Protein kinases are a big family of enzymes, which trigger the phosphorylation of target protein substrates. Among these kinases, P38 kinase which is mediated in the regulation of Interleukin-I (IL-1) and Tumor Necrosis factor α (TNF-α) as pro-inflammatory cytokines secreted by macrophage and monocytes in response of inflammatory stimuli as Lipopolysaccharide (LPS). High expression of TNF-α is implicated in triggering many diseases as rheumatoid arthritis (RA), osteoarthritis, inflammatory bowel disease [7] .

In 2009, Pettus, L, H. Patented a new set of pyrido[2,3-d]pyridazine-2(1H)-one derivatives which are useful in prophylaxis and treatment of protein kinase mediated inflammation and related diseases e.g. rheumatoid arthritis, pulmonary diseases, pain. Among these compounds, compounds 1a, b and 2 showed the most potent activity in treatment of P38-kinase mediated diseases [7] . In 2011, R. M. Tynebor et al., Merck sharp & dohme, developed a new class of p38α selective pyridopyridazin-6-one series from the p38 α/β dual inhibitor with highselectivity, potency and with favor of suppressing lipopolysaccharide (LPS) induced TNF α production (proinflammatory cytokines) which resulting in effective therapeutically response against autoimmune driven diseases as rheumatoid arthritis. Form this new class, compound 3 is one of the most active derivatives in suppressing TNF α levels in LPS rodent model [8] . One year later, the same authors studied the structural activity relationship for the previously mentioned pyridopyridazin-6-one scaffold. They discovered compound 4 which has subnanomolar p38α activity with moderate p38β isoform selectivity [15] (Figure 3).

Pain is an unpleasant sensory and emotional experience associated with potential or actual tissue damage [16] . Pain represents a major symptom of many pathological conditions and millions of people worldwide must use analgesic agents in order to treat different types of pain. In 1998, H. Sladowska et al. synthesized a set of 4-aminosubstituted-2,6,7-trimethyl-1,5-dioxo-1,2,5,6-tetrahydropyrido[3,4-d]pyridazines which significantly inhibit the spontaneous locomotors activity and decrease the excitatory action of amphetamine and exerted analgesic properties in mice. From this set of compounds, compounds 5a-c showed strong analgesic activity as assayed in the “writhing syndrome” test. While, compounds 5a,b suppressed spontaneous locomotors activity [17] . In 2009, W. Pakulska et al. synthesized a new series of N-(dimethylamino) ethyl pyridopyridazinones. The compounds have been assayed for their analgesic effect in the hot-plate, tail-flick, and writhing tests [9] . Among them, compounds 6a,b exhibited potent analgesic activity [9] . While, compound 7 showed a potent analgesic and anti-inflammatory activity [18] (Figure 4).

Hypertension is a leading risk factor for cardiovascular disease development like coronary disease, myocardial infarction or stroke being also a recognized contributor to sudden death as well as to congestive cardiac failure

and renal insufficiency. Therefore, antihypertensive agents are of critical importance in order to achieve a better control of blood pressure. In 1975, Nishikawa et al. have synthesized a series of substituted-pyridopyridazine derivatives. The compounds have excellent diuretic activity and different characters from known diuretics such as thiazides, carbonic anhydrase inhibitors or furosemide. Among these compounds, Compound 8 showed an excellent diuretic activity [19] .

Endralazine “compound 9” is clinically approved direct acting vasodilator which resembles Hydralazine in chemical structure. Despite the close relationship, Endralazine is about five times more potent than Hydralazine and that allowing lower dosage usage. Thus, less immunological response would be achieved [3] [4] (Figure 5).

In 2001, A. Mitchinson et al., Merck Sharp & Dohme Limited, filed a patent for a new series of pyrido[2,3-d] pyridazine. These derivatives are a selective ligands for GABA_A receptors and therefore they are used in treatment a variety of disorders of the central nervous system. Such as, anxiety, panic disorder, animal phobia, social phobia, psychotic disorder. Such as, schizophrenia. Compound 10 showed the highest activity. It has a Ki value for displacement of [3H]-flumazenil from the α2 and/or α3 and/or α5 subunit of the human GABA_A receptor of 100 nM [5]. 2 years later, the same company filed another patent for a new class of substituted pyrido[2,3-c] py- ridazine derivatives as a useful treatment in the therapy of neorolgical disorders. Among these series of pyrido[2,3-c]pyridazine, compound 11 has a Ki value for displacement of [3H]-flumazenil from the α2 and/or α3 and/or α5 subunit of the human GABA_A receptor less than 100 nM [20] (Figure 4). 3 years later, the inventors of the first patent, developed a novel derivatives of 2,3,8-trisubstituted pyrido[2,3-d]pyridazine, Compounds 10 and 12, with two different synthetic methods. The new derivatives revealed high affinity ligands for the GABA_A receptor benzodiazepine binding site [21] (Figure 6).

