Anti-inflammatory and bronchodilator properties of RP 73401, a novel and selective phosphodiesterase type IV inhibitor

Assessment of PDE4 Inhibitor-Induced Hypothermia as a Correlate of Nausea in Mice

Simple Summary

Type 4 cAMP-phosphodiesterases (PDE4s) comprise a family of four isoenzymes, PDE4A to D, that hydrolyze and inactivate the second messenger cAMP. Non/PAN-selective PDE4 inhibitors, which inhibit all four PDE4 subtypes simultaneously, produce many promising therapeutic benefits, such as anti-inflammatory or cognition- and memory-enhancing effects. However, unwanted side effects, principally, nausea, diarrhea, and emesis, have long hampered their clinical and commercial success. Targeting individual PDE4 subtypes has been proposed for developing drugs with an improved safety profile, but which PDE4 subtype(s) is/are actually responsible for nausea and emesis remains ill-defined. Based on the observation that nausea is often accompanied by hypothermia in humans and other mammals, we used the measurement of core body temperatures of mice as a potential correlate of nausea induced by PDE4 inhibitors in humans. We find that selective inactivation of any of the four PDE4 subtypes did not change the body temperature of mice, suggesting that PAN-PDE4 inhibitor-induced hypothermia (and hence nausea in humans) requires the simultaneous inhibition of multiple PDE4 subtypes. This finding contrasts with prior reports that proposed PDE4D as the subtype mediating these side effects of PDE4 inhibitors and suggests that subtype-selective inhibitors that target any individual PDE4 subtype, including PDE4D, may not cause nausea.

Abstract

Treatment with PAN-PDE4 inhibitors has been shown to produce hypothermia in multiple species. Given the growing body of evidence that links nausea and emesis to disturbances in thermoregulation in mammals, we explored PDE4 inhibitor-induced hypothermia as a novel correlate of nausea in mice. Using knockout mice for each of the four PDE4 subtypes, we show that selective inactivation of individual PDE4 subtypes per se does not produce hypothermia, which must instead require the concurrent inactivation of multiple (at least two) PDE4 subtypes. These findings contrast with the role of PDE4s in shortening the duration of α₂-adrenoceptor-dependent anesthesia, a behavioral surrogate previously used to assess the emetic potential of PDE4 inhibitors, which is exclusively affected by inactivation of PDE4D. These different outcomes are rooted in the distinct molecular mechanisms that drive these two paradigms; acting as a physiologic α₂-adrenoceptor antagonist produces the effect of PDE4/PDE4D inactivation on the duration of α₂-adrenoceptor-dependent anesthesia, but does not mediate the effect of PDE4 inhibitors on body temperature in mice. Taken together, our findings suggest that selective inhibition of any individual PDE4 subtype, including inhibition of PDE4D, may be free of nausea and emesis.

Inhibition of type 4 cAMP-phosphodiesterases (PDE4s) in mice induces hypothermia via effects on behavioral and central autonomous thermoregulation

Inhibitors of Type 4 cAMP-phosphodiesterases (PDE4s) exert a number of promising therapeutic benefits, including potent anti-inflammatory, memory- and cognition-enhancing, metabolic, and antineoplastic effects. We report here that treatment with a number of distinct PDE4 inhibitors, including Rolipram, Piclamilast, Roflumilast and RS25344, but not treatment with the PDE3-selective inhibitor Cilostamide, induces a rapid (10-30 min), substantial (-5 °C) and long-lasting (up to 5 h) decrease in core body temperature of C57BL/6 mice; thus, identifying a critical role of PDE4 also in the regulation of body temperature. As little as 0.04 mg/kg of the archetypal PDE4 inhibitor Rolipram induces hypothermia. As similar or higher doses of Rolipram were used in a majority of published animal studies, most of the reported findings are likely paralleled by, or potentially impacted by hypothermia induced by these drugs. We further show that PDE4 inhibition affects central body temperature regulation and acts by lowering the cold-defense balance point of behavioral (including posture and locomotion) and autonomous (including cutaneous tail vasodilation) cold-defense mechanisms. In line with the idea of an effect on central body temperature regulation, hypothermia is induced by moderate doses of various brain-penetrant PDE4 inhibitors, but not by similar doses of YM976, a PDE4 inhibitor that does not efficiently cross the blood-brain barrier. Finally, to begin delineating the mechanism of drug-induced hypothermia, we show that blockade of D2/3-type dopaminergic, but not β-adrenergic, H1-histaminergic or opiate receptors, can alleviate PDE4 inhibitor-induced hypothermia. We thus propose that increased D2/3-type dopaminergic signaling, triggered by PDE4 inhibitor-induced and cAMP-mediated dopamine release in the thermoregulatory centers of the hypothalamus, is a significant contributor to PDE4 inhibitor-induced hypothermia.

PAN-selective inhibition of cAMP-phosphodiesterase 4 (PDE4) induces gastroparesis in mice

Inhibitors of cAMP-phosphodiesterase 4 (PDE4) exert a number of promising therapeutic benefits, but adverse effects, in particular emesis and nausea, have curbed their clinical utility. Here, we show that PAN-selective inhibition of PDE4, but not inhibition of PDE3, causes a time- and dose-dependent accumulation of chow in the stomachs of mice fed ad libitum without changing the animals' food intake or the weight of their intestines, suggesting that PDE4 inhibition impairs gastric emptying. Indeed, PDE4 inhibition induced gastric retention in an acute model of gastric motility that traces the passage of a food bolus through the stomach over a 30 minutes time period. In humans, abnormal gastric retention of food is known as gastroparesis, a syndrome predominated by nausea (>90% of cases) and vomiting (>80% of cases). We thus explored the abnormal gastric retention induced by PDE4 inhibition in mice under the premise that it may represent a useful correlate of emesis and nausea. Delayed gastric emptying was produced by structurally distinct PAN-PDE4 inhibitors including Rolipram, Piclamilast, Roflumilast, and RS25344, suggesting that it is a class effect. PDE4 inhibitors induced gastric retention at similar or below doses commonly used to induce therapeutic benefits (e.g., 0.04 mg/kg Rolipram), thus mirroring the narrow therapeutic window of PDE4 inhibitors in humans. YM976, a PAN-PDE4 inhibitor that does not efficiently cross the blood-brain barrier, induced gastroparesis only at significantly higher doses (≥1 mg/kg). This suggests that PDE4 inhibition may act in part through effects on the autonomic nervous system regulation of gastric emptying and that PDE4 inhibitors that are not brain-penetrant may have an improved safety profile. The PDE4 family comprises four subtypes, PDE4A, B, C, and D. Selective ablation of any of these subtypes in mice did not induce gastroparesis per se, nor did it protect from PAN-PDE4 inhibitor-induced gastroparesis, indicating that gastric retention may result from the concurrent inhibition of multiple PDE4s. Thus, potentially, any of the four PDE4 subtypes may be targeted individually for therapeutic benefits without inducing nausea or emesis. Acute gastric retention induced by PDE4 inhibition is alleviated by treatment with the widely used prokinetic Metoclopramide, suggesting a potential of this drug to alleviate the side effects of PDE4 inhibitors. Finally, given that the cause of gastroparesis remains largely idiopathic, our findings open the possibility that a physiologic or pathophysiologic downregulation of PDE4 activity/expression may be causative in a subset of patients.

Anti‑inflammatory effect of ciclamilast in an allergic model involving the expression of PDE4B

The present study aimed to investigate the potent inhibitory effects and possible biochemical basis of the novel phosphodiesterase 4 (PDE4) inhibitor ciclamilast, which is a derivative of piclamilast (RP 73401), on PDE4 and allergic inflammation. Ciclamilast was orally administered to allergic rats, their lungs and bronchoalveolar lavage fluid (BALF) were harvested, and their levels of inflammation and goblet cell hyperplasia, particularly cAMP‑PDE activity, and expression and distribution of PDE4 subtypes were determined. The results suggested that oral administration of ciclamilast significantly reduced the total leukocyte number and eosinophil number in BALF and suppressed lung histology changes, including the infiltration of inflammatory cells into the perivascular and peribronchial spaces, structural changes and goblet cell hyperplasia. For eosinophil infiltration, ciclamilast exhibited improved selectivity compared with piclamilast. Furthermore, ciclamilast significantly inhibited the upregulated activity of cAMP‑PDE and showed improved selective inhibition of the protein expression of PDE4B than piclamilast in a dose‑dependent manner. The mRNA expression of PDE4D was significantly increased in allergic rats, but PDE4B was not. PDE4B was mainly distributed in the cytoplasm, whereas PDE4D was mainly distributed in the cell membrane. The improved anti‑inflammatory activity of ciclamilast compared with piclamilast may be due to its higher level of inhibition of the activity, mRNA and protein expression of PDE4, particularly its effect on PDE4B.

