 g026
Ijms 26 03806 g027
Figure 27. Structure of tricyclic pyridazinones 106 and 107.
Figure 27. Structure of tricyclic pyridazinones 106 and 107.
Ijms 26 03806 g027
Ijms 26 03806 g028
Figure 28. Structure of diazepam (108) and benzothiepino[5,4-c]pyridazine-3(2H)-one-7-oxide 109.
Figure 28. Structure of diazepam (108) and benzothiepino[5,4-c]pyridazine-3(2H)-one-7-oxide 109.
Ijms 26 03806 g028
Ijms 26 03806 g029
Figure 29. Structure of thieno[2′,3′:2,3]thiepino[4,5-c]pyridazine-3(2H)-ones 110 and 111.
Figure 29. Structure of thieno[2′,3′:2,3]thiepino[4,5-c]pyridazine-3(2H)-ones 110 and 111.
Ijms 26 03806 g029
Ijms 26 03806 g030
Figure 30. Structure of thienocinnolinone 112 and pyrido-thiopyrano-pyridazinone 113.
Figure 30. Structure of thienocinnolinone 112 and pyrido-thiopyrano-pyridazinone 113.
Ijms 26 03806 g030
Ijms 26 03806 g031
Figure 31. Structure of 10-hydroxy-7,8-dihydropyrazino[1′,2′:1,5]pyrrolo[2,3-d]pyridazine-1,9(2H,6H)-dione 114.
Figure 31. Structure of 10-hydroxy-7,8-dihydropyrazino[1′,2′:1,5]pyrrolo[2,3-d]pyridazine-1,9(2H,6H)-dione 114.
Ijms 26 03806 g031
Ijms 26 03806 g032
Figure 32. Structure of thieno[3,2-h]cinnolinones 115 and 116.
Figure 32. Structure of thieno[3,2-h]cinnolinones 115 and 116.
Ijms 26 03806 g032
Ijms 26 03806 g033
Figure 33. Structure of pyrazolopyrimidopyridazinones 117–119.
Figure 33. Structure of pyrazolopyrimidopyridazinones 117–119.
Ijms 26 03806 g033
Ijms 26 03806 g034
Figure 34. Structure of pyridazino[4,5-b]indole-1-acetamides 120–124.
Figure 34. Structure of pyridazino[4,5-b]indole-1-acetamides 120–124.
Ijms 26 03806 g034
Ijms 26 03806 g035
Figure 35. Structure of thieno-cycloalkyl-pyridazinones 125–130.
Figure 35. Structure of thieno-cycloalkyl-pyridazinones 125–130.
Ijms 26 03806 g035
Ijms 26 03806 g036
Figure 36. Structure of thieno[3,2-b]pyrrole[3,2-d]pyridazinone 131 and pyridazino[4,5-b]indol-4-one 132–135.
Figure 36. Structure of thieno[3,2-b]pyrrole[3,2-d]pyridazinone 131 and pyridazino[4,5-b]indol-4-one 132–135.
Ijms 26 03806 g036
Table 1. Anti-inflammatory activity of compounds 2–8 in comparison to ASA [16,17,18].
Table 1. Anti-inflammatory activity of compounds 2–8 in comparison to ASA [16,17,18].
Compd.mg/KgCarrageenin Rat Paw Edema Inhibition %Compd.mg/KgCarrageenin Rat Paw Edema Inhibition %
250
100		22
36		650
100		38
49
350
100		34
42		7-
100		-
41
450
100		20
33		8-
100		-
39
550
100		41
49		ASA50
100		31
43
Table 2. Cardiovascular-related properties of compounds 9–29 and 33–48.
Table 2. Cardiovascular-related properties of compounds 9–29 and 33–48.
Gen. Formula/Ref.Compd.R, R1, n, 4-4a BondBiological Activity (In Vivo/In Vitro)
Ijms 26 03806 i001
[23,24]		9R = 7-NH2antihypertensive, antithrombotic, antiaggregating, anti-inflammatory activity
10R = 7-NHCOCH3antihypertensive, antithrombotic, antiaggregating, anti-inflammatory, antiulcer activity
118-NO2antithrombotic activity
128-NH2antithrombotic activity
138-NHCOCH3antithrombotic activity
149-NHCOCH3antithrombotic activity
159-NH2data not available
167-OCH3data not available
177-OHantithrombotic activity
187-NHCH3antihypertensive, antithrombotic activity
197-N(CH3)COCH3antihypertensive, antithrombotic activity
207-N(CH3)COCH2CH3antihypertensive, antithrombotic activity
Ijms 26 03806 i002
[25,26]		21n = 1, R = 8-NH2, R1 = H, 4-4a single bondantihypertensive, inotropic, antithrombotic activity
