It is known that inhibitors of phosphodiesterase 4 (PDE 4) are useful in the treatment of inflammatory and allergic processes such as asthma, non-steroidal antiinflammatory drugs-induced gastrointestinal damage and atopic dermatitis.

EP-A-85,840 discloses a series of triazolo-phthalazine derivatives of formula: which are useful as anxiolytic agents.

We have now found that the presence of a pyridine ring instead of the benzo ring in the above structure, provides new compounds which inhibit cyclic phosphodiesterases, in particular type 4 cyclic phosphodiesterases and have a very low emetic activity (10-100 times less active than rolipram in inducing emesis in dogs).

Accordingly, the present invention provides a compound which is a heterocycle of formula (I):

wherein: R1 represents a hydrogen atom or a -(CH2)m-Y group, wherein m is an integer from 0 to 4 and Y represents an alkyl, haloalkyl (preferably trifluoromethyl), alkoxy, alkoxycarbonyl, C3-C7 cycloalkyl, norbornyl (preferably 2- norbornyl) or phenylalkenyl group, or an aromatic group (preferably phenyl or pyridyl) which aromatic group Y may optionally be substituted by one or more halogen atoms; R2 represents an aromatic group (preferably phenyl, naphthyl or thienyl) which aromatic group may optionally be substituted by one or more halogen atoms or alkyl, alkoxy, C3-C6 cycloalkoxy, methylenedioxy, nitro, dialkylamino or trifluoromethyl groups; and R3 represents a hydrogen or halogen atom (preferably chloro) or an alkyl group, and pharmaceutically acceptable salts thereof.

The alkyl, haloalkyl, alkenyl or alkynyl groups and moieties, such as in the alkoxy groups, mentioned in relation to the groups R1 - R3 in compounds of the invention are usually "lower" alkyl, that is containing up to 6 and particularly up to 4 carbon atoms, the hydrocarbon chain being branched or straight. Examples of alkyl groups and moieties are CH3, C2Hs, C3H7, i-C3H7, n-C4Hg, i-C4Hg, isoamyl and neopentyl.

When any of the groups, such as R1 or R2 has a chiral centre, the compounds of formula (I) exhibit optical isomerism and the isomers are within the scope of the present invention.

Examples of Rl are the preferred alkyl groups mentioned above, cyclopropyl, cyclopropylmethyl, cyclobutyl, cyclobutylmethyl, cyclopentyl and cyclopenthylmethyl.

Examples of R2 are phenyl, 3-chlorophenyl, 4-chlorophenyl, 3-fluorophenyl, 4-fluorophenyl and 3-nitrophenyl.

Examples of R3 are hydrogen, alkyl or chloro, preferably in the 8- or 9- positions.

The most preferred compounds of the invention are 6-(4-fluorophenyl)-3-isobutyl-1,2,4-triazolo[4,3- b]pyrido[3,2-d]pyridazine, 3-cyclopropylmethyl-6-(3- <BR> <BR> <BR> nitrophenyl)-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine, <BR> <BR> <BR> 3-cyclopropyl-6-phenyl-1,2,4-triazolo[4,3-b]pyrido[3,2- d]pyridazine, and 3-cyclobutylmethyl-6-(3-nitrophenyl)- 1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine.

According to a further feature of the present invention, the heterocyclic compounds of formula (I) can be prepared from the corresponding hydrazine derivative of formula (II): wherein R2 and R3 are as defined above, by reaction with a reactive derivative of a carboxylic acid of the general formula (III): HOOC - R1 (III) wherein R1 is as defined above. The reactive derivative of the said carboxylic acid may be, for example, a halide (preferably chloride), an anhydride or a mixed anhydride.

The reaction is preferably carried out in an inert organic solvent such as methylene chloride, dioxane or tetrahydrofuran, in the presence of an organic nitrogen- containing base, e.g. triethylamine and at a temperature between -100C and +600C. In the reaction, the corresponding hydrazide of general formula (IV) is first formed: wherein R1, R2 and R3 are as defined above. A suspension of this hydrazide (IV) in an organic solvent such as dioxane, tetrahydrofuran, isopropanol or n-butanol, is heated, for example at the boiling point of the solvent, to give the corresponding heterocyclic compound of formula (I).

