Biarylcarboxylic acids and -amides: inhibition of phosphodiesterase type IV versus [3H]rolipram binding activity and their relationship to emetic behavior in the ferret

An Overview of PDE4 Inhibitors in Clinical Trials: 2010 to Early 2022

Since the early 1980s, phosphodiesterase 4 (PDE4) has been an attractive target for the treatment of inflammation-based diseases. Several scientific advancements, by both academia and pharmaceutical companies, have enabled the identification of many synthetic ligands for this target, along with the acquisition of precise information on biological requirements and linked therapeutic opportunities. The transition from pre-clinical to clinical phase was not easy for the majority of these compounds, mainly due to their significant side effects, and it took almost thirty years for a PDE4 inhibitor to become a drug i.e., Roflumilast, used in the clinics for the treatment of chronic obstructive pulmonary disease. Since then, three additional compounds have reached the market a few years later: Crisaborole for atopic dermatitis, Apremilast for psoriatic arthritis and Ibudilast for Krabbe disease. The aim of this review is to provide an overview of the compounds that have reached clinical trials in the last ten years, with a focus on those most recently developed for respiratory, skin and neurological disorders.

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.

The Molecular Biology of Phosphodiesterase 4 Enzymes as Pharmacological Targets: An Interplay of Isoforms, Conformational States, and Inhibitors

The phosphodiesterase 4 (PDE4) enzyme family plays a pivotal role in regulating levels of the second messenger cAMP. Consequently, PDE4 inhibitors have been investigated as a therapeutic strategy to enhance cAMP signaling in a broad range of diseases, including several types of cancers, as well as in various neurologic, dermatological, and inflammatory diseases. Despite their widespread therapeutic potential, the progression of PDE4 inhibitors into the clinic has been hampered because of their related relatively small therapeutic window, which increases the chance of producing adverse side effects. Interestingly, the PDE4 enzyme family consists of several subtypes and isoforms that can be modified post-translationally or can engage in specific protein-protein interactions to yield a variety of conformational states. Inhibition of specific PDE4 subtypes, isoforms, or conformational states may lead to more precise effects and hence improve the safety profile of PDE4 inhibition. In this review, we provide an overview of the variety of PDE4 isoforms and how their activity and inhibition is influenced by post-translational modifications and interactions with partner proteins. Furthermore, we describe the importance of screening potential PDE4 inhibitors in view of different PDE4 subtypes, isoforms, and conformational states rather than testing compounds directed toward a specific PDE4 catalytic domain. Lastly, potential mechanisms underlying PDE4-mediated adverse effects are outlined. In this review, we illustrate that PDE4 inhibitors retain their therapeutic potential in myriad diseases, but target identification should be more precise to establish selective inhibition of disease-affected PDE4 isoforms while avoiding isoforms involved in adverse effects. SIGNIFICANCE STATEMENT: Although the PDE4 enzyme family is a therapeutic target in an extensive range of disorders, clinical use of PDE4 inhibitors has been hindered because of the adverse side effects. This review elaborately shows that safer and more effective PDE4 targeting is possible by characterizing 1) which PDE4 subtypes and isoforms exist, 2) how PDE4 isoforms can adopt specific conformations upon post-translational modifications and protein-protein interactions, and 3) which PDE4 inhibitors can selectively bind specific PDE4 subtypes, isoforms, and/or conformations.

New insights on the arylpiperazinylalkyl pyridazinone ET1 as potent antinociceptive and anti-inflammatory agent

Pyridazine derivatives, such as arylpiperazinylalkyl pyridazinones, display antinociceptive effects to thermal and chemical stimuli. Here, we extended our previous knowledge on the pharmacological profile of 4-amino-6-methyl-2-(3-(4-(4-methylcyclohexa-1,3-dien-1-yl)piperazin-1-yl)propyl)-5-vinylpyridazin-3(2H)-one, here referred as ET1, paving the way for the comprehension of its complete mechanism of action. To this aim, we have evaluated the mouse behavioural responses in several animal models of pain, the effect of ET1 in the murine model of zymosan-induced paw oedema and air-pouch, assessing the cytokines and the cellular phenotype and finally, an in vitro radioligand binding study was performed on a panel of 30 different receptors. In the formalin test, ET1 reduced both neurogenic and inflammatory phase of nociception induced by the aldehyde. Similarly, ET1 strongly reduced paw licking response in the capsaicin test, the abdominal stretching in the writhing test and the carrageenan-induced thermal hyperalgesia. ET1 also evoked a long-lasting reduction of thermal hyperalgesia. Furthermore, ET1 produced a long-lasting anti-inflammatory effect in the zymosan-induced mouse paw oedema and air-pouch through the selective inhibition of inflammatory monocytes recruitment and the modulation of IL-1β, IL-6, TNF-α and MCP-1. Binding experiments confirmed an inhibitory effect on adrenergic α_1A, α_1B and α_2A receptors subtypes and, for the first time, a moderate affinity was observed for the following receptors: histamine H₁, imidazoline I₂, sigma non-opioid intracellular receptor 1 and σ2. These results prompt ET1 as a potent analgesic and anti-inflammatory agent, and support the possibility that it may be suitable for clinical applications in a wide-range of inflammatory-based diseases.

Development of the Neuroimmune Modulator Ibudilast for the Treatment of Alcoholism: A Randomized, Placebo-Controlled, Human Laboratory Trial

Current directions in medication development for alcohol use disorder (AUD) emphasize the need to identify novel molecular targets and efficiently screen new compounds aimed at those targets. Ibudilast (IBUD) is a neuroimmune modulator that inhibits phosphodiesterase-4 and -10 and macrophage migration inhibitory factor and was recently found to reduce alcohol intake in rats by ∼50%. To advance medication development for AUD, the present study consists of a randomized, crossover, double-blind, placebo-controlled laboratory study of IBUD in nontreatment-seeking individuals with current (ie, past month) mild-to-severe AUD. This study tested the safety, tolerability, and initial human laboratory efficacy of IBUD (50 mg b.i.d.) on primary measures of subjective response to alcohol as well as secondary measures of cue- and stress-induced changes in craving and mood. Participants (N=24) completed two separate 7-day intensive outpatient protocols that included daily visits for medication administration and testing. Upon reaching a stable target dose of IBUD (or matched placebo), participants completed a stress-exposure session (day 5; PM), an alcohol cue-exposure session (day 6; AM), and an i.v. alcohol administration session (day 6; PM). Participants stayed overnight after the alcohol administration, and discharge occurred on day 7 of the protocol. Medication conditions were separated by a washout period that was ⩾7 days. IBUD was well tolerated; however, there were no medication effects on primary measures of subjective response to alcohol. IBUD was associated with mood improvements on the secondary measures of stress exposure and alcohol cue exposure, as well as reductions in tonic levels of craving. Exploratory analyses revealed that among individuals with higher depressive symptomatology, IBUD attenuated the stimulant and mood-altering effects of alcohol as compared with placebo. Together, these findings extend preclinical demonstrations of the potential utility of IBUD for the treatment of AUD and suggest that depressive symptomatology should be considered as a potential moderator of efficacy for pharmacotherapies with neuroimmune effects, such as IBUD.

