Efficacious inhaled PDE4 inhibitors with low emetic potential and long duration of action for the treatment of COPD

Designing inhaled small molecule drugs for severe respiratory diseases: an overview of the challenges and opportunities

INTRODUCTION

Inhaled drugs offer advantages for the treatment of respiratory diseases over oral drugs by delivering the drug directly to the lung, thus improving the therapeutic index. There is an unmet medical need for novel therapies for lung diseases, exacerbated by a multitude of challenges for the design of inhaled small molecule drugs.

AREAS COVERED

The authors review the challenges and opportunities for the design of inhaled drugs for respiratory diseases with a focus on new target discovery, medicinal chemistry, and pharmacokinetic, pharmacodynamic, and toxicological evaluation of drug candidates.

EXPERT OPINION

Inhaled drug discovery is facing multiple unique challenges. Novel biological targets are scarce, as is the guidance for medicinal chemistry teams to design compounds with inhalation-compatible features. It is exceedingly difficult to establish a PK/PD relationship given the complexity of pulmonary PK and the impact of physical properties of the drug substance on PK. PK, PD and toxicology studies are technically challenging and require large amounts of drug substance. Despite the current challenges, the authors foresee that the design of inhaled drugs will be facilitated in the future by our increasing understanding of pathobiology, emerging medicinal chemistry guidelines, advances in drug formulation, PBPK models, and in vitro toxicology assays.

Phosphodiesterase inhibitors and lung diseases

Phosphodiesterase enzymes (PDE) have long been known as regulators of cAMP and cGMP, second messengers involved in various signaling pathways and expressed in a variety of cell types implicated in respiratory diseases such as airway smooth muscle and inflammatory cells making them a key target for the treatment of lung diseases as chronic obstructive pulmonary disease (COPD), asthma, cystic fibrosis, and pulmonary hypertension (PH). The first reported PDE inhibitor was the xanthine, theophylline, described as a non-specific PDE inhibitor and whilst this drug is effective, it also has a range of unwanted side effects. In an attempt to improve the therapeutic window of xanthines, a number of selective PDE inhibitors have been developed for the treatment of respiratory diseases with only the selective PDE 4 inhibitor, roflumilast, being approved for the treatment of severe COPD. However, roflumilast also has a very narrow therapeutic window due to a number of important doses limiting side effects, particularly in the gastrointestinal tract. However, there continues to be research carried out in this field to identify improved selective PDE inhibitors, both by targeting other PDE subtypes (e.g., PDE 7 found in a number of inflammatory and immune cells) and through development of selective PDE inhibitors for pulmonary administration to reduce systemic exposure and improve the side effect profile. This approach has been exemplified by the development of ensifentrine, a dual PDE 3-PDE 4 inhibitor, an inhaled drug that has recently completed two successful Phase III clinical trials in patients with COPD.

Acid-Promoted Direct C-H Carbamoylation at the C-3 Position of Quinoxalin-2(1H)-ones with Isocyanide in Water

Described herein is a concise and practical direct amidation at the C-3 position of quinoxalin-2(1H)-ones through an acid-promoted carbamoylation with isocyanide in water. In this conversion, environmentally friendly water and commercial inexpensive isocyanide were used as a solvent and carbamoylation reagent, respectively. This study not only provides a green and efficient strategy for the construction of 3-carbamoylquinoxalin-2(1H)-one derivatives that can be applied to the synthesis of druglike structures but also expands the application of isocyanide in organic chemistry.

Inhaled Phosphodiesterase Inhibitors for the Treatment of Chronic Obstructive Pulmonary Disease

Phosphodiesterase (PDE) 4 inhibitors prevent the metabolism of cyclic adenosine monophosphate, thereby reducing inflammation. Inhaled PDE4 inhibitors aim to restrict systemic drug exposure to enhance the potential for clinical benefits (in the lungs) versus adverse events (systemically). The orally administered PDE4 inhibitor roflumilast reduces exacerbation rates in the subgroup of chronic obstructive pulmonary disease patients with a history of exacerbations and the presence of chronic bronchitis, but can cause PDE4 related adverse effects due to systemic exposure. CHF6001 is an inhaled PDE4 inhibitor, while inhaled ensifentrine is an inhibitor of both PDE3 and PDE4; antagonism of PDE3 facilitates smooth muscle relaxation and hence bronchodilation. These inhaled PDE inhibitors have both reported positive findings from early phase clinical trials, and have been well tolerated. Longer term trials are needed to firmly establish the clinical benefits of these drugs.