Identification of novel small-molecule inhibitors of Zika virus infection

The recent re-emergence of Zika virus (ZIKV), a member of the Flaviviridae family, has become a global emergency and a serious public health threat worldwide. ZIKV infection causes severe neuroimmunopathology and is particularly harmful to the developing fetuses of infected pregnant women causing various developmental abnormalities. Currently, there are no effective methods of preventing or treating ZIKV infection, and new treatment options are urgently needed. Therefore, we have used an in vitro plaque assay to screen a limited proprietary library of small organic compounds and identified highly bioactive leads, with the most active analogs showing activity in low picomolar range. Identified "hits" possess certain common structural features that can be used in the design of the next generation(s) of ZIKV inhibitors. Collectively, our findings suggest that identified compounds represent excellent template(s) for the development of inexpensive and orally available anti-Zika drugs.

Tumour growth inhibition and anti-angiogenic effects using curcumin correspond to combined PDE2 and PDE4 inhibition

Vascular endothelial growth factor (VEGF) plays a major role in angiogenesis by stimulating endothelial cells. Increase in cyclic AMP (cAMP) level inhibits VEGF-induced endothelial cell proliferation and migration. Cyclic nucleotide phosphodiesterases (PDEs), which specifically hydrolyse cyclic nucleotides, are critical in the regulation of this signal transduction. We have previously reported that PDE2 and PDE4 up-regulations in human umbilical vein endothelial cells (HUVECs) are implicated in VEGF-induced angiogenesis and that inhibition of PDE2 and PDE4 activities prevents the development of the in vitro angiogenesis by increasing cAMP level, as well as the in vivo chicken embryo angiogenesis. We have also shown that polyphenols are able to inhibit PDEs. The curcumin having anti-cancer properties, the present study investigated whether PDE2 and PDE4 inhibitors and curcumin could have similar in vivo anti-tumour properties and whether the anti-angiogenic effects of curcumin are mediated by PDEs. Both PDE2/PDE4 inhibitor association and curcumin significantly inhibited in vivo tumour growth in C57BL/6N mice. In vitro, curcumin inhibited basal and VEGF-stimulated HUVEC proliferation and migration and delayed cell cycle progression at G0/G1, similarly to the combination of selective PDE2 and PDE4 inhibitors. cAMP levels in HUVECs were significantly increased by curcumin, similarly to rolipram (PDE4 inhibitor) and BAY-60-550 (PDE2 inhibitor) association, indicating cAMP-PDE inhibitions. Moreover, curcumin was able to inhibit VEGF-induced cAMP-PDE activity without acting on cGMP-PDE activity and to modulate PDE2 and PDE4 expressions in HUVECs. The present results suggest that curcumin exerts its in vitro anti-angiogenic and in vivo anti-tumour properties through combined PDE2 and PDE4 inhibition.

Design and synthesis of new 8-anilide theophylline derivatives as bronchodilators and antibacterial agents

Theophylline derivatives have long been recognized as potent bronchodilators for the relief of acute asthma. Recently, it was found that bacterial infection has a role in asthma pathogenesis. The present work involves the design and synthesis of 8-substituted theophylline derivatives as bronchodilators and antibacterial agents. The chemical structures of these compounds were elucidated by IR, (1)H-NMR, mass spectrometry, and elemental analyses. The bronchodilator activity was evaluated using acetylcholine-induced bronchospasm in guinea pigs, and most of the compounds showed significant anti-bronchoconstrictive activity in comparison with standard aminophylline. In addition, the antibacterial activity of all the target compounds was investigated in vitro against Gram-positive and Gram-negative bacteria using ampicillin as a reference drug. Results showed that some of the tested compounds possessed significant antibacterial activity. A pharmacophore model was computed to obtain useful insight into the essential structural features of bronchodilator activity. A structure activity relationship was also discussed.

Phosphodiesterase inhibitors: history of pharmacology

The first pharmacological investigations of phosphodiesterase (PDE) inhibitors were developed with the clinical efficacies of drugs isolated from coffee, cacao and tea but only later their relevant ingredients were identified as xanthines that act as PDE. With its diuretic, inotropic and bronchodilating clinical efficacy, use of theophylline anticipated the clinical goals, which were later approached with the first-generation of weakly selective PDE inhibitors in the period from 1980 to 1990. Pharmacological and clinical research with these early compounds provided a vast pool of information regarding desired and adverse actions - although most of these new drugs had to be discontinued due to severe adverse effects. The pharmacological models for cardiac, vascular and respiratory indications were analysed for their PDE isoenzyme profiles, and when biochemical and molecular biological approaches expanded our knowledge of the PDE superfamily, the purified isoenzymes that were now available opened the door for more systematic studies of inhibitors and for generation of highly selective isoenzyme-specific drugs. The development of simple screening models and clinically relevant indication models reflecting the growing knowledge about pathomechanisms of disease are summarised here for today's successful application of highly selective PDE3, PDE4 and PDE5 inhibitors. The interplay of serendipitous discoveries, the establishment of intelligent pharmacological models and the knowledge gain by research results with new substances is reviewed. The broad efficacies of new substances in vitro, the enormous biodiversity of the PDE isoenzyme family and the sophisticated biochemical pharmacology enabled Viagra to be the first success story in the field of PDE inhibitor drug development, but probably more success stories will follow.

Design and synthesis of some new theophylline derivatives with bronchodilator and antibacterial activities

Methylxanthines especially theophylline have been recognized as potent bronchodilators for the relief of acute asthma for over 65 years. Recently, it was found that bacterial infection plays a role in asthma pathogenesis. Accordingly, the present work involves the synthesis of 6-(4-(un)substituted phenyl)thiazolo[2,3-f]theophyllines 2a-g and different series of 8-(1,2,4-triazol-3-ylmethylthio)theophyllines 6-9. The chemical structures of the target compounds were proved by IR, (1)H NMR, (13)C NMR, EI-MS and HRMS spectroscopic techniques along with elemental analyses. The bronchodilator activity of fifteen compounds was determined in vivo by acetylcholine induced bronchospasm in anaesthetized guinea pigs. Results revealed that all compounds showed moderate to good activity; in addition, five compounds exhibited a bronchodilator activity nearly similar to that of aminophylline as a standard. The antibacterial activity of all the target compounds was investigated in vitro against both Gram-positive and Gram-negative bacterial strains. Results revealed that some compounds showed more potent antibacterial activity than ampicillin as a standard. Acute toxicity study for four target compounds revealed that none of these derivatives showed significant toxicity up to 300 mg/kg. It was found that compound 8c combined both promising bronchodilator and antibacterial activities. This compound could be subjected for further investigations as a new possible candidate in the treatment of bronchial asthma.