Crystal structure of an apremilast ethanol hemisolvate hemihydrate solvatomorph

The title compound, C22H24N2O7S·0.5C2H5OH·0.5H2O {systematic name: (S)-4-acetamido-2-[1-(3-eth-oxy-4-meth-oxy-phen-yl)-2-(methyl-sulfon-yl)eth-yl]iso-indo-line-1,3-dione ethanol hemisolvate hemihydrate}, is a novel solvatomorph of apremilast (AP), which is an inhibitor of phosphodiesterase 4 (PDE4) and is indicated for the treatment of adult patients with active psoriatic arthritis. The asymmetric unit contains one mol-ecule of AP and disordered mol-ecules of ethanol and water, both with half occupancy. The dihedral angle between the planes of the phenyl ring and the iso-indole ring is 67.9 (2)°. Extensive intra- and inter-molecular hydrogen bonds help to stabilize the mol-ecular conformation and sustain the crystal packing.

The PDE4 cAMP-Specific Phosphodiesterases: Targets for Drugs with Antidepressant and Memory-Enhancing Action

The PDE4 cyclic nucleotide phosphodiesterases are essential regulators of cAMP abundance in the CNS through their ability to regulate PKA activity, the phosphorylation of CREB, and other important elements of signal transduction. In pre-clinical models and in early-stage clinical trials, PDE4 inhibitors have been shown to have antidepressant and memory-enhancing activity. However, the development of clinically-useful PDE4 inhibitors for CNS disorders has been limited by variable efficacy and significant side effects. Recent structural studies have greatly enhanced our understanding of the molecular configuration of PDE4 enzymes, especially the "long" PDE4 isoforms that are abundant in the CNS. The new structural data provide a rationale for the development of a new generation of PDE4 inhibitors that specifically act on long PDE4 isoforms. These next generation PDE4 inhibitors may also be capable of targeting the interactions of select long forms with their "partner" proteins, such as RACK1, β-arrestin, and DISC1. They would therefore have the ability to affect cAMP levels in specific cellular compartments and target localized cellular functions, such as synaptic plasticity. These new agents might also be able to target PDE4 populations in select regions of the CNS that are implicated in learning and memory, affect, and cognition. Potential therapeutic uses of these agents could include affective disorders, memory enhancement, and neurogenesis.

Pigs and Ferrets as Models in Toxicology and Biological Safety Assessment

Both the pig and the ferret are used as animal models in toxicology and drug development, with the pig having been subject to a significant increase in usage (particularly in Europe) over the last 5 years. These two species are quite different from primates and dogs, the “standard” nonrodent models. This article seeks to both survey the rationales for and extent of use of the pig and ferret, and to highlight consideration and key factors in their use in studies.

Identification and in vivo evaluation of a fluorine-18 rolipram analogue, [(18) F]MNI-617, as a radioligand for PDE4 imaging in mammalian brain

Phosphodiesterase (PDE) 4 is the most prevalent PDE in the central nervous system (CNS) and catalyzes hydrolysis of intracellular cAMP, a secondary messenger. By therapeutic inhibition of PDE4, intracellular cAMP levels can be stabilized, and the symptoms of psychiatric and neurodegenerative disorders including depression, memory loss and Parkinson's disease can be ameliorated. Radiotracers targeting PDE4 can be used to study PDE4 density and function, and evaluate new PDE4 therapeutics, in vivo in a non-invasive way, as has been shown using the carbon-11 labeled PDE4 inhibitor R-(-)-rolipram. Herein we describe a small series of rolipram analogs that contain fluoro- or iodo-substituents that could be used as fluorine-18 PET or iodine-123 SPECT PDE4 radiotracers. This series was evaluated with an in vitro binding assay and a 4-(fluoromethyl) derivative of rolipram, MNI-617, was identified, with a five-fold increase in affinity for PDE4 (Kd  = 0.26 nM) over R-(-)-rolipram (Kd  = 1.6 nM). A deutero-analogue d2 -[(18) F]MNI-617 was radiolabeled and produced in 23% yield with high (>5 Ci/µmol) specific activity and evaluated in non-human primate, where it rapidly entered the brain, with SUVs between 4 and 5, and with a distribution pattern consistent with that of PDE4.

CHF6001 II: a novel phosphodiesterase 4 inhibitor, suitable for topical pulmonary administration--in vivo preclinical pharmacology profile defines a potent anti-inflammatory compound with a wide therapeutic window

CHF6001 [(S)-3,5-dichloro-4-(2-(3-(cyclopropylmethoxy)-4-(difluoromethoxy)phenyl)-2-(3-(cyclopropylmethoxy)-4-(methylsulfonamido)benzoyloxy)ethyl)pyridine 1-oxide] is a novel phosphodiesterase 4 (PDE4) inhibitor designed for use in pulmonary diseases by inhaled administration. Intratracheal administration of CHF6001 to ovalbumin-sensitized Brown-Norway rats suppressed the antigen-induced decline of lung functions (ED50 = 0.1 µmol/kg) and antigen-induced eosinophilia (ED50 = 0.03 µmol/kg) when administered (0.09 μmol/kg) up to 24 hours before antigen challenge, in agreement with CHF6001-sustained lung concentrations up to 72 hours after intratracheal treatment (mean residence time 26 hours). Intranasal, once daily administration of CHF6001 inhibited neutrophil infiltration observed after 11 days of tobacco smoke exposure in mice, both upon prophylactic (0.15-0.45 µmol/kg per day) or interventional (0.045-0.45 µmol/kg per day) treatment. CHF6001 was ineffective in reversing ketamine/xylazine-induced anesthesia (a surrogate of emesis in rat) up to 5 µmol/kg administered intratracheally, a dose 50- to 150-fold higher than anti-inflammatory ED50 observed in rats. When given topically to ferrets, no emesis and nausea were evident up to 10 to 20 µmol/kg, respectively, whereas the PDE4 inhibitor GSK-256066 (6-[3-(dimethylcarbamoyl)phenyl]sulfonyl-4-(3-methoxyanilino)-8-methylquinoline-3-carboxamide) induced nausea at 1 µmol/kg intratracheally. A 14-day inhalation toxicology study in rats showed a no-observed-adverse-effect level dose of 4.4 µmol/kg per day for CHF6001, lower than the 0.015 μmol/kg per day for GSK-256066. CHF6001 was found effective and extremely well tolerated upon topical administration in relevant animal models, and may represent a step forward in PDE4 inhibition for the treatment of asthma and chronic obstructive respiratory disease.