New Avenues for Phosphodiesterase Inhibitors in Asthma

Introduction

Phosphodiesterases (PDEs) are isoenzymes ubiquitously expressed in the lungs where they catalyse cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (GMP), which are fundamental second messengers in asthma, thereby regulating the intracellular concentrations of these cyclic nucleotides, their signaling pathways and, consequently, myriad biological responses. The superfamily of PDEs is composed of 11 families with a distinct substrate specificity, molecular structure and subcellular localization. Experimental studies indicate a possible role in asthma mainly for PDE3, PDE4, PDE5 and PDE7. Consequently, drugs that inhibit PDEs may offer novel therapeutic options for the treatment of this disease.

Areas Covered

In this article, we describe the progress made in recent years regarding the possibility of using PDE inhibitors in the treatment of asthma.

Expert Opinion

Many data indicate the potential benefits of PDE inhibitors as an add-on treatment especially in severe asthma due to their bronchodilator and/or anti-inflammatory activity, but no compound has yet reached the market as asthma treatment mainly because of their limited tolerability. Therefore, there is a growing interest in developing new PDE inhibitors with an improved safety profile. In particular, the research is focused on the development of drugs capable of interacting simultaneously with different PDEs, or to be administered by inhalation. CHF 6001 and RPL554 are the only molecules that currently are under clinical development but there are several new agents with interesting pharmacological profiles. It will be stimulating to assess the impact of such agents on individual treatable traits in specially designed studies.

Access to 2-pyridinylamide and imidazopyridine from 2-fluoropyridine and amidine hydrochloride

Under catalyst-free conditions, an efficient method to synthesize 2-pyridinylamides has been developed, and the protocol uses inexpensive and readily available 2-fluoropyridine and amidine derivatives as the starting materials. Simultaneously, the copper-catalysed approach to imidazopyridine derivatives has been established with high chemoselectivity and regiospecificity. The results suggest that the nitrogen-heterocycles containing iodide substituents can also be compatible for the reaction via the cascade Ullmann-type coupling, and the nucleophilic substitution reaction provides the target products in a one-pot manner.

Inhaled Phosphodiesterase 4 (PDE4) Inhibitors for Inflammatory Respiratory Diseases

PDE4 inhibitors can suppress a variety of inflammatory cell functions that contribute to their anti-inflammatory actions in respiratory diseases like chronic obstructive pulmonary disease (COPD) and asthma. The systemically delivered PDE4 inhibitor roflumilast has been approved for use in a subset of patients with severe COPD with chronic bronchitis and a history of exacerbations. Use of systemically delivered PDE4 inhibitors has been limited by systemic side effects. Inhaled PDE4 inhibitors have been considered as a viable alternative to increase tolerability and determine the maximum therapeutic potential of PDE4 inhibition in respiratory diseases.

Exploring the Role of Water Molecules in the Ligand Binding Domain of PDE4B and PDE4D: Virtual Screening Based Molecular Docking of Some Active Scaffolds

BACKGROUND

The phosphodiesterase (PDE) is a superfamily represented by four genes: PDE4A, B,C, and D which cause the hydrolysis of phosphodiester bond of cAMP to yield inactive AMP. c-AMP catalyzing enzyme is predominant in inflammatory and immunomodulatory cells. Therapy to treat Chronic Obstructive Pulmonary Disease (COPD) with the use of PDE4 inhibitors is highly envisaged.

OBJECTIVE

A molecular docking experiment with large dataset of diverse scaffolds has been performed on PDE4 inhibitors to analyze the role of amino acid responsible for binding and activation of the secondary transmitters. Apart from the general docking experiment, the main focus was to discover the role of water molecules present in the ligand-binding domain.

METHODS

All the compounds were docked in the PDE4B and PDE4D active cavity to produce the free binding energy scores and spatial disposition/orientation of chemical groups of inhibitors around the cavity. Under uniform condition, the experiments were carried out with and without water molecules in the LBD. The exhaustive study was carried out on the Autodock 4.2 software and explored the role of water molecules present in the binding domain.

RESULTS

In presence of water molecule, Roflumilast has more binding affinity (-8.48 Kcal/mol with PDE4B enzyme and -8.91 Kcal/mol with PDE4D enzyme) and forms two hydrogen bonds with Gln443 and Glu369 and amino acid with PDE4B and PDE4D enzymes respectively. While in absence of water molecule its binding affinity has decreased (-7.3 Kcal/mol with PDE4B enzyme and -5.17 Kcal/mol with PDE4D enzyme) as well as no H-bond interactions were observed. Similar observation was made with clinically tested molecules.

