Exploration of SAR features by modifications of thiazoleacetic acids as CRTH2 antagonists

Update on the status of DP2 receptor antagonists; from proof of concept through clinical failures to promising new drugs

INTRODUCTION

The identification of PGD2 as the cognate ligand for the DP2 (formerly CRTH2) receptor and the apparent role of that receptor in allergic disease has led to considerable interest in the development of DP2 receptor antagonists for the treatment of asthma. Around 20 DP2 receptor antagonists have progressed into development.

AREAS COVERED

The DP2 antagonists in clinical development and those whose development has been discontinued are discussed in detail. This article highlights the former and examines the available clinical data in respect of both groups of antagonists. It draws upon data that are available from clinical trial registries as well as data that have been presented.

EXPERT OPINION

The unpromising clinical outcomes obtained with setipiprant, vidupiprant and AZD-1981, and the reason why development of OC-459 appears stalled, are all considered. An assessment of the nine DP2 antagonists currently in clinical development highlights the apparent advantages of ADC-3680 and MK-1029. The scope for licensing opportunities in this field is also highlighted.

Comprehensive review in current developments of imidazole-based medicinal chemistry

Imidazole ring is an important five-membered aromatic heterocycle widely present in natural products and synthetic molecules. The unique structural feature of imidazole ring with desirable electron-rich characteristic is beneficial for imidazole derivatives to readily bind with a variety of enzymes and receptors in biological systems through diverse weak interactions, thereby exhibiting broad bioactivities. The related research and developments of imidazole-based medicinal chemistry have become a rapidly developing and increasingly active topic. Particularly, numerous imidazole-based compounds as clinical drugs have been extensively used in the clinic to treat various types of diseases with high therapeutic potency, which have shown the enormous development value. This work systematically gives a comprehensive review in current developments of imidazole-based compounds in the whole range of medicinal chemistry as anticancer, antifungal, antibacterial, antitubercular, anti-inflammatory, antineuropathic, antihypertensive, antihistaminic, antiparasitic, antiobesity, antiviral, and other medicinal agents, together with their potential applications in diagnostics and pathology. It is hoped that this review will be helpful for new thoughts in the quest for rational designs of more active and less toxic imidazole-based medicinal drugs, as well as more effective diagnostic agents and pathologic probes.

CRTH2 Antagonists

Prostaglandin D2 (PGD2) plays a key role in many of the physiological markings of allergic inflammation including vasodilation, bronchoconstriction, vascular permeability and lymphocyte recruitment. The action of this molecule is elicited through its two primary receptors, DP and CRTH2. Activation of CRTH2 leads to lymphocyte chemotaxis, potentiation of histamine release from basophils, production of inflammatory cytokines (IL-4, IL-5 and IL-13) by Th2 cells, eosinophil degranulation and prevention of Th2 cell apoptosis. As such, antagonism of CRTH2 has been reported to ameliorate the symptoms associated with various allergen challenge animal models including murine antigen induced lung inflammation, murine cigarette smoke induced lung inflammation, murine allergic rhinitis, guinea pig PGD2-induced airflow obstruction, guinea pig airway hyper-responsiveness, sheep airway hyper-responsiveness and murine contact hypersensitivity. CRTH2 antagonists fall into four broad categories: tricyclic ramatroban analogues, indole acetic acids, phenyl/phenoxy acetic acids and non-acid-containing tetrahydroquinolines. Numerous CRTH2 antagonists have been advanced into the clinic and early reports from two Phase II trials suggest promising activity in the alleviation of atopic symptoms.

Optimization of phenylacetic acid derivatives for balanced CRTH2 and DP dual antagonists

Our first generation CRTH2 and DP dual antagonists, represented by AMG 009, are more potent toward the CRTH2 receptor than to the DP receptor. Here we report our efforts in the discovery of CRTH2 and DP dual antagonists with more balanced potencies to both receptors, such as compound 15.

Optimization of phenylacetic acid derivatives for CRTH2 and DP selective antagonism

We have previously reported that optimization of a series of phenylacetic acid derivatives led to the discovery of CRTH2 and DP dual antagonists, such as AMG 009 and AMG 853. During the optimization process, we discovered that minor structural modifications also afforded potent and selective CRTH2 or DP antagonists. Here we report the structure-activity relationship that led to the discovery of selective CRTH2 antagonists such as 2 and 17, and selective DP antagonists, such as 4 and 5.