Inhibition of phosphodiesterase 4 reduces ethanol intake and preference in C57BL/6J mice

Some anti-inflammatory medications reduce alcohol consumption in rodent models. Inhibition of phosphodiesterases (PDE) increases cAMP and reduces inflammatory signaling. Rolipram, an inhibitor of PDE4, markedly reduced ethanol intake and preference in mice and reduced ethanol seeking and consumption in alcohol-preferring fawn-hooded rats (Hu et al., 2011; Wen et al., 2012). To determine if these effects were specific for PDE4, we compared nine PDE inhibitors with different subtype selectivity: propentofylline (nonspecific), vinpocetine (PDE1), olprinone, milrinone (PDE3), zaprinast (PDE5), rolipram, mesopram, piclamilast, and CDP840 (PDE4). Alcohol intake was measured in C57BL/6J male mice using 24-h two-bottle choice and two-bottle choice with limited (3-h) access to alcohol. Only the selective PDE4 inhibitors reduced ethanol intake and preference in the 24-h two-bottle choice test. For rolipram, piclamilast, and CDP840, this effect was observed after the first 6 h but not after the next 18 h. Mesopram, however, produced a long-lasting reduction of ethanol intake and preference. In the limited access test, rolipram, piclamilast, and mesopram reduced ethanol consumption and total fluid intake and did not change preference for ethanol, whereas CDP840 reduced both consumption and preference without altering total fluid intake. Our results provide novel evidence for a selective role of PDE4 in regulating ethanol drinking in mice. We suggest that inhibition of PDE4 may be an unexplored target for medication development to reduce excessive alcohol consumption.

PDE4 as a target for cognition enhancement

INTRODUCTION

The second messengers cAMP and cGMP mediate fundamental aspects of brain function relevant to memory, learning, and cognitive functions. Consequently, cyclic nucleotide phosphodiesterases (PDEs), the enzymes that inactivate the cyclic nucleotides, are promising targets for the development of cognition-enhancing drugs.

AREAS COVERED

PDE4 is the largest of the 11 mammalian PDE families. This review covers the properties and functions of the PDE4 family, highlighting procognitive and memory-enhancing effects associated with their inactivation.

EXPERT OPINION

PAN-selective PDE4 inhibitors exert a number of memory- and cognition-enhancing effects and have neuroprotective and neuroregenerative properties in preclinical models. The major hurdle for their clinical application is to target inhibitors to specific PDE4 isoforms relevant to particular cognitive disorders to realize the therapeutic potential while avoiding side effects, in particular emesis and nausea. The PDE4 family comprises four genes, PDE4A-D, each expressed as multiple variants. Progress to date stems from characterization of rodent models with selective ablation of individual PDE4 subtypes, revealing that individual subtypes exert unique and non-redundant functions in the brain. Thus, targeting specific PDE4 subtypes, as well as splicing variants or conformational states, represents a promising strategy to separate the therapeutic benefits from the side effects of PAN-PDE4 inhibitors.

The absence of mrp4 has no effect on the recruitment of neutrophils and eosinophils into the lung after LPS, cigarette smoke or allergen challenge

The multidrug resistance protein 4 (Mrp4) is an ATP-binding cassette transporter that is capable of exporting the second messenger cAMP from cells, a process that might regulate cAMP-mediated anti-inflammatory processes. However, using LPS- or cigarette smoke (CS)-inflammation models, we found that neutrophil numbers in the bronchoalveolar lavage fluid (BALF) were similar in Mrp4^−/− and Mrp4^+/+ mice treated with LPS or CS. Similarly, neutrophil numbers were not reduced in the BALF of LPS-challenged wt mice after treatment with 10 or 30 mg/kg of the Mrp1/4 inhibitor MK571. The absence of Mrp4 also had no impact on the influx of eosinophils or IL-4 and IL-5 levels in the BALF after OVA airway challenge in mice sensitized with OVA/alum. LPS-induced cytokine release in whole blood ex vivo was also not affected by the absence of Mrp4. These data clearly suggest that Mrp4 deficiency alone is not sufficient to reduce inflammatory processes in vivo. We hypothesized that in combination with PDE4 inhibitors, used at suboptimal concentrations, the anti-inflammatory effect would be more pronounced. However, LPS-induced neutrophil recruitment into the lung was no different between Mrp4^−/− and Mrp4^+/+ mice treated with 3 mg/kg Roflumilast. Finally, the single and combined administration of 10 and 30 mg/kg MK571 and the specific breast cancer resistance protein (BCRP) inhibitor KO143 showed no reduction of LPS-induced TNFα release into the BALF compared to vehicle treated control animals. Similarly, LPS-induced TNFα release in murine whole blood of Mrp4^+/+ or Mrp4^−/− mice was not reduced by KO143 (1, 10 µM). Thus, BCRP seems not to be able to compensate for the absence or inhibition of Mrp4 in the used models. Taken together, our data suggest that Mrp4 is not essential for the recruitment of neutrophils into the lung after LPS or CS exposure or of eosinophils after allergen exposure.

The therapeutic profile of rolipram, PDE target and mechanism of action as a neuroprotectant following spinal cord injury