CONCLUSION

In protein-ligand binding interactions, appropriate selection of water molecules facilitated the ligand binding, which eventually enhances the efficiency as well as the efficacy of ligand binding.

4-Amino-7,8-dihydro-1,6-naphthyridin-5(6 H)-ones as Inhaled Phosphodiesterase Type 4 (PDE4) Inhibitors: Structural Biology and Structure-Activity Relationships

Rational design of a novel template of naphthyridinones rapidly led to PDE4 inhibitors with subnanomolar enzymatic potencies. X-ray crystallography confirmed the binding mode of this novel template. We achieved compounds with double-digit picomolar enzymatic potencies through further structure-based design by targeting both the PDE4 enzyme metal-binding pocket and occupying the solvent-filled pocket. A strategy for lung retention and long duration of action based on low aqueous solubility was followed. In vivo efficacies were measured in a rat lung neutrophilia model by suspension microspray and dry powder administration. Suspension microspray of potent compounds showed in vivo efficacy with a clear dose-response. Despite sustained lung levels, dry powder administration performed much less well and without proper dose-response, highlighting clear differences between the two formulations. This indicates a deficiency in the low aqueous solubility strategy for long duration lung efficacy.

Inhibition of phosphodiesterase 4 by FCPR16 protects SH-SY5Y cells against MPP+-induced decline of mitochondrial membrane potential and oxidative stress

Phosphodiesterase 4 (PDE4) is a promising target for the treatment of Parkinson's disease (PD). However, the underlying mechanism has not yet been well elucidated. Additionally, most of current PDE4 inhibitors produce severe nausea and vomiting response in patients, which limit their clinical application. FCPR16 is a novel PDE4 inhibitor with little emetic potential. In the present study, the neuroprotective effect and underlying mechanism of FCPR16 against cellular apoptosis induced by 1-methyl-4-phenylpyridinium (MPP⁺) were examined in SH-SY5Y cells. FCPR16 (12.5–50 μM) dose-dependently reduced MPP⁺-induced loss of cell viability, accompanied by reductions in nuclear condensation and lactate dehydrogenase release. The level of cleaved caspase 3 and the ratio of Bax/Bcl-2 were also decreased after treatment with FCPR16 in MPP⁺-treated cells. Furthermore, FCPR16 (25 μM) significantly suppressed the accumulation of reactive oxygen species (ROS), prevented the decline of mitochondrial membrane potential (Δψm) and attenuated the expression of malonaldehyde level. Further studies disclosed that FCPR16 enhanced the levels of cAMP and the exchange protein directly activated by cAMP (Epac) in SH-SY5Y cells. Western blotting analysis revealed that FCPR16 increased the phosphorylation of cAMP response element-binding protein (CREB) and protein kinase B (Akt) down-regulated by MPP⁺ in SH-SY5Y cells. Moreover, the inhibitory effects of FCPR16 on the production of ROS and Δψm loss could be blocked by PKA inhibitor H-89 and Akt inhibitor KRX-0401. Collectively, these results suggest that FCPR16 attenuates MPP⁺-induced dopaminergic degeneration via lowering ROS and preventing the loss of Δψm in SH-SY5Y cells. Mechanistically, cAMP/PKA/CREB and Epac/Akt signaling pathways are involved in these processes. Our findings indicate that FCPR16 is a promising pre-clinical candidate for the treatment of PD and possibly other oxidative stress-related neuronal diseases.

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FCPR16 protected SH-SY5Y cells against MPP⁺-induced apoptosis.

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FCPR16 attenuated Δψm loss and ROS generation in SH-SY5Y cells treated with MPP⁺.

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FCPR16 activated cAMP/PKA/CREB and Epac/Akt signaling pathways in SH-SY5Y cells.

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Blocking cAMP/PKA/CREB or Epac/Akt pathways canceled the protective role of FCPR16.

Discovery and Optimization of Thiazolidinyl and Pyrrolidinyl Derivatives as Inhaled PDE4 Inhibitors for Respiratory Diseases

Phosphodiesterase 4 (PDE4) is a key cAMP-metabolizing enzyme involved in the pathogenesis of inflammatory disease, and its pharmacological inhibition has been shown to exert therapeutic efficacy in chronic obstructive pulmonary disease (COPD). Herein, we describe a drug discovery program aiming at the identification of novel classes of potent PDE4 inhibitors suitable for pulmonary administration. Starting from a previous series of benzoic acid esters, we explored the chemical space in the solvent-exposed region of the enzyme catalytic binding pocket. Extensive structural modifications led to the discovery of a number of heterocycloalkyl esters as potent in vitro PDE4 inhibitors. (S*,S**)-18e and (S*,S**)-22e, in particular, exhibited optimal in vitro ADME and pharmacokinetics properties and dose-dependently counteracted acute lung eosinophilia in an experimental animal model. The optimal biological profile as well as the excellent solid-state properties suggest that both compounds have the potential to be effective topical agents for treating respiratory inflammatory diseases.