Benzodiazepinone Derivatives as CRTH2 Antagonists

Multiple CRTH2 antagonists are currently evaluated in human clinical trials for asthma and chronic obstructive pulmonary disease (COPD). During our lead optimization for CRTH2 antagonists, an observation of an intramolecular hydrogen bond in ortho-phenylsulfonamido benzophenone derivatives led to the design and synthesis of conformationally constrained benzodiazepinones as potent CRTH2 antagonists. The benzodiazepinones are 2 orders of magnitude more potent than the original flexible bisaryl ethers in our binding assay. Selected benzodiazepinones, such as compound 6, were also potent in the human eosinophil shape change assay. Analysis of the rigid conformations of these benzodiazepinones and ortho-phenylsulfonamido benzophenones provided an explanation for the structure-activity relationship and revealed the possible bound conformations to CRTH2, which may be useful for building a pharmacophore model of CRTH2 antagonists.

Discovery of AMG 853, a CRTH2 and DP Dual Antagonist

Prostaglandin D2 (PGD2) plays a key role in mediating allergic reactions seen in asthma, allergic rhinitis, and atopic dermatitis. PGD2 exerts its activity through two G protein-coupled receptors (GPCRs), prostanoid D receptor (DP or DP1), and chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2 or DP2). We report the optimization of a series of phenylacetic acid derivatives in an effort to improve the dual activity of AMG 009 against DP and CRTH2. These efforts led to the discovery of AMG 853 (2-(4-(4-(tert-butylcarbamoyl)-2-(2-chloro-4-cyclopropylphenyl sulfonamido)phenoxy)-5-chloro-2-fluorophenyl)acetic acid), which is being evaluated in human clinical trials for asthma.

DP(2) receptor antagonists in development

IMPORTANCE OF THE FIELD

In asthmatic lung, allergen challenge leads to the production of large quantities of (prostaglandin D(2)) PGD(2) which both contracts human airway tissue and stimulates an inflammatory response. The identification of PGD(2) as the cognate ligand for a second specific receptor, the DP(2) receptor and the limited inhibition of its pro-inflammatory effects by TP or DP(1) receptor antagonists provide the stimulus to identify, and characterize, selective DP(2) antagonists. This has led to considerable interest in the development of such agents, stimulated by promising initial clinical data.

AREAS COVERED IN THIS REVIEW

The 10 DP(2) antagonists reported to be in clinical development are considered in as much detail as possible from available information. Reported preclinical efforts are also considered and contrasted to more advanced agents.

WHAT THE READER WILL GAIN

A comprehensive overview of the state of the art in the development of DP(2) antagonists for the treatment of asthma, and knowledge of the companies which are currently actively seeking to develop such agents.

TAKE HOME MESSAGE

Considerable progress has been made in the development of selective DP(2) antagonists and initial indications are that they could prove a useful new option in the treatment of asthma. More comprehensive clinical results that will shortly become available will further clarify their therapeutic potential and also indicate the possibility of their use in the treatment of chronic obstructive pulmonary disease.

Novel CRTH2 antagonists: a review of patents from 2006 to 2009

IMPORTANCE OF THE FIELD

The receptor CRTH2 (also known as DP₂) is an important mediator of the inflammatory effects of prostaglandin D₂ and has attracted much attention as a therapeutic target for the treatment of conditions such as asthma, COPD, allergic rhinitis and atopic dermatitis.

AREAS COVERED IN THIS REVIEW

The validation of CRTH2 as a therapeutic target and the early antagonists are summarized, CRTH2 antagonists published in the patent literature from 2006 to 2009 are comprehensively covered and a general update on the recent progress in the development of CRTH2 antagonists for the treatment of inflammatory diseases is provided.

WHAT THE READER WILL GAIN

Insight into the validation of CRTH2 as a therapeutic target, a comprehensive overview of the development of new CRTH2 ligands between 2006 and 2009, and a general overview of the state of the art.

TAKE HOME MESSAGE

Many diverse potent CRTH2 antagonists are now available, and several are in or on the way into the clinic. It is still early to draw final conclusions, but preliminary results give reason for optimism, and the prospect that we will see new CRTH2 antagonists reaching the market for the treatment of asthma, rhinitis, atopic dermatitis and/or COPD seems good.