The extent of damage following spinal cord injury (SCI) can be reduced by various neuroprotective regimens that include maintaining levels of cyclic adenosine monophosphate (cyclic AMP), via administration of the phosphodiesterase 4 (PDE4) inhibitor Rolipram. The current study sought to determine the optimal neuroprotective dose, route and therapeutic window for Rolipram following contusive SCI in rat as well as its prominent PDE target and putative mechanism of protection. Rolipram or vehicle control (10% ethanol) was given subcutaneously (s.c.) daily for 2 wk post-injury (PI) after which the preservation of oligodendrocytes, neurons and central myelinated axons was stereologically assessed. Doses of 0.1 mg/kg to 1.0 mg/kg (given at 1 h PI) increased neuronal survival; 0.5 mg to 1.0 mg/kg protected oligodendrocytes and 1.0 mg/kg produced optimal preservation of central myelinated axons. Ethanol also demonstrated significant neuronal and oligo-protection; though the preservation provided was significantly less than Rolipram. Subsequent use of this optimal Rolipram dose, 1.0 mg/kg, via different routes (i.v., s.c. or oral, 1 h PI), demonstrated that i.v. administration produced the most significant and consistent cyto- and axo- protection, although all routes were effective. Examination of the therapeutic window for i.v. Rolipram (1.0 mg/kg), when initiated between 1 and 48 h after SCI, revealed maximal neuroprotection at 2 h post-SCI, although the protective efficacy of Rolipram could still be observed when administration was delayed for up to 48 h PI. Importantly, use of the optimal Rolipram regimen significantly improved locomotor function after SCI as measured by the BBB score. Lastly we show SCI-induced changes in PDE4A, B and D expression and phosphorylation as well as cytokine expression and immune cell infiltration. We demonstrate that Rolipram abrogates SCI-induced PDE4B1 and PDE4A5 production, PDE4A5 phosphorylation, MCP-1 expression and immune cell infiltration, while preventing post-injury reductions in IL-10. This work supports the use of Rolipram as an acute neuroprotectant following SCI and defines an optimal administration protocol and target for its therapeutic application.

Harnessing the clinical efficacy of phosphodiesterase 4 inhibitors in inflammatory lung diseases: dual-selective phosphodiesterase inhibitors and novel combination therapies

Phosphodiesterase (PDE) 4 inhibitors have been in development as a novel anti-inflammatory therapy for more than 20 years, with asthma and chronic obstructive pulmonary disease (COPD) being primary indications. Despite initial optimism, only one selective PDE4 inhibitor, roflumilast (Daxas (®)), has been approved for use in humans and available in Canada and the European Union in 2011 for the treatment of a specific population of patients with severe COPD. In many other cases, the development of PDE4 inhibitors of various structural classes has been discontinued due to lack of efficacy and/or dose-limiting adverse events. Indeed, for many of these compounds, it is likely that the maximum tolerated dose is either subtherapeutic or at the very bottom of the efficacy dose-response curve. Thus, a significant ongoing challenge that faces the pharmaceutical industry is to synthesize compounds with therapeutic ratios that are superior to roflumilast. Several strategies are being considered, but clinically effective compounds with an optimal pharmacophore have not, thus far, been reported. In this chapter, alternative means of harnessing the clinical efficacy of PDE4 inhibitors are described. These concepts are based on the assumption that additive or synergistic anti-inflammatory effects can be produced with inhibitors that target either two or more PDE families or with a PDE4 inhibitor in combination with other anti-inflammatory drugs such as a glucocorticoid.

Design, synthesis, and evaluation of 2-aryl-7-(3',4'-dialkoxyphenyl)-pyrazolo[1,5-a]pyrimidines as novel PDE-4 inhibitors

Described herein is design, synthesis, and biological evaluation of novel series of 2-aryl-7-(3',4'-dialkoxyphenyl)-pyrazolo[1,5-a]pyrimidines acting as inhibitors of type 4 phosphodiesterase (PDE4) which is known as a good target for the treatment of asthma and COPD. For this purpose, structure optimization was conducted with the aid of structure-based drug design using the known X-ray crystallography. Also, biological effects of these compounds on the target enzyme were evaluated by using in vitro assays, leading to the potent and selective PDE-4 inhibitor (IC(50)<10nM).

Phosphodiesterase type 4 inhibitors: potential in the treatment of multiple sclerosis?

Phosphodiesterases (PDEs) are involved in the regulation of intracellular levels of the second messengers cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). These enzymes hydrolyse the cyclic nucleotides to the corresponding nucleoside 5'-monophosphates. Nine PDE subtypes have been identified; these differ in their substrate specificity and mode of activation. The type 4 PDE (PDE(4)) hydrolyses cAMP, is activated by elevated levels of cAMP, and is inhibited by rolipram. Inhibition of enzyme activity has been shown to modulate the activity of cells of the immune system. The production of tumour necrosis factor (TNF)(alpha) by activated monocytes and macrophages is inhibited, and cytokine secretion and proliferation of type 1 T helper cells are suppressed. Both immune cell activation and their concomitant induction of cytokine secretion are implicated in multiple sclerosis (MS), which is the major demyelinating disease of the central nervous system. Studies with the selective PDE(4) inhibitor rolipram in experimental autoimmune encephalomyelitis (an animal model of MS) in mice, rats and nonhuman primates have demonstrated the efficacy of the compound in this disease model, suggesting that PDE(4) inhibitors could ameliorate the clinical course of MS. Unfortunately, clinical trials with PDE(4) inhibitors revealed the major adverse effects of these drugs, namely nausea and vomiting. However, novel PDE(4) inhibitors, which target only a subpopulation of PDE(4) enzymes, may provoke fewer adverse effects. The efficacy of a PDE(4) inhibitor in MS still needs to be demonstrated in a well designed clinical trial.

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.

Peripheral phosphodiesterase 4 inhibition produced by 4-[2-(3,4-Bis-difluoromethoxyphenyl)-2-[4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropan-2-yl)-phenyl]-ethyl]-3-methylpyridine-1-oxide (L-826,141) prevents experimental autoimmune encephalomyelitis

Administration of phosphodiesterase 4 (PDE4) inhibitors suppresses the pathogenesis associated with experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). In the present study, we compared the effects of rolipram and 4-[2-(3,4-bis-difluoromethoxyphenyl)-2-[4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropan-2-yl)-phenyl]-ethyl]-3-methylpyridine-1-oxide (L-826,141), a novel nonbrain penetrant PDE4 inhibitor, on the onset and severity of clinical signs in a chronic, nonrelapsing/remitting model of EAE. Both rolipram (10 mg/kg p.o.) and L-826,141 (3 mg/kg p.o.) reduced the severity of EAE relative to controls, whereas L-826,141 (3 mg/kg p.o.) also delayed disease onset. To assess whether L-826,141 prevented EAE progression after the first signs of clinical onset, rolipram (10 mg/kg p.o.) or L-826,141 (3 or 30 mg/kg p.o.) were administered 24 h after the first signs of EAE were observed. Only L-826,141 at a dose of 30 mg/kg p.o. significantly decreased the clinical severity of EAE compared with vehicle controls. Immunohistochemical detection of the neuronal activity marker Fos confirmed that L-826,141 did not reach concentrations in the central nervous system sufficient to activate central neurons. Lipopolysaccharide-induced tumor necrosis factor-alpha in whole blood and plasma concentrations of L-826,141 revealed that only the 30-mg/kg dose resulted in levels sufficient to produce a near complete inhibition of PDE4 activity in immune cells. Taken together, these results demonstrate that peripheral PDE4 inhibition, produced by L-826,141, prevents the progression of EAE after the first onset of clinical signs, and suggest that similar compounds may have clinical efficacy in the treatment of MS.