The hydrazine derivative of formula (II) may be prepared by: 1) reacting a hydrazone of formula (V):

wherein R2 and R3 are as defined above and R4 is an alkyl group, with a phosphorus halide or phosphorus oxyhalide (preferably phosphorus oxychloride), to form the intermediate compound of formula (VI): wherein R2 and R3 are as defined above and X is a chlorine or bromine atom; 2) reacting compound (VI) with an alkyl carbazate (preferably t-butyl carbazate) of formula (VII): H2N-NH-COOR5 (Vll) wherein R5 is an alkyl group, to give the alkoxycarbonylhydrazine derivative (VIII):

wherein R2, R3 and R5 are as defined above; and 3) treating compound (VIII) with hydrogen chloride in an anhydrous solvent as ethanol.

The reaction between the hydrazone of formula (V) and a phosphorus halide or phosphorus oxyhalide is carried out with an excess of reagent at a temperature from 800C to 1200C, then removed the excess of reagent and poured into cold water. In this way the compound (VI) is obtained.

The reaction of (VI) with the alkyl carbazate of formula (VII) to obtain the corresponding alkoxycarbonylhydrazine derivative (VIII), is preferably carried out in the presence of an organic solvent as tetrahydrofuran or dioxan at a temperature of from 60"C to the boiling point of the reaction medium.

The alkoxycarbonylhydrazine derivative (VIII) may, for example, be transformed into the hydrazine derivative (II) at room temperature in hydrogen chloride-ethanol saturated solution.

The hydrazone derivatives of formula (V) are known compounds which can be prepared from the corresponding 2- acylnicotinic acid by known methods described in the literature.

The inhibition of cyclic nucleotide phosphodiesterase 4 from guinea-pig hearts was performed using 96-well microtiter plates as described by Verghese et al., (Molecular Pharmacology, 47, 1164-1171 (1995)).

The results from such test are shown in Table 1.

TABLE 1 Compound * PDE4 IC50 (µM) A 10 6 2 7 0.3 12 3 31 0.2 47 0.7 55 0.2 60 0.1 61 2 109 0.04 112 0.7 113 0.2 (*) See structures in Table 2.

Compound A is 3-isobutyl-6-phenyl-1,2,4-triazolo[3,4-a] phthalazine, a compound included in EP-A-85,840.

As it can be seen from Table 1, the compounds of formula (I) are cyclic phosphodiesterase inhibitors, in particular type 4 cyclic AMP phosphodiesterase inhibitors. The compounds are also capable of blocking the production of some pro-inflammatory cytokines such as, for example, TNFa.

Thus, they can be used in the treatment of allergic, inflammatory and immunological diseases, as well as those diseases or conditions where the blockade of pro- inflammatory cytokines or the selective inhibition of PDE 4 could be of benefit.

These diseases states include asthma, rheumatoid

arthritis, osteoarthritis, osteoporosis, bone-formation disorders, glomerulonephritis, multiple sclerosis, Graves ophtalmopathy, myasthenia gravis, insulin-dependent diabetes mellitus, graft rejection, gastrointestinal disorders such as ulcerative colitis or Crohn disease, septic shock, adult distress respiratory syndrome, and skin diseases such as atopic dermatitis, contact dermatitis, acute dermatomyositis and psoriasis.

They can also be used as improvers of cerebrovascular function as well as in the treatment of other CNS related diseases such as dementia, Alzheimer' s disease, depression, and as nootropic agents.

The compounds of the present invention are also of benefit when administered in combination with other drugs such as steroids and immunosuppressive agents, such as cyclosporin A, rapamycin or T-cell receptor blockers. In this case the administration of the compounds allows a reduction of the dosage of the other drugs, thus preventing the appearance of the undesired side effects associated with both steroids and immunosuppressants.

