Synthesis of 4-(8-benzo[1,2,5]oxadiazol-5-yl-[1,7]naphthyridine-6-yl)-benzoic acid: a potent and selective phosphodiesterase type 4D inhibitor

PDE4D: A Multipurpose Pharmacological Target

Phosphodiesterase 4 (PDE4) enzymes catalyze cyclic adenosine monophosphate (cAMP) hydrolysis and are involved in a variety of physiological processes, including brain function, monocyte and macrophage activation, and neutrophil infiltration. Among different PDE4 isoforms, Phosphodiesterases 4D (PDE4Ds) play a fundamental role in cognitive, learning and memory consolidation processes and cancer development. Selective PDE4D inhibitors (PDE4Dis) could represent an innovative and valid therapeutic strategy for the treatment of various neurodegenerative diseases, such as Alzheimer's, Parkinson's, Huntington's, and Lou Gehrig's diseases, but also for stroke, traumatic brain and spinal cord injury, mild cognitive impairment, and all demyelinating diseases such as multiple sclerosis. In addition, small molecules able to block PDE4D isoforms have been recently studied for the treatment of specific cancer types, particularly hepatocellular carcinoma and breast cancer. This review overviews the PDE4DIsso far identified and provides useful information, from a medicinal chemistry point of view, for the development of a novel series of compounds with improved pharmacological properties.

Advances in the development of phosphodiesterase-4 inhibitors

Phosphodiesterase 4 (PDE4) hydrolyzes cyclic adenosine monophosphate (cAMP) and plays a vital roles in many biological processes. PDE4 inhibitors have been widely studied as therapeutics for the treatment of various diseases, including asthma, chronic obstructive pulmonary disease (COPD) and psoriasis. Many PDE4 inhibitors have progressed to clinical trials and some have been approved as therapeutic drugs. Although many PDE4 inhibitors have been approved to enter clinical trials, however, the development of PDE4 inhibitors for the treatment of COPD or psoriasis has been hampered by their side effects of emesis. Herein, this review summarizes advances in the development of PDE4 inhibitors over the last ten years, focusing on PDE4 sub-family selectivity, dual target drugs, and therapeutic potential. Hopefully, this review will contribute to the development of novel PDE4 inhibitors as potential drugs.

Furazans in Medicinal Chemistry

Incorporation of heterocycles into drug molecules can enhance physical properties and biological activity. A variety of heterocyclic groups is available to medicinal chemists, many of which have been reviewed in detail elsewhere. Oxadiazoles are a class of heterocycle containing one oxygen and two nitrogen atoms, available in three isomeric forms. While the 1,2,4- and 1,3,4-oxadiazoles have seen widespread application in medicinal chemistry, 1,2,5-oxadiazoles (furazans) are less common. This Review provides a summary of the application of furazan-containing molecules in medicinal chemistry and drug development programs from analysis of both patent and academic literature. Emphasis is placed on programs that reached clinical or preclinical stages of development. The examples provided herein describe the pharmacology and biological activity of furazan derivatives with comparative data provided where possible for other heterocyclic groups and pharmacophores commonly used in medicinal chemistry.

Quinoxaline-Based Scaffolds Targeting Tyrosine Kinases and Their Potential Anticancer Activity

Quinoxaline derivatives, also called benzopyrazines, are an important class of heterocyclic compounds. Quinoxalines have drawn great attention due to their wide spectrum of biological activities. They are considered as an important basis for anticancer drugs due to their potential activity as protein kinase inhibitors. In this review, we focus on the chemistry of the quinoxaline derivatives, the strategies for their synthesis, their potential activities against various tyrosine kinases, and on the structure-activity relationship studies reported to date.

Preparation of Tyrian purple (6,6'-dibromoindigo): past and present

Over the past century, various synthetic approaches have been suggested to the most famous dye of antiquity, Tyrian purple (6,6′-dibromoindigo). These synthetic routes have been exhaustively surveyed and critically evaluated from the perspective of convenience, cost, safety and yield.

The discovery and synthesis of highly potent subtype selective phosphodiesterase 4D inhibitors

The SAR study of a series of 6-aryloxymethyl-8-aryl substituted quinolines is described. Optimization of the series led to the discovery of compound 26b, a highly potent (IC50=0.6 nM) and selective PDE4D inhibitor with a 75-fold selectivity over the A, B, and C subtypes and over 18,000-fold selectivity against other PDE family members. Rat pharmacokinetics and tissue distribution are also summarized.

Synthetic studies on novel benzimidazolopeptides with antimicrobial, cytotoxic and anthelmintic potential

Four substituted benzimidazolyl-benzoic/salicylic acids 5-8 were synthesized by interaction of 5,6-dimethyl-/6-nitrobenzimidazoles with diazotized substituted/unsubstituted aminobenzoic acids in the presence of cupric chloride. The coupling of compounds 5-8 with different amino acid ester hydrochlorides/dipeptide/tripeptide/tetrapeptide methyl esters afforded novel benzimidazolopeptide derivatives 5a-f, 6a-h, 7a-g and 8a-g. The structures of all newly synthesized compounds were established on the basis of analytical, IR, (1)H NMR, (13)C NMR and mass spectral data. Selected peptide ester derivatives were further hydrolyzed by using lithium hydroxide (LiOH) to yield corresponding acid derivatives 5b(a)-d(a), 6e(a)-g(a), 7c(a)-e(a) and 8e(a)-g(a). All peptide derivatives were screened for their antimicrobial, anthelmintic and cytotoxic activities. Almost all newly synthesized benzimidazolopeptides have shown moderate to good anthelmintic activity against all three earthworm species and good antimicrobial activity against pathogenic fungal strains Candida albicans and Aspergillus niger, gram negative bacterial strains Pseudomonas aeruginosa and Escherichia coli. Compounds 8g and 8g(a) possessed significant cytotoxic activity against Dalton's lymphoma ascites (DLA) and Ehrlich's ascites carcinoma (EAC) cell lines.

A new chemical tool for exploring the role of the PDE4D isozyme in leukocyte function

Nicotinamide (2) is a potent and selective inhibitor of the PDE4D isozyme and as a chemical tool selectively blocks eosinophil mediator release and chemotaxis thus linking the role of PDE4D to eosinophil function.

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.

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.

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.

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.