Antidepressant-like effects of PDE4 inhibitors mediated by the high-affinity rolipram binding state (HARBS) of the phosphodiesterase-4 enzyme (PDE4) in rats

RATIONALE

Phosphodiesterase-4 (PDE4) has two conformation states based on rolipram binding, the high-affinity rolipram binding state (HARBS) and the low-affinity rolipram binding state (LARBS); their functions remain to be fully explained.

OBJECTIVE

Experiments were carried out to determine the roles of the HARBS and LARBS in the mediation of antidepressant-like effects on behavior.

MATERIALS AND METHODS

Two animal models sensitive to antidepressant drugs, the forced-swim test (FST), and the differential-reinforcement-of-low-rate (DRL) 72-s operant schedule, were used to examine the antidepressant-like effects of rolipram, CDP840, and piclamilast, PDE4 inhibitors that interact differentially with the HARBS and LARBS, and MEM1018 and MEM1091, two novel PDE4 inhibitors. Drug discrimination vs rolipram and rolipram competition binding assays also were carried out.

RESULTS

In the FST, rolipram and piclamilast, both at 0.1 mg/kg, produced an antidepressant-like effect, i.e., reduced immobility and increased swimming, whereas, 1 mg/kg of CDP840 or 0.5 mg/kg of MEM1018 or MEM1091 was required to produce a similar effect. Consistent with this, only rolipram and piclamilast produced antidepressant-like effects in rats under the DRL schedule of reinforcement, as evidenced by decreased response rates and increased reinforcement rates. In addition, in rats trained to discriminate rolipram from its vehicle, only rolipram and piclamilast substituted. Finally, [(3)H]rolipram and [(3)H]piclamilast binding analysis revealed that CDP840 and the two novel PDE4 inhibitors MEM1018 and MEM1091 exhibited a lower affinity for the HARBS than did rolipram.

CONCLUSION

These results suggest that the HARBS of PDE4 is the primary conformation important for antidepressant-like effects on behavior.

Mesenteritis precedes vasculitis in the rat mesentery after subacute administration of a phosphodiesterase type 4 inhibitor

Inhibitors of phosphodiesterase type 4 (PDE4) are currently exploited as potent drugs for pulmonary diseases. Some PDE4 inhibitors induce necrotizing panarteritis in the mesentery of rats, comparable to spontaneous polyarteritis nodosa in rats and vascular alterations that are induced by various vasoactive compounds, such as fenoldopam and inhibitors of PDE3. The mechanism of toxicity is unknown. In order to investigate the development of arteritis in the splanchnic vasculature of rats, a time-course study was performed with high doses of a compound (BYK169171), specifically inhibiting PDE4. Rats were treated orally for 1-28 days, and alterations in the mesentery were evaluated by histology, morphometry, and immunohistology. As early as 3 days after the onset of treatment, a mesenteritis was found, characterized by macrophage infiltration, fibroblast proliferation, neovascularization, and loss of adipocytes. Incidence and severity of the mesenteritis were low during the first 2 weeks of treatment, but increased with duration of treatment, finally affecting 2/3 of all animals. A segmental necrotizing panarteritis was detected in some rats treated for 21 or 28 days, but always followed a mesenteritis, whereas many animals with mesenteric inflammation did not have vascular lesions. We postulate that PDE4 inhibitors do not cause a primary vasculitis/arteritis in rats, but induce a non-purulent inflammation as the predominant initial toxic effect in the mesentery. This renders their toxic effect distinct from that of PDE3 inhibitors.

Effects of cilomilast, a selective phosphodiesterase 4 inhibitor, on esophageal motility and ph, and orocecal and colonic transit: Two single-center, randomized, double-blind, placebo-controlled, two-part crossover studies in healthy volunteers

BACKGROUND

Phase IIb studies have reported that cilomilast, a selective phosphodiesterase 4 inhibitor being developed for the treatment of chronic obstructive pulmonary disease, is associated with gastrointestinal (GI) adverse effects (AEs) in a small proportion (approximately 5%) of individuals.

OBJECTIVES

The aims of these 2 studies were to investigate the effects of cilomilast 15 mg BID on: (1) lower esophageal sphincter pressure (LESP) and esophageal body motility and pH (study 1); and (2) orocecal and whole-gut transit times (OCTT and WGTT, respectively) (study 2) in healthy volunteers.

METHODS

These 2 randomized, double-blind, placebo-controlled, 2-part crossover studies were conducted at the Neurogastroenterology Unit, Wythenshawe Hospital, Manchester, United Kingdom (study 1) and GlaxoSmithKline, Harlow, United Kingdom (study 2). In study 1, subjects were randomly assigned to receive either cilomilast (15 mg BID) or matched placebo (control) for 7 days (13 doses; subjects were not given the evening dose on day 7), and in study 2, cilomilast (15 mg BID) or matched placebo (control) for 9 days (18 doses) in each of 2 treatment periods. After study drug administration, combined esophageal motility and pH were recorded for 2 hours before and 4 hours after the administration of a standardized meal (2400 kJ [573 kcal]). Sequences of 6 consecutive 5-mL water swallows (separated by 20 seconds) were carried out 60 and 90 minutes (fasting) and 150, 180, 210, 240, 300, and 360 minutes (fed) after study drug administration. OCTT was determined from the increase in breath hydrogen after the meal. WGTT was determined from the time taken to excrete at least 16 of 20 ingested radiopaque markers, ingested as 2 capsules, each containing 10 radiopaque markers, with 240 mL of water. AEs were elicited at specified times throughout each session using nonleading questions, spontaneously reported AEs, and diary cards.

RESULTS

Study 1 enrolled 20 subjects (11 men, 9 women; age range, 20-52 years). Study 2 enrolled 16 subjects (10 men, 6 women; age range, 19-48 years). No clinically significant differences in the amplitude (mean difference in postprandial-preprandial AUC0-t/t, 6.09 mm Hg; 95% CI, -10.66 to 22.84), duration (difference, -0.08 second; 95% CI, -0.54 to 0.37), or velocity of propagation (difference, 0.90 cm/s; 95% CI, -0.66 to 2.46) of esophageal contractions, LESP (difference, -0.39 mm Hg; 95% CI, -5.23 to 4.45), or preprandial or postprandial percentage time pH<4 (median differences: preprandial, 0.47% [95% CI, -0.45 to 1.27]; postprandial, -0.005% [95% CI, -1.30 to 6.27]) were found with cilomilast compared with placebo. No significant differences in OCTT (difference, -0.37 hour; 95% CI, -1.59 to 0.84) or WGTT (difference, -2.96 hours; 95% CI, -20.76 to 14.84) were found with cilomilast compared with controls. In both studies, the most frequently reported AEs with cilomilast use were nausea (8/18 in study 1 and 3/16 in study 2) and headache (8/18 in study 1 and 6/16 in study 2); however, these were generally of mild to moderate intensity. Overall, GI AEs did not correlate with changes in GI motility.