GSK256066, an exceptionally high-affinity and selective inhibitor of phosphodiesterase 4 suitable for administration by inhalation: in vitro, kinetic, and in vivo characterization

Oral phosphodiesterase (PDE) 4 inhibitors such as roflumilast have established the potential of PDE4 inhibition for the treatment of respiratory diseases. However, PDE4 inhibitor efficacy is limited by mechanism-related side effects such as emesis and nausea. Delivering the inhibitor by the inhaled route may improve therapeutic index, and we describe 6-({3-[(dimethylamino)carbonyl]phenyl}sulfonyl)-8-methyl-4-{[3-methyloxy) phenyl]amino}-3-quinolinecarboxamide (GSK256066), an exceptionally high-affinity inhibitor of PDE4 designed for inhaled administration. GSK256066 is a slow and tight binding inhibitor of PDE4B (apparent IC(50) 3.2 pM; steady-state IC(50) <0.5 pM), which is more potent than any previously documented compound, for example, roflumilast (IC(50) 390 pM), tofimilast (IC(50) 1.6 nM), and cilomilast (IC(50) 74 nM). Consistent with this, GSK256066 inhibited tumor necrosis factor α production by lipopolysaccharide (LPS)-stimulated human peripheral blood monocytes with 0.01 nM IC(50) (compared with IC(50) values of 5, 22, and 389 nM for roflumilast, tofimilast, and cilomilast, respectively) and by LPS-stimulated whole blood with 126 pM IC(50). GSK256066 was highly selective for PDE4 (>380,000-fold versus PDE1, PDE2, PDE3, PDE5, and PDE6 and >2500-fold against PDE7), inhibited PDE4 isoforms A-D with equal affinity, and had a substantial high-affinity rolipram binding site ratio (>17). When administered intratracheally to rats, GSK256066 inhibited LPS-induced pulmonary neutrophilia with ED(50) values of 1.1 μg/kg (aqueous suspension) and 2.9 μg/kg (dry powder formulation) and was more potent than an aqueous suspension of the corticosteroid fluticasone propionate (ED(50) 9.3 μg/kg). Thus, GSK256066 has been demonstrated to have exceptional potency in vitro and in vivo and is being clinically investigated as a treatment for chronic obstructive pulmonary disease.

Conserved expression and functions of PDE4 in rodent and human heart

PDE4 isoenzymes are critical in the control of cAMP signaling in rodent cardiac myocytes. Ablation of PDE4 affects multiple key players in excitation–contraction coupling and predisposes mice to the development of heart failure. As little is known about PDE4 in human heart, we explored to what extent cardiac expression and functions of PDE4 are conserved between rodents and humans. We find considerable similarities including comparable amounts of PDE4 activity expressed, expression of the same PDE4 subtypes and splicing variants, anchoring of PDE4 to the same subcellular compartments and macromolecular signaling complexes, and downregulation of PDE4 activity and protein in heart failure. The major difference between the species is a fivefold higher amount of non-PDE4 activity in human hearts compared to rodents. As a consequence, the effect of PDE4 inactivation is different in rodents and humans. PDE4 inhibition leads to increased phosphorylation of virtually all PKA substrates in mouse cardiomyocytes, but increased phosphorylation of only a restricted number of proteins in human cardiomyocytes. Our findings suggest that PDE4s have a similar role in the local regulation of cAMP signaling in rodent and human heart. However, inhibition of PDE4 has ‘global’ effects on cAMP signaling only in rodent hearts, as PDE4 comprises a large fraction of the total cardiac PDE activity in rodents but not in humans. These differences may explain the distinct pharmacological effects of PDE4 inhibition in rodent and human hearts.

Cytokine-dependent balance of mitogenic effects in primary human lung fibroblasts related to cyclic AMP signaling and phosphodiesterase 4 inhibition

Interleukin-1beta (IL-1beta) and basic fibroblast growth factor (bFGF) are important regulators of proliferation, and their expression is increased in lungs of patients with asthma, idiopathic pulmonary fibrosis (IPF), or chronic obstructive pulmonary disease (COPD). We investigated the effect of IL-1beta and bFGF on proliferation of human lung fibroblasts and the role of COX-2, PGE(2), and cAMP in this process. Furthermore, the effect of phosphodiesterase (PDE) 3 and 4 inhibition was analyzed. In primary human lung fibroblasts low concentrations of IL-1beta (<10 pg/ml) potentiated the bFGF-induced DNA synthesis, whereas higher concentrations revealed antiproliferative effects. Higher concentrations of IL-1beta-induced COX-2 mRNA and protein associated with an increase in PGE(2) and cAMP, and all of these parameters were potentiated by bFGF. The PDE4 inhibitor piclamilast concentration-dependently reduced proliferation by a partial G1 arrest. The PDE3 inhibitor motapizone was inactive by itself but enhanced the effect of the PDE4 inhibitor. This study demonstrates that bFGF and IL-1beta act in concert to fine-tune lung fibroblast proliferation resulting in amplification or reduction. The antiproliferative effect of IL-1beta is likely attributed to the induction of COX-2, which is further potentiated by bFGF, and the subsequent generation of PGE(2) and cAMP. Inhibition of PDE4 inhibition (rather than PDE3) may diminish proliferation of human lung fibroblasts and therefore could be useful in the therapy of pathological remodeling in lung diseases.

Differential effects of PDE4 inhibitors on cortical neurons and T-lymphocytes

Inhibitors of PDE4 (cAMP-specific phosphodiesterase) induce side effects, including nausea and emesis, that limit their therapeutic potential. We investigated the function of two catalytically active conformations of PDE4 (a low-affinity conformer detected by conventional cAMP hydrolytic activity and a high-affinity conformer detected by [(3)H]rolipram binding) in neuronal cells. We assessed enhancement of beta-adrenoceptor-mediated cAMP accumulation in cortical neurons in vitro by eleven PDE4 inhibitors with diverse biochemical profiles. The compounds tested have a wide inhibition range of PDE4 catalytic activity and [(3)H]rolipram binding. Inhibition potency for PDE4 catalytic activity and [(3)H]rolipram binding for each compound was different. Potency in augmentation of cAMP correlated significantly with the inhibitory effect on [(3)H]rolipram binding, but not with that against PDE4 catalytic activity. On the other hand, the inhibitory effect on proliferation of T-lymphocytes of the same PDE4 inhibitors correlated both with inhibition of PDE4 catalytic activity and with inhibition of [(3)H]rolipram binding. These findings indicate that the high affinity PDE4 conformer exists at a high level in cortical neurons and is important in the regulation of cAMP. Furthermore, the relative contributions of the two PDE4 conformers in cell function may cause different PDE4 inhibitor effects on cortical neurons and T-lymphocytes.

Correlation between emetic effect of phosphodiesterase 4 inhibitors and their occupation of the high-affinity rolipram binding site in Suncus murinus brain

We employed an ex vivo [(3)H]rolipram binding experiment to elucidate the mechanism of emetic activity of phosphodiesterase 4 inhibitors. In Suncus murinus (an insectivore used for evaluation of emesis), emetic potential as well as ability to occupy the high-affinity rolipram binding site in brain membrane fraction in vivo were determined for phosphodiesterase 4 inhibitors. In vitro, [(3)H]rolipram bound to the membrane fraction of S. murinus brain with high affinity and its value was comparable to that for rat brain (K(d)=3.6 nM and 3.5 nM, respectively). The test compounds included denbufylline, rolipram, piclamilast, CDP840 and KF19514, each of which possessed similar affinities for the rolipram binding sites in both S. murinus and rat brain. In S. murinus, these compounds induced emesis via intraperitoneal administration. Their ED(50) values were as follows: denbufylline (1.4 mg/kg), rolipram (0.16 mg/kg), piclamilast (1.8 mg/kg), CDP840 (20 mg/kg), and KF19514 (0.030 mg/kg). In addition, these compounds occupied the high-affinity rolipram binding site in vivo as detected by dose-dependent reduction in capacity of ex vivo [(3)H]rolipram binding in brain membrane fractions. A clear correlation was observed between dose required to induce emesis and that to occupy the high-affinity rolipram binding site for individual phosphodiesterase 4 inhibitors. We conclude that the emetic effect of phosphodiesterase 4 inhibitors is caused at least in part via binding to the high-affinity rolipram binding site in brain in vivo.

Selective inhibitors for phosphodiesterase 3 and 4 in antigen-induced increase of cough reflex sensitivity in guinea pigs

Effects of the selective phosphodiesterase 3 (PDE3) inhibitor olprinone and the selective PDE4 inhibitor SB207499 were investigated on antigen-induced increase of cough reflex sensitivity and normal cough response to capsaicin in guinea pigs. Number of coughs elicited by inhalation of capsaicin (10(-8), 10(-6) and 10(-4)M) was counted 24h after an antigen challenge in actively sensitized guinea pigs, and then bronchoalveolar lavage (BAL) was performed. Olprinone (1 or 10mg/kg) or SB207499 (1 or 10mg/kg) was given twice intraperitoneally: One hour after the antigen challenge and 1h before the capsaicin provocation. SB207499, but not olprinone, significantly reduced both antigen-induced increase of cough response to inhaled capsaicin in sensitized animals and normal cough response in non-sensitized animals. SB207499, but not olprinone, also reduced the antigen-induced BAL eosinophilia. PDE4 inhibitors may be useful in treating cough associated with airway allergy via inhibiting both allergic airway inflammation and the common pathway of cough reflex.