The compounds of the invention have also shown their efficacy in blocking, after preventive and/or curative treatment, the erosive and ulcerogenic effects induced by a variety of etiological agents, such as antiinflammatory drugs (steroidal or non-steroidal antiinflammatory agents), stress, ammonia, ethanol and concentrated acids. They can be used alone or in combination with antacids and/or antisecretory drugs in the preventive and/or curative treatment of gastrointestinal pathologies like drug-induced ulcers, peptic ulcers, H. Pylori-related ulcers, esophagitis and gastro-esophageal reflux disease.

They can also be used in the treatment of pathological situations where damage to the cells or tissues is produced

through conditions like anoxia or the production of an excess of free radicals. Examples of such beneficial effects are the protection of cardiac tissue after coronary artery occlusion or the prolongation of cell and tissue viability when the compounds of the invention are added to preserving solutions intended for storage of transplant organs or fluids such as blood or sperm. They are also of benefit on tissue repair and wound healing.

The present invention also provides a heterocyclic compound of formula (I) for use in a method of treatment of the human or animal body by therapy, particularly for use as a PDE 4 inhibitor or to block the production of a pro- inflammatory cytokine such as TNFa.

The present invention additionally provides a pharmaceutical composition which comprises, as active ingredient, at least one heterocyclic compound of formula (I), and a pharmaceutically acceptable carrier or diluent.

Preferably the compositions are in a form suitable for oral, inhalation, rectal, transdermal, nasal, topical or parenteral administration.

The pharmaceutically-acceptable carriers or diluents which are admixed with the active compound or compounds to form the compositions of this invention are well known per se and the actual excipients used depend inter alia on the intended method of administration of the compositions.

Compositions of this invention are preferably adapted for administration per os. The compositions for oral administration may take the form of tablets, capsules, lozenges or effervescent granules or liquid preparations such as elixirs, syrups or suspensions, all containing one or more compounds of the invention. Such preparations may be made by methods well known in the art, for instance by mixing the heterocyclic compound of formula (I) with the

pharmaceutically acceptable carrier or diluent.

The diluents which may be used in the preparation of the compositions include those liquid and solid diluents which are compatible with the active ingredient, together with colouring or flavouring agents if desided. Tablets or capsules may conveniently contain from 1 to 100 mg and preferably from 5 to 50 mg of active ingredient. The compounds may also be incorporated into pellets coated with appropriate natural or synthetic polymers known in the art to produce sustained release characteristics or incorporated with polymers into tablet form to produce the same characteristics.

The liquid compositions adapted for oral use may be in the form of solutions, suspensions or aerosols. The solutions may be aqueous or aqueous-alcoholic solutions in association with, for example, sucrose or sorbitol to form a syrup. The suspensions may comprise an insoluble or microencapsulated form of an active compound of the invention in association with water and other acceptable solvents together with a suspending agent or flavouring agent.

Compositions for inhalation administration may be in the form of solutions, suspensions or micronized powder, contained in an appropriate inhaler.

Compositions for parenteral injection may be prepared, which may or may not be freeze-dried and which may be dissolved in water or an appropriate parenteral injection fluid.

In human therapy, the doses of the heterocyclic compound depend on the desired effect and duration of the treatment; adult doses are generally from lmg to 100 mg per day. In general the physician will decide the posology, taking into account the age and weight of the patient being treated.

The following Examples further illustrate the invention.