CONCLUSION

The results of these 2 studies suggest that cilomilast was not associated with significant changes in esophageal motility and pH or GI transit in these healthy volunteers.

Phosphodiesterase 4 inhibitor rolipram potentiates the inhibitory effect of calcitonin on osteoclastogenesis

To assess the combination effect of calcitonin and the phosphodiesterase 4 inhibitor rolipram on osteoclastogenesis, adherent cell-depleted bone marrow cells from mouse tibia and femur (ACD-BMCs), which were cultured in the presence of 25 ng/ml colony-stimulating factor 1 (CSF-1) and 100 ng/ml soluble receptor activator of NF-kappaB ligand (sRANKL), were utilized. Calcitonin inhibited formation of tartrate-resistant acid phosphatase-positive multinucleated cells, as mature osteoclasts, by 70% even at 20 pM, whereas rolipram (10 microM) scarcely affected osteoclast formation; in contrast, the combination of both agents led to significant inhibition of multinucleation and pit formation ability of osteoclasts. The combined administration of calcitonin and rolipram attenuated calcitonin receptor mRNA expression in comparison to treatment with either agent alone, whereas expression of RANK and CSF-1 receptor mRNAs was unchanged. Alone, these agents scarcely elevated intracellular cyclic AMP (cAMP) concentration; however, combination treatment with both agents significantly increased cAMP concentration in osteoclast progenitors and osteoclasts. The combination effect was abolished by H-89, an inhibitor of protein kinase A. It appears that rolipram inhibited hydrolysis of cAMP formed by calcitonin in cells and potentiated the inhibitory effect of calcitonin on osteoclastogenesis. The escape phenomenon following calcitonin treatment may also be prevented by concomitant treatment with the phosphodiesterase 4 inhibitor.

Highly potent PDE4 inhibitors with therapeutic potential

The hypothesis that the dose-limiting side effects of PDE4 inhibitors could be mediated via the central nervous system prompted us to design and synthesize a hydrophilic piperidine analog to improve the side effect profile of Ariflo 1, which is an orally active second-generation PDE4 inhibitor. During evaluation of various water-soluble piperidine analogs, 2a-b, 11b-14b, and 17a showed therapeutic potential in cross-species comparison studies. The following three findings were obtained: (1) The hydroxamic acid group, a well known metal chelator, caused a marked increase of inhibitory activity. (2) Water-soluble piperidine analogs lacked the configurational isomerism of Ariflo 1 without loss of inhibitory activity. (3) Replacement of the 4-methoxy residue with a difluoromethoxy residue led to an increase of in vivo potency. Structure-activity relationships are presented. Single-dose rat pharmacokinetic data for 11b, 12b, and 17a are also presented.

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.

Cyclic nucleotide phosphodiesterases and their role in immunomodulatory responses: advances in the development of specific phosphodiesterase inhibitors

The activity of phosphodiesterases (PDEs) is associated with a wide variety of diseases and an intense effort toward the development of specific PDEs inhibitors has been generated for the last years. They are the enzymes responsible for the hydrolysis of intracellular cyclic adenosine and guanosine monophosphate, and their complexity, as well as their different functional role, makes these enzymes a very attractive therapeutic target. This review is focused on the role of PDEs played on immunomodulatory processes and the advance on the development of specific inhibitors, covering PDEs mainly related to the regulation of autoimmune processes, PDE4 and PDE7. The review also highlights the novel structural classes of PDE4 and PDE7 inhibitors, and the therapeutic potential that combined PDE4/PDE7 inhibitors offer as immunomodulatory agents.

Orally active PDE4 inhibitor with therapeutic potential

Based on the promising results obtained by the clinical trial of Ariflo, further optimization of the spatial arrangement of the three pharmacophores (the carboxylic acid moiety, nitrile moiety and 3-cyclopentyloxy-4-methoxyphenyl moiety) in the structure of Ariflo 1 was attempted using a bicyclo[3 ?3 ?0]octane template with more stereochemical diversity than the cyclohexane template of Ariflo 1. Biological evaluation of the decyanated analogs and further optimization of the cyclopentyloxy moiety of 2a-b were also performed. Among the compounds tested, 2a, 7a-b and 12a were found to be orally active and were estimated to have therapeutic potential based on cross-species and same-species comparisons. The structure-activity relationships (SARs) of these compounds were investigated and pharmacokinetic data for 2a and 7b were also obtained by single-dose studies in rats.

Searching for a Reliable Orientation of Ligands in Their Binding Site: Comparison between a Structure-Based (Glide) and a Ligand-Based (FIGO) Approach in the Case Study of PDE4 Inhibitors

Two 3D QSAR Grid/Golpe models, differing in the alignment criterion of the studied phosphodiesterase 4 (PDE4) inhibitors, were compared. The docking-guided alignment, obtained by exploiting the known 3D structure of the PDE4, was used to test and validate the field-fit alignment solution proposed by FIGO procedure. The analysis of the direct (docking) and indirect (FIGO) superposition methodologies occurs through the comparison of the respective PLS coefficient maps. The inclusion in the FIGO algorithm of factors related to the hydrophobicity and shape of the molecules leads to promising results, making the new FIGO algorithm a valid alternative in the molecule overlay, particularly when the 3D structure of the target is unknown.