Roflumilast inhibits leukocyte-endothelial cell interactions, expression of adhesion molecules and microvascular permeability

BACKGROUND AND PURPOSE

The present study addressed the effects of the investigational PDE4 inhibitor roflumilast on leukocyte-endothelial cell interactions and endothelial permeability in vivo and in vitro.

EXPERIMENTAL APPROACH

In vivo, intravital video-microscopy was used to determine effects of roflumilast p.o. on leukocyte-endothelial cell interactions and microvascular permeability in rat mesenteric venules. In vitro, the effects of roflumilast N-oxide, the active metabolite of roflumilast in humans, and other PDE4 inhibitors on neutrophil adhesion to tumour necrosis factor alpha (TNFalpha)-activated human umbilical vein endothelial cells (HUVEC), E-selectin expression and thrombin-induced endothelial permeability was evaluated. Flow cytometry was used to determine the effect of roflumilast on N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced CD11b upregulation on human neutrophils.

KEY RESULTS

In vivo, roflumilast, given 1 h before lipopolysaccharide (LPS), dose-dependently reduced leukocyte-endothelial cell interactions in rat mesenteric postcapillary venules. It also diminished histamine-induced microvascular permeability. Immunohistochemical analyses revealed that roflumilast prevented LPS-induced endothelial P- and E-selectin expression. In vitro, roflumilast N-oxide concentration-dependently suppressed neutrophil adhesion to TNFalpha-activated HUVEC and CD11b expression on fMLP-stimulated neutrophils. It also reduced TNFalpha-induced E-selectin expression on HUVEC, when PDE3 activity was blocked. HUVEC permeability elicited by thrombin was concentration-dependently suppressed by roflumilast N-oxide. While roflumilast N-oxide was as potent as roflumilast at inhibiting stimulated endothelial cell and neutrophil functions, both compounds were significantly more potent than the structurally unrelated PDE4 inhibitors, rolipram or cilomilast.

CONCLUSIONS AND IMPLICATIONS

These findings further support earlier observations on the inhibition of inflammatory cell influx and protein extravasation by roflumilast in vivo.

Characterization of inhibitors of phosphodiesterase 1C on a human cellular system

Different inhibitors of the Ca(2+)/calmodulin-stimulated phosphodiesterase 1 family have been described and used for the examination of phosphodiesterase 1 in cellular, organ or animal models. However, the inhibitors described differ in potency and selectivity for the different phosphodiesterase family enzymes, and in part exhibit additional pharmacodynamic actions. In this study, we demonstrate that phosphodiesterase 1C is expressed in the human glioblastoma cell line A172 with regard to mRNA, protein and activity level, and that lower activities of phosphodiesterase 2, phosphodiesterase 3, phosphodiesterase 4 and phosphodiesterase 5 are also present. The identity of the phosphodiesterase 1C activity detected was verified by downregulation of the mRNA and protein through human phosphodiesterase 1C specific small interfering RNA. In addition, the measured K(m) values (cAMP, 1.7 microm; cGMP, 1.3 microm) are characteristic of phosphodiesterase 1C. We demonstrate that treatment with the Ca(2+) ionophore ionomycin increases intracellular Ca(2+) in a concentration-dependent way without affecting cell viability. Under conditions of enhanced intracellular Ca(2+) concentration, a rapid increase in cAMP levels caused by the adenylyl cyclase activator forskolin was abolished, indicating the involvement of Ca(2+)-activated phosphodiesterase 1C. The reduction of forskolin-stimulated cAMP levels was reversed by phosphodiesterase 1 inhibitors in a concentration-dependent way. Using this cellular system, we compared the cellular potency of published phosphodiesterase 1 inhibitors, including 8-methoxymethyl-3-isobutyl-1-methylxanthine, vinpocetine, SCH51866, and two established phosphodiesterase 1 inhibitors developed by Schering-Plough (named compounds 31 and 30). We demonstrate that up to 10 microm 8-methoxymethyl-3-isobutyl-1-methylxanthine and vinpocetine had no effect on the reduction of forskolin-stimulated cAMP levels by ionomycin, whereas the more selective and up to 10 000 times more potent phosphodiesterase 1 inhibitors SCH51866, compound 31 and compound 30 inhibited the ionomycin-induced decline of forskolin-induced cAMP at nanomolar concentrations. Thus, our data indicate that SCH51866 and compounds 31 and 30 are effective phosphodiesterase 1 inhibitors in a cellular context, in contrast to the weakly selective and low-potency phosphodiesterase inhibitors 8-methoxymethyl-3-isobutyl-1-methylxanthine and vinpocetine. A172 cells therefore represent a suitable system in which to study the cellular effect of phosphodiesterase 1 inhibitors. 8-Methoxymethyl-3-isobutyl-1-methylxanthine and vinpocetine seem not to be suitable for the study of phosphodiesterase 1-mediated functions.

Inhibition of TGF-beta induced lung fibroblast to myofibroblast conversion by phosphodiesterase inhibiting drugs and activators of soluble guanylyl cyclase

Pulmonary fibroblast to myofibroblast conversion is a pathophysiological feature of idiopathic pulmonary fibrosis and COPD. This conversion is induced by transforming growth factor (TGF)-beta derived from epithelial cells as well as activated macrophages that have infiltrated the lung. Preventing this conversion might be a favourable therapeutic approach. Within this study we examined the activity of different members of the phosphodiesterase (PDE) family in primary human lung fibroblasts and various lung fibroblast cell lines both before and after TGF-beta induced differentiation to myofibroblasts as reflected by the expression of alpha-smooth muscle actin. We showed that the predominant PDE activities in lung fibroblasts are attributed to PDE5, PDE1 and to a smaller extent to PDE4. cyclic GMP (cGMP)-hydrolyzing activity declines by about half after differentiation to myofibroblasts in all pulmonary fibroblasts investigated, which is accompanied by a down-regulation of PDE5 protein. Lung fibroblast to myofibroblast differentiation is blocked by treatment with the PDE4 inhibitor piclamilast alone, depending on the TGF-beta concentration applied, and in combination with prostaglandin E(2) (PGE(2)) in a synergistic manner. Despite the high PDE5 activity the PDE5 inhibitor sildenafil by itself as well as in combination with brain natriuretic peptide or the nitric oxide-donor DETA-NONOate shows no inhibiting effects. However, combining sildenafil with the guanylyl cyclase (GC) activator BAY58-2667 and ODQ (which sensitizes GC for activation by BAY58-2667) suppressed TGF-beta induced differentiation. In summary, our data indicate that drugs interfering with the cyclic AMP (cAMP)-as well as with the NO-cGMP-pathway offer the therapeutic opportunity to prevent the differentiation of pulmonary fibroblasts to myofibroblasts in lung fibrosis.

The role of PDE4 in pulmonary inflammation and goblet cell hyperplasia in allergic rats

Phosphodiesterase 4 (PDE4) has been suggested to a critical factor in the pathogenesis of inflammation by metabolizing cAMP in human leukocytes, endothelium and epithelium. The present study aimed at evaluating the PDE4 activity and expression, the relationship between the inflammation and cAMP- activity in the lungs, and potential interventions of PDE inhibitors and antiinflammatory drugs in the reduction of lung inflammation and goblet cell hyperplasia in allergic rats. The total leukocyte number and eosinophil number in bronchoalveolar lavegar fluid and infiltration of inflammatory cells in the perivascular and peribronchial spaces, structure changes and goblet cell hyperplasia in the OVA-sensitized and challenged allergic rats. A significant correlation was observed between the increases in cAMP-PDE activity and inflammation in the lung. Those OVA-induced changes were prevented by pretreatment with PDE inhibitor in a dose-related patterns and with glucocorticosteriod. We found an increase in the proportion of PDE4 and PDE4 gene expression, while a decrease in the proportion of PDE3 in the lung of the allergic rats. Incubation with different PDE inhibitors down-regulated OVA-induced cAMP hydrolysis. Our data suggest that PDE4C may play an important role in the airway inflammation, remodeling and goblet cell hyperplasia after repeated challenge of sensitized rats.

