Current Medicinal Chemistry - Volume 18, Issue 13, 2011

This special issue of Current Medicinal Chemistry focuses on the chemistry and biology of nicotinamide adenine dinucleotide (NAD and NADP). In recent years, NAD(P)-utilizing enzymes have been extensively investigated and implicated in a wide variety of diseases. Thus, understanding of the role of NAD(P) in medical disorders may lead to potential NAD(P)-based therapeutics. NAD(P) is not only a key component of redox reactions but also participates in a diverse range of cellular processes that are crucial for the cell health and replication. There is a growing interest in identification of small molecules as modulators of NAD(P)-dependent enzymes implicated in a variety of diseases. In the first chapter, Felczak and Pankiewicz reviewed a number of conformational and structural factors that might affect (improve) the affinity of various inhibitors to the NAD-binding domain. They discussed potential selectivity of NAD(P)-like molecules toward human IMPdehydrogenase (IMPDH) and other target enzymes, which is crucial for potential application of NAD analogues as therapeutic agents. Hedstrom at al. focused on bacterial IMPDHs as potential target for the development of new antibiotics. These enzymes share only 20-40% of amino acids sequences with the human enzyme. The identification of the structural features in IMPDHs from a wide variety of pathogenic bacteria is described as well as the discovery of an inhibitor against Helicobacter pylori. Sauve and co-workers reviewed biochemical activities of seven human sirtuin isoforms called SIRT1-7 and their roles in biology. These enzymes are involved in post-translational protein modifications, including NAD-dependent histone deacetylation. Strategies for how sirtuins can be targeted by small molecules are discussed. Chen described early sirtuin inhibitors that mimic NAD+ or substrate peptides as well as new structures identified by high-throughput and in silico screenings. His review outlines inhibitor chemotypes, and their biological evaluations, highlighting strategies to enhance inhibition, and selectivity among isoforms. Burgos reviewed the role of nicotinamide phosphoribosyltransferase (NAMPT), the enzyme which catalyzes the first step in the salvage pathway of NAD biosynthesis. The first NAMPT inhibitors entered clinical trials; however, it is clear that better understanding of the catalytic mechanism of NAMPT may permit the design of improved NAMPT inhibitors as potential drugs against cancer. Jayaram and co-workers focused on nicotinamide mononucleotide adenylyltransferease (NMNAT), an enzyme present in all organisms and involved in the biosynthesis of NAD from ATP and nicotinamide mononucleotide (NMN). The role of NMNAT in the regulation of NAD levels in the cell and consequently the enzyme effects on performance of NAD-utilizing enzymes are discussed. Cappellacci at al. described the design, chemistry, and potential therapeutic applications of inhibitors of the two last enzymes in NAD and NADP biosynthesis, i.e. NMNAT and NAD kinase. They concluded that the recognized role of NADKs and NMNATs in pathological conditions like cancer, and neuro-degenerative diseases makes these enzymes excellent targets for drug discovery. Also, bacterial NADK and NaMNAT are promising new targets for developing novel antibiotics. Finally, all chapters indicate enormous potential for innovative research in this field. NAD-based therapeutics are still in the early stage of development but the NAD-related field progresses faster than ever before.

A large number of enzymes that use nicotinamide adenine dinucleotide NAD or its phosphorylated form NADP as a cofactor or substrate were found to play an important role in the growth and reproduction of living organisms. NAD(P)-dependent and NAD(P)- utilizing enzymes [NAD(P)-addicted?] have been extensively investigated and implicated in a wide variety of diseases. NAD, generally considered a key component involved in redox reactions, has been found to participate in a broad spectrum of cellular processes, including signal transduction, DNA repair, and post-translational protein modifications. The reduced form of NADP, i.e. NADPH, guards the cell against oxidative stress and it has been suggested that suppression of NADPH oxidase activity could result in anti-angiogenesis and anticancer effects. Consequently, small molecule NAD(P)-based inhibitors that selectively bind at the NAD(P)-binding domain of the targeted enzyme have been designed for novel treatment of medical disorders. The NAD(P)-binding domain is modular in nature; it can be divided into three sub-sites, the nicotinamide monophosphate (NMN) binding sub-site (N sub-site), the adenosine monophosphate (AMP) binding sub-site (A sub-site), and the pyrophosphate binding sub-site (P sub-site or P-groove). Each sub-site plays an important role in securing proper and tight binding; however, each has its own requirements. In this review we discuss a number of conformational and structural factors that might affect (improve) the affinity of various inhibitors to these sub-sites, as well as to the whole binding domain. We have focused on potential selectivity of NAD(P)-like molecules toward targeted enzymes and their potential application in biology and medicine.

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes the first committed step of guanosine 5'-monophosphate (GMP) biosynthesis, and thus regulates the guanine nucleotide pool, which in turn governs proliferation. Human IMPDHs are validated targets for immunosuppressive, antiviral and anticancer drugs, but as yet microbial IMPDHs have not been exploited in antimicrobial chemotherapy. Selective inhibitors of IMPDH from Cryptosporidium parvum have recently been discovered that display anti-parasitic activity in cell culture models of infection. X-ray crystal structure and mutagenesis experiments identified the structural features that determine inhibitor susceptibility. These features are found in IMPDHs from a wide variety of pathogenic bacteria, including select agents and multiply drug resistant strains. A second generation inhibitor displays antibacterial activity against Helicobacter pylori, demonstrating the antibiotic potential of IMPDH inhibitors.

Since the discovery in 2000 that the yeast sirtuin called “Sir2” catalyzes NAD+ dependent histone deacetylation, a wave of research has focused on evaluating the biochemical and biological functions of sirtuins. Sirtuins are activated by low calorie diets in numerous organisms and are found throughout biology in species from archaea to humans. There are seven human sirtuin isoforms called SIRT1-SIRT7. The biochemical functions of SIRT1, SIRT2, SIRT3, SIRT5 and SIRT6 have been reported and NAD+ dependent deacetylase activities confirmed. In some instances the biological target substrates for each isoform have been identified, helping to connect distinct biological processes to sirtuin regulation. This knowledge has informed potential drug design strategies that target distinct sirtuin isoforms. This review presents current knowledge of biochemical activities of SIRT1-7 in humans and the biological consequences of these sirtuin activities. Regulatory principles that govern sirtuin deacetylation activity in cells are discussed as well as strategies for how sirtuins can be targeted by small molecules. Finally, this review updates research on pharmacologic sirtuin activation and allostery on sirtuins and considers new developments for detection and isolation of sirtuins in complex mixtures.

As members of Class III histone deacetylases (HDACs), sirtuins use stoichiometric nicotinamide adenine dinucleotide (NAD+) to remove the acetyl group from N-acetyl-lysines of histones or non-histone proteins. Sirtuins have been implicated in metabolic diseases, cancer, and neurodegenerative diseases, constituting a promising target for drug discovery. While the early sirtuin inhibitors mimicked NAD+ or substrate peptides, high-throughput and in silico screenings have identified a wide range of core structures, many of which have been subjected to medicinal chemistry efforts. This review outlines inhibitor chemotypes, and their chemical modifications and biological evaluations, highlighting strategies to enhance inhibitory activity and selectivity among isoforms.

Nicotinamide phosphoribosyltransferase (NAMPT) catalyzes the first reversible step in NAD biosynthesis and nicotinamide (NAM) salvage. The enzyme is designed for efficient capture of nicotinamide by coupling of ATP hydrolysis to assist in extraordinary NAM binding affinity and formation of nicotinamide mononucleotide (NMN). NAMPT provides the mechanism to replenish the NAD pool in human metabolism. In addition to its role in redox biochemistry, NAD fuels the sirtuins (SIRTs) to regulate transcription factors involved in pathways linked to inflammation, diabetes and lifespan. NAMPT-mediated lifespan expansion has caused a focus on the catalytic mechanism, regulation and inhibition of NAMPT. Structural, mechanistic and inhibitor design all contribute to a developing but yet incomplete story of NAMPT function. Although the first generation of NAMPT inhibitors has entered clinical trials, disappointing outcomes suggest more powerful and specific inhibitors will be needed. Understanding the ATP-linked mechanism of NAMPT and the catalytic site machinery may permit the design of improved NAMPT inhibitors as more efficient drugs against cancer.

Nicotinamide mononucleotide adenylyltransferease (NMNAT), a rate-limiting enzyme present in all organisms, reversibly catalyzes the important step in the biosynthesis of NAD from ATP and NMN. NAD and NADP are used reversibly in anabolic and catabolic reactions. NAD is necessary for cell survival in oxidative stress and DNA damage. Based on their localization, three different NMNAT's have been recognized, NMNAT-1 (homohexamer) in the nucleus (chromosome 1 p32-35), NMNAT-2 (homodimer) in the cytoplasm (chromosome 1q25) and NMNAT-3 (homotetramer) in the mitochondria. NMNAT also catalyzes the metabolic conversion of potent antitumor prodrugs like tiazofurin and benzamide riboside to their active forms which are analogs of NAD. NAD synthase- NMNAT acts as a chaperone to protect against neurodegeneration, injury-induced axonal degeneration and also correlates with DNA synthesis during cell cycle. Since its activity is rather low in tumor cells it can be exploited as a source for therapeutic targeting. Steps involved in NAD synthesis are being utilized as targets for chemoprevention, radiosensitization and therapy of wide range of diseases, such as cancer, multiple sclerosis, neurodegeneration and Huntington's disease.

Nicotinamide adenine dinucleotide (NAD+) has a crucial role in many cellular processes, both as a coenzyme for redox reactions and as a substrate to donate ADP-ribose units. Thus, enzymes involved in NAD+ metabolism are attractive targets for drug discovery against a variety of human diseases. Herein we focus on two of them: NMN/NaMN adenylyltransferase (NMNAT) and NAD kinase (NADK). NMNAT is a key enzyme in all organisms catalyzing coupling of ATP and NMN or NaMN yielding NAD or NaAD, respectively. NADKs are ubiquitous enzymes involved in the last step of the biosynthesis of NADP. They phosphorylate NAD to produce NADP using ATP (or inorganic polyphosphates) in the presence of Mg2+. No other pathway of NADP biosynthesis has been found in prokaryotic or eukaryotic cells. In this review we provide a comprehensive summary of NMNAT and NADK inhibitors highlighting their chemical modifications by different synthetic approaches, and structure-activity relationships depending on their potential therapeutic applications

As a well-established class, macrolide antibiotics continue to enjoy a remarkable interest within pharmaceutical industry. Several stunning breakthroughs in semi-synthetic study of erythromycin A (EMA) contribute to the important role played by the macrolide class in search for new anti-infectious agents. Earlier structural modifications of EMA to address the issue of acid instability resulted in the first breakthrough in search for anti-infectious agents derived from EMA. Clarithromycin (CAM) and azithromycin (AZM) are two representative antibacterials commercialized during this period. Afterwards, continued research on the modifications of EMA to combat bacterial resistance culminated in the second breakthrough in this field. Telithromycin and cethromycin are two innovative antibacterials discovered in this period for treating community-acquired pneumonia (CAP). Recently, further structural modifications of EMA generate promising antibacterials endlessly, which will hopefully arouse another breakthrough in the near future. In this review, we will give an account of these breakthroughs and discuss the future directions of semi-synthetic research on EMA. In particular, the design and synthesis of some distinguished or promising antibacterials derived from EMA will be highlighted.

In many physiopathological conditions, the cell controls its proper dysfunction via activation of the unfolded protein response to restore efficient protein synthesis and folding in the endoplasmic reticulum. However, whether the aim of unfolded protein response is to promote the cell survival, it can also lead to induction of cell death and then affect the cell fate. Recently, endoplasmic reticulum stress appeared to be critical for acute as well as chronic diseases including neurodegeneration, cardiac disease, cancer, obesity, type 2 diabetes, and ischemia/reperfusion injury. Therefore, inhibition of the endoplasmic reticulum stress could constitute a promising therapeutic strategy to limit cellular damage in pathologies such as hepatic ischemia/reperfusion.

Cyclin dependent kinases (CDKs) are a family of proteins involved in the regulation of cell cycle progression and attractive targets in oncology. The regulation of CDKs activities is achieved by their association with cyclin partners and kinases, phosphatases and specific inhibitors. Different CDKs complexes exert their functions at different phases. CDK1 is a master modulator in the initiation and transition process through mitosis of the cell cycle. Previous studies have shown that loss of CDK1 activity or the aberrant expression of CDK1 involved in G2 phase arrest and many tumor types, thereby validating CDK1 as a therapeutic target. Therefore, a surge of interest has been devoted to searching for potent CDK1 inhibitors as effective chemotherapeutic agents. Herein we focus, in this review, mainly on the studies about the structure, functions and different structure classes of potent CDK1 inhibitors.