EXAMPLE 1 a) A mixture of t-butoxycarbonylhydrazone of 2- benzoylnicotinic acid (45 g; 13.2 mols) in phosphorus oxychloride (500 ml) was boiled under reflux for one hour, then the excess of phosphorus oxychloride was removed under reduced pressure, the residue treated with ice-water and extracted twice with methylene chloride. The organic solution was washed with 4% sodium bicarbonate aqueous solution, with brine and after drying (Na2SO4), the solvent removed in vacuo. The obtained solid was collected with a mixture of diethyl ether-petrol ether 1:1 to give 5-chloro- 8-phenylpyrido[2,3-d]pyridazine as a red solid, (25.4 g; 80% yield). b) To a suspension of the above compound (18.2; 0.075 mols) in anhydrous tetrahydrofuran (180 ml), t-butyl carbazate (10.0 g; 0.075 mols) was added and the mixture was boiled under reflux for one hour. After cooling the crystallized solid was collected by filtration when 5-t- <BR> <BR> <BR> <BR> butoxyCarbonylhydrazino-8-phenylpyrido[2,3-d]pyridazine was obtained (28.5 g). This compound was solved in ethanol (150 ml), hydrogen chloride in ethanol saturated solution (100 ml) was added and the resulting mixture stirred at room temperature for 15 hours. A solid was formed which was collected by filtration and washed with diethyl ether to give 5-hydrazino-8-phenylpyrido[2,3-d]pyridazine dihydrochloride (21.6 g; 92% yield). c) To a suspension of 5-hydrazino-8-phenylpyrido[2,3- d]pyridazine dihydrochloride (1.24 g; 0.004 mols) in methylene chloride (30 ml), triethylamine (1.9 ml; 0.013 mols) was added, then stirred at room temperature for 15 minutes and pivaloyl chloride (0.5 ml; 0.0044 moles) slowly

added. After stirring at room temperature for two hours, water (30 ml) was added, the formed yellow solid, collected by filtration and washed with diethyl ether to give the intermediate hydrazide. This compound was suspended in n- butanol (30 ml), boiled under reflux for 15 hours and on cooling, crystallized a white solid which was collected by filtration and washed with diethyl ether. The obtained solid was purified by flash column chromatography with silica gel and methylene chloride-ethanol-ammonium hydroxide 200:8:1 as eluent. 3-t-butyl-6-phenyl-1,2,4-triazolo[4,3-b]pyrido[3,2- d]pyridazine was obtained (0.83 g; 69% yield), m.p. 188.1 (determined by Differential Scanning Calorimetry, Perkin- Elmer DSC-7 (compound 8 in Table 2).

The heterocyclic compounds of formula (I) in Table 2 were prepared according to the processes disclosed in this Example, but with the appropriate starting materials.

TABLE 2 Compound R R2 Ra m.p. OC 1 H C6H5 H 215.8 2 CH3 " 215.9 3 C2H5 " 194.1 4 C3H7 " 168.1 5 i-C3H7 176.8 6 n-C4Hg 162.9 7 i-C4Hg 179.7 8 t-C4Hg 188.1 9 n-C5H11 " " 137.4

Compound R1 R2 R3 m.p. °C 10 neopentyl 216.3 11 t-amyl 153 12 cyclopropyl 244.3 13 cyclobutyl 218 14 cyclopentyl 202.4 15 cyclohexyl 196.3 16 cyclopropyl-CH2 " " 195 17 cyclobutyl-CH2 " " 183 18 cyclopentyl-CH2 " " 192 19 cyclohexyl-CH2 " " 212.8 20 2-norbornyl-CH2 " " 217 21 C6H5 304.1 22 C6H5-CH2 192 23 C6H5-CH2CH2 " " 176 24 C6H5-CH=CH 278 25 CF3 192.5 26 H3CO-CH2 159 27 2-C1C6H4 206 28 4-pyridyl 333.4 29 CH3 4-FC6H4 276 30 n-C4Hg 111 31 i-C4Hg 135 32 t-C4Hg 195 33 neopentyl 216 34 cyclopropyl " " 245 35 cyclohexyl " 177 36 cyclopropyl -CH2 160 37 cyclobutyl-CH2 132 38 cyclopentyl-CH2 162 39 2-norbornyl-CH2 " " 161 40 C6H5-CH=CH " 272 41 C2H5OOC-CH2 " 185 42 i-C4H9 3-FC6H4 " 147 43 neopentyl 190 44 cyclopropyl " 222 45 cyclopropyl-CH2 " 174 46 cyclobutyl-CH2 139

Compound R1 R2 R3 m.p. °C 47 cyclopentyl-CH2 " " 145 48 i-C4H9 2-FC6H4 " 202 49 t-C4H9 " " 212 50 neopentyl 235 51 cyclopropyl " 262 52 cyclopropyl-CH2 " 224 53 i-C4H9 4-C1C6H4 133 54 cyclopropyl 208 55 i-C4H9 3-C1C6H4 113 56 t-C4Hg " 160 57 neopentyl 177 58 t-amyl 150 59 cyclopropyl 189 60 cyclopropyl-CH2 136 61 cyclobutyl-CH2 156 62 cyclopentyl-CH2 147 63 i-C4H9 2-ClC6H4 182 64 neopentyl 216 65 cyclopropyl 198 66 i-C4H9 4-BrC6H4 135 67 neopentyl 204 68 cyclopropyl 208 69 cyclopropyl-CH2 " " 140 70 cyclopentyl-CH2 187 71 2-norbornyl-CH2 " " 174 72 i-C4H9 3-BrC6H4 " 152 73 t-C4Hg 160 74 neopentyl " 177 75 cyclopropyl 186 76 cyclopentyl-CH2 " " 143 77 i-C4H9 3,4-diClC6H3 143 78 neopentyl 215 79 i-C4H9 3-CH3C6H4 " 119 80 cyclopropyl 206 81 i-C4H9 2-CH3C6H4 " 147 82 neopentyl 191 83 cyclopropyl 200

Compound R1 R2 R3 m.p. "C 84 i-C4Hg 3,4-diCH3C6H3 165 85 neopentyl n n 184 86 cyclopropyl 182 87 cyclohexyl " 211 88 cyclopentyl-CH2 n n 144 89 i-C4Hg 3-CF3C6H4 n 139 90 cyclopropyl 172 91 cyclopentyl-CR2 n n 141 92 i -C4Hg 4 -CH3OC6H4 n ' 177 93 cyclopropyl n a 164 94 i-C4H9 3-CH3OC6H4 n 119 95 neopentyl 155 96 cyclopropyl " 192 97 i-C4Hg 2 -CH3OC6H4 n 181 98 cyclopropyl 211 99 n 3,4-diCR3OC6H3 n 177 100 i-C4H9 e 158 101 t-C4Hg n 251 102 neopentyl n 208 103 cyclopropyl 208 104 i-C4Hg HzCOe 193 105 tC4Hg n n 210 106 neopentyl n n 219 107 cyclopropyl n az 162 108 i-C3H7 3-NQC6H4 n 176 109 i-C4R n n 178 110 neopentyl n n 229 111 cyclopropyl n n 234 112 cyclopropyl-CR2 n n 164 113 cyclobutyl-CH2 n n 150 114 cyclopentyl-CH2 n 183 115 cyclopropyl 3- (CH3) 2NC6H4 213

Compound R1 R2 R3 m.p. °C 116 i-C4Hg 2-naphthyl " 140 117 cyclopropyl " " 212 118 i-C4Hg 2-thienyl " 196 119 cyclopropyl " " 214 120 i-C4Hg 3-thienyl " 166 121 cyclopropyl " " 183 122 i-C4H9 C6H5 8-H3C 170 123 neopentyl " " 221 124 cyclopropyl " " 185 125 cyclopentyl-CH@ " " 163 126 2-norbornyl-CH2 " 193 127 i-C4Hg " 9-C1 174 128 cyclopropyl n 149 129 cyclopropyl-CH2 " " 175 130 cyclopentyl-CH2 " " 175 The following Examples illustrate pharmaceutical compositions according to the invention.

EXAMPLE 2 3,000 inhalation-flasks each containing 40 mg of 3-t- butyl-6-phenyl-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine (active compound) were prepared as follows: Active compound 120 g Sorbitan trioleate 4 g propellent q.s. 60 1 Procedure The microcrystalline suspension prepared with these ingredients was introduced in the inhalation-flasks at a volume of 20 ml per flask with a filling machine. The flasks

were furnished with an appropriate valve which released 0.2 ml of suspension for each activation (0.4 mg of active compound).

EXAMPLE 3 15,000 capsules each containing 20 mg of 3-t-butyl-6- phenyl-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine (active compound) were prepared from the following formulation: Active compound 300 g Sodium carboxymethyl starch 330 g Talc 195 g Hydrogenated castor oil 165 g Corn starch 495 g Procedure The above ingredients were sieved through a 60 mesh sieve, then mixed in a suitable mixer and filled into 15,000 gelatine capsules.