Roflumilast inhibition of pulmonary leukotriene production and bronchoconstriction in ovalbumin-sensitized and -challenged Guinea pigs

This study investigated the effects of roflumilast, a PDE4 inhibitor, on slow-reacting substance of anaphylaxis (SRS-A)-mediated bronchoconstriction and pulmonary leukotriene (LT) release in ovalbumin (OVA)-sensitized and -challenged guinea pigs. Animals were treated with roflumilast orally (0.04, 0.12, 0.4, or 4 mg/kg) or placebo 1 hour before OVA challenge. Bronchoconstriction was quantified by measuring airway conductance (Gaw) and dynamic lung compliance (Cdyn). Roflumilast significantly attenuated the decrease in Gaw (50% inhibitory dose [ID50] = 0.33 mg/kg) and Cdyn (ID50 = 0.25 mg/kg) in a dose-dependent manner and significantly inhibited Cys-LT (ID50 = 0.06 mg/kg) and LTB4 (ID50 = 0.05 mg/kg) release versus placebo-treated animals. Roflumilast did not affect LTD4-induced bronchoconstriction. These findings support the role of roflumilast as an anti-inflammatory treatment for asthma.

The therapeutic potential of PDE4 inhibitors

Phosphodiesterase enzymes are responsible for the inactivation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Phosphodiesterase 4 (PDE4) is a cAMP specific phosphodiesterase expressed in inflammatory cells such as eosinophils. Inhibition of PDE4 results in an elevation of cAMP in these cells, which in turn downregulates the inflammatory response. The anti-inflammatory effects of PDE4 inhibitors have been well documented both in vitro and in vivo in a variety of animal models. The potential use of PDE4 inhibitors as anti-inflammatory agents for the treatment of asthma and other inflammatory disorders has received considerable attention from the pharmaceutical industry, but to date, there are no selective PDE4 inhibitors on the market. Early PDE4 inhibitors, typified by rolipram, suffered from dose-limiting side effects, including nausea and emesis, which severely restricted their therapeutic utility. Second generation compounds, including CDP840 and SB207499 (Ariflo), have been identified with reduced side effect liability. Recent evidence suggests a correlation between side effects and the ability of compounds to bind at the so-called high affinity rolipram binding site (HPDE), whilst beneficial effects appear to correlate with binding at the catalytic site. A number of companies are actively pursuing compounds which exhibit improved affinity for the catalytic site and reduced affinity for the HPDE, in the expectation that this will provide compounds with an improved therapeutic index.

PDE4 inhibitors as new anti-inflammatory drugs: effects on cell trafficking and cell adhesion molecules expression

Phosphodiesterase 4 (PDE4) is a major cyclic AMP-hydrolyzing enzyme in inflammatory and immunomodulatory cells. The wide range of inflammatory mechanisms under control by PDE4 points to this isoenzyme as an attractive target for new anti-inflammatory drugs. Selective inhibitors of PDE4 have demonstrated a broad spectrum of anti-inflammatory activities including the inhibition of cellular trafficking and microvascular leakage, cytokine and chemokine release from inflammatory cells, reactive oxygen species production, and cell adhesion molecule expression in a variety of in vitro and in vivo experimental models. The initially detected side effects, mainly nausea and emesis, appear at least partially overcome by the 'second generation' PDE4 inhibitors, some of which like roflumilast and cilomilast are in the later stages of clinical development for treatment of chronic obstructive pulmonary disease. These new drugs may also offer opportunities for treatment of other inflammatory diseases.

Simultaneous determination of YM-64227, a phosphodiesterase type 4 inhibitor, and its ?ve metabolites in dog plasma by high-performance liquid chromatography with ?uorescence detection

We developed and validated a reversed-phase high-performance liquid chromatographic method with fluorescence detection for the simultaneous determination of YM-64227 [4-cyclohexyl-1-ethyl-7-methylpyrido(2,3-d)pyrimidin-2-(1H)-one], a novel and selective phosphodiesterase type 4 inhibitor, and its fi ve hydroxylated metabolites in dog plasma. The plasma samples were extracted with tert-butyl methyl ether under alkali conditions. The analytes were well separated on a phenyl ethyl column (5 microm, 250 x 4.6 mm i.d.), opreating at 40 degrees C and using an acetonitrile-acetic acid gradient at a fl ow rate of 1.0 mL/min. The fluorescence signal was monitored at an excitation and emission wavelength of 330 and 400 nm, respectively. No interfering peak was observed at the retention time of YM-64227, its metabolites or the internal standard. The validated quantitation range of the method was 0.4-200 ng/mL for all analytes using 0.5 mL of the plasma sample. The recovery of analytes in the extraction process was more than 65.5%. The intra- and inter-assay precision was less than 5.1 and 12.6%, respectively, and the intra- and inter-assay accuracy ranged from -8.1 to 11.8% and -8.0 to 9.9%, respectively. Using this assay, the plasma concentration of YM-64227 and metabolites can be determined after the oral administration of YM-64227 to beagle dogs.

In vivo efficacy in airway disease models of N-(3,5-dichloropyrid-4-yl)-[1-(4-fluorobenzyl)-5-hydroxy-indole-3-yl]-glyoxylic acid amide (AWD 12-281), a selective phosphodiesterase 4 inhibitor for inhaled administration

N-(3,5-Dichloro-pyrid-4-yl)-[1-(4-fluorobenzyl)-5-hydroxy-indole-3-yl]-glyoxylic acid amide (AWD 12-281) is a highly potent and selective phosphodiesterase 4 (PDE4) inhibitor that was designed to have a metabolic profile that was optimized for topical administration. The aim of the current study was to explore the pharmacological profile of intratracheally administered AWD 12-281 in different models of asthma and chronic obstructive pulmonary disease (COPD) in comparison with steroids. To assess the anti-inflammatory potential of AWD 12-281, the antigen-induced cell infiltration in bronchoalveolar lavage fluid (BALF) of Brown Norway rats was determined. AWD 12-281 (ID50 of 7 microg/kg i.t.) as well as beclomethasone (0.1microg/kg i.t.) suppresses late-phase eosinophilia when administered intrapulmonary. Furthermore, AWD 12-281 has also strong anti-inflammatory properties when tested in lipopolysaccharide-induced acute lung neutrophilia in Lewis rats (ID50 of 0.02 microg/kg i.t.), ferrets (ID50 of 10 microg/kg i.t.), and domestic pigs (2-4 mg/pig i.t. or 1 mg/kg i.v.). In pigs, AWD 12-281 was as effective as beclomethasone (0.4 mg/pig i.t.) and dexamethasone (0.28 mg/kg i.v.), although at 3 to 10 times the dosage. The bronchodilatory activity of AWD 12-281 was assessed in sensitized guinea pigs. AWD 12-281 (1.5 mg/kg i.t., 1-h pretreatment) inhibited allergen-induced bronchoconstriction by 68% (parameter airway resistance). In sensitized BP-2 mice AWD 12-281 abolished the allergen-induced bronchial hyperresponsiveness and eosinophilia in BALF, showing dose dependence. When given orally, i.v. or i.t., AWD 12-281 has a considerably lower emetic potential than cilomilast in ferrets and roflumilast in pigs. When given topically by inhalation, no emesis could be induced in dogs up to the highest feasible dose (15 mg/kg in 50% lactose blend). These results indicate that AWD 12-281 is a unique potential new drug for the topical treatment of asthma and COPD.