Discovery of BRL 50481 [3-(N,N-dimethylsulfonamido)-4-methyl-nitrobenzene], a Selective Inhibitor of Phosphodiesterase 7: In Vitro Studies in Human Monocytes, Lung Macrophages, and CD8+T-Lymphocytes

The biochemical and pharmacological characteristics in human proinflammatory cells of BRL 50481 [3-(N,N-dimethylsulfonamido)-4-methyl-nitrobenzene], a novel and selective inhibitor of phosphodiesterase (PDE) 7, are described. BRL 50481 inhibited the activity of hrPDE7A1 expressed in baculovirus-infected Spodoptera frugiperda 9 cells in a competitive manner (Ki value of 180 nM) and was 416 and 1884 times less potent against PDE3 and 38 and 238 times less potent against PDE4 at a substrate concentration of 1 microM and 50 nM cAMP, respectively. Western blotting identified HSPDE7A1 but not HSPDE7A2 in three human cell types that are implicated in the pathogenesis of chronic obstructive lung disease, namely, CD8+ T-lymphocytes, monocytes, and lung macrophages. BRL 50481 had no effect on the proliferation of CD8+ T-lymphocytes and only marginally (approximately 2-11%) reduced the generation of tumor necrosis factor (TNF)alpha from blood monocytes and lung macrophages. However, in the presence of BRL 50481 the inhibitory effect of rolipram was enhanced on all three cell types. The expression of HSPDE7A1 was increased in a time-dependent manner in monocytes that were "aged" in culture medium. Under this condition, BRL 50481 now inhibited TNFalpha generation in a concentration-dependent manner. In aged monocytes, rolipram, Org 9935 (a PDE3 inhibitor), and prostaglandin E2 inhibited TNFalpha generation in a concentration-dependent manner and interacted additively with BRL 50481. BRL 50481 is the first fully documented PDE7 inhibitor that has acceptable selectivity for in vitro studies. Furthermore, although BRL 50481 had only a modest inhibitory effect per se on the proinflammatory cells studied, it acted at least additively with other cAMP-elevating drugs, especially when HSPDE7A1 was up-regulated.

Highly potent PDE4 inhibitors with therapeutic potential

Based on the hypothesis that the dose-limiting side effects of PDE4 inhibitors could be mediated via the central nervous system (CNS), design and synthesis of a hydrophilic analogue is considered to be one approach to improving the side-effect profile of Ariflo 1. Water-soluble piperidine derivatives were found to possess therapeutic potential.

Effects of dialkoxylphenyl compounds with oxime group on macrophage function and the proliferation of lymphocytes

Dialkoxyphenyl compounds have been reported to possess anti-inflammatory activity through inhibition of phosphodieseterase (PDE) type IV. In this study, a series of derivatives of dialkoxyphenyl compounds with an oxime group, which is generally known to be one of the biologically active functional groups, were prepared and evaluated for their ability to inhibit the production of inflammatory mediators in activated macrophages and the proliferation of lymphocytes. The structure-activity relationship (SAR) study with 12 compounds on tumour necrosis factor (TNF)-alpha inhibition, analysed by the oxime geometry and different size of spacers between the oxime and phenyl group, indicated that there might be at least three possible hydrogen bonding sites in the inhibitor binding pocket of PDE IV. Of them, compound 6 clearly displayed the highest inhibitory effect on in-vitro TNF-alpha production from lipopolysaccharide (LPS)-stimulated RAW264.7 cells. Compound 6 also suppressed in-vivo TNF-alpha release from LPS-primed mice, a level comparable with that of the standard PDE IV inhibitor, rolipram. In addition, oxime compounds also significantly inhibited both nitric oxide production from activated RAW264.7 cells and T lymphocyte proliferation elicited by concanavalin A but not IL-2. The data suggest that the oxime group may act as a functional group, capable of interacting with the inhibitor-binding pocket of target PDE IV. Therefore, it is conceivable that compound 6 may have the potential either to be developed as a new anti-inflammatory drug or to be used to develop more potent analogues.

Synthesis and biological evaluation of neutrophilic inflammation inhibitors

In several non-infectious human diseases, such as ulcerous colitis, rheumatoid arthritis, chronic obstructive pulmonary disease (COPD), the extravasal recruitment of neutrophils plays a crucial role in the development of tissue damage, which, when persistent, can lead to the irreversible organ dysfunction. The neutrophil activation is controlled by a number of intracellular pathways, particularly by a cAMP-dependent protein kinase A (PKA) which also acts on phosphodiesterase IV (PDE4) gene stimulating the synthesis of this enzyme, able to transform cAMP to inactive AMP. PDE4 inhibitors enhance intracellular cAMP and decrease inflammatory cell activation. Several 3-cyclopentyloxy-4-methoxybenzaldehyde and 3-cyclopentyloxy-4-methoxybenzoic acid derivatives were synthesized and studied by us to evaluate their ability to inhibit the superoxide anion production in human neutrophils. These compounds were found able to inhibit the neutrophil activation and some of them increased the cAMP level on tumor necrosis factor-alpha-stimulated neutrophils. Moreover, they also inhibited selectively the human PDE4 enzyme, although they are less potent than the reference compound Rolipram. We report here synthesis, biological studies and some SAR considerations concerning the above mentioned compounds.

Phosphodiesterase-4 inhibitors in the treatment of inflammatory lung disease

Phosphodiesterases (PDE) belong to an important family of proteins that regulate the intracellular levels of cyclic nucleotide second messengers. Targeting PDE with selective inhibitors may offer novel therapeutic strategies in the treatment of various conditions, and in the context of respiratory disease these include asthma and chronic obstructive pulmonary disease (COPD). The rationale for such an approach stems, in part, from the clinical efficacy of theophylline, an orally active drug that is purportedly a nonselective PDE inhibitor. In addition, intracellular cyclic adenosine monophosphate (cAMP) levels regulate the function of many of the cells thought to contribute to the pathogenesis of respiratory diseases such as asthma and COPD, and these cells also selectively express PDE4. This has offered pharmaceutical companies the opportunity to selectively targeting these enzymes for the treatment of these diseases. Finally, the success of targeting PDE5 in the treatment of erectile dysfunction provides clinical proof of concept for the targeting of PDE in disease. Whether a 'Viagra' of the airways can be found for the treatment of asthma and COPD remains to be seen, but positive results from recent clinical studies examining the efficacy of selective PDE4 inhibitors such as cilomilast and roflumilast offer some optimism. However, one of the major issues to be resolved is the tolerability profile associated with this drug class that is a consequence of PDE4 inhibition. While cilomilast and roflumilast have low emetic potential they are not free from emesis and various strategies are being investigated in the hope of developing a PDE4 inhibitor without this adverse effect.