22n = 1, R = 8-NHCOCH3, R1 = H, 4-4a single bondantihypertensive, inotropic, antithrombotic activity
23n = 1, R = 8-NHCOCH3, 4-4a double bondantihypertensive, inotropic, antithrombotic activity
24n =2, R = 9-NH2, R1 = H, 4-4a single bondantithrombotic activity
25n = 2, R = 9-NHCOCH3, R1 = H, 4-4a single bondantithrombotic activity
26n =1, R = H, R1 = CH2OH, 4-4a single bondantihypertensive activity
27n = 1, R = 8-OCH3, R1 = CH2OH, 4-4a single bondantihypertensive, antithrombotic activity
28n = 1, R = 9-OCH3, R1 = CH2OH, 4-4a single bondantithrombotic activity
29n = 1, R = H, R1 = CH3, 4-4a single bondantithrombotic activity
Ijms 26 03806 i003
[29]		33R = 9-NH2antiaggregating, hypotensive, antihypertensive activity
34R = 9-NHCOCH3antiaggregating, hypotensive, antihypertensive activity
Ijms 26 03806 i004
[32]		35R = Hantiulcer activity
36R = 8-NH2antithrombotic activity
37R = 8-NHCOCH3antihypertensive, antithrombotic activity
38R = 9-OCH3antiulcer activity
39R = 8,9-OCH3antiulcer activity
Ijms 26 03806 i005
[33,34]		40R = R1 = H, 4-4a single bondantithrombotic activity
41R = 8-NHCOCH3, R1 = H, 4-4a single bondantithrombotic activity
42R = 9-NHCOCH3, R1 = H, 4-4a single bondhypotensive activity
43R = 9-NHCOCH3, 4-4a double bondhypotensive activity, antiaggregating activity
46R = H, R1 = CH3 4-4a single bondantihypertensive, antiaggregating activity
47R = 8-NHCOCH3, R1 = CH3, 4-4a single bondhypotensive, antiaggregating activity
48R = 9-NHCOCH3, R1 = CH3, 4-4a single bondhypotensive, antiaggregating activity
Ijms 26 03806 i006
[33]		44R = 7-NHCOCH3hypotensive, antiaggregating activity
45R = 8-NHCOCH3hypotensive, antihypertensive activity, antiaggregating activity
Table 3. Antiulcer properties of compounds 55–63.
Table 3. Antiulcer properties of compounds 55–63.
Gen. Formula/Ref.Compd.X, R1, 4-4a BondRBiological Activity (In Vivo)
Ijms 26 03806 i007
[37]		55X = O, 4-4a double bondHantiulcer activity (ethanol and ASA model), antisecretory activity
56X = CH2, 4-4a double bondHantiulcer activity (ethanol model)
57X = CH2, R1 = CH3, 4-4a single bondHantiulcer activity (ethanol model)
58X = O, R1 = H, 4-4a single bondIjms 26 03806 i008antiulcer activity (ethanol model)
59X = O, 4-4a double bondIjms 26 03806 i009antiulcer activity (ethanol and ASA model)
Ijms 26 03806 i010
[38]		604-4a double bond7,8-(OCH3)2 antiulcer activity (ethanol and indomethacine model), antisecretory activity
614-4a double bond6,9-(OCH3)2antiulcer activity (ethanol and indomethacine model)
62R1 = H, 4-4a single bond7,8-(OCH3)2antiulcer activity (ethanol and indomethacine model)
63R1 = H, 4-4a single bond6,9-(OCH3)2antiulcer activity (ethanol and indomethacine model)
Table 4. Anticancer properties of selected tricyclic pyridazinones.
Table 4. Anticancer properties of selected tricyclic pyridazinones.
Compd./Ref.Human Cancer Cell Line/TargetCompd./Ref.Human Cancer Cell Line/Target
Ijms 26 03806 i011
[52]		colon (LoVo and LoVo/DX),
murine leukemia (L1210 and L1210/CDDP)		Ijms 26 03806 i012
[65]		hepatocellular (Hub-7), colorectal (Caco2), breast (MDA-MB-231)
DYRKIA inhibitor
Ijms 26 03806 i013
[53]		renal (ACHN), leukemia (MOLT-4), non-small-cell lung (NCI-H460), colon (HCT-116), CNS (SF-295)Ijms 26 03806 i014
[66]		breast (MDA-MB-231, MCF-7), CNS (U-87, IMR-32)
PI3K inhibitor (in silico study).
Ijms 26 03806 i015
[55]		non-small-cell lung (EKVX, HOP-95), CNS (SNB-75)Ijms 26 03806 i016
[67]		breast (MCF-7)
PI3K inhibitor (in silico study).