Pharmacology of a new cyclic nucleotide phosphodiesterase type 4 inhibitor, V11294

V11294 is a new cyclic nucleotide phosphodiesterase type 4 (PDE4) inhibitor of the rolipram class. In this report we present the pharmacological profile of V11294. V11294 inhibited PDE4 isolated from human lung with IC(50) 405 nM, compared to 3700 nM for rolipram. In contrast, V11294 inhibition of human PDE3 and PDE5 occurred only at concentrations greater than 100,000 nM. Like rolipram, V11294 inhibited PDE4D more potently than other PDE4 subtypes. V11294, when incubated with human anticoagulated whole blood in vitro, or administered to mice, caused increased cAMP concentration, consistent with inhibition of PDE4. V11294 inhibited lectin-induced proliferation and lipopolysaccharide-induced TNFalpha synthesis by human adherent monocytes in vitro and inhibited lipopolysaccharide-induced TNFalpha synthesis in mice. V11294 caused relaxation of guinea pig isolated trachea and inhibited allergen-induced bronchoconstriction and eosinophilia in guinea pigs at doses of 1 and 3 mg/kg, p.o. In ferrets, V11294 was not emetogenic at doses up to 30 mg/kg, p.o., despite plasma concentration reaching 10-fold the IC(50) for PDE4. In contrast, rolipram induced severe retching and vomiting at 10 mg/kg, p.o. In conclusion, V11294 is an orally active PDE4 inhibitor that exhibits antiinflammatory activity in vitro, and in vivo at doses that are not emetogenic.

Relaxant effect of YM976, a novel phosphodiesterase 4 inhibitor, on bovine tracheal smooth muscle

Effects of 4-(3-chlorophenyl)-1,7-diethylpyrido[2,3-d]pyrimidin-2(1H)-one (YM976), a novel and selective phosphodiesterase type 4 inhibitor, on tension and adenosine 3',5'-cyclic monophosphate (cAMP) content of bovine tracheal smooth muscle were compared with those of rolipram and theophylline. YM976, rolipram and theophylline relaxed the tracheal preparations contracted with histamine in a concentration-dependent manner. The relaxant effects of YM976 and rolipram were more potent than those of theophylline. These phosphodiesterase inhibitors-induced relaxations were dramatically diminished when tracheal smooth muscle was contracted with methacholine instead of histamine. Pretreatment of the tracheal preparations with YM976 (10 microM) or rolipram (10 microM), but not with theophylline (1 mM), shifted the concentration-response curves for contractile responses to histamine; however, the same procedure failed to affect concentration-response relationships for methacholine-induced contractions. At 1 and 10 microM, both YM976 and rolipram increased the tissues cAMP content. These results suggest that YM976 relaxes tracheal smooth muscle, probably through the cAMP-dependent mechanism.

Molecular docking of competitive phosphodiesterase inhibitors

Mammalian phosphodiesterases types 3 and 4 (PDE3 and PDE4) hydrolyze cAMP and are essential for the regulation of this intracellular second messenger. These enzymes share structural and biochemical similarities, but each can be distinguished by its sensitivity to isoenzyme-specific, substrate-competitive inhibitors. We present a model configuration for the PDE4 substrate (cAMP) and a PDE4-specific inhibitor (rolipram) within the active site of the enzyme. The docked models were also used to examine the structural consequences of mutations that confer resistance to rolipram and other PDE4-specific inhibitors. The proposed rolipram-binding configuration is consistent with the substrate-competitive nature of inhibition and also provides a structural basis for the observed specificity of binding to the R- versus S-enantiomer. For mutations that render the enzyme rolipram-insensitive, there was generally an inverse relationship between the magnitude of the drug resistance and the distance of the altered residue from the predicted binding site. We observed a direct correlation between the net loss of protein residue interactions (van der Waals contacts and hydrogen bond interactions) and the degree of rolipram resistance. The positions of several drug sensitivity-determinant residues define a surface leading to the substrate- and drug-binding sites, suggesting a possible approach channel leading to the enzyme active site. The binding of other PDE4 inhibitors (high- and low-affinity) was also modeled and used to predict the involvement of residues that were not previously implicated in pharmacological interactions.

Bronchodilatory and anti-inflammatory properties of inhaled selective phosphodiesterase inhibitors in a guinea pig model of allergic asthma

In a guinea pig model of allergic asthma, we investigated the effects of the selective phosphodiesterase inhibitors rolipram (phosphodiesterase 4-selective), Org 9935 (phosphodiesterase 3-selective) and Org 20241 (dual phosphodiesterase 4/phosphodiesterase 3-selective), administered by aerosol inhalation in approximately equipotent bronchodilatory doses, on allergen-induced early and late asthmatic reactions, airway hyperreactivity and airway inflammation. Using ovalbumin-sensitized non-challenged animals, different nebulizer concentrations of each inhibitor were tested for their protective effects against histamine-induced bronchoconstriction. Inhalation of 2.5 mM rolipram, 100 mM 4,5-dihydro-6-(5,6-dimethoxybenzo[b]thien-2-yl-5-methyl-3(2H)pyridazinone (Org 9935) and 10 and 100 mM N-hydroxy-4-(3,4-dimethoxyphenyl)-thiazole-2-carboximidamide HCl (Org 20241) provided a similar, 1.8-fold (P<0.01), 2.0-fold (P<0.05), and 1.8- and 1.9-fold (P<0.05) protection, respectively. The duration of these bronchoprotective effects were different, the rate of decline being faster with rolipram and the lower Org 20241 concentration than with Org 9935 and the higher concentration of Org 20241. All compounds strongly protected against the immediate allergen-induced bronchoconstriction and significantly (P<0.05) diminished the overall early asthmatic reaction from 0 to 6 h following allergen-provocation. The severity of the late asthmatic reaction was also significantly inhibited by rolipram (P<0.05) and Org 9935 (P<0.05). Allergen-induced airway hyperreactivity to inhaled histamine after the early reaction, at 6 h after ovalbumin challenge, was strongly reduced by rolipram (P<0.05) and completely prevented by the two other phosphodiesterase inhibitors; in addition, airway hyperreactivity after the late asthmatic reaction, at 24 h, was abolished in all treatment groups. Bronchoalveolar lavage performed at 24 h after allergen challenge revealed no inhibition of eosinophil infiltration in the rolipram-treated animals, whereas inhalation of Org 9935 and the higher-but not the lower-concentration of Org 20241 strongly reduced the influx of these cells. Eosinophil peroxidase activity in the lavage fluid tended to be diminished in all treatment groups but significance was not reached with the exception of the lower concentration of Org 20241. Infiltration of lymphocytes and macrophages was significantly inhibited by Org 9935 only (P<0.05 and P<0.01, respectively), whereas neutrophil influx was not significantly affected. The results indicate that inhalation of phosphodiesterase 3-, phosphodiesterase 4- and dual phosphodiesterase 3/phosphodiesterase 4-selective inhibitors afford protection against acute histamine- and allergen-induced bronchoconstriction and prevent the development of airway hyperreactivity both after the early and late asthmatic reaction predominantly through inhibition of phosphodiesterase 4; in contrast, for significant reduction of eosinophil infiltration, both phosphodiesterase 3 and phosphodiesterase 4 inhibition seems to be required.

Administration of pentoxifylline during allergen sensitization dissociates pulmonary allergic inflammation from airway hyperresponsiveness

Asthma, a chronic inflammatory disease characterized by intermittent, reversible airflow obstruction and airway hyperresponsiveness (AHR), is classically characterized by an excess of Th2 cytokines (IL-13, IL-4) and depletion of Th1 cytokines (IFN-gamma, IL-12). Recent studies indicating an important role for Th1 immunity in the development of AHR with allergic inflammation suggest that Th1/Th2 balance may be important in determining the association of AHR with allergic inflammation. We hypothesized that administration of pentoxifylline (PTX), a phosphodiesterase inhibitor known to inhibit Th1 cytokine production, during allergen (OVA) sensitization and challenge would lead to attenuation of AHR in a murine model of allergic pulmonary inflammation. We found that PTX treatment led to attenuation of AHR when administered at the time of allergen sensitization without affecting other hallmarks of pulmonary allergic inflammation. Attenuation of AHR with PTX treatment was found in the presence of elevated bronchoalveolar lavage fluid levels of the Th2 cytokine IL-13 and decreased levels of the Th1 cytokine IFN-gamma. PTX treatment during allergen sensitization leads to a divergence of AHR and pulmonary inflammation following allergen challenge.