Allergic rhinitis and allergic processes in general are mediated by histamine, an intracellular chemical messenger which is released from several cells and specifically by mast cells. In 2008, S. Vile, Glaxo Group Limited, patented a new class of substituted derivatives of pyrido[3,4-d]pyridazine 13a-c. The new compounds were found to have antihistaminic action which could be useful in treatment of inflammatory and allergic diseases of the respiratory tract such as bronchitis, allergic rhinitis, asthma, chronic obstructive pulmonary disease. Compound 13a showed a potent H1 receptor antagonist. It exhibited a longer duration of action than Azelastine [6] (Figure 7).

Bronchial asthma is a lung disease characterized by airway obstruction, inflammation and hyperresponsiveness [22] .

In 1995 R. Whilhelm, reported a series of pyrido-[2,3-d]pyridazinones as potent and selective Type IV Phosphodiesterase Inhibitors (14a-c). These inhibitors are promising drugs in the treatment of asthma, inflammation [11] .

In 1997, V. Dal Piaz et al. synthesized a group of heterocyclic-fused pyridazinones as selective phosphodiesterase (PDE) IV inhibitors. Among these heterocycles, Compound 15, pyrido[2,3-d]pyridazinone, exhibited a good balance of potency and selectivity versus PDE IV and it displayed an affinity for Rolipram binding site by 2 orders of magnitude lower than Rolipram [23] (Figure 8).

Diabetes mellitus is a chronic multifactorial disease characterized by a high blood glucose concentration (hyperglycemia). This complex disorder resulting from insulin deficiency (type I diabetes) or insulin resistance (type II diabetes), disturbs the metabolism of carbohydrates, fats and proteins giving rise to not only acute but also long- term complications. More than 90% of diabetic patients suffer from type II diabetes, i.e. non-exogenous insulin dependent diabetes [24] .

Increased glucose flux through the sorbitol pathway, which is mediated by the enzyme aldose reductase, has been implicated in the pathogenesis of diabetic complications. So, the discovery and development of aldose reductase inhibitors (ARIs) as potential therapeutic agents for alleviating these complications was important aim. In 1991, Mylari et al. synthesized a series of pyrido[2,3-d]pyridazin-5-yl)acetic acid, its ester 16a-c, 17a,b and evaluated their activity for inhibition of aldose reductase. Compound 16b showed potent aldose reductase inhibitory activity [12] (Figure 9).

Tuberculosis (TB) is a potentially serious infectious disease that mainly affects lungs. In 1967, Kakimoto found that some derivatives of pyrido[2,3-d]pyridazine have potent anti-tuberculousis. Compounds 18 showed the best activity against tuberclebacilli than other aza phthlazine derivatives [25] .

In 2005, J. Stanasiuk, developed another set of pyrido[3,4-d]pyridazine derivatives which have antimycobacterial activity. Compound 19 showed the strongest activity [13] (Figure 10).

Cancer consists in an uncontrolled proliferation of abnormal cells. These mutated cells invade surrounding tissues and sometimes migrate through the blood or lymph spreading to other body organs causing metastases, which are the major cause of death from cancer. Although a significant proportion of cancers can be cured by surgery combined with radiotherapy or chemotherapy, especially if they are detected early, this disease is currently a leading cause of death in developed countries [26] .

In 2009, J. Kaizerman et al. synthesized a new Pyrido pyridazine derivatives which showed anticancer activity. Those pyridopyridazine derivatives inhibit cascade of signals which regulate proliferation. Inhibition of proliferation could be achieved by inhibition of cyclin-dependent kinases. A survey of the literature reveals that the vast majority of kinase inhibitor scaffolds consist of planar heterocycles that present both key hydrogen bond donating and hydrogen bond accepting. Compounds 20a-d showed strong anticancer activity [10] .

Recently, 2014, P. Selvakumar et al. constructed a series of pyridopyridazin-3(2H)-one derivatives. Compounds 21a-c showed potent anticancer activity against MCF-7 breast cancer cell line relative to Doxorubicin as the standard drug. The antitumor potency is belonged to the electron withdrawing substituents in the aromatic ring besides the hydrazine incorporated in those derivatives [27] . This will open new avenue to elaborate potential anticancer agents against breast cancer (Figure 11).

Infectious diseases caused by microorganisms, bacteria, fungi and viruses have registered significant growth in recent years. The objective of microorganisms is to survive and many of them have determinants of resistance which can be expressed phenotypically as needed [28] .

Elassar et al. studied various anti-microbial biological effects of some newly synthesized fused pyridine derivatives. Among that, Compound 22 showed potential antimicrobial activity [14] (Figure 12).