Subacute toxicity and toxicokinetics of CJ-10882, a type IV phosphodiesterase inhibitor, after 4-week repeated oral administration in dogs

The subacute toxicity and toxicokinetics of a type IV phosphodiesterase inhibitor, CJ-10882, were evaluated after single (on the 1st day) and 4-week (on the 27th day) oral administration of the drug, in doses of 0 (to serve as a control), 2, 10 and 50 mg/kg/day, to male and female dogs (n=3 for male and female dogs for each dose). During the test period, clinical signs, mortality, body weight, food consumption, ophthalmoscopy, urinalysis, hematology, serum biochemistry, gross findings, organ weight and histopathology were examined. The 4-week repeated oral doses of CJ-10882 resulted in salivation, vomiting, and atrophy of the thymus. The absolute toxic dose was 50 mg/kg/day and the level at which no adverse effects were observed was 2 mg/kg/day for male and female dogs. There were no significant gender differences in the pharmacokinetic parameters of CJ-10882 for each dose after both single and 4-week oral administration. The pharmacokinetic parameters of CJ-10882 were dose independent after a single oral administration; the time to reach a peak plasma concentration (T(max)) and the dose-normalized area under the plasma concentration-time curve from time zero to 8 h in plasma (AUC(0-8 h)) were not significantly different among three doses. The accumulation of CJ-10882 after 4-week oral administration was not notable at the toxic dose of 50 mg/kg/day. For example, after 4-week administration, the dose-normalized AUC(0-8 h) value at 50 mg/kg/day (0.132 microg h/ml) was not significantly greater than that at 10 mg/kg/day (0.131 microg h/ml). After 4-week oral administration, the dose-normalized C(max) and AUC(0-8 h) at 50 mg/kg/day were not significantly higher and greater, respectively, than those after the single oral administration.

Orally active PDE4 inhibitors with therapeutic potential

Based on the successful results in the clinical trial of Ariflo, further optimization of the spatial arrangement of the three pharmacophores (carboxylic acid moiety, nitrile moiety and 3-cyclopentyl-4-methoxyphenyl moiety) in the structure of Ariflo 1 was attempted using a bicyclo[3.3.0]octane template instead of a cyclohexane template. As a result, 2a, 7a and 7b were found to be orally active and were predicted to have an improved therapeutic potential based on evaluation by cross-species and same-species comparisons. Structure-activity relationships (SARs) of these compounds are also discussed.

New orally active PDE4 inhibitors with therapeutic potential

Structural optimization of pyrazolopyridine derivative 2, which is one of the newly discovered chemical leads for PDE4 inhibitors from our in-house library, was carried out successfully. The process of discovery of new orally active PDE4 inhibitors, which are expected to possess therapeutic potential, is presented and their structure-activity relationships are discussed.

Antidepressant-induced increase in high-affinity rolipram binding sites in rat brain: dependence on noradrenergic and serotonergic function

The effects of antidepressant treatment on the high- and low-affinity rolipram binding sites on type 4 phosphodiesterase (PDE4) were determined; previous work had shown that repeated antidepressant treatment increases the overall expression of PDE4. Rats were administered different doses of the antidepressant drugs desipramine or fluoxetine, or saline, for 1, 7, or 14 days. [3H]Rolipram and [3H]piclamilast were used to assess the high-affinity rolipram binding sites (HARBS) and low-affinity rolipram binding sites (LARBS) on PDE4 in the hippocampus and cerebral cortex. Repeated, but not acute, treatment with the antidepressants increased [3H]rolipram binding to membrane fractions in a dose-dependent manner; the HARBS component of [3H]piclamilast binding also was increased by these treatments. By contrast, the LARBS component of [3H]piclamilast binding was not altered. [3H]Rolipram and [3H]piclamilast binding to the cytosolic fractions of rat cerebral cortex and hippocampus was not altered by the antidepressant treatments. 6-Hydroxydopamine (6-OHDA; 300 microg i.c.v.) and 5,7-dihydroxytryptamine (5,7-DHT; 200 microg i.c.v.) were used to lesion noradrenergic and serotonergic neurons, respectively. The effects of desipramine, but not fluoxetine, on [3H]rolipram and [3H]piclamilast binding to rat hippocampal membranes were blocked by the 6-OHDA-induced lesion. By contrast, the effects of fluoxetine, but not desipramine, were reduced by the 5,7-DHT-induced lesion. This indicates that the up-regulation of the HARBS by desipramine and fluoxetine requires the integrity of noradrenergic and serotonergic neurons, respectively. Collectively, these results suggest that antidepressants, although acting through different pathways, may eventually lead to the regulation of components of the cAMP signal transduction system.

Inhibition of osteoclastogenesis by a phosphodiesterase 4 inhibitor XT-611 through synergistic action with endogenous prostaglandin E2

We examined the effect of a phosphodiesterase 4 (PDE4) inhibitor, 3,4-dipropyl-4,5,7,8-tetrahydro-3H-imidazo[1,2-i]-purin-5-one (XT-611) on osteoclast formation in three different mouse bone-marrow cell (BMC) culture systems. We confirmed that selective inhibitors of PDE4, including XT-611, among several PDE inhibitors decreased osteoclast formation in the BMC culture system. XT-611 also inhibited osteoclast formation in co-culture of mouse bone-marrow stromal cell line ST2 and adherent cell-depleted (ACD)-BMCs. However, it did not inhibit osteoclastogenesis in culture of ACD-BMCs alone in the presence of macrophage-colony stimulating factor (M-CSF) and soluble receptor activator of NF-kappaB ligand (sRANKL). XT-611 significantly increased prostaglandin E(2) (PGE(2)) production from ST2 cells and, in combination with PGE(2), synergistically increased cAMP concentration in osteoclast progenitors. In the ST2 co-culture system, XT-611 did not influence the expression of RANKL, osteoprotegerin and RANK mRNAs. By combined treatment with XT-611 and PGE(2) of ACD-BMCs, osteoclast multinucleation was clearly inhibited with decrease in the expression of calcitonin receptor mRNA, while the expression of RANK and c-fms (an M-CSF receptor) mRNAs was unchanged. These results indicate that the PDE4 inhibitor inhibits osteoclastogenesis by acting on osteoclast progenitors synergistically with PGE(2) secreted from stromal cells, but not by influencing the cell-to-cell interaction between stromal cells and osteoclast progenitors.

Improvement of therapeutic index of phosphodiesterase type IV inhibitors as anti-Asthmatics

A new series of catechol hydrazines was synthesized and their structure-activity relationship (SAR) was analyzed for developing an effective phosphodiesterase 4 (PDE4) inhibitor as an anti-asthmatic drug candidate. Among the (E)-Analogues tested using in vitro assays, 5CC showed a strong PDE4 inhibitory activity and a significantly improved rolipram binding profile compared with rolipram, a prototype PDE4 inhibitor. Moreover, from in-vivo asthma model, we observed that (E)-Analogue 5CC had a good efficacy against guinea-pig respiratory tract inflammation and bronchoconstriction, along with a remarkably reduced emetic side effect, compared with rolipram. Conclusively, (E)-Analogue 5CC seems to be a promising candidate for the development of anti-asthmatic PDE4 inhibitors.