Synthesis and structure-activity relationship of N-arylrolipram derivatives as inhibitors of PDE4 isozymes

Structure activity studies of N-phenylrolipram derivatives have led to the identification of highly potent PDE4 inhibitors. The potential of these inhibitors for cellular activity was routinely assessed in an assay of fMLP induced oxidative burst in human eosinophils. Since first generation PDE4 inhibitors have been plagued with a number of unwanted side effects, parallel structure activity studies for competition with the [3H]-rolipram binding site in rat brain were performed. In this fashion 5-[4-(3-cyclopentyloxy-4-methoxyphenyl)-2-oxo-pyrrolidin-1-yl]-3-(3-methoxybenzyloxy)benzoic acid N',N'-dimethylhydrazide (22) was identified as a potent inhibitor of PDE4 which exhibits >1000 fold selectivity versus PDE3, and is a nanomolar inhibitor in all the cellular assays tested. Studies on the stereoselectivity of PDE4 inhibition of this class of rolipram based compounds revealed, that for example (S)-11 is a more potent inhibitor than (R)-11. This effect can also be observed in primary human cells where the (S)-enantiomer is about 10 fold more potent than the corresponding (R)-enantiomer.

Disease activated drugs: a new concept for the treatment of asthma

Disease activated drugs (DAD) are pro-drugs of one active principle or combinations of two drugs, which have a proven efficacy for the treatment of the target disease. In opposition to pro-drugs, DAD are activated in inflamed but not normal tissues. Due to the disease specific activation, the amount of locally released drug(s) should be related directly to the severity of the inflammation. To test this concept in asthma a PDE4 inhibitor, an isoquinoline derivative, was chemically derivatized into pro-drugs or combined with corticosteroids. These new compounds were more readily cleaved into active PDE4 inhibitor, in bronchoalveolar lavage fluid (BALF) from Brown-Norway rats with lung inflammation than in BALF from rats without airway inflammation. The DAD concept (local selective release and improved therapeutic window) was validated in vivo using the inhibition of methacholine induced bronchoconstriction in guinea pigs with or without ozone induced lung inflammation. An example of DAD hydrolysis (isoquinoline-dexamethasone) was also examined in BALF from asthmatics and healthy volunteers.

Effects of several glucocorticosteroids and PDE4 inhibitors on increases in total lung eosinophil peroxidase (EPO) levels following either systemic or intratracheal administration in sephadex- or ovalbumin-induced inflammatory models

Representative glucocorticosteroids (GCS) and phosphodiesterase IV (PDE4) inhibitors were compared in several models of pulmonary inflammation ranging in severity. Lung tissue eosinophil peroxidase (EPO) levels rather than bronchoalveolar lavage fluid (BALF) EPO or eosinophil percentages were used to indicate eosinophil recruitment after intratracheal instillation of sephadex beads in rats or nebulized ovalbumin in sensitized guinea pigs. A single oral or intratracheal administration of a GCS was effective against mild and robust sephadex-induced eosinophilia whereas the PDE4 inhibitors evaluated appeared more effective in the milder sephadex models. The GCS were also more effective against sephadex-induced than ovalbumin-induced eosinophilia. The effectiveness of the GCS and PDE4 inhibitors improved when the severity of the ovalbumin-induced eosinophilia was decreased. Multiple day dosing also improved activity. These studies indicated that activity was influenced greatly by administration protocols, the severity of the inflammatory response and possibly the method used for estimating eosinophil recruitment.

Quantitation of N-(3,5-dichloropyrid-4-yl)-3-cyclopentyloxy-4-methoxybenzamide and 4-amino-3,5-dichloropyridine in rat and mouse plasma by LC/MS/MS

The metabolism of N-(3,5-dichloropyrid-4-yl)-3-cyclopentyloxy-4-methoxybenzamide (RP73401), a phosphodiesterase IV (PDE IV) inhibitor is extensive (unpublished); however, until recently, studies for this compound did not report 4-amino-3,5-dichloropyridine (ADCP) as a metabolite either in vitro or in vivo. This prompted a reinvestigation into the metabolism of RP73401 in rats and mice using mass spectrometry. The results of the reinvestigation confirmed that 4-amino-3,5-dichloropyridine was formed via the metabolism of RP73401 both in vitro and in vivo. In order to further investigate RP73401 hydrolysis in vivo, a liquid chromatography/mass spectrometry assay was developed and validated for the simultaneous determination of RP73401 and ADCP in rat and mouse plasma. The method used Waters Oasis HLB brand solid phase extraction cartridges to isolate the analytes (RP73401 and ADCP) and internal standard from the plasma. HPLC chromatographic separation was achieved using a Zorbax SB C18 HPLC column and detection was accomplished using positive ion atmospheric pressure chemical ionization tandem mass spectroscopy in multiple reaction monitoring (MRM) mode. The assay was developed and validated over the range of 0.5-100 ng ml(-1) for RP73401 and 5-500 ng ml(-1) for ADCP using 0.050 ml of plasma. The assay proved to be sensitive, accurate, precise and specific for RP73401 and ADCP. Intraday and interday quality control results routinely showed accuracy and precision to be within +/- 20%. This LC/MS/MS method was subsequently employed to investigate the hydrolysis of RP73401 in the rat and mouse, and determine the effects of tri-o-tolyl phosphate (TOTP, a carboxylesterase inhibitor) preadministration on the hydrolysis reaction in the rat.

Effects of inhaled glaucine on pulmonary responses to antigen in sensitized guinea pigs

The alkaloid (S)-(+)-1,2,9,10-tetramethoxyaporphine (glaucine) is a phosphodiesterase 4 inhibitor with bronchodilator and anti-inflammatory activity in vitro. In this study, we examined the in vivo effects of glaucine on an animal model of asthma. In ovalbumin sensitized guinea pigs, inhaled glaucine (10 mg ml(-1), 3 min) inhibited the acute bronchoconstriction produced by aerosol antigen (antigen response was 256+/-42 and 95+/-14 cm H(2)O l(-1) s(-1) in control and glaucine-treated animals, respectively; P<0.05). Pretreatment with glaucine (10 mg ml(-1), 10 min inhalation, 30 min pre- and 3 h post-antigen exposure) markedly reduced airway hyperreactivity to histamine, eosinophil lung accumulation, and increased eosinophil peroxidase activity in bronchoalveolar lavage fluid 24 h after exposure of conscious guinea pigs to aerosol antigen. In addition, inhaled glaucine (5-10 mg ml(-1), 3 min) inhibited the microvascular leakage produced after inhaled antigen at all airway levels. These data support the potential interest of phosphodiesterase 4 inhibitors in asthma treatment.

The inhaled administration of KF19514, a phosphodiesterase 4 and 1 inhibitor, prevents antigen-induced lung inflammation in guinea pigs

We examined in this study the effect of KF19514, a phosphodiesterase 4 and 1 inhibitor, on antigen-induced lung inflammation by inhaled administration in guinea-pigs. It was previously reported that inhaled KF19514 prevented antigen-induced bronchoconstriction and platelet-activating factor (PAF)-induced lung inflammation. In fact, a variety of factors other than PAF are related to lung inflammation in real subjects with asthma. Guinea-pigs were actively sensitized by exposure to ovalbumin (OA). Fifteen to 20 days later, the guinea pigs were challenged by exposure to aerosols of five successively increasing concentrations of OA (0.01, 0.1, 0.5, 1 and 5 mg/ml). Bronchoalveolar lavages (BALs) were performed 24 h after the antigen challenge, and airway hyperresponsiveness to acetylcholine (ACh) was studied 24 h after the challenge by measuring lung resistance and dynamic compliance. Ovalbumin antigen challenge produced a marked and significant eosinophil accumulation in the BAL fluids and airway hyperresponsiveness to ACh 24 h after the challenge. Inhaled KF19514 (0.01-0.1%) inhibited the eosinophil accumulation significantly and dose-dependently but inhaled rolipram (0.01-0.1%) and aminophylline (0.1-1%) did not. In addition, the development of airway hyperresponsiveness was prevented by inhaled KF19514 (0.01%) but not by inhaled rolipram (0.01%) and aminophylline (0.1%). Based on these data, KF19514 was suggested to be a promising drug in the treatment of asthma by local administration to the lung.

Phosphodiesterase 4 inhibitors and the treatment of asthma: where are we now and where do we go from here?

Research conducted over the last 20 years has established that inflammation of the airways is central to the airway dysfunction that characterises asthma. Typically, the airway wall is infiltrated by a variety of cells including mast cells, eosinophils and T lymphocytes, which have deviated towards a T(H)2 phenotype. Together, these cells release a plethora of mediators including interleukin (IL)-4, IL-5, granulocyte/macrophage colony-stimulating factor and eotaxin which ultimately cause the histopathology and symptoms of asthma. Glucocorticosteroids are the only drugs currently available that effectively impact upon this inflammation and resolve, to a greater or lesser extent, compromised lung function. However, steroids are nonselective and generally unsuitable for paediatric use. New drugs are clearly required. One group of potential therapeutic agents for asthma are inhibitors of cyclic AMP-specific phosphodiesterase (PDE), of which theophylline may be considered a prototype. It is now known that PDE is a generic term which refers to at least 11 distinct enzyme families that hydrolyse cAMP and/or cGMP. Over the last decade, inhibitors of PDE4 (a cAMP-specific family that negatively regulates the function of almost all pro-inflammatory and immune cells, and exerts widespread anti-inflammatory activity in animal models of asthma) have been developed with the view to reducing the adverse effects profile associated with non-selective inhibitors such as theophylline. Such is the optimism regarding PDE4 as a viable therapeutic target that more than 100 PDE4 inhibitor patent applications have been filed since 1996 by 13 major pharmaceutical companies. This article reviews the progress of PDE4 inhibitors as anti-inflammatory agents, and identifies problems that have been encountered by the pharmaceutical industry in the clinical development of these drugs and what strategies are being considered to overcome them.

A comparison of the inhibitory activity of PDE4 inhibitors on leukocyte PDE4 activity in vitro and eosinophil trafficking in vivo

1. Phosphodiesterase (PDE) 4 inhibitors have been shown to inhibit eosinophil PDE4 activity in vitro and accumulation of eosinophils in experimental airways inflammation. However, direct effects on eosinophil trafficking have not been studied in detail and it is not known if activity in vitro translates into efficacy in vivo. In the present study, we compared the activity of five PDE4 inhibitors in vitro and against trafficking of (111)In-eosinophils in cutaneous inflammation in the guinea-pig. 2. The rank order of potency for inhibition of PDE4 activity in guinea-pig eosinophil, neutrophil and macrophage, and human neutrophil lysates was RP73401 > SB207499 >CDP840 > rolipram > LAS31025. On TNFalpha production by human PBMC, all inhibitors with the exception of rolipram showed potency similar to their effect on neutrophil lysates. 3. In a brain cerebellum binding assay, the rank order of potency at displacing [3H]-rolipram was RP73401 > rolipram > SB207499 > CDP840 > LAS30125. 4. Trafficking of (111)In-eosinophils to skin sites injected with PAF, ZAP or antigen in sensitized sites was inhibited by oral administration of all PDE4 inhibitors. The rank order of potency was RP73401 = rolipram > LAS31025 > SB207499 > CDP840. 5. With the exception was RP73401, which was the most potent compound in all assays, there was no clear relationship between activity of PDE4 inhibitors in vitro and capacity to inhibit eosinophil trafficking in vivo. Thus, we conclude that in vitro activity of PDE4 inhibitors does not predict in vivo efficacy in an experimental model of eosinophil trafficking.

Phosphodiesterase 4 inhibitors as airways smooth muscle relaxant agents: synthesis and biological activities of triazine derivatives

A series of triazine derivatives was synthesized. The compounds were evaluated for tracheal smooth muscle relaxant and type 4 phosphodiesterase inhibitory activities. A highly significant correlation was observed between the two effects. Two compounds exhibited potent relaxant activity (EC50: 17 and 24 nM) and might be useful for the treatment of asthma.

Identification of inhibitor specificity determinants in a mammalian phosphodiesterase

Mammalian phosphodiesterase types 3 and 4 (PDE3 and PDE4) hydrolyze cAMP and are essential for the regulation of this intracellular second messenger in many cell types. Whereas these enzymes share structural and biochemical similarities, each can be distinguished by its sensitivity to isozyme-specific inhibitors. By using a series of chimeric enzymes, we have localized the region of PDE4 that confers sensitivity to selective inhibitors. This inhibitor specificity domain lies within a short sequence at the carboxyl terminus of the catalytic domain of the protein, consistent with the competitive nature of inhibition by these compounds. Surprisingly, the identified region also includes some of the most highly conserved residues among PDE isoforms. A yeast-based expression system was used for the isolation and characterization of mutations within this area that confer resistance to the PDE4-specific inhibitor rolipram. Analysis of these mutants indicated that both conserved and unique residues are required for isoform-specific inhibitor sensitivity. In some cases, combined point mutations contribute synergistically to the reduction of sensitivity (suppression of IC50). We also report that several mutations display differential sensitivity changes with respect to distinct structural classes of inhibitors.

N-arylrolipram derivatives as potent and selective PDE4 inhibitors

Derivatization of rolipram led to the identification of 3-[4-(3-cyclopentyloxy-4-methoxyphenyl)-2-oxo-pyrrolidin-1-yl]-5-( 3- methoxybenzyloxy)-benzoic acid N',N'-dimethylhydrazide (4), a potent and selective inhibitor of PDE4, which inhibits the activation of human leukocytes with pIC50 values in the range of 7.3-7.8, and blocks antigen induced eosinophilia in Brown Norway rats at a dose of 1 mg/kg (i.t.).

The synthesis and biological evaluation of a novel series of indole PDE4 inhibitors I

This communication describes the synthesis and in vitro evaluation of a novel potent series of phosphodiesterase type (IV) (PDE4) inhibitors. The compounds described contain an indole moiety which replaces the 'rolipram-like' 3-methoxy-4-cyclopentoxy motif. Several of the compounds presented possess low nanomolar IC50's for PDEIV inhibition. In vivo activities determined from measurement of serum TNF-alpha levels in LPS challenged mice (mouse endotoxemia model) are also reported.

Comparison of recombinant human PDE4 isoforms: interaction with substrate and inhibitors

Four cyclic-nucleotide phosphodiesterase (PDE) genes belonging to the PDE4 family (PDE4A, 4B, 4C and 4D) have been identified. All four isogenes, including several deletions and alterations of the amino, carboxyl and central catalytic domains, were expressed in insect cells. Lysates were characterised for enzyme activity by using the Km for substrate and the EC50 for activation by the cofactor Mg2+. The catalytic domain alone appears to be sufficient for the normal enzymatic function of PDE4 proteins. Substrate affinity varied by less than 2-fold between catalytic-domain forms of the PDE4A, 4B and 4D isogenes and the long forms (PDE4A5, PDE4B1 and PDE4D3). The affinity for Mg2+ varied by less than 4-fold between long and catalytic-domain forms of PDE4A and 4B. The catalytic-domain form of PDE4D, however, had a 12-fold lower affinity for Mg2+ that was restored by including a portion of the amino-terminal domain, upstream conserved region-2 (UCR2). This result suggests that the Mg2+-binding site of PDE4D involves the UCR2 region. Inhibition of the PDE4 proteins by synthetic compounds is apparently affected differently by the domains. For PDE4B, the catalytic domain is sufficient for interactions with the inhibitors studied: IBMX, trequinsin, rolipram, TVX 2706, RP 73401 and RS-25344. For PDE4D the catalytic-domain form is less sensitive than the long form to inhibition by RS-25344, rolipram and TVX 2706, by 1463-, 11-and 12-fold, respectively. Addition of UCR2 to the catalytic-domain form of PDE4D restored all the lost sensitivities. The catalytic-domain form of PDE4A showed a reduced inhibitor affinity with RS-25344 and TVX 2706 by 77- and 90-fold, respectively. Both catalytic-domain and long forms of PDE4 isogenes interacted with equal affinity with the non-specific inhibitors IBMX and trequinsin, as well as the very potent PDE4-specific inhibitor RP 73401. Other potent and specific PDE4 inhibitors, such as rolipram, RS-25344 or TVX 2706, appear to utilize non-catalytic domain interactions with PDE4D and 4A to supplement those within the catalytic domains. These observations suggest a different relation between amino and catalytic domains in PDE4D relative to PDE4B. We therefore propose a model to illustrate these isogene-specific PDE4 domain interactions with substrate, inhibitors and the co-factor Mg2+. The model for PDE4D is also discussed in relation to changes in the activation curve for Mg2+ and sensitivity to RS-25344 that accompany phosphorylation of the long form by protein kinase A.