Emerging pharmaceutical therapies for COPD

COPD, for which cigarette smoking is the major risk factor, remains a worldwide burden. Current therapies provide only limited short-term benefit and fail to halt progression. A variety of potential therapeutic targets are currently being investigated, including COPD-related proinflammatory mediators and signaling pathways. Other investigational compounds target specific aspects or complications of COPD such as mucus hypersecretion and pulmonary hypertension. Although many candidate therapies have shown no significant effects, other emerging therapies have improved lung function, pulmonary hypertension, glucocorticoid sensitivity, and/or the frequency of exacerbations. Among these are compounds that inhibit the CXCR2 receptor, mitogen-activated protein kinase/Src kinase, myristoylated alanine-rich C kinase substrate, selectins, and the endothelin receptor. Activation of certain transcription factors may also be relevant, as a large retrospective cohort study of COPD patients with diabetes found that the peroxisome proliferator-activated receptor γ (PPARγ) agonists rosiglitazone and pioglitazone were associated with reduced COPD exacerbation rate. Notably, several therapies have shown efficacy only in identifiable subgroups of COPD patients, suggesting that subgroup identification may become more important in future treatment strategies. This review summarizes the status of emerging therapeutic pharmaceuticals for COPD and highlights those that appear most promising.

New and developing non-adrenoreceptor small molecule drugs for the treatment of asthma

INTRODUCTION

Inhaled corticosteroids (ICS) alone or in combination with an inhaled long-acting beta₂-agonist (LABA) are the preferred long-term treatment for adults and adolescents with symptomatic asthma. Additional drugs include leukotriene-receptor antagonists, slow-release theophylline and the long-acting muscarinic antagonist (LAMA) tiotropium (approved in 2015). There is a need for more effective therapies, as many patients continue to have poorly controlled asthma. Areas covered: New and developing long-acting non-adrenoreceptor synthetic drugs for the treatment of symptomatic chronic asthma despite treatment with an ICS alone or combined with a LABA. Data was reviewed from studies published up until November 2016. Expert opinion: Tiotropium improves lung function and has a modest effect in reducing exacerbations when added to ICS alone or ICS and LABA. The LAMAs umeclidinium and glycopyrronium are under development in fixed dose combination with ICS and LABA. Novel small molecule drugs, such as CRTH2 receptor antagonists, PDE₄ inhibitors, protein kinase inhibitors and nonsteroidal glucocorticoid receptor agonists and 'off-label' use of licensed drugs, such as macrolides and statins are under investigation for asthma, although their effectiveness in clinical practice is not established. To better achieve the goal of developing effective novel small molecule drugs for asthma will require greater understanding of mechanisms of disease and the different phenotypes and endotypes of asthma.

Targeted Delivery of an Anti-inflammatory PDE4 Inhibitor to Immune Cells via an Antibody-drug Conjugate

Phosphodiesterase 4 (PDE4) inhibitors are approved for the treatment of some moderate to severe inflammatory conditions. However, dose-limiting side effects in the central nervous system and gastrointestinal tract, including nausea, emesis, headache, and diarrhea, have impeded the broader therapeutic application of PDE4 inhibitors. We sought to exploit the wealth of validation surrounding PDE4 inhibition by improving the therapeutic index through generation of an antibody-drug conjugate (ADC) that selectively targets immune cells through the CD11a antigen. The resulting ADC consisted of a human αCD11a antibody (based on efalizumab clone hu1124) conjugated to an analog of the highly potent PDE4 inhibitor GSK256066. Both the human αCD11a ADC and a mouse surrogate αCD11a ADC (based on the M17 clone) rapidly internalized into immune cells and suppressed lipololysaccharide (LPS)-induced TNFα secretion in primary human monocytes and mouse peritoneal cells, respectively. In a carrageenan-induced air pouch inflammation mouse model, treatment with the ADC significantly reduced inflammatory cytokine production in the air pouch exudate. Overall, these results provide compelling evidence for the feasibility of delivering drugs with anti-inflammatory activity selectively to the immune compartment via CD11a and the development of tissue-targeted PDE4 inhibitors as a promising therapeutic modality for treating inflammatory diseases.