Ijms 26 03806 i017
[57]		STAT3 inhibitorIjms 26 03806 i018
[70]		PARP-1 inhibitor
clinical use: breast cancer
Ijms 26 03806 i019
[59]		PKM2 activatorIjms 26 03806 i020
[71]		PARP-1 inhibitor
clinical use: melanoma
Ijms 26 03806 i021
[60]		PKR activatorIjms 26 03806 i022
[72]		PARP-1 inhibitor
Ijms 26 03806 i023
[61]		hepatocellular (Hub-7), colorectal (Caco2, HCT-116), breast (MDA-MB-231), prostate (PC3), lung (NCI-H727). PI3Kα inhibitorIjms 26 03806 i024
[74]		leukemia (HL-60TB, K-562, RPMI-8226), colon (HCT-116, HT29), melanoma (MDA-MB-435), ovarian (OVCAR-3), breast (MDA-MB468)
DHFR inhibitor
Table 5. ALR2 inhibition data of compounds 96–107.
Table 5. ALR2 inhibition data of compounds 96–107.
(Gen.) Formula/Ref.Compd. RR1ALR2 IC50 μM or (% Inhibn.)
Ijms 26 03806 i025
[81,82] 		96HIjms 26 03806 i02612.6
977,9-(CH3)2Ijms 26 03806 i027(31, 48 μM)
98HIjms 26 03806 i02811.4
998,9-(OCH3)2Ijms 26 03806 i029(35, 79 μM)
1007,9-(CH3)2Ijms 26 03806 i03017.4
1017-OCH3Ijms 26 03806 i0314.25
Ijms 26 03806 i032
[84] 		103H 7.6
1048-CH3 18.0
1058-Cl 31.4
Ijms 26 03806 i033
[83]		102 0.15
Ijms 26 03806 i034
[85]		106Ijms 26 03806 i035 0.42
107Ijms 26 03806 i036 0.046
Table 6. BZR-binding affinity of compounds 109–113.
Table 6. BZR-binding affinity of compounds 109–113.
(Gen.) Formula/Ref.Compd.X, n, R, R1[3H]Diazepam
Ki (nM)
Ijms 26 03806 i037
[87]		109 (Y-23684) 42.0
Ijms 26 03806 i038
[87,88]		110X = SO2, n = 2, R = CH2CH3,
R1 = Cl		1.1
111X = SO2, n = 2, R = Br, R1 = Cl0.61
112X = CH2, n = 1,
R = CH(OH)CH3, R1 = CH3		6.4
Ijms 26 03806 i039
[89]		113 695.0 ([3H]Flumazenil)
Table 7. Biological properties of representative tricyclic pyridazinones previously described.
Table 7. Biological properties of representative tricyclic pyridazinones previously described.
Compd./Ref.TargetCompd./Ref.Target
Ijms 26 03806 i040
[95]		MMP-8 inhibitorIjms 26 03806 i041
[106] 		cholinesterase (AChE, BChE) inhibitor
Ijms 26 03806 i042
[98]		PDE5 inhibitorIjms 26 03806 i043
[106] 		cholinesterase (AChE, BChE) inhibitor
Ijms 26 03806 i044
[101]		hA1 antagonistIjms 26 03806 i045
[106] 		AChE inhibitor, 5-HT6
Ijms 26 03806 i046
[103]		TSPO Ijms 26 03806 i047
[108] 		PKM2 activator
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Asproni, B.; Pinna, G.A.; Corona, P.; Coinu, S.; Piras, S.; Carta, A.; Murineddu, G. Therapeutic Potential of Tricyclic Pyridazinone-Based Molecules: An Overview. Int. J. Mol. Sci. 2025, 26, 3806. https://doi.org/10.3390/ijms26083806

AMA Style

Asproni B, Pinna GA, Corona P, Coinu S, Piras S, Carta A, Murineddu G. Therapeutic Potential of Tricyclic Pyridazinone-Based Molecules: An Overview. International Journal of Molecular Sciences. 2025; 26(8):3806. https://doi.org/10.3390/ijms26083806

Chicago/Turabian Style

Asproni, Battistina, Gérard A. Pinna, Paola Corona, Silvia Coinu, Sandra Piras, Antonio Carta, and Gabriele Murineddu. 2025. "Therapeutic Potential of Tricyclic Pyridazinone-Based Molecules: An Overview" International Journal of Molecular Sciences 26, no. 8: 3806. https://doi.org/10.3390/ijms26083806

APA Style

Asproni, B., Pinna, G. A., Corona, P., Coinu, S., Piras, S., Carta, A., & Murineddu, G. (2025). Therapeutic Potential of Tricyclic Pyridazinone-Based Molecules: An Overview. International Journal of Molecular Sciences, 26(8), 3806. https://doi.org/10.3390/ijms26083806

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