PDE5 inhibitors and their applications

PDE5 belongs to a superfamily of enzymes that catalyzes the hydrolysis of cyclic nucleotides cAMP and cGMP to the corresponding 5-nucleoside monophosphate. PDE5 takes part in many physiological and pathological functions, therefore selective PDE5 inhibitors are potentially useful for a variety of pathologies. At the present, PDE5 inhibitors available on the market have been used for the treatment of erectile dysfunction but, at the same time, are in clinical trials investigating other pathologies such as pulmonary arterial hypertension, coronary vasospasm, benign prostatic hyperplasia etc. This review analyzes the PDE5 inhibitors currently in clinical use, the drugs in clinical trials and the most representative chemical classes published in literature in this last decade.

Synthesis and complete 1H, 13C and 15N NMR assignment of substituted isoxazolo[3,4-d]pyridazin-7(6H)-ones

The synthesis and complete assignment of all hydrogen, carbon and nitrogen NMR signals of several new isoxazolo[3,4-d]pyridazin-7(6H)-ones is reported. The spectroscopic characterization is extended to previously described analogues.

Selective ACAT inhibitors as promising antihyperlipidemic, antiathero-sclerotic and anti-Alzheimer drugs

Inhibition of ACAT, the enzyme which catalyses the intracellular formation of cholesteryl esters, is a very attractive target for the treatment of hypercholesterolaemia and atherosclerosis. However, in the past years many ACAT inhibitors gave disappointing results in clinical trials showing very low efficacy. In addition, their development was affected by the adrenotoxicity observed in many compounds. The discovery of two isoforms of the enzyme, namely ACAT1 and ACAT2, with different substrate specificity and different potential function, offers a precious information for planning selective inhibitors with reduced secondary effects. Today some potent, bioavailable and non adrenotoxic ACAT inhibitors are under clinical evaluation. Amongst others, a very promising compound is Avasimibe, presently in phase III clinical trials as anti-hyperlipidemic and anti-atherosclerotic agent. Finally, ACAT inhibitors have recently been proposed for the treatment of Alzheimer's disease.

5,6-Diphenylpyridazine derivatives as acyl-CoA:cholesterol acyltransferase inhibitors

Alkyl-5,6-diphenylpyridazine derivatives combining several main features of ACAT inhibitors, such as a long alkyl side chain linked to a heterocycle and the o-diphenyl system, were synthesized and tested. Moreover, modeling studies on representative terms were performed. Some compounds displayed ACAT inhibition in the micromolar range, both on the enzyme isolated from rat liver microsomes and in cell-free homogenate of murine macrophages.

Phenylpiperazinylalkylamino substituted pyridazinones as potent alpha(1) adrenoceptor antagonists

QSAR models have been used for designing a series of compounds characterized by a N-phenylpiperazinylalkylamino moiety linked to substituted pyridazinones, which have been synthesized. Measurements of the binding affinities of the new compounds toward the alpha(1a)-, alpha(1b)-, and alpha(1d)-AR cloned subtypes as well as the 5-HT(1A) receptor have been done validating, at least in part, the estimations of the theoretical models. This study provides insight into the structure activity relationships of the alpha(1)-ARs ligands and their alpha(1)-AR/5-HT(1A) selectivity.

Phosphodiesterase 4 inhibitors, structurally unrelated to rolipram, as promising agents for the treatment of asthma and other pathologies

An increase of cyclic adenosine and guanosine monophosphate (cAMP and cGMP) level can be achieved by inhibition of phosphodiesterases (PDEs), which are the enzymes responsible for the conversion of these second messengers into the corresponding 5-monophosphate inactive counterparts. The high heterogeneity in PDE families and in their tissue distribution, as well as their different functional role, make these enzymes very attractive targets for medicinal chemists. The PDE 4 family is particularly abundant in immunocompetent cells, where an increase of cAMP leads to the inhibition of the synthesis and release of pro-inflammatory mediators, cytokines and active oxygen species. Moreover PDE 4 inhibitors are able to reduce bronchial smooth muscle tone in vitro and show bronchodilatory effects in vivo. Thus, the current therapy for asthma, which is based on a combination of beta(2) agonists and corticosteroids, could be replaced by treatment with PDE 4 inhibitors. This review mainly covers PDE 4 inhibitors structurally related to xanthines and Nitraquazone, which appear to be very attractive models for the synthesis of novel PDE 4 inhibitors potentially useful for the treatment of asthma, chronic pulmonary obstructive disease and some autoimmune diseases. These compounds could be devoid of the central side-effects (nausea, vomiting, headache) of the archetypal Rolipram, which hampered its development as a drug. The review also highlights the novel structural classes of PDE 4 inhibitors recently reported in the literature.

Antinociceptive activity of a 3(2H)-pyridazinone derivative in mice

The antinociceptive activity of a 3(2H)-pyridazinone derivative (18a) was investigated in mice. 18a administered at doses which did not change either motor coordination or locomotor activity was able to induce antinociceptive effects in four nociceptive tests, the hot plate test, the tail flick test, the writhing test, and the formalin test. In the hot plate and tail flick test, 18a-induced antinociception was observed both after intraperitoneal administration and after intracerebroventricular injection thus indicating 18a has a central site of action. The pretreatment with the opioid antagonist naloxone, the alpha2-antagonist yohimbine or the GABA(B) antagonist CGP 35348 did not change 18a-induced antinociception in the hot plate test and in the tail flick test. Pretreatment with nicotinic antagonist mecamylamine did not change 18a effects either. A reversion of the 18a effects was observed after pretreatment with the muscarinic antagonists atropine and pirenzepine. Binding experiments revealed that 18a binds to muscarinic receptors, suggesting that 18a antinociception is mediated by central muscarinic receptors. The above findings together with the lack of parasympathomimetic cholinergic side effects indicate useful clinical application for this compound.

Isoxazolo-[3,4-d]-pyridazin-7-(6H)-one as a potential substrate for new aldose reductase inhibitors

The isoxazolo-[3,4-d]-pyridazin-7-(6H)-one (2) and its corresponding open derivatives 5-acetyl-4-amino-(4-nitro)-6-substituted-3(2H)pyridazinones (3, 4) were used as simplified substrates for the synthesis of new aldose reductase inhibitors with respect to the previously reported 5, 6-dihydrobenzo[h]cinnolin-3(2H)one-2 acetic acids (1). Moreover, a few derivatives lacking the 5-acetyl group were prepared. Several compounds derived from 2 displayed inhibitory properties comparable to those of Sorbinil. In this class the presence at position 6 of a phenyl carrying an electron-withdrawing substituent proved to be beneficial, independently from its position on the ring (5g,j-l). Acetic acid derivatives were more effective than propionic and butyric analogues. On the contrary, all the monocyclic compounds (6-8) were either inactive or only weakly active. The 3-methyl-4-(p-chlorophenyl)isoxazolo-[3,4-d]-pyridazin-7-(6H )-one acetic acid (5g), which proved to be the most potent derivative, was also investigated in molecular modeling studies, to assess possible similarities in its interaction with the enzyme, with respect to the model 1.

Isoxazolo-[3,4-d]-pyridazin-7-(6H)-ones and their corresponding 4,5-disubstituted-3-(2H)-pyridazinone analogues as new substrates for alpha1-adrenoceptor selective antagonists: synthesis, modeling, and binding studies

A series of phenylpiperazinylalkyl moieties were attached to monocyclic or bicyclic substituted pyridazinones and the new compounds tested for their affinity towards alpha1-adrenoceptor and its alpha1a, alpha1b and alpha1d subtypes, as well as serotonin 5-HT1A receptor. Several analogues (5, 6, 9, and 10) showed remarkable potency and selectivity towards alpha1a, and alpha1d with respect to alpha1b subtype. None of the test compounds exhibited significant affinity for 5-HT1A receptor. Finally, on the basis of the alpha1-AR subtypes 3D models recently proposed, we have elaborated theoretical interaction models for the new compounds. The theoretical study allowed us to predict the affinity of the new compounds as well as to infer the structural/dynamics determinants of their interaction with the three alpha1-AR subtypes.

Chiral resolution and absolute configuration of the enantiomers of 5-acetyl-2-methyl-4-methylsulfinyl-6-phenyl-3(2H)-pyridazinone and evaluation of their platelet aggregation inhibitory activity

In a series of 5-acyl-6-phenyl-2,4-substituted-3(2H)-pyridaziones the derivative 1a, with a sulfur stereogenic center, had the most potent activity as human platelet aggregation inhibitor. The resolution of rac-1a was successfully performed by chiral chromatography on Chiralcel OD-R, OD-H, and Chiralpak AD columns and scaled up to a preparative level. The absolute configuration of (-)-(S)-1a was determined by X-ray crystallographic analysis. In vitro human platelet aggregation inhibitory activity was evaluated. Both the enantiomers showed IC50 values in the same micromolar range, but the (-)-(S) isomer was slightly more potent [(S)/(R) potency ratio was 4/1].

Indenopyridazinone derivatives as potential antisecretory and antiulcer agents

A series of substituted indenopyridazinones (4b-h) has been synthesized and tested for their antisecretory and antiulcer activity, in comparison with ranitidine, as reference drug. While the monomethoxy (4b-d), as well as the benzyloxy (4f) and the 6,9-dimethoxy (4g) derivatives were found to be devoid of overt antisecretory properties, the 9-methoxy (4e) was weakly active. The most interesting compound of this class was the 7,8-dimethoxy substituted (4h), which at an oral dose of 30 mg/kg still retains a significant activity. The dihydroderivatives of 4g,h (compounds 1g,h) were more active (1g) or comparable (1h) to the parent compounds, thus proving that the 4, 4a-double bond, which was an essential requirement in the analogues 2 and 3, is not necessary in this new series. The disubstituted derivatives (1g,h; 4g,h) were also tested as antiulcer agents, in two different models. All compounds were able to prevent haemorragic lesions induced in rats by 90% ethanol in a dose dependent manner. In the indomethacin model they still showed a significant activity, though lower than in the previous test. Attempts have been made to elucidate their mechanism of action.

Novel heterocyclic-fused pyridazinones as potent and selective phosphodiesterase IV inhibitors

A series of 6-aryl-4,5-heterocyclic-fused pyridazinones were designed and synthesized as selective phosphodiesterase (PDE) IV inhibitors. Biological evaluation of these compounds demonstrated a good selectivity profile toward the PDE IV family and greatly attenuated affinity for the Rolipram high-affinity binding site that seems to be responsible for undesiderable side effects. Structure-activity relationships (SARs) studies showed that the presence of an ethyl group at pyridazine N-2 is associated with the best potency and selectivity profile.

Study of the antisecretory and antiulcer mechanisms of a new indenopirydazinone derivative in rats

The present study investigates the antisecretory and antiulcer mechanisms of a new indenopyridazinone derivative previously reported to inhibit gastric acid secretion in pylorus-ligated rats and to prevent gastric ulcerations induced by indomethacin or ethanol in the same animal species. The new compound was tested on the acid hypersecretion induced by histamine, pentagastrin and bethanechol in in vivo and in vitro experimental models. Furthermore, its influence on the mucosal layer adhering the gastric wall in indomethacin-treated rats was considered. Ranitidine was selected as a reference drug. The results obtained demonstrated that the new molecule, at variance with ranitidine, exerts antiulcer activity mainly enhancing the gastric mucosal integrity and simultaneously inhibiting the gastric acid hypersecretion evoked exclusively by cholinergic pulses. Therefore, an involvement of a neuronal rather than an effectorial mechanism has been suggested. Due to these mechanisms of action it clearly differentiates from ranitidine and its possible application in the peptic disease resistant to H2-blockers could be speculated.

Synthesis of 4,5-functionalized-2-methyl-6-(substituted aryl)-3 (2H)-pyridazinones: a new group of potent platelet aggregation inhibitors

A series of 4,5-functionalized-2-methyl-6-(substituted phenyl)-3 (2H)-pyridazinones were synthesized and evaluated as platelet aggregation inhibitors in human platelet rich plasma (PRP). The new products generally displayed significant higher activity with respect to the corresponding unsubstituted aryl compounds. Compounds 27 and 31 appeared of particular interest, being their IC50s in the submicromolar range. Structure-activity relationships (SARs) are discussed.

Synthesis and evaluation as platelet aggregation inhibitors of 6-phenyl-2,4-substituted-3(2H)-pyridazinones and their rigid analogues benzo[h]cinnolin-3,5-diones

A series of 5-acyl-6-phenyl-2,4-substituted-3(2H)-pyridazinones and analogous benzo[h]cinnolin-3,5-diones with reduced flexibility has been prepared and evaluated as human platelet aggregation inhibitors. The 4-methylsulfoxide 13b was the most potent compound of the series (IC50 = 1.2 microM). SAR studies have shown the primary importance of an electronegative substituent at position 4 and an acetyl group at position 5 of the pyridazine system for potent platelet aggregation inhibitory activity. Biological tests performed on a group of representative compounds showed these products have not effects on prostaglandins, thromboxanes and nitric oxide biosynthetic pathways. Some of synthesized compounds produced a moderate increase of cAMP level in platets which does not depend on the adenylate-cyclase stimulation. Tests performed on human platelet PDE III have shown that these compounds are not inhibitors of this enzyme.

4-substituted-5-acetyl-2-methyl-6-phenyl-3(2H)pyridazinones as PGE2 and IL-1 release inhibitors from mouse adherent macrophages

A series of 4,5-functionalized 3(2H)-pyridazinones were evaluated as prostaglandin E2 (PGE2) and interleukin-1 (IL-1) release inhibitors from mouse adherent macrophages. Among the tested compounds only 2b was found to be devoid of activity in both the PGE2 and IL-1 tests, whereas the other compounds, showed a significant dose-dependent activity. Compounds 2a, 3 and 4 were able to inhibit PGE2 better than IL-1 release from stimulated macrophages. Compound 4, which showed an IC50 = 5.5 microM in the IL-1 test, appears to be a promising agent in this cell inflammation model. Structure-activity relationship (SAR) studies demonstrated the importance of the presence of a substituent characterized by a positive sigma constant at position 4 of the pyridazine system.

Synthesis and evaluation of in vitro antitumor activity of some substituted 5-pyridazinyl-styrylketones

A series of twenty pyridazinyl-styrilketones, substituted at position 4 with linear or cyclic tertiary amino groups, were synthesized and evaluated in vitro as antitumor agents against 60 human tumor cell-lines. Moderate activity and differential cell sensitivity were found for several of the compounds. Cell-line XF-498L (panel:Brain) showed differential cell sensitivity towards compound 5b (more than 1000 times the mean sensitivity of all cell-lines).

5-Acyl-6-aryl-4-nitro-3(2H)pyridazinones and related 4-amino compounds: synthesis and pharmacological evaluation

Several 4-nitro- and 4-amino-5-acyl-6-aryl-3(2H)pyridazinones were prepared and their in vitro and ex vivo antiaggregatory properties were evaluated. 4-Nitro derivatives 3 generally showed good activity in vitro towards arachidonic acid (AA)-induced human blood platelet aggregation. The 4-amino compound 4a, which has weak in vitro activity, exhibited antiplatelet activity, particularly on adenosine dephosphate (ADP)-induced aggregation ex vivo in rabbit. Moreover, the same compound was shown to be active in platelet-activating factors (PAF)-induced rat paw hyperalgesia and to be endowed with low acute oral toxicity. The 4-amino derivatives 4a-m and the other pyridazinones 5-9 administered orally to rats were also found to be more potent antiinflammatory agents than acetyl salicylic acid (ASA). Compounds 3a and 4a, tested in vitro on lipopolysaccharide (LPS)-stimulated rat peritoneal macrophages, were seen to be active in the inhibition of prostaglandin E2 (PGE2) production and interleukin-1 activity. Structure-activity relationship studies in the series of antiaggregating pyridazinones 3 have shown the primary importance of the nitro and acetyl substituents at positions 4 and 5, respectively. Hydrophobic substituents at position 2 were also required for better activity.

New 6-cyano-3-phenyl-1-,2,3,4-tetrahydro-1,2-diazepin-5-ones: synthesis and in vitro antitumor activity evaluation

A series of 6-cyano-3-phenyl-1,2,3,4-tetrahydro-1,2-diazepin-5-ones were synthesized and evaluated in vitro as antitumor agents against several human tumor cell lines. Moderate activity and differential cellular sensitivity were found for a few of the tested compounds.

1,2,3,4,-Tetrahydro-1,2-diazepine derivatives: synthesis and evaluation of antileukemic activity

A series of 3-aryl or etheroaryl-6-cyano-7-phenyl-1,2,3,4-tetrahydro-1,2-diazepin- 5-ones was synthesized and evaluated for its antitumor activity against P388 leukemic tumor system in mice. None of the tested compounds showed significant antitumor properties.

Gravimetric determination of chlorhexidine using tetraphenylborate ion

A precise and accurate gravimetric procedure was developed for the determination of chlorhexidine diacetate, digluconate, or dihydrochloride. Sodium tetraphenylborate solution was the precipitant in an acidic medium (pH 1). Tablets containing both chlorhexidine diacetate and benzocaine also were assayed.