Novel approaches to the management of noneosinophilic asthma

Noneosinophilic airway inflammation occurs in approximately 50% of patients with asthma. It is subdivided into neutrophilic or paucigranulocytic inflammation, although the proportion of each subtype is uncertain because of variable cut-off points used to define neutrophilia. This article reviews the evidence for noneosinophilic inflammation being a target for therapy in asthma and assesses clinical trials of licensed drugs, novel small molecules and biologics agents in noneosinophilic inflammation. Current symptoms, rate of exacerbations and decline in lung function are generally less in noneosinophilic asthma than eosinophilic asthma. Noneosinophilic inflammation is associated with corticosteroid insensitivity. Neutrophil activation in the airways and systemic inflammation is reported in neutrophilic asthma. Neutrophilia in asthma may be due to corticosteroids, associated chronic pulmonary infection, altered airway microbiome or delayed neutrophil apoptosis. The cause of poorly controlled noneosinophilic asthma may differ between patients and involve several mechanism including neutrophilic inflammation, T helper 2 (Th2)-low or other subtypes of airway inflammation or corticosteroid insensitivity as well as noninflammatory pathways such as airway hyperreactivity and remodelling. Smoking cessation in asthmatic smokers and removal from exposure to some occupational agents reduces neutrophilic inflammation. Preliminary studies of 'off-label' use of licensed drugs suggest that macrolides show efficacy in nonsmokers with noneosinophilic severe asthma and statins, low-dose theophylline and peroxisome proliferator-activated receptor gamma (PPARγ) agonists may benefit asthmatic smokers with noneosinophilic inflammation. Novel small molecules targeting neutrophilic inflammation, such as chemokine (CXC) receptor 2 (CXCR2) antagonists reduce neutrophils, but do not improve clinical outcomes in studies to date. Inhaled phosphodiesterase (PDE)4 inhibitors, dual PDE3 and PDE4 inhibitors, p38MAPK (mitogen-activated protein kinase) inhibitors, tyrosine kinase inhibitors and PI (phosphoinositide) 3kinase inhibitors are under development and these compounds may be of benefit in noneosinophilic inflammation. The results of clinical trials of biological agents targeting mediators associated with noneosinophilic inflammation, such as interleukin (IL)-17 and tumor necrosis factor (TNF)-α are disappointing. Greater understanding of the mechanisms of noneosinophilic inflammation in asthma should lead to improved therapies.

A short review on structure and role of cyclic-3',5'-adenosine monophosphate-specific phosphodiesterase 4 as a treatment tool

Cyclic nucleotide phosphodiesterases (PDEs) are known as a super-family of enzymes which catalyze the metabolism of the intracellular cyclic nucleotides, cyclic-3',5'-adenosine monophosphate (cAMP), and cyclic-3',5'-guanosine monophosphate that are expressed in a variety of cell types that can exert various functions based on their cells distribution. The PDE4 family has been the focus of vast research efforts over recent years because this family is considered as a prime target for therapeutic intervention in a number of inflammatory diseases such as asthma, chronic obstructive pulmonary disease, and rheumatoid arthritis, and it should be used and researched by pharmacists. This is because the major isoform of PDE that regulates inflammatory cell activity is the cAMP-specific PDE, PDE4. This review discusses the relationship between PDE4 and its inhibitor drugs based on structures, cells distribution, and pharmacological properties of PDE4 which can be informative for all pharmacy specialists.

Phosphodiesterase inhibitors for the treatment of asthma and chronic obstructive pulmonary disease

Xanthines like theophylline have long been recognised as being effective drugs for the treatment of asthma and chronic obstructive pulmonary disease (COPD). They are of interest as they possess both anti-inflammatory and bronchodilator activity in the same molecule. Since the discovery of phosphodiesterases (PDEs) in the late 1950s, it has been suggested that xanthines work, in part, by acting as non-selective PDE inhibitors. However, it has also been suggested that the ability of xanthines to non-selectively inhibit PDEs contributes to their many unwanted side effects, thus limiting their use since the arrival of inhaled drugs with more favourable safety profiles. As our understanding of PDEs has improved over the last 30 years, and with the recognition that the distribution of different PDEs varies across different cell types, this family of enzymes has been widely investigated as targets for novel drugs. In particular, PDE3 in airway smooth muscle and PDE4 and PDE7 in inflammatory cells have been targeted to provide new bronchodilators and anti-inflammatory agents, respectively. This review discusses the progress made in this field over the last decade in the development of selective PDE inhibitors to treat COPD and asthma.