Zaprinast is (M&B 22,948; 2-o-propoxy-phenyl-8-azapurin-6-one) is a selective inhibitor of cyclic GMP (cGMP)-dependent phosphodiesterases, with vasodilation activity in a variety of species and tissues. The potency of zaprinast as a vasorelaxant varies with the species, the tissue, and the presence and absence of endothelium. Zaprinast apparently loses all or most of its vasorelaxant capacity when arteries have been denuded of the endothelial cell layer. Alternatively, the vasorelaxant effects of zaprinast can be attenuated using methylene blue, an inhibitor of soluble guanylate cyclase and hemoglobin inhibitor of nitric oxide synthesis. Therefore, zaprinast-induced relaxations appear to be endothelium-dependent. In human platelets zaprinast, at a dose of 10 mkM, caused a modest (20%) inhibition of aggregation as well as a small increase in cGMP content. In anesthetized rats, zaprinast dose-dependently lowered mean arterial blood pressure (MAP), an effect that correlated well with increased levels of plasma cGMP. Zaprinast decreased mean arterial pressure, the reduction being inversely related to zaprinast concentration. Zaprinast had no effect on heart rate, but increased cardiac output, urinary output, urinary sodium excretion, as well as renal blood flow. Oral administration of zaprinast to spontaneously hypertensive rats at a dose of 200 mg/kg/day normalized blood pressure. In normotensive rats, however, no changes in blood pressure were observed with the same treatment. In an initial placebo-controlled, double-blind crossover trial, 10 mg of zaprinast was effective in reducing exercise-, but not histamine-induced bronchoconstriction in adult asthmatics. However, in a similar study with asthmatic children, zaprinast was ineffective in preventing exercise-induced bronchoconstriction. Since then the clinical development of zaprinast has been discontinued.

Enprofylline is a xanthine derivative that shares theophylline's bronchodilator properties. It can be considered a relatively selective, though not potent adenosine A2B receptors antagonist. Enprofylline is used in asthma, chronic obstructive pulmonary disease, and in the management of cerebrovascular insufficiency, sickle cell disease, and diabetic neuropathy. Long-term enprofylline administration may be associated with the elevation in liver enzyme levels and unpredictable blood levels.

Siguazodan is a selective inhibitor of phosphodiesterase 3. It caused significant increases in cardiac output and stroke volume. It is orally active inotropic/vasodilator agent with a sustained duration in vivo. Siguazodan has potential utility in the treatment of congestive heart failure. Siguazodan has anti-platelet actions over the same concentration range that it is an inotrope and vasodilator. Siguazodan caused bronchodilation. In combination with phosphodiesterase 4, it may be useful in the therapy of asthma.

Boehringer Ingelheim is developing BI-671800, an orally administered treatment for seasonal allergic rhinitis and asthma. BI-671800 is an antagonist of the PGD2 receptor, CRTH2. PGD2 stimulates bronchoconstriction and allergic airway inflammation in animal models. Inhibition of CRTH2 may reduce airway inflammatory cells, IL -4, -5, -13 production, serum IgE and airway hyper reactivity. Treatment with BI-671800 in poorly controlled asthmatic patients receiving FP was associated with a significant improvement in FEV1. BI-671800 was well tolerated at a total daily dose of 800 mg for 6 weeks.

Oglemilast (GRC-3886), is a potent and selective PDE4 inhibitor (IC50: 2.5 nM (PDE4B) and 1.7 nM (PDE4D)). Oglemilast is in phase II clinical trials for the treatment of asthma and chronic obstructive pulmonary disease (COPD). Oglemilast was originally developed by Glenmark Pharmaceuticals, and licensed to Forest (acquired by Actavis in 2014) for the rights in North America in 2004. Teijin Pharma obtained the rights of the compound in Japan in 2005.

Senicapoc (ICA-17043) is a blocker of Gardos channel, a calcium-activated potassium channel, in the red blood cell. Preclinical studies and studies in transgenic models of Sickle cell disease (SCD) show that inhibition of potassium efflux through the Gardos channel is associated with an increased haemoglobin level, decreased dense cells and decreased hemolysis. Senicapoc is well tolerated when administered to SCD patients and produces dose-dependent increases in hemoglobin and decreases in markers of hemolysis. Senicapoc exerts anti-fibrotic and anti-inflammatory activities. Senicapoc has previously been in Phase III and Phase II clinical trials for the treatment of SCD and asthma, respectively.

Piriprost (U-60, 257) is a structural analog of prostaglandin I2 (PGI2) with low IP receptor-mediated activity. It inhibits 5-LO (5-lipoxygenase). Piriprost inhibits the release of histamine and leukotrienes, implicating its role in inflammation and allergic responses. However, it was shown, that piriprost did not influence the airway responses after allergen in asthma. Nevertheless, even more, the drug was irritant to the respiratory tract than was placebo.

Cromakalim is an ATP-sensitive potassium (KATP) channel opener, which was studied for the treatment of gastric ulcer, hypertension and preventing a cardiomyopathy. But the development of this drug was discontinued due to heart lesions found in monkey chronic toxicity studies.

Cinalukast (Ro 24-5913 ) is a selective leukotriene D4 receptor antagonist originated by Roche. Cinalukast inhibits the actions of Leukotriene D4 at the cysteinyl leukotriene receptor, CysLT1, in the human airway. Leukotriene receptor occupation has been correlated with the pathophysiology of asthma, including airway edema and altered cellular activity associated with the inflammatory process, which contributes to the signs and symptoms of asthma. Cinalukast had been investigated for the treatment of asthma, but that study was discontinued.

Atizoram (CP-80633) is a phosphodiesterase IV inhibitor with bronchodilatory and antiinflammatory properties. It was in phase II trials with Pfizer in the US for the treatment of asthma, atopic dermatitis and psoriasis. Development of atizoram has been discontinued in the asthma indication and no recent development has been reported for the other indications. CP-80633 inhibits PDE4 isozymes (human lung IC50 = 1.27 uM) in the absence of effects on PDE1, PDE2, PDE3 and PDE5 isozymes (IC50 > 100 uM). It exhibits no significant selectivity for any single cloned PDE4A, B